How Do We Terraform Saturn’s Moons?

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present our guide to terraforming Saturn’s Moons. Beyond the inner Solar System and the Jovian Moons, Saturn has numerous satellites that could be transformed. But should they be?

Around the distant gas giant Saturn lies a system of rings and moons that is unrivaled in terms of beauty. Within this system, there is also enough resources that if humanity were to harness them – i.e. if the issues of transport and infrastructure could be addressed – we would be living in an age a post-scarcity. But on top of that, many of these moons might even be suited to terraforming, where they would be transformed to accommodate human settlers.

As with the case for terraforming Jupiter’s moons, or the terrestrial planets of Mars and Venus, doing so presents many advantages and challenges. At the same time, it presents many moral and ethical dilemmas. And between all of that, terraforming Saturn’s moons would require a massive commitment in time, energy and resources, not to mention reliance on some advanced technologies (some of which haven’t been invented yet).

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How Do We Terraform Jupiter’s Moons?

Continuing with our “Definitive Guide to Terraforming“, Universe Today is happy to present to our guide to terraforming Jupiter’s Moons. Much like terraforming the inner Solar System, it might be feasible someday. But should we?

Fans of Arthur C. Clarke may recall how in his novel, 2010: Odyssey Two (or the movie adaptation called 2010: The Year We Make Contact), an alien species turned Jupiter into a new star. In so doing, Jupiter’s moon Europa was permanently terraformed, as its icy surface melted, an atmosphere formed, and all the life living in the moon’s oceans began to emerge and thrive on the surface.

As we explained in a previous video (“Could Jupiter Become a Star“) turning Jupiter into a star is not exactly doable (not yet, anyway). However, there are several proposals on how we could go about transforming some of Jupiter’s moons in order to make them habitable by human beings. In short, it is possible that humans could terraform one of more of the Jovians to make it suitable for full-scale human settlement someday.

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The Next Generation of Exploration: The NEOCam Mission

In February of 2014, NASA put out the call for submissions for the thirteenth mission of their Discovery Program. In keeping with the program’s goal of mounting low-cost, highly focused missions to explore the Solar System, the latest program is focused on missions that look beyond Mars to new research goals. On September 30th, 2015, five semifinalists were announced, which included proposals for sending probes back to Venus, to sending orbiters to study asteroids and Near-Earth Objects.

Among the proposed NEO missions is the Near Earth Object Camera, or NEOCam. Consisting of a space-based infrared telescope designed to survey the Solar System for potentially hazardous asteroids, the NEOCam would be responsible for discovering and characterizing ten times more near-Earth objects than all NEOs that have discovered to date.

If deployed, NEOCam will begin discovering approximately one million asteroids in the Main Belt and thousands of comets in the course of its 4 year mission. However, the primary scientific goal of NEOCam is to discover and characterize over two-thirds of the asteroids that are larger that 140 meters, since it is possible some of these might pose a threat to Earth someday.

The NEOCam space telescope will survey the regions of space closest to the Earth's orbit, where potentially hazardous asteroids are most likely to be found. NEOCam will use infrared light to characterize their physical properties such as their diameters. (Image credit: NASA/JPL-Caltech)
Artist’s concept of the NEOCam spacecraft, a proposed mission for NASA’s Discovery program that would search for potentially hazardous near-Earth asteroids. Credit: NASA/JPL-Caltech

The technical term is Potentially Hazardous Objects (PHO), and it applies to near-Earth asteroids/comets that have an orbit that will allow them to make close approaches to Earth. And measuring more than 140 meters in diameter, they are of sufficient size that they could cause significant regional damage if they struck Earth.

In fact, a study conducted in 2010 through the Imperial College of London and Purdue University found that an asteroid measuring 50-meters across with a density of 2.6 grams per cubic centimeter and a speed of 12.7 kps could generate 2.9 Megatons of airburst energy once it passed through our atmosphere. To put that in perspective, that’s the equivalent of about nine W87 thermonuclear warheads!

By comparison, the meteor that appeared over the small Russian community of Chelyabinsk in 2013 measured only 20 meters across. Nevertheless, the explosive airbust caused by it entering our atmosphere generated only 500 kilotons of energy,  creating a zone of destruction tens of kilometers wide and injuring 1,491 people. One can imagine without much effort how much worse it would have been had the explosion been six times as big!

What’s more, as of August 1st, 2015, NASA has listed a total of 1,605 potentially hazardous asteroids and 85 near-Earth comets. Among these, there are 154 PHAs believed to be larger than one kilometer in diameter. This represents a tenfold increase in discoveries since the end of the 1990s, which is due to several astronomical surveys being performed (as well as improvements in detection methods) over the past two and a half decades.

The NEOCam sensor (right) is the lynchpin for the proposed Near Earth Object Camera, or NEOCam, space mission (left). Credit: NASA/JPL-Caltech
The NEOCam sensor (right) is the lynchpin for the proposed Near Earth Object Camera, or NEOCam, space mission (left). Credit: NASA/JPL-Caltech

As a result, monitoring and characterizing which of these objects is likely to pose a threat to Earth in the future has been a scientific priority in recent years. It is also why the U.S. Congress passed the “George E. Brown, Jr. Near-Earth Object Survey Act” in 2005. Also known as the “NASA Authorization Act of 2005”, this Act of Congress mandated that NASA identify 90% of all NEOs that could pose a threat to Earth.

If deployed, NEOCam will monitor NEOs from the Earth–Sun L1 Lagrange point, allowing it to look close to the Sun and see objects inside Earth’s orbit. To this, NEOCam will rely on a single scientific instrument: a 50 cm diameter telescope that operates at two heat-sensing infrared wavelengths, to detect the even the dark asteroids that are hardest to find.

By using two heat-sensitive infrared imaging channels, NEOCam can also make accurate measurements of NEO and gain valuable information about their sizes, composition, shapes, rotational states, and orbits. As Dr. Amy Mainzer, the Principal Investigator of the NEOCam mission,  explained:

“Everyone wants to know about asteroids hitting the Earth; NEOCam is designed to tackle this issue. We expect that NEOCam will discover about ten times more asteroids than are currently known, plus millions of asteroids in the main belt between Mars and Jupiter. By conducting a comprehensive asteroid survey, NEOCam will address three needs: planetary defense, understanding the origins and evolution of our solar system, and finding new destinations for future exploration.”

Dr. Mainzer is no stranger to infrared imaging for the sake of space exploration. In addition to being the Principal Investigator on this mission and a member of the Jet Propulsion Laboratory, she is also the Deputy Project Scientist for the Wide-field Infrared Survey Explorer (WISE) and the Principal Investigator for the NEOWISE project to study minor planets.

She has also appeared many times on the History Channel series The Universe, the documentary featurette “Stellar Cartography: On Earth”, and serves as the science consultant and host for the live-action PBS Kids series Ready Jet Go!, which will be debuting in the winter of 2016. Under her direction, the NEOCam mission will also study the origin and ultimate fate of our solar system’s asteroids, and finding the most suitable NEO targets for future exploration by robots and humans.

Proposals for NEOCam have been submitted a total of three times to the NASA Discovery Program – in 2006, 2010, and 2015, respectively. In 2010, NEOCam was selected to receive technology development funding to design and test new detectors optimized for asteroid and comet detection and discovery. However, the mission was ultimately overruled in favor of the Mars InSight Lander, which is scheduled for launch in 2016.

As one of the semifinalists for Discovery Mission 13, the NEOCam mission has received $3 million for year-long studies to lay out detailed mission plans and reduce risks. In September of 2016, one or two finalist will be selected to receive the program’s budget of $450 million (minus the cost of a launch vehicle and mission operations), and will launch in 2020 at the earliest.

In related news, NASA has confirmed that the asteroid known as 86666 (2000 FL10) will be passing Earth tomorrow. No need to worry, though. At its closest approach, the asteroid will still be at a distance of 892,577 km (554,000 mi) from Earth. Still, every passing rock underlines the need for knowing more about NEOs and where they might be headed one day!

A Mission to a Metal World: The Psyche Mission

In their drive to set exploration goals for the future, NASA’s Discovery Program put out the call for proposals for their thirteenth Discovery mission in February 2014. After reviewing the 27 initial proposals, a panel of NASA and other scientists and engineers recently selected five semifinalists for additional research and development, one or two of which will be launching by the 2020s.

With an eye to Venus, near-Earth objects and asteroids, these missions are looking beyond Mars to address other questions about the history and formation of our Solar System. Among them is the proposed Psyche mission, a robotic spacecraft that will explore the metallic asteroid of the same name – 16 Psyche – in the hopes of shedding some light on the mysteries of planet formation.

Discovered by Italian astronomer Annibale de Gasparis on March 17th, 1852 – and named after a Greek mythological figure – Psyche is one the ten most-massive asteroids in the Asteroid Belt. It is also the most massive M-type asteroid, a special class pertaining to asteroids composed primarily of nickel and iron.

For some time, scientists have speculated that this metallic asteroid is in fact the survivor of a protoplanet. In this scenario, a violent collision with a planetesimal stripped off Psyche’s outer, rocky layers, leaving behind only the dense, metallic interior. This theory is supported by estimates of Psyche’s bulk density, spectra, and radar surface properties; all of which show it to be an object unlike any others in the Belt.

Promotional artwork for the proposed Psyche mission. Credit: Peter Rubin/JPL-CALTECH.
Promotional artwork for the proposed Psyche mission. Credit: Peter Rubin/JPL-CALTECH.

