I look forward to all the future missions that NASA is going to be sending out in the Solar System. Here, check this out. You can use NASA’s website to show you all the future missions. Here’s everything planned for the future, here’s everything going to Mars.
Now, let’s look and see what missions are planned for the outer planets of the Solar System, especially Uranus and Neptune. Oh, that’s so sad… there’s nothing.
Uranus, seen by Voyager 2. Image credit: NASA/JPL
It’s been decades since humanity had an up close look at Uranus and Neptune. For Uranus, it was Voyager 2, which swept through the system in 1986. We got just a few tantalizing photographs of the ice giant planet and it’s moons.
Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL
What’s that?
Oberon, as imaged by the Voyager 2 probe during its flyby on Jan. 24, 1986. Credit: NASA
What’s going on there?
Color composite of the Uranian satellite Miranda, taken by Voyager 2 on Jan. 24, 1986, from a distance of 147,000 km (91,000 mi). Credit: NASA/JPL
What are those strange features? Sorry, insufficient data.
And then Voyager 2 did the same, zipping past Neptune in 1989.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Check this out.
Neptune’s largest moon Triton photographed on August 25, 1989 by Voyager 2. Credit: NASA
What’s going here on Triton? Wouldn’t you like to know more? Well, too bad! You can’t it’s done, that’s all you get.
Don’t get me wrong, I’m glad we’ve studied all these other worlds. I’m glad we’ve had orbiters at Mercury, Venus, everything at Mars, Jupiter, and especially Saturn. We’ve seen Ceres and Vesta, and the Moon up close. We even got a flyby of Pluto and Charon.
It’s time to go back to Uranus and Neptune, this time to stay.
And I’m not the only one who feels this way.
Scientists at NASA recently published a report called the Ice Giant Mission Study, and it’s all about various missions that could be sent to explore Uranus, Neptune and their fascinating moons.
The team of scientists who worked on the study considered a range of potential missions to the ice giants, and in the end settled on four potential missions; three that could go to Uranus, and one headed for Neptune. Each of them would cost roughly $2 billion.
Uranus is closer, easier to get to, and the obvious first destination of a targeted mission. For Uranus, NASA considered three probes.
The first idea is a flyby mission, which will sweep past Uranus gathering as much science as it can. This is what Voyager 2 did, and more recently what NASA’s New Horizons did at Pluto. In addition, it would have a separate probe, like the Cassini and Galileo missions, that would detach and go into the atmosphere to sample the composition below the cloudtops. The mission would be heavy and require an Atlas V rocket with the same configuration that sent Curiosity to Mars. The flight time would take 10 years.
NASA’s Curiosity Mars Science Laboratory (MSL) rover blasts off for Mars atop a stunningly beautiful Atlas V rocket. Credit: Ken Kremer – kenkremer.com
The main science goal of this mission would be to study the composition of Uranus. It would make some other measurements of the system as it passed through, but it would just be a glimpse. Better than Voyager, but nothing like Cassini’s decade plus observations of Saturn.
I like where this is going, but I’m going to hold out for something better.
The next idea is an orbiter. Now we’re talking! It would have all the same instruments as the flyby and the detachable probe. But because it would be an orbiter, it would require much more propellant. It would have triple the launch mass of the flyby mission, which means a heavier Atlas V rocket. And a slightly longer flight time; 12 years instead of 10 for the flyby.
Because it would remain at Uranus for at least 3 years, it would be able to do an extensive analysis of the planet and its rings and moons. But because of the atmospheric probe, it wouldn’t have enough mass for more instruments. It would have more time at Uranus, but not a much better set of tools to study it with.
Okay, let’s keep going. The next idea is an orbiter, but without the detachable probe. Instead, it’ll have the full suite of 15 scientific instruments, to study Uranus from every angle. We’re talking visible, doppler, infrared, ultraviolet, thermal, dust, and a fancy wide angle camera to give us those sweet planetary pictures we like to see.
Study Uranus? Yes please. But while we’re at it, let’s also sent a spacecraft to Neptune.
The labeled ring arcs of Neptune as seen in newly processed data. The image spans 26 exposures combined into a equivalent 95 minute exposure, and the ring trace and an image of the occulted planet Neptune is added for reference. (Credit: M. Showalter/SETI Institute).
As part of the Ice Giants Study, the researchers looked at what kind of missions would be possible. In this case, they settled on a single recommended mission. A huge orbiter with an additional atmospheric probe. This mission would be almost twice as massive as the heaviest Uranus mission, so it would need a Delta IV Heavy rocket to even get out to Neptune.
As it approached Neptune, the mission would release an atmospheric probe to descend beneath the cloudtops and sample what’s down there. The orbiter would then spend an additional 2 years in the environment of Neptune, studying the planet and its moons and rings. It would give us a chance to see its fascinating moon Triton up close, which seems to be a captured Kuiper Belt Object.