In addition, this composition of 16 Psyche is strikingly similar to that of Earth’s metal core. Given that astronomers think that larger planets like Venus, Earth and Mars formed from the collision and merger of smaller worlds, Psyche could be the remains of a protoplanet that did not get to create a larger body.

Had such a planetesimal been struck by a large enough object, it would have been able to lose its lower-mass exterior while keeping its core intact. Thus, studying this 250 km (155 mile) wide body, offers a unique opportunity to learn more about the interiors of planets and large moons, whose cores are hidden beneath many miles of rock.

Dr. Linda Elkins-Tanton of Arizona State University’s School of Earth and Space Exploration is the Principle Investigator of this mission. As she and her team stated in their mission proposal paper, which was originally submitted as part of the 45th Lunar and Planetary Science Conference (2014):

“This mission would be a journey back in time to one of the earliest periods of planetary accretion, when the first bodies were not only differentiating, but were being pulverized, shredded, and accreted by collisions. It is also an exploration, by proxy, of the interiors of terrestrial planets and satellites today: we cannot visit a metallic core any other way.

“For all of these reasons, coupled with the relative accessibility to low- cost rendezvous and orbit, Psyche is a superb target for a Discovery-class mission that would characterize its geology, shape, elemental composition, magnetic field , and mass distribution.”

The huge metal asteroid Psyche may have a strong remnant magnetic field. Credit: Damir Gamulin/Ben Weiss
The huge metal asteroid Psyche may have a strong remnant magnetic field. Credit: Damir Gamulin/Ben Weiss

A robotic mission to Pysche would also help astronomers learn more about metal worlds, a type of solar system object that scientists know very little about. But perhaps the greatest reason to study 16 Psyche is the fact that it is unique. So far, this body is the only metallic core-like body that has been discovered in the Solar System.

The proposed spacecraft would orbit Psyche for six months, studying its topography, surface features, gravity, magnetism, and other characteristics. The mission would also be cost-effective and quick to launch, since it is largely based on technology that went into the making of NASA’s Dawn probe. Currently in orbit around Ceres, the Dawn mission has demonstrated the effectiveness of many new technologies, not the least of which was the xenon ion thruster.

The Psyche orbiter mission was selected as one of the Discovery Program’s five semifinalists on September 30th, 2015. Each proposal has received $3 million for year-long studies to lay out detailed mission plans and reduce risks. One or two finalist will be selected to receive the program’s budget of $450 million (minus the cost of a launch vehicle and mission operations) and will launch in 2020 at the earliest.

The Planet Neptune

Neptune photographed by Voyage. Image credit: NASA/JPL

Neptune is the eight planet from our Sun, one of the four gas giants, and one of the four outer planets in our Solar System. Since the “demotion” of Pluto by the IAU to the status of a dwarf planet – and/or Plutoid and Kuiper Belt Object (KBO) – Neptune is now considered to be the farthest planet in our Solar System.

As one of the planets that cannot be seen with the naked eye, Neptune was not discovered until relatively recently. And given its distance, it has only been observed up close on one occasion – in 1989 by the Voyager 2 spaceprobe. Nevertheless, what we’ve come to know about this gas (and ice) giant in that time has taught us much about the outer Solar System and the history of its formation.

Discovery and Naming:

Neptune’s discovery did not take place until the 19th century, though there are indications that it was observed before long that. For instance, Galileo’s drawings from December 28th, 1612, and January 27th, 1613, contained plotted points which are now known to match up with the positions of Neptune on those dates. However, in both cases, Galileo appeared to have mistaken it for a star.

1821, French astronomer Alexis Bouvard published astronomical tables for the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, which led Bouvard to hypothesize that an unknown body was perturbing Uranus’ orbit through gravitational interaction.

New Berlin Observatory at Linden Street, where Neptune was discovered observationally. Credit: Leibniz-Institut für Astrophysik Potsdam
New Berlin Observatory at Linden Street, where Neptune was discovered observationally. Credit: Leibniz-Institute for Astrophysics Potsdam

In 1843, English astronomer John Couch Adams began work on the orbit of Uranus using the data he had and produced several different estimates in the following years of the planet’s orbit. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations, which he shared with Johann Gottfried Galle of the Berlin Observatory. Galle confirmed the presence of a planet at the coordinates specified by Le Verrier on September 23rd, 1846.

The announcement of the discovery was met with controversy, as both Le Verrier and Adams claimed responsibility. Eventually, an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, a re-evaluation by historians in 1998 of the relevant historical documents led to the conclusion that Le Verrier was more directly responsible for the discovery and deserves the greater share of the credit.

Claiming the right of discovery, Le Verrier suggested the planet be named after himself, but this met with stiff resistance outside of France. He also suggested the name Neptune, which was gradually accepted by the international community. This was largely because it was consistent with the nomenclature of the other planets, all of which were named after deities from Greco-Roman mythology.

Neptune’s Size, Mass and Orbit:

With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus. The planet has a very minor eccentricity of 0.0086, and orbits the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.

A size comparison of Neptune and Earth. Credit: NASA
A size comparison of Neptune and Earth. Credit: NASA

Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).

Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.

Neptune’s orbit also has a profound impact on the region directly beyond it, known as the Kuiper Belt (otherwise known as the “Trans-Neptunian Region”). Much in the same way that Jupiter’s gravity dominates the Asteroid Belt, shaping its structure, so Neptune’s gravity dominates the Kuiper Belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune’s gravity, creating gaps in the Kuiper belt’s structure.

There also exists orbits within these empty regions where objects can survive for the age of the Solar System. These resonances occur when Neptune’s orbital period is a precise fraction of that of the object – meaning they complete a fraction of an orbit for every orbit made by Neptune. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance.

Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune’s orbit regularly, the 2:3 resonance ensures they can never collide.

Neptune has a number of known trojan objects occupying both the Sun–Neptune L4 and L5 Lagrangian Points – regions of gravitational stability leading and trailing Neptune in its orbit. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured.

Neptune’s Composition:

Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.

The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”.  Meanwhile, the atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa – or about 100,000 times that of Earth’s atmosphere.

Composition of Neptune. Image credit: NASA
Composition of Neptune. Image credit: NASA

Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle.

Neptune’s Atmosphere:

At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.

Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.

Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.

In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka)
A modified color/contrast image emphasizing Neptune’s atmospheric features, including wind speed. Credit Erich Karkoschka)

For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.

Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.

This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.

The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter life span than Jupiter’s.

Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL

The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot. This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot.

The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.

Neptune’s Moons:

Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (S/2004 N 1 is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.

They range in distance from 48,227 km (Naiad) to 117,646 km (Proteus) from Neptune, and all but the outermost two (S/2004 N 1, and Proteus) orbit Neptune slower than its orbital period of 0.6713 days. Based on observational data and assumed densities, these moons range in size and mass from 96 x 60 x 52 km and 1.9 x 1017 kg (Naiad) to 436 x 416 x 402 km and 50.35 x 1017 kg (Proteus).

This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, nearly 4.8 billion km (3 billion miles) from Earth. Credit: NASA, ESA, and M. Showalter (SETI Institute).
This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, nearly 4.8 billion km (3 billion miles) from Earth. Credit: NASA, ESA, and M. Showalter (SETI Institute).

With the exception of Larissa and Proteus (which are largely rounded) all of Neptune’s inner moons are believed to be elongated in shape. Their spectra also indicates that they are made from water ice contaminated by some very dark material, probably organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.

Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.

In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.

In terms of size and mass, the irregular moons are relatively consistent, ranging from approximately 40 km in diameter and 4 x 1016 kg in mass (Psamathe) to 62 km and 16 x 1016 kg for Halimede. Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons. Between these two and the other irregular moons, four major differences have been noted.

First of all, they are the largest two known irregular moons in the Solar System. Triton itself is almost an order of magnitude larger than all other known irregular moons and comprises more than 99.5% of all the mass known to orbit Neptune (including the planet’s rings and thirteen other known moons).

Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS

Secondly, they both have atypically small semi-major axes, with Triton’s being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton’s orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite

With a mean diameter of around 2700 km and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.

Triton follows a retrograde and quasi-circular orbit, and is composed largely of nitrogen, methane, carbon dioxide and water ices. With a geometric albedo of more than 70% and a Bond albedo as high as 90%, it is also one of the brightest objects in the Solar System. The surface has a reddish tint, owning to the interaction of ultraviolet radiation and methane, causing tholins.

Triton is also one of the coldest moons in the Solar System, with surface temperature of about 38 K (-235.2 °C). However, owing to the moon being geologically active (which results in cryovolcanism) and surface temperature variations that cause sublimation, Triton is one of only two moons in the Solar System that has a substantial atmosphere. Much like it’s surface, this atmosphere is composed primarily of nitrogen with small amounts of methane and carbon monoxide, and with an estimated pressure of about 14 nanobar.

Triton has a relatively high density of about 2 g/cm3 indicating that rocks constitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There also may be a layer of liquid water deep inside Triton, forming a subterranean ocean. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.

Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), Triton is grouped with the planet’s irregular moons (see below). In addition, it is believed to be a captured object, possibly a dwarf planet that was once part of the Kuiper Belt. At the same time, these orbital characteristics are the reason why Triton experiences tidal deceleration. and will eventually spiral inward and collide with the planet in about 3.6 billion years.

Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton’s capture. Water ice has been spectroscopically detected on its surface. Nereid shows large, irregular variations in its visible magnitude, which are probably caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.