Unfortunately there’s no perfect grand tour trajectory available to us any more, where a single spacecraft could visit all the large planets in the Solar System. Missions to Uranus and Neptune will have to be separate, however, if NASA’s Space Launch System gets going, it could carry probes for both destinations and launch them together.
The goal of these missions is the science. We want to understand the ice giants of the outer Solar System, which are quite different from both the inner terrestrial planets and the gas giants Jupiter and Saturn.
The Solar System. Credit: NASA
The gas giants are mostly hydrogen and helium, like the Sun. But the ice giants are 65% water and other ices made from methane and ammonia. But it’s not like they’re big blobs of water, or even frozen water. Because of their huge gravity, the ice giants crush this material with enormous pressure and temperature.
What happens when you crush water under this much pressure? It would all depend on the temperature and pressure. There could be different types of ice down there. At one level, it could be an electrically conductive soup of hydrogen and oxygen, and then further down, you might get crystallized oxygen with hydrogen ions running through it.
Hailstones made of diamond could form out of the carbon-rich methane and fall down through the layers of the planets, settling within a molten carbon core. What I’m saying is, it could be pretty strange down there.
We know that ice giants are common in the galaxy, in fact, they’ve made up the majority of the extrasolar planets discovered so far. By better understanding the ones we have right here in our own Solar System, we can get a sense of the distant extrasolar planets turning up. We’ll be better able to distinguish between the super earths and mini-neptunes.
Artist’s impression of the Milky Way’s 100 billion exoplanets. Credit: NASA, ESA, and M. Kornmesser (ESO)
Another big question is how these planets formed in the first place. In their current models, most planetary astronomers think these planets had very short time windows to form. They needed to have massive enough cores to scoop up all that material before the newly forming Sun’s solar wind blasted it all out into space. And yet, why are these kinds of planets so common in the Universe?
The NASA mission planners developed a total of 12 science objectives for these missions, focusing on the composition of the planets and their atmospheres. And if there’s time, they’d like to know about how heat moves around, their constellations of rings and moons. They’d especially like to investigate Neptune’s moons Triton, which looks like a captured Kuiper Belt Object, as it orbits in the reverse direction from all the other moons in the Solar System.
In terms of science, the two worlds are very similar. But because Neptune has Triton. If I had to choose, I’d go with a Neptune mission.
Neptune and its large moon Triton as seen by Voyager 2 on August 28th, 1989. (Credit: NASA).
Are you excited? I’m excited. Here’s the bad news. According to NASA, the best launch windows for these missions would be 2029 or 2034. And that’s just the launch time, the flight time is an additional decade or more on top of that. In other words, the first photos from a Uranus flyby could happen in 2039 or 2035, while orbiters could arrive at either planet in the 2040s. I’m sure my future grandchildren will enjoy watching these missions arrive.
But then, we have to keep everything in perspective. NASA’s Cassini mission was under development in the 1980s. It didn’t launch until 1997, and it didn’t get to Saturn until 2004. It’s been almost 20 years since that launch, and almost 40 years since they started working on it.
I guess we need to be more patient. I can be patient.
The interior structure of Neptune. Credit: Moscow Institute of Physics and Technology
Since it’s discovery in the mid-19th century, Neptune has consistently been a planet of mystery. As the farthest planet from our Sun, it has only been visited by a single robotic mission. And there are still many unanswered questions about what kind of mechanics power its interior. Nevertheless, what we have learned about the planet in the course of the past few decades is considerable.
For example, thanks to the Voyager 2 probe and multiple surveys using Earth-based instruments, scientists have managed to gain a pretty good understanding of Neptune’s structure and composition. In addition to knowing what makes up its atmosphere, planetary models have also predicted what the interior of the planet looks like. So just what is Neptune made of?
Structure and Composition:
Neptune, like the rest of the gas giant planets in the Solar System, can be broken up into various layers. The composition of Neptune changes depending on which of these layers you’re looking at. The outermost layer of Neptune is the atmosphere, forming about 5-10% of the planet’s mass, and extending up to 20% of the way down to its core.
Composition and interior structure of Neptune. Credit: NASA
Beneath the atmosphere is the planet’s large mantle. This is a superheated liquid region where temperatures can reach as high as 2,000 to 5,000 K (1727 – 4727 °C; 3140 – 8540 °F). 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”.
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. Like the other gas/ice giants, Neptune is believed to have a solid core, the composition of which is still subject to guesswork. However, the theory that it is rocky and metal-rich is consistent with current theories of planet formation.
In accordance with these theories, 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.
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.
Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons
Neptune’s Atmosphere:
Neptune’s 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. 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.
Color and contrast-modified image that emphasizes Neptune’s atmospheric features. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. 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
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.