Neptune’s Ring System:

Neptune has five rings, all of which are named after astronomers who made important discoveries about the planet – Galle, Le Verrier, Lassell, Arago, and Adams. The rings are composed of at least 20% dust (with some containing as much as 70%) while the rest of the material consists of small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size.

The Galle ring was named after Johann Gottfried Galle, the first person to see the planet using a telescope; and at 41,000–43,000 km, it is the nearest of Neptune’s rings.  The La Verrier ring – which is very narrow at 113 km in width – is named after French astronomer Urbain Le Verrier, the planet’s co-founder.

At a distance of between 53,200 and 57,200 km from Neptune (giving it a width of 4,000 km) the Lassell ring is the widest of Neptune’s rings. This ring is named after William Lassell, the English astronomer who discovered Triton just seventeen days after Neptune was discovered. The Arago ring is 57,200 kilometers from the planet and less than 100 kilometers wide. This ring section is named after Francois Arago, Le Verrier’s mentor and the astronomer who played an active role in the dispute over who deserved credit for discovering Neptune.

The outer Adams ring was named after John Couch Adams, who is credited with the co-discovery of Neptune. Although the ring is narrow at only 35 kilometers wide, it is the most famous of the five due to its arcs. These arcs accord with areas in the ring system where the material of the rings is grouped together in a clump, and are the brightest and most easily observed parts of the ring system.

Although the Adams ring has five arcs, the three most famous are the “Liberty”, “Equality”, and “Fraternity” arcs. Scientists have been traditionally unable to explain the existence of these arcs because, according to the laws of motion, they should distribute the material uniformly throughout the rings. However, stronomers now estimate that the arcs are corralled into their current form by the gravitational effects of Galatea, which sits just inward from the ring.

The rings of Neptune as seen from Voyager 2 during the 1989 flyby. (Credit: NASA/JPL).
The rings of Neptune as seen from Voyager 2 during the 1989 flyby. Credit: NASA/JPL

The rings of Neptune are very dark, and probably made of organic compounds that have been altered due to exposition to cosmic radiation. This is similar to the rings of Uranus, but very different to the icy rings around Saturn. They seem to contain a large quantity of micrometer-sized dust, similar in size to the particles in the rings of Jupiter.

It’s believed that the rings of Neptune are relatively young – much younger than the age of the Solar System, and much younger than the age of Uranus’ rings. Consistent with the theory that Triton was a KBO that was seized, by Neptune’s gravity, they are believed to be the result of a collision between some of the planet’s original moons.


The Voyager 2 probe is the only spacecraft to have ever visited Neptune. The spacecraft’s closest approach to the planet occurred on August 25th, 1989, which took place at a distance of 4,800 km (3,000 miles) above Neptune’s north pole. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton – similar to what had been done for Voyager 1s encounter with Saturn and its moon Titan.

The spacecraft performed a near-encounter with the moon Nereid before it came to within 4,400 km of Neptune’s atmosphere on August 25th, then passed close to the planet’s largest moon Triton later the same day. The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the center and tilted in a manner similar to the field around Uranus.

Neptune’s rotation period was determined using measurements of radio emissions and Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered during the flyby, and the planet was shown to have more than one ring.

While no missions to Neptune are currently being planned, some hypothetical missions have been suggested. For instance, a possible Flagship Mission has been envisioned by NASA to take place sometime during the late 2020s or early 2030s. Other proposals include a possible Cassini-Huygens-style “Neptune Orbiter with Probes”, which was suggested back in 2003.

Another, more recent proposal by NASA was for Argo – a flyby spacecraft that would be launched in 2019, which would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton, which would be investigated around 2029.

With its icy-blue color, liquid surface, and wavy weather patterns, Neptune was appropriately named after the Roman god of the sea. And given its distance from our planet, there is still a great deal that remains to be learned about it. In the coming decades, one can only hope that a mission to the outer Solar System and/or Kuiper Belt includes a flyby of Neptune.

We have many interesting articles about Neptune here at Universe Today. Below is a comprehensive list for your viewing (and possibly researching) pleasure!

Characteristics of Neptune:

Position and Movement of Neptune:

Neptune’s Moon and Rings:

History of Neptune:

Neptune’s Surface and Structure:

Solar System Guide

The Solar System. Image Credit: NASA

The Universe is a very big place, and we occupy a very small corner of it. Known as the Solar System, our stomping grounds are not only a tiny fraction of the Universe as we know it, but is also a very small part of our galactic neighborhood (aka. the Milky Way Galaxy). When it comes right down to it, our world is just a drop of water in an endless cosmic sea.

Nevertheless, the Solar System is still a very big place, and one which is filled with its fair share of mysteries. And in truth, it was only within the relatively recent past that we began to understand its true extent. And when it comes to exploring it, we’ve really only begun to scratch the surface.


With very few exceptions, few people or civilizations before the era of modern astronomy recognized the Solar System for what it was. In fact, the vast majority of astronomical systems posited that the Earth was a stationary object and that all known celestial objects revolved around it. In addition, they viewed it as being fundamentally different from other stellar objects, which they held to be ethereal or divine in nature.

Although there were some Greek, Arab and Asian astronomers during Antiquity and the Medieval period who believed that the universe was heliocentric in nature (i.e. that the Earth and other bodies revolved around the Sun) it was not until Nicolaus Copernicus developed his mathematically predictive model of a heliocentric system in the 16th century that it began to become widespread.

The first star party? Galileo shows of the sky in Saint Mark's square in Venice. Note the lack of adaptive optics. (Illustration in the Public Domain).
Galileo (1564 – 1642) would often show people how to use his telescope to view the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain

During the 17th-century, scientists like Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance that the Earth revolves round the Sun. The development of theories like gravity also led to the realization that the other planets are governed by the same physical laws as Earth.

The widespread use of the telescope also led to a revolution in astronomy. After Galileo discovered the moons of Jupiter in 1610, Christian Huygens would go on to discover that Saturn also had moons in 1655. In time, new planets would also be discovered (such as Uranus and Neptune), as well as comets (such as Halley’s Comet) and the Asteroids Belt.

By the 19th century, three observations made by three separate astronomers determined the true nature of the Solar System and its place the universe. The first was made in 1839 by German astronomer Friedrich Bessel, who successfully measured an apparent shift in the position of a star created by the Earth’s motion around the Sun (aka. stellar parallax). This not only confirmed the heliocentric model beyond a doubt, but revealed the vast distance between the Sun and the stars.

In 1859, Robert Bunsen and Gustav Kirchhoff (a German chemist and physicist) used the newly invented spectroscope to examined the spectral signature of the Sun. They discovered that it was composed of the same elements as existed on Earth, thus proving that Earth and the heavens were composed of the same elements.

With parallax technique, astronomers observe object at opposite ends of Earth's orbit around the Sun to precisely measure its distance. CREDIT: Alexandra Angelich, NRAO/AUI/NSF.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

Then, Father Angelo Secchi  – an Italian astronomer and director at the Pontifical Gregorian University – compared the spectral signature of the Sun with those of other stars, and found them to be virtually identical. This demonstrated conclusively that our Sun was composed of the same materials as every other star in the universe.

Further apparent discrepancies in the orbits of the outer planets led American astronomer Percival Lowell to conclude that yet another planet, which he referred to as “Planet X“, must lie beyond Neptune. After his death, his Lowell Observatory conducted a search that ultimately led to Clyde Tombaugh’s discovery of Pluto in 1930.

Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the MIT discovered the Trans-Neptunian Object (TNO) known as (15760) 1992 QB1. This would prove to be the first of a new population, known as the Kuiper Belt, which had already been predicted by astronomers to exist at the edge of the Solar System.

Further investigation of the Kuiper Belt by the turn of the century would lead to additional discoveries. The discovery of Eris and other “plutoids” by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers would lead to the Great Planet Debate – where IAU policy and the convention for designating planets would be contested.

Structure and Composition:

At the core of the Solar System lies the Sun (a G2 main-sequence star) which is then surrounded by four terrestrial planets (the Inner Planets), the main Asteroid Belt, four gas giants (the Outer Planets), a massive field of small bodies that extends from 30 AU to 50 AU from the Sun (the Kuiper Belt). The system is then surrounded a spherical cloud of icy planetesimals (the Oort Cloud) that is believed to extend to a distance of 100,000 AU from the Sun into the Interstellar Medium.

The Sun contains 99.86% of the system’s known mass, and its gravity dominates the entire system. Most large objects in orbit around the Sun lie near the plane of Earth’s orbit (the ecliptic) and most planets and bodies rotate around it in the same direction (counter-clockwise when viewed from above Earth’s north pole). The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at greater angles to it.

It’s four largest orbiting bodies (the gas giants) account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.

Sun and Planets
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson

Astronomers sometimes informally divide this structure into separate regions. First, there is the Inner Solar System, which includes the four terrestrial planets and the Asteroid Belt. Beyond this, there’s the outer Solar System that includes the four gas giant planets. Meanwhile, there’s the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune (i.e. Trans-Neptunian Objects).

Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites (or moons). In the case of the four giant planets, there are also planetary rings – thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. The terrestrial planets of the Inner Solar System are composed primarily of silicate rock, iron and nickel. Beyond the Asteroid Belt, planets are composed mainly of gases (such as hydrogen, helium) and ices – like water, methane, ammonia, hydrogen sulfide and carbon dioxide.

Objects farther from the Sun are composed largely of materials with lower melting points. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (hence why they are sometimes referred to as “ice giants”) and the numerous small objects that lie beyond Neptune’s orbit.

Together, gases and ices are referred to as volatiles. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, which lies roughly 5 AU from the Sun. Within the Kuiper Belt, objects and planetesimals are composed mainly of these materials and rock.

Formation and Evolution:

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud composed of hydrogen, helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System (known as the pre-solar nebula) collapsed, conservation of angular momentum caused it to rotate faster.

The center, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.

Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the frost line).

The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved.

At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium. This will occur roughly 5 billion years from now and mark the end of the Sun’s main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present.

The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion and the Sun will burn helium for a time, after which nuclear reactions in the core will start to dwindle.

At this point, the Sun’s outer layers will move away into space, leaving a white dwarf – an extraordinarily dense object that will have half the original mass of the Sun, but will be the size of Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.

Inner Solar System:

In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.

For starters, the inner planets are rocky and terrestrial, composed mostly of silicates and metals, whereas the outer planets are gas giants. The inner planets are also much more closely spaced than their outer Solar System counterparts. In fact, the radius of the entire region is less than the distance between the orbits of Jupiter and Saturn.

Generally, inner planets are smaller and denser than their counterparts, and have few to no moons or rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.

The terrestrial inner planets are composed largely of refractory minerals such as the silicates, which form their crusts and mantles, and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather. All of them have impact craters and tectonic surface features as well, such as rift valleys and volcanoes.

Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. Its magnetic field is only about 1% that of Earth’s, and it’s very thin atmosphere means that it is hot during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out. It has no moons of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System.

Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other gases. Dense clouds within Venus’ atmosphere are composed of sulphuric acid and other corrosive compounds, with very little water. Much of Venus’ surface is marked with volcanoes and deep canyons – the biggest of which is over 6400 km (4,000 mi) long.

Earth is the third inner planet and the one we know best. Of the four terrestrial planets, Earth is the largest, and the only one that currently has liquid water, which is necessary for life as we know it. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight and warmth in, which is also essential for life to survive.

Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only one we have, is comprised of a mixture of various rocks and minerals.

Mars, as it appears today, Credit: NASA
Mars, as it appears today, Credit: NASA

Mars is the fourth and final inner planet, and is also known as the “Red Planet” due to the oxidization of iron-rich materials that form the planet’s surface. Mars also has some of the most interesting terrain features of any of the terrestrial planets. These include the largest mountain in the Solar System (Olympus Mons) which rises some 21,229 m (69,649 ft) above the surface, and a giant canyon called Valles Marineris – which is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi).

Much of Mars’ surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.

Mars’ thin atmosphere has led some astronomers to believe that the surface water that once existed there might have actually taken liquid form, but has since evaporated into space. The planet has two small moons called Phobos and Deimos.

Outer Solar System:

The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas that have rings and plenty of moons. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers that the solar system was bigger than previously thought.

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute

Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot – a raging storm that has happening for the past 400 years at least.

Saturn is best known for its prominent ring system – seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years).

Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. The only spacecraft to visit this planet was the Voyager 2 spacecraft in 1986.

Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. NASA’s Voyager 2 spacecraft also visited this planet and its system by 1989 during its transit of the outer Solar System.

How many moons are there in the Solar System? Image credit: NASA
How many moons are there in the Solar System? Image credit: NASA

Trans-Neptunian Region:

There have been more than a thousand objects discovered in the Kuiper Belt, and it’s theorized that there are as many as 100,000 objects larger than 100 km in diameter. Given to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine.

However, spectrographic studies conducted of the region since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice – a composition they share with comets. Initial studies also confirmed a broad range of colors among KBOs, ranging from neutral grey to deep red.

This suggests that their surfaces are composed of a wide range of compounds, from dirty ices to hydrocarbons. In 1996, Robert H. Brown et al. obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of Pluto (as well as Neptune’s moon Triton) in that it possessed large amounts of methane ice.

Water ice has been detected in several KBOs, including 1996 TO66, 38628 Huya and 20000 Varuna. In 2004, Mike Brown et al. determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs, 50000 Quaoar. Both of these substances would have been destroyed over the age of the Solar System, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.

Keeping Pluto company out in the Kuiper belt are many other objects worthy of mention. Quaoar, Makemake, Haumea, Orcus and Eris are all large icy bodies in the Belt and several of them even have moons of their own. These are all tremendously far away, and yet, very much within reach.

Oort Cloud and Farthest Regions:

The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.

Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.


Our knowledge of the Solar System also benefited immensely from the advent of robotic spacecraft, satellites, and robotic landers. Beginning in the mid-20th century, in what was known as “The Space Age“, manned and robotic spacecraft began exploring planets, asteroids and comets in the Inner and Outer Solar System.

All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all the planets. In the case of landers and rovers, tests have been performed on the soils and atmospheres of some.

Sputnik 1
Photograph of a Russian technician putting the finishing touches on Sputnik 1, humanity’s first artificial satellite. Credit: NASA/Asif A. Siddiqi

The first artificial object sent into space was the Soviet satellite Sputnik 1, which was launched in space in 1957, successfully orbited the Earth for months, and collected information on the density of the upper atmosphere and the ionosphere. The American probe Explorer 6, launched in 1959, was the first satellite to capture images of the Earth from space.

Robotic spacecraft conducting flybys also revealed considerable information about the planet’s atmospheres, geological and surface features. The first successful probe to fly by another planet was the Soviet Luna 1 probe, which sped past the Moon in 1959. The Mariner program resulted in multiple successful planetary flybys, consisting of the Mariner 2 mission past Venus in 1962, the Mariner 4 mission past Mars in 1965, and the Mariner 10 mission past Mercury in 1974.

By the 1970’s, probes were being dispatched to the outer planets as well, beginning with the Pioneer 10 mission which flew past Jupiter in 1973 and the Pioneer 11 visit to Saturn in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.

Launched on January 19th, 2006, the New Horizons probe is the first man-made spacecraft to explore the Kuiper Belt. This unmanned mission flew by Pluto in July 2015. Should it prove feasible, the mission will also be extended to observe a number of other Kuiper Belt Objects (KBOs) in the coming years.

Orbiters, rovers, and landers began being deployed to other planets in the Solar System by the 1960’s. The first was the Soviet Luna 10 satellite, which was sent into lunar orbit in 1966. This was followed in 1971 with the deployment of the Mariner 9 space probe, which orbited Mars, and the Soviet Venera 9 which orbited Venus in 1975.

The Galileo probe became the first artificial satellite to orbit an outer planet when it reached Jupiter in 1995, followed by the CassiniHuygens probe orbiting Saturn in 2004. Mercury and Vesta were explored by 2011 by the MESSENGER and Dawn probes, respectively, with Dawn establishing orbit around the asteroid/dwarf planet Ceres in 2015.

The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, probes have landed on or impacted on the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3 and Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn’s moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005.

Curiosity Rover snapped this self portrait mosaic with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop where the robot conducted historic first sample drilling inside the Yellowknife Bay basin, on Feb. 8 (Sol 182) at lower left in front of rover. The photo mosaic was stitched from raw images snapped on Sol 177, or Feb 3, 2013, by the robotic arm camera - accounting for foreground camera distortion. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer (
Curiosity Rover self portrait mosaic, taken with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop in Feb. 2013. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer

To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first robotic rover to land on another planet was the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was Sojourner, which traveled 500 meters across the surface of Mars in 1997, followed by Spirit (2004), Opportunity (2004), and Curiosity (2012).

Manned missions into space began in earnest in the 1950’s, and was a major focal point for both the United States and Soviet Union during the “Space Race“. For the Soviets, this took the form of the Vostok program, which involved sending manned space capsules into orbit.

The first mission – Vostok 1 – took place on April 12th, 1961, and was piloted by Soviet cosmonaut Yuri Gagarin (the first human being to go into space). On June 6th, 1963, the Soviets also sent the first woman – Valentina Tereshvoka – into space as part of the Vostok 6 mission.

In the US, Project Mercury was initiated with the same goal of placing a crewed capsule into orbit. On May 5th, 1961, astronaut Alan Shepard went into space aboard the Freedom 7 mission and became the first American (and second human) to go into space.

After the Vostok and Mercury programs were completed, the focus of both nations and space programs shifted towards the development of two and three-person spacecraft, as well as the development of long-duration spaceflights and extra-vehicular activity (EVA).

Bootprint in the moon dust from Apollo 11. Credit: NASA
Bootprint in the moon dust from Apollo 11. Credit: NASA

This took the form of the Voshkod and Gemini programs in the Soviet Union and US, respectively. For the Soviets, this involved developing a two to three-person capsule, whereas the Gemini program focused on developing the support and expertise needed for an eventual manned mission to the Moon.

These latter efforts culminated on July 21st, 1969 with the Apollo 11 mission, when astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon. As part of the Apollo program, five more Moon landings would take place through 1972, and the program itself resulted in many scientific packages being deployed on the Lunar surface, and samples of moon rocks being returned to Earth.

After the Moon Landing took place, the focus of the US and Soviet space programs then began to shift to the development of space stations and reusable spacecraft. For the Soviets, this resulted in the first crewed orbital space stations dedicated to scientific research and military reconnaissance – known as the Salyut and Almaz space stations.

The first orbital space station to host more than one crew was NASA’s Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then.

Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA
Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA

The United States’ Space Shuttle, which debuted in 1981, became the only reusable spacecraft to successfully make multiple orbital flights. The five shuttles that were built (Atlantis, Endeavour, Discovery, Challenger, Columbia and Enterprise) flew a total of 121 missions before being decommissioned in 2011.

During their history of service, two of the craft were destroyed in accidents. These included the Space Shuttle Challenger – which exploded upon take-off on Jan. 28th, 1986 – and the Space Shuttle Columbia which disintegrated during re-entry on Feb. 1st, 2003.

In 2004, then-U.S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. These goals have since been maintained by the Obama administration, and now include plans for an Asteroid Redirect mission, where a robotic craft will tow an asteroid closer to Earth so a manned mission can be mounted to it.

All the information gained from manned and robotic missions about the geological phenomena of other planets – such as mountains and craters – as well as their seasonal, meteorological phenomena (i.e. clouds, dust storms and ice caps) have led to the realization that other planets experience much the same phenomena as Earth. In addition, it has also helped scientists to learn much about the history of the Solar System and its formation.

As our exploration of the Inner and Outer Solar System has improved and expanded, our conventions for categorizing planets has also changed. Our current model of the Solar System includes eight planets (four terrestrial, four gas giants), four dwarf planets, and a growing number of Trans-Neptunian Objects that have yet to be designated. It also contains and is surrounded by countless asteroids and planetesimals.

Given its sheer size, composition and complexity, researching our Solar System in full detail would take an entire lifetime. Obviously, no one has that kind of time to dedicate to the topic, so we have decided to compile the many articles we have about it here on Universe Today in one simple page of links for your convenience.

There are thousands of facts about the solar system in the links below. Enjoy your research.

The Solar System:

Theories about the Solar System:


Anything EXTREME!:

Solar System Stuffs:

What is the Asteroid Belt?

In the 18th century, observations made of all the known planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) led astronomers to discern a pattern in their orbits. Eventually, this led to the Titius–Bode Law, which predicted the amount of space between the planets. In accordance with this law, there appeared to be a discernible gap between the orbits of Mars and Jupiter, and investigation into it led to a major discovery.

In addition to several larger objects being observed, astronomers began to notice countless smaller bodies also orbiting between Mars and Jupiter. This led to the creation of the term “asteroid”, as well as “Asteroid Belt” once it became clear just how many there were. Since that time, the term has entered common usage and become a mainstay of our astronomical models.


In 1800, hoping to resolve the issue created by the Titius-Bode Law, astronomer Baron Franz Xaver von Zach recruited 24 of his fellow astronomers into a club known as the “United Astronomical Society” (sometimes referred to the as “Stellar Police”). At the time, its ranks included famed astronomer William Herschel, who had discovered Uranus and its moons in the 1780s.

Ironically, the first astronomer to make a discovery in this regions was Giuseppe Piazzi – the chair of astronomy at the University of Palermo – who had been asked to join the Society but had not yet received the invitation. On January 1st, 1801, Piazzi observed a tiny object in an orbit with the exact radius predicted by the Titius-Bode law.

Ceres (left, Dawn image) compared to Tethys (right, Cassini image) at comparative scale sizes. (Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and NASA/JPL-Caltech/SSI. Comparison by J. Major.)
Ceres (left, Dawn image) compared to Tethys (right, Cassini image) at comparative scale sizes. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA and NASA/JPL-Caltech/SSI. Comparison by J. Major.

Initially, he believed it to be a comet, but ongoing observations showed that it had no coma. This led Piazzi to consider that the object he had found – which he named “Ceres” after the Roman goddess of the harvest and patron of Sicily – could, in fact, be a planet. Fifteen months later, Heinrich Olbers ( a member of the Society) discovered a second object in the same region, which was later named 2 Pallas.

In appearance, these objects seemed indistinguishable from stars. Even under the highest telescope magnifications, they did not resolve into discs. However, their rapid movement was indicative of a shared orbit. Hence, William Herschel suggested that they be placed into a separate category called “asteroids” – Greek for “star-like”.

By 1807, further investigation revealed two new objects in the region, 3 Juno and 4 Vesta; and by 1845, 5 Astraea was found. Shortly thereafter, new objects were found at an accelerating rate, and by the early 1850s, the term “asteroids” gradually came into common use. So too did the term “Asteroid Belt”, though it is unclear who coined that particular term. However, the term “Main Belt” is often used to distinguish it from the Kuiper Belt.

One hundred asteroids had been located by mid-1868, and in 1891 the introduction of astrophotography by Max Wolf accelerated the rate of discovery even further. A total of 1,000 asteroids were found by 1921, 10,000 by 1981, and 100,000 by 2000. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars. Credit: Wikipedia Commons
The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars. Credit: Wikipedia Commons


Despite common perceptions, the Asteroid Belt is mostly empty space, with the asteroids spread over a large volume of space. Nevertheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more. Over 200 asteroids are known to be larger than 100 km in diameter, and a survey in the infrared wavelengths has shown that the asteroid belt has 0.7–1.7 million asteroids with a diameter of 1 km (0.6 mi) or more.

Located between Mars and Jupiter, the belt ranges from 2.2 to 3.2 astronomical units (AU) from the Sun and is 1 AU thick. Its total mass is estimated to be 2.8×1021 to 3.2×1021 kilograms – which is equivalent to about 4% of the Moon’s mass. The four largest objects – Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea – account for half of the belt’s total mass, with almost one-third accounted for by Ceres alone.

The main (or core) population of the asteroid belt is sometimes divided into three zones, which are based on what is known as Kirkwood Gaps. Named after Daniel Kirkwood, who announced in 1866 the discovery of gaps in the distance of asteroids, these describe the dimensions of an asteroid’s orbit based on its semi-major axis.

Within this scheme, there are three zones. Zone I lies between the 4:1 resonance and 3:1 resonance Kirkwood gaps, which are 2.06 and 2.5 AU from the Sun respectively. Zone II continues from the end of Zone I out to the 5:2 resonance gap, which is 2.82 AU from the Sun. Zone III extends from the outer edge of Zone II to the 2:1 resonance gap at 3.28 AU.

The asteroid belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 AU), and the outer belt formed by those asteroids closer to Jupiter’s orbit.

The asteroids that have a radius of 2.06 AU from the Sun can be considered the inner boundary of the asteroid belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion at 1.67 AU) or ejected by its gravitational perturbations in the early history of the Solar System.

The temperature of the Asteroid Belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73 °C) at 2.2 AU down to 165 K (-108 °C) at 3.2 AU. However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.


Much like the terrestrial planets, most asteroids are composed of silicate rock while a small portion contains metals such as iron and nickel. The remaining asteroids are made up of a mix of these, along with carbon-rich materials. Some of the more distant asteroids tend to contain more ices and volatiles, which includes water ice.

Vesta seen from the Earth-orbit based Hubble Space Telescope in 2007 (left) and up close with the Dawn spacecraft in 2011. Hubble Credit: NASA, ESA, and L. McFadden (University of Maryland). Dawn Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Photo Combination: Elizabeth Howell
Vesta seen from the Earth-orbit based Hubble Space Telescope in 2007 (left) and up close with the Dawn spacecraft in 2011. Hubble Credit: NASA, ESA, and L. McFadden (University of Maryland). Dawn Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. Photo Combination: Elizabeth Howell

The Main Belt consists primarily of three categories of asteroids: C-type, or carbonaceous asteroids; S-type, or silicate asteroids; and M-type, or metallic asteroids. Carbonaceous asteroids are carbon-rich, dominate the belt’s outer regions, and comprise over 75% of the visible asteroids. Their surface composition is similar to that of carbonaceous chondrite meteorites while their spectra is similar to what the early Solar System’s is believed to be.

S-type (silicate-rich) asteroids are more common toward the inner region of the belt, within 2.5 AU of the Sun. These are typically composed of silicates and some metals, but not a significant amount of carbonaceous compounds. This indicates that their materials have been modified significantly over time, most likely through melting and reformation.

M-type (metal-rich) asteroids form about 10% of the total population and are composed of iron-nickel and some silicate compounds. Some are believed to have originated from the metallic cores of differentiated asteroids, which were then fragmented from collisions. Within the asteroid belt, the distribution of these types of asteroids peaks at a semi-major axis of about 2.7 AU from the Sun.

There’s also the mysterious and relatively rare V-type (or basaltic) asteroids. This group takes their name from the fact that until 2001, most basaltic bodies in the Asteroid Belt were believed to have originated from the asteroid Vesta. However, the discovery of basaltic asteroids with different chemical compositions suggests a different origin. Current theories of asteroid formation predict that the V-type asteroids should be more plentiful, but 99% of those that have been predicted are currently missing.

Families and Groups:

Approximately one-third of the asteroids in the asteroid belt are members of an asteroid family. These are based on similarities in orbital elements – such as semi-major axis, eccentricity, orbital inclinations, and similar spectral features, all of which indicate a common origin. Most likely, this would have involved collisions between larger objects (with a mean radius of ~10 km) that then broke up into smaller bodies.

This artist's conception shows how families of asteroids are created. Credit: NASA/JPL-Caltech
This artist’s conception shows how families of asteroids are created. Credit: NASA/JPL-Caltech

Some of the most prominent families in the asteroid belt are the Flora, Eunomia, Koronis, Eos, and Themis families. The Flora family, one of the largest with more than 800 known members, may have formed from a collision less than a billion years ago. Located within the inner region of the Belt, this family is made up of S-type asteroids and accounts for roughly 4-5% of all Belt objects.

The Eunomia family is another large grouping of S-type asteroids, which takes its name from the Greek goddess Eunomia (goddess of law and good order). It is the most prominent family in the intermediate asteroid belt and accounts for 5% of all asteroids.

The Koronis family consists of 300 known asteroids which are thought to have been formed at least two billion years ago by a collision. The largest known, 208 Lacrimosa, is about 41 km (25 mi) in diameter, while an additional 20 more have been found that are larger than 25 km in diameter.

The Eos (or Eoan) family is a prominent family of asteroids that orbit the Sun at a distance of 2.96 – 3.03 AUs, and are believed to have formed from a collision 1-2 billion years ago. It consists of 4,400 known members that resemble the S-type asteroid category. However, the examination of Eos and other family members in the infrared show some differences with the S-type, thus why they have their own category (K-type asteroids).

Asteroids we've seen up close show cratered surfaces similar to yet different from much of the cratering on comets. Credit: NASA
Asteroids we’ve seen up close show cratered surfaces similar to yet different from much of the cratering on comets. Credit: NASA

The Themis asteroid family is found in the outer portion of the asteroid belt, at a mean distance of 3.13 AU from the Sun.  This core group includes the asteroid 24 Themis (for which it is named) and is one of the more populous asteroid families. It is made up of C-type asteroids with a composition believed to be similar to that of carbonaceous chondrites and consists of a well-defined core of larger asteroids and a surrounding region of smaller ones.

The largest asteroid to be a true member of a family is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise, the HED meteorites may also have originated from Vesta as a result of this collision.

Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to a few hundred micrometers. This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Three prominent bands of dust have been found within the asteroid belt – which have similar orbital inclinations as the Eos, Koronis, and Themis asteroid families – and so are possibly associated with those groupings.


Originally, the Asteroid Belt was thought to be the remnants of a much larger planet that occupied the region between the orbits of Mars and Jupiter. This theory was originally suggested by Heinrich Olbders to William Herschel as a possible explanation for the existence of Ceres and Pallas. However, this hypothesis has since fallen out of favor for a number of reasons.

Artist's impression of the early Solar System, where collision between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech
Artist’s impression of the early Solar System, where collisions between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech

First, there is the amount of energy it would have required to destroy a planet, which would have been staggering. Second, there is the fact that the entire mass of the Belt is only 4% that of the Moon.  Third, the significant chemical differences between the asteroids do not point towards them having been once part of a single planet.

Today, the scientific consensus is that, rather than fragmenting from a progenitor planet, the asteroids are remnants from the early Solar System that never formed a planet at all. During the first few million years of the Solar System’s history, when gravitational accretion led to the formation of the planets, clumps of matter in an accretion disc coalesced to form planetesimals. These, in turn, came together to form planets.

However, within the region of the Asteroid Belt, planetesimals were too strongly perturbed by Jupiter’s gravity to form a planet. These objects would continue to orbit the Sun as before, occasionally colliding and producing smaller fragments and dust.

During the early history of the Solar System, the asteroids also melted to some degree, allowing elements within them to be partially or completely differentiated by mass. However, this period would have been necessarily brief due to their relatively small size, and likely ended about 4.5 billion years ago, in the first tens of millions of years of the Solar System’s formation.

Though they are dated to the early history of the Solar System, the asteroids (as they are today) are not samples of its primordial self. They have undergone considerable evolution since their formation, including internal heating, surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites. Hence, the Asteroid Belt today is believed to contain only a small fraction of the mass of the primordial belt.

Computer simulations suggest that the original asteroid belt may have contained as much mass as Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt a million years after its formation, leaving behind less than 0.1% of the original mass. Since then, the size distribution of the asteroid belt is believed to have remained relatively stable.

When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a “snow line” below the freezing point of water. Essentially, planetesimals formed beyond this radius were able to accumulate ice, some of which may have provided a water source of Earth’s oceans (even more so than comets).


The asteroid belt is so thinly populated that several unmanned spacecraft have been able to move through it; either as part of a long-range mission to the outer Solar System, or (in recent years) as a mission to study larger Asteroid Belt objects. In fact, due to the low density of materials within the Belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.

Artist's concept of the Dawn spacecraft arriving at Vesta. Image credit: NASA/JPL-Caltech
Artist’s concept of the Dawn spacecraft arriving at Vesta. Image credit: NASA/JPL-Caltech

The first spacecraft to make a journey through the asteroid belt was the Pioneer 10 spacecraft, which entered the region on July 16th, 1972. As part of a mission to Jupiter, the craft successfully navigated through the Belt and conducted a flyby of Jupiter (which culminated in December of 1973) before becoming the first spacecraft to achieve escape velocity from the Solar System.

At the time, there were concerns that the debris would pose a hazard to the Pioneer 10 space probe. But since that mission, 11 additional spacecraft passed through the Asteroid Belt without incident. These included Pioneer 11, Voyager 1 and 2, Ulysses, Galileo, NEAR, Cassini, Stardust, New Horizons, the ESA’s Rosetta, and most recently, the Dawn spacecraft.

For the most part, these missions were part of missions to the outer Solar System, where opportunities to photograph and study asteroids were brief. Only the Dawn, NEAR and JAXA’s Hayabusa missions have studied asteroids for a protracted period in orbit and at the surface. Dawn explored Vesta from July 2011 to September 2012 and is currently orbiting Ceres (and sending back many interesting pictures of its surface features).

And someday, if all goes well, humanity might even be in a position to begin mining the asteroid belt for resources – such as precious metals, minerals, and volatiles. These resources could mined be from an asteroid and then used in space of in-situ utilization (i.e. turning them into construction materials and rocket propellant), or brought back to Earth.

It is even possible that humanity might one day colonize larger asteroids and establish outposts throughout the Belt. In the meantime, there’s still plenty of exploring left to do, and quite possibly millions of more objects out there to study.

We have written many articles about the asteroid belt for Universe Today. Here’s Where Do Asteroids Come From?, Why the Asteroid Belt Doesn’t Threaten Spacecraft, and Why isn’t the Asteroid Belt a Planet?.

Also, be sure to learn which is the Largest Asteroid in the Solar System, and about the asteroid named after Leonard Nimoy. And here’s 10 Interesting Facts about Asteroids.

We also have many interesting articles about the Dawn spacecraft’s mission to Vesta and Ceres, and asteroid mining.

To learn more, check out NASA’s Lunar and Planetary Science Page on asteroids, and the Hubblesite’s News Releases about Asteroids.

Astronomy Cast also some interesting episodes about asteroids, like Episode 55: The Asteroid Belt and Episode 29: Asteroids Make Bad Neighbors.


The Dwarf Planet Ceres

The Asteroid Belt is a pretty interesting place. In addition to containing between 2.8 and 3.2 quintillion metric tons of matter, the region is also home to many minor planets. The largest of these, known as Ceres, is not only the largest minor planet in the Inner Solar System, but also the only body in this region to be designated as a “dwarf planet” by the International Astronomical Union (IAU).

Due to its size and shape, when it was first observed, Ceres was thought to be a planet. While this belief has since been revised, Ceres is alone amongst objects in the Asteroid Belt in that it is the only object massive enough to have become spherical in shape. And like most of the dwarf planets in our Solar System, its status remains controversial, and our knowledge of it limited.

Discovery and Naming:

Ceres was discovered by Giuseppe Piazzi on January 1st, 1801, while searching for zodiacal stars. However, the existence of Ceres had been predicted decades before by Johann Elert Bode, a German astronomer who speculated that there had to be a planet between Mars and Jupiter. The basis for this assumption was the now defunct Bode-Titus law, which was first proposed by Johann Daniel Titius in 1766.

This law stated that there existed a regular pattern in the semi-major axes of the orbits of known planets, the only exception of which was the large gap between Mars and Jupiter. In an attempt to resolve this, in 1800, German astronomer Franz Xaver von Zach sent requests to twenty-four experienced astronomers (dubbed the “Celestial Police”) to combine their their efforts to located this missing planet.

Comparison of HST and Dawn FC images of Ceres taken nearly 11 years apart. Credit: NASA.
Comparison of HST and Dawn FC images of Ceres taken nearly 11 years apart. Credit: NASA.

One of these astronomers was Giuseppe Piazzi at the Academy of Palermo, who had made the discovery shortly before his invitation to join the group had arrived. At the time of his discovery, he mistook it for a comet, but subsequent observations led him to declare that it could be something more. He officially shared his observations with friends and colleagues by April of 1801, and sent the information to von Zach to be published in September.

Unfortunately, due to its change in its apparent position, Ceres was too close to the Sun’s glare to be visible to astronomers. It would not be until the end of the year that it would be spotted again, thanks in large part to German astronomer Carl Freidrich Gauss and the predictions he made of its orbit. On December 31st, von Zach and his colleague Heinrich W.M. Olbers found Ceres near the position predicted by Gauss, and thus recovered it.

Piazzi originally suggesting naming his discovery Cerere Ferdinandea, after the Roman goddess of agriculture Ceres (Cerere in Italian) and King Ferdinand of Sicily. The name Ferdinand was dropped in other nations, but Ceres was eventually retained. Ceres was also called Hera for a short time in Germany; whereas in Greece, it is still called Demeter after the Greek equivalent of the Roman goddess Ceres.


The classification of Ceres has changed more than once since its discovery, and remains the subject of controversy. For example, Johann Elert Bode – a contemporary of Piazzi –  believed Ceres to be the “missing planet” he had proposed to exist between Mars and Jupiter. Ceres was assigned a planetary symbol, and remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno, and 4 Vesta) until the mid-19th century.

Ceres compared to asteroids visited to date, including Vesta, Dawn's mapping target in 2011. Image by NASA/ESA. Compiled by Paul Schenck.
Ceres compared to asteroids visited to date, including Vesta, Dawn’s mapping target in 2011. Credit: NASA/ESA/Paul Schenck.

As other objects were discovered in the neighborhood of Ceres, it was realized that Ceres represented the first of a new class of objects. In 1802, with the discovery of 2 Pallas, William Herschel coined the term asteroid (“star-like”) for these bodies. As the first such body to be discovered, Ceres was given the designation 1 Ceres under the modern system of minor-planet designations.

By the 1860s, the existence of a fundamental difference between asteroids such as Ceres and the major planets was widely accepted, though a precise definition of “planet” was never formulated. The 2006 debate surrounding Eris, Pluto, and what constitutes a planet led to Ceres being considered for reclassification as a planet.

The definition that was adopted on August 24th, 2006 carried the requirements that a planet have sufficient mass to assume hydrostatic equilibrium, be in orbit around a star and not be a satellite, and have cleared the neighborhood around its orbit. As it is, Ceres does not dominate its orbit, but shares it with the thousands of other asteroids, and constitutes only about a third of the mass of the Asteroid Belt. Bodies like Ceres that met some of these qualification, but not all, were instead classified as “dwarf planets”.

In addition to the controversy surrounding the use of this term, there is also the question of whether or not Ceres status as a dwarf planet means that it can no longer be considered an asteroid. The 2006 IAU decision never addressed whether Ceres is an asteroid or not. In fact, the IAU has never defined the word ‘asteroid’ at all, having preferred the term ‘minor planet’ until 2006, and the terms ‘small Solar System body’ and ‘dwarf planet’ thereafter.

Size, Mass and Orbit:

Early observations of Ceres were only able to calculate its size to within an order of magnitude. In 1802, English astronomer William Herschel underestimated its diameter as 260 km, whereas in 1811 Johann Hieronymus Schröter overestimated it as 2,613 km. Current estimates place its mean radius at 473 km, and its mass at roughly 9.39 × 1020 kg (the equivalent of 0.00015 Earths or 0.0128 Moons).

Size comparison of Vesta, Eros and Ceres and Eros
Size comparison of Vesta, Eros and Ceres. Credit: NASA/JPL

With this mass, Ceres comprises approximately a third of the estimated total mass of the asteroid belt (which is in turn approximately 4% of the mass of the Moon). The next largest objects are Vesta, Pallas and Hygiea, which have mean diameters of more than 400 km and masses of 2.6 x 1020 kg, 2.11 x 1020 kg, and 8.6 ×1019 kg respectively. The mass of Ceres is large enough to give it a nearly spherical shape, which  makes it unique amongst objects and minor planets in the Asteroid Belt.

Ceres follows a slightly inclined and moderately eccentric orbit, ranging from 2.5577 AU (382.6 million km) from the Sun at perihelion and 2.9773 AU (445.4 million km) at aphelion. It has an orbital period of 1,680 Earth days (4.6 years) and takes 0.3781 Earth days (9 hours and 4 minutes) to complete a sidereal rotation.

Composition and Atmosphere:

Based on its size and density (2.16 g/cm³), Ceres is believed to be differentiated between a rocky core and an icy mantle. Based on evidence provided by the Keck telescope in 2002, the mantle is estimated to be 100 km-thick, and contains up to 200 million cubic km of water – which is more fresh water than exists on Earth. Infrared data on the surface also suggests that Ceres may have an ocean beneath its icy mantle.

If true, it is possible that this ocean could harbor microbial extraterrestrial life, similar to what has been proposed about Mars, Titan, Europa and Enceladus. It has further been hypothesized that ejecta from Ceres could have sent microbes to Earth in the past.

Other possible surface constituents include iron-rich clay minerals (cronstedtite) and carbonate minerals (dolomite and siderite), which are common minerals in carbonaceous chondrite meteorites. The surface of Ceres is relatively warm, with the maximum temperature estimated to reach approximately 235 K (-38 °C, -36 °F) when the Sun is overhead.

Assuming the presence of sufficient antifreeze (such as ammonia), the water ice would become unstable at this temperature. Therefore, it is possible that Ceres may have a tenuous atmosphere caused by outgassing from water ice on the surface. The detection of significant amounts of hydroxide ions near Ceres’ north pole, which is a product of water vapor dissociation by ultraviolet solar radiation, is another indication of this.

However, it was not until early 2014 that several localized mid-latitude sources of water vapor were detected on Ceres. Possible mechanisms for the vapor release include sublimation from exposed surface ice (as with comets), cryovolcanic eruptions resulting from internal heat, and subsurface pressurization. The limited amount of data suggests that the vaporization is more consistent with cometary-style sublimation.


Multiple theories exist as to the origin of Ceres. On the one hand, it is widely believed that Ceres is a surviving protoplanet which formed 4.57 billion year ago in the Asteroid Belt. Unlike other inner Solar System protoplanets, Ceres neither merged with others to form a terrestrial planet and avoided being ejected from the Solar System by Jupiter. However, there is an alternate theory that proposes that Ceres formed in the Kuiper belt and later migrated to the asteroid belt.

The geological evolution of Ceres is dependent on the heat sources that were available during and after its formation, which would have been provided by friction from planetesimal accretion and decay of various radionuclides. These are thought to have been sufficient to allow Ceres to differentiate into a rocky core and icy mantle soon after its formation. This icy surface would have gradually sublimated, leaving behind various hydrated minerals like clay minerals and carbonates.

Today, Ceres appears to be a geologically inactive body, with a surface sculpted only by impacts. The presence of significant amounts of water ice in its composition is what has led scientists to the possible conclusion that Ceres has or had a layer of liquid water in its interior.


Until recently, very few direct observations had been made of Ceres and little was known about its surface features. In 1995, the Hubble Space Telescope captured high-resolutions images that showed a dark spot in the surface that was thought to be a crater – and nicknamed “Piazzi” after its founder.

The near-infrared images taken by the Keck telescope in 2002 showed several bright and dark features moving with Ceres’s rotation. Two of the dark features had circular shapes and were presumed to be craters. One was identified as the “Piazzi” feature, while the other was observed to have a bright central region. In 2003 and 2004, visible-light images were taken by Hubble during a full rotation that showed 11 recognizable surface features, the natures of which are yet undetermined.

With the launch of the Dawn mission, with which NASA intends to conduct a nearly decade-long study of Ceres and Vesta, much more has been learned about this dwarf planet. For instance, after achieving orbit around the asteroid in March of 2015, Dawn revealed a large number of surface craters with low relief, indicating that they mark a relatively soft surface, most likely made of water ice.

Several bright spots have also been observed by Dawn, the brightest of which (“Spot 5”) is located in the middle of an 80 km (50 mi) crater called Occator. These bright features have an albedo of approximately 40% that are caused by a substance on the surface, possibly ice or salts, reflecting sunlight. A haze periodically appears above Spot 5, supporting the hypothesis that some sort of outgassing or sublimating ice formed the bright spots.

The Dawn spacecraft also noted the presence of a towering 6 kilometer-tall mountain (4 miles or 20,000 feet) in early August, 2015. This mountain, which is roughly pyramidal in shape and protrudes above otherwise smooth terrain, appears to be the only mountain of its kind on Ceres.

Like so many celestial bodies in our Solar System, Ceres is a mystery that scientists and astronomers are working to slowly unravel. In time, our exploration of this world will likely teach us much about the history and evolution of our Solar System, and may even lead to the discovery of life beyond Earth.

We have many interesting articles on Ceres here at Universe Today. For example, here are some articles on the many bright spots captured by the Dawn probe, and what they likely are.

And here are some articles on the Asteroid Belt and Why it Isn’t a Planet.

For more information, check out NASA’s Dawn – Ceres and Vesta and Dwarf Planets: Overview.

Mysterious Bright Spots and Pyramidal Mountain Star in Dawn’s Daunting Flyover of Ceres: Video

Video caption: Take a tour of weird Ceres! Visit a 2-mile-deep crater and a 4-mile-tall mountain in the video narrated by mission director Marc Rayman. Get your red/blue glasses ready for the finale – a global view of the dwarf planet in 3D. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI/PSI

Mysterious bright spots and a pyramidal shaped mountain star in a daunting new flyover video of dwarf planet Ceres created from imagery gathered by NASA’s history making Dawn mission – the first ever to visit any dwarf planet which simultaneously ranks as the largest world in the main asteroid belt residing between Mars and Jupiter.

Ceres was nothing more than a fuzzy blob to humankinds most powerful telescopes like the Hubble Space Telescope (HST), until the probe swooped in this year and achieved orbit on March 6, 2015.

The newly released, stunning video takes takes you on a tour like none before for a global cruise over the most fascinating features on Ceres – including the 2-mile-deep (4-km-deep) crater dubbed Occator and a towering 4-mile-tall (6 kilometer-tall) mountain as tall as any in North America.

The spectacular flyover animation was generated from high resolution images taken by Dawn’s framing camera during April and May and is narrated by Marc Rayman, Dawn Chief Engineer and Mission Director of NASA’s Jet Propulsion Laboratory, Pasadena, California.

The video concludes with a 3D view, so you’ll need to whip out your handy red/blue glasses for the finale – a global view of the dwarf planet in 3D.

From the orbital altitude at that time ranging from about 8,400 miles (13,600 kilometers) to 2,700 miles (4,400 kilometers), the highest-resolution regions on Ceres have a resolution of 1,600 feet (480 meters) per pixel.

Pockmarked Ceres is an alien world unlike any other in our solar system, replete with unexplained bright spots and craters of many sizes, large and small.

Occatur has captured popular fascination world-wide because the 60 miles (90 kilometers) diameter crater is rife with a host of the bodies brightest spots and whose nature remains elusive to this day, nearly half a year after Dawn arrived in orbit this past spring.

“Now, after a journey of 3.1 billion miles (4.9 billion kilometers) and 7.5 years, Dawn calls Ceres, home,” says Rayman.

The crater is named after the Roman agriculture deity of harrowing, a method of pulverizing and smoothing soil.

Dawn is an international science mission managed by NASA and equipped with a trio of science instruments from the US, Germany and Italy. The framing camera was provided by the Max Planck Institute for Solar System Research, Göttingen, Germany and the German Aerospace Center (DLR).

The visible and infrared mapping spectrometer (VIR), provided by Italy is an imaging spectrometer that examines Ceres in visible and infrared light.

Dawn’s science team is using the instruments to investigate the light reflecting from Occator at different wavelengths.

From a distance, the crater appeared to be home to a duo of bright spots that looked like a pair of eyes. As Dawn moves ever closer, they became more resolved and now are split into dozens of smaller bright spots.

Global view of Ceres uses data collected by NASA's Dawn mission in April and May 2015.  The highest-resolution parts of the map have a resolution of 1,600 feet (480 meters) per pixel.  Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI/PSI
Global view of Ceres uses data collected by NASA’s Dawn mission in April and May 2015. The highest-resolution parts of the map have a resolution of 1,600 feet (480 meters) per pixel. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI/PSI

Although some early speculation centered on the spots possibly being consistent with water ice or salts, newly gathered data “has not found evidence that is consistent with ice. The spots’ albedo -¬ a measure of the amount of light reflected -¬ is also lower than predictions for concentrations of ice at the surface,” according to the scientists.

“The science team is continuing to evaluate the data and discuss theories about these bright spots at Occator,” said Chris Russell, Dawn’s principal investigator at the University of California, Los Angeles, in a statement.

“We are now comparing the spots with the reflective properties of salt, but we are still puzzled by their source. We look forward to new, higher-resolution data from the mission’s next orbital phase.”
Occator lies in Ceres northern hemisphere.

The huge pyramidal mountain lies farther to the southeast of Occator – at 11 degrees south, 316 degrees east.

Based on the latest calculations, the mountain sits about 4 miles (6 kilometers) high, with respect to the surface around it. That make it roughly the same elevation as Mount McKinley in Denali National Park, Alaska, the highest point in North America.

Among the highest features seen on Ceres so far is a mountain about 4 miles (6 kilometers) high, which is roughly the elevation of Mount McKinley in Alaska's Denali National Park.  Vertical relief has been exaggerated by a factor of five to help understand the topography. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI
Among the highest features seen on Ceres so far is a mountain about 4 miles (6 kilometers) high, which is roughly the elevation of Mount McKinley in Alaska’s Denali National Park. Vertical relief has been exaggerated by a factor of five to help understand the topography. Credits: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/LPI

The Texas-sized world is slightly smaller than previously thought. Based on new measurements from Dawn, Ceres’ average diameter to 584 miles (940 kilometers), compared to earlier estimates of 590 miles (950 kilometers).

Dawn made history in March when it simultaneously became the first probe from Earth to reach Ceres as well as the first spacecraft to orbit two extraterrestrial bodies.

It had previously visited Vesta. After achieving orbit in July 2011, Dawn became the first spacecraft from Earth to orbit a body in the main Asteroid Belt.

In sharp contrast to rocky Vesta, Ceres is an icy world.

Scientists believe that Ceres may harbor an ocean of subsurface liquid water as large in volume as the oceans of Earth below a thick icy mantle despite its small size – and thus could be a potential abode for life. Overall Ceres is estimated to be about 25% water by mass.

“We really appreciate the interest in our mission and hope they are as excited as we have been about these scientific surprises,” Russell told Universe Today.

“Since we are only just beginning our investigation, I expect that there will be more surprises. So please stick with us!”

As Dawn spirals down to a lower orbit of about 1,200 miles (1,900 km) above Ceres (and then even lower) using its ion engines, new answers and new mysteries are sure to be forthcoming.

“There are many other features that we are interested in studying further,” said Dawn science team member David O’Brien, with the Planetary Science Institute, Tucson, Arizona.

“These include a pair of large impact basins called Urvara and Yalode in the southern hemisphere, which have numerous cracks extending away from them, and the large impact basin Kerwan, whose center is just south of the equator.”

The mission is expected to last until at least June 2016 depending upon fuel reserves.

Dawn was launched on September 27, 2007 by a United Launch Alliance (ULA) Delta II Heavy rocket from Space Launch Complex-17B (SLC-17B) at Cape Canaveral Air Force Station, Florida.

Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.

Ken Kremer

Dawn Does Dramatic Fly Over of Ceres, Enters Lower Mapping Orbit: Video

Video caption: This new video animation of Ceres was created from images taken by NASA’s Dawn spacecraft at altitudes of 8,400 miles (13,600 kilometers) and 3,200 miles (5,100 kilometers) away. Vertical dimension has been exaggerated by a factor of two and a star field added. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Scientists leading NASA’s Dawn mission to dwarf planet Ceres have just released a brand new animated video showing a dramatic fly over of the heavily cratered world featuring its mysterious bright spots whose exact origin and nature remain elusive.

Meanwhile, the venerable probe has just successfully entered its new and lower mapping orbit on June 3 from which researchers hope to glean hordes of new data to unravel the secrets of the bright spots and unlock the nature of Ceres origin and evolution.

Pockmarked Ceres is an alien world unlike any other in our solar system.

“Dawn completed the maneuvering to reach its second mapping orbit and stopped ion-thrusting on schedule. Since May 9, the spacecraft has reduced its orbital altitude from 8,400 miles (13,600 kilometers) to 2,700 miles (4,400 kilometers),” reported Marc Rayman, Dawn Chief Engineer/ Mission Director of NASA’s Jet Propulsion Laboratory, Pasadena, California.

“As Dawn flew 2,700 miles (4,400 kilometers) over Ceres’ north pole on June 5 that marked the beginning of the new mapping phase, and Dawn began taking photos and making other measurements on schedule.”

Each orbit of Dawn around Ceres at this second science mapping orbit lasts 3.1 days.

The new video was created by the research team based on observations of Ceres that were taken from Dawn’s initial mapping orbit, at an altitude of 8,400 miles (13,600 kilometers), as well as the most recent navigational images taken from 3,200 miles (5,100 kilometers), according to NASA.

It is based on data from over 80 images captured by Dawn’s framing cameras which were provided The German Aerospace Center (DLR) and Max Planck Institute for Solar System Research in Göttingen, Germany.

The images were used to provide a three-dimensional video view. The vertical dimension is exaggerated by a factor of two in the video.

“We used a three-dimensional terrain model that we had produced based on the images acquired so far,” said Dawn team member Ralf Jaumann of the German Aerospace Center (DLR), in Berlin.

“They will become increasingly detailed as the mission progresses — with each additional orbit bringing us closer to the surface.”

Imagery of the mysterious bright spots show them to seemingly be sheets of many spots of water ice, and not just single huge patches. The famous duo of ice spots are located inside the middle of a 57 miles (92 kilometers) wide crater situated in Ceres northern hemisphere.

Dawn is an international science mission managed by NASA’s Jet Propulsion Laboratory, Pasadena, California. The trio of science instruments are from the US, Germany and Italy.

The framing camera was provided by the Max Planck Institute for Solar System Research, Göttingen, Germany and the German Aerospace Center (DLR).

This view of Ceres was taken by Dawn spacecraft on May 23 and shows finer detail becoming visible on the dwarf planet. The spacecraft snapped the image at a distance of 3,200 miles (5,100 kilometers) with a resolution of 1,600 feet (480 meters) per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
This view of Ceres was taken by Dawn spacecraft on May 23 and shows finer detail becoming visible on the dwarf planet. The spacecraft snapped the image at a distance of 3,200 miles (5,100 kilometers) with a resolution of 1,600 feet (480 meters) per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dawn will spend most if June at this second mapping orbit before firing up the ion engines and spiraling yet lower for a mission expected to last until at least June 2016.

Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.

Ken Kremer

Dawn’s spiral descent from its first mapping orbit (RC3) to its second (survey). The two mapping orbits are shown in green. The color of Dawn’s trajectory progresses through the spectrum from blue, when it began ion-thrusting on May 9, to red, when ion-thrusting concludes on June 3. The red dashed sections show where Dawn is coasting, mostly for telecommunications. The first two coast periods include OpNav 8 and 9. Image credit: NASA/JPL-Caltech
Dawn’s spiral descent from its first mapping orbit (RC3) to its second (survey). The two mapping orbits are shown in green. The color of Dawn’s trajectory progresses through the spectrum from blue, when it began ion-thrusting on May 9, to red, when ion-thrusting concludes on June 3. The red dashed sections show where Dawn is coasting, mostly for telecommunications. The first two coast periods include OpNav 8 and 9. Image credit: NASA/JPL-Caltech