Exploration:
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 1‘s 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.
Given its distance from Earth, it is no secret why the Trans-Neptunian region remains mysterious to us. In the coming decades, several proposed missions are expected to travel there and explore its rich population of icy bodies and the giant planet for which it is named. From these studies, we are likely to learn a great deal about Neptune and the history of the Solar System.
Special Guest:
Tim Blais is the founder of A Capella Science, an “educational and utterly nerdy online video project.” You can find his videos online on YouTube at A Capella Science.
We use a tool called Trello to submit and vote on stories we would like to see covered each week, and then Fraser will be selecting the stories from there. Here is the link to the Trello WSH page (http://bit.ly/WSHVote), which you can see without logging in. If you’d like to vote, just create a login and help us decide what to cover!
Announcements:
On Friday, May 12, the WSH will welcome authors Michael Summers and James Trefil to the show to discuss their new book, Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets and the New Search for Life Beyond Our Solar System. In anticipation of their appearance, the WSH Crew is pleased to offer our viewers a chance to win one of two hard cover copies of Exoplanets. Two winners will be drawn live by @fraser during our show on May 12th. To enter for a chance to win a copy of Exoplanets, send an email to: [email protected] with the Subject: Exoplanets. Be sure to include your name and email address in the body of your message so that we can contact the winners afterward. All entries must be electronically postmarked by 23:59 EST on May 10, 2017, in order to be eligible. No purchase necessary. Two winners will be selected at random from all eligible entries. Good luck!
If you’d like to join Fraser and Paul Matt Sutter on their Tour to Iceland in February 2018, you can find the information at astrotouring.com.
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Here on Earth, a year lasts roughly 365.25 days, each of which lasts 24 hours long. During the course of a single year, our planet goes through some rather pronounced seasonal changes. This is the product of our orbital period, our rotational period, and our axial tilt. And when it comes to the other planets in our Solar System, much the same is true.
Consider Neptune. As the eight and farthest planet from the Sun, Neptune has an extremely wide orbit and a comparatively slow orbital velocity. As a result, a year on Neptune is very long, lasting the equivalent of almost 165 Earth years. Combined with its extreme axial tilt, this also means that Neptune experiences some rather extreme seasonal changes.
Orbital Period:
Neptune orbits our Sun at an average distance (semi-major axis) of 4,504.45 million km (2,798.656 million mi; 30.11 AU). Because of its orbital eccentricity (0.009456), this distance varies somewhat, ranging from 4,460 million km (2,771 million mi; 29.81 AU) at its closest (perihelion) to 4,540 million km (2,821 million mi; 30.33 AU) at its farthest (aphelion).
The orbit of Neptune and the other outer Solar planets, as well as the ice-rich Kuiper Belt that lies just beyond it. Credit: NASA
With an average orbital speed of 5.43 km/s, it takes Neptune 164.8 Earth years (60,182 Earth days) to complete a single orbital period. This means, in effect, that a year on Neptune lasts as long as about 165 years here on Earth. However, given its rotational period of 0.6713 Earth days (16 hours 6 minutes 36 seconds), a year on Neptune works out to 89,666 Neptunian solar days.
Given that Neptune was discovered in 1846, humanity has only known about its existence for 171 years (at the time of this article’s writing). That means that since its discovery, the planet has only completed a single orbital period (which ended in 2010) and is only seven years into its second. This orbital period will be complete by 2179.
Orbital Resonance:
Because of its location in the outer Solar System, Neptune’s orbit has a profound impact on the neighboring Kuiper Belt. This region, which is similar (but significantly larger) than the Main Asteroid Belt, consists of many small icy worlds and objects that extends from Neptune’s orbit (at 30 AU) to a distance of about 55 AU from the Sun.
Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign
So much as Jupiter’s gravity has dominated the Asteroid Belt, affecting its structure and occasionally kicking asteroids and planetoids into the inner Solar System, Neptune’s gravity dominates the Kuiper Belt. This has led to the creation of gaps in the belt, empty regions where objects have achieved an orbital resonance with Neptune.
Within these gaps, objects have a 1:2, 2:3 or 3:4 resonance with Neptune, meaning they complete one orbit of the Sun for every two completed by Neptune, two for every three, or three for every four. The over 200 known objects that exist in the 2:3 resonance (the most populous) are known as plutinos, since Pluto is the largest of them.
Although Pluto crosses Neptune’s orbit on a regular basis, their 2:3 orbital resonance ensures they can never collide. On occasion, Neptune’s gravity also causes icy bodies to be kicked out of the Kuiper Belt. Many of these then travel to the Inner Solar System, where they become comets with extremely long orbital periods.
Neptune’s largest satellite, Triton, is believed to have once been a Kuiper Belt Object (KBO) – and Trans-Neptunian Object (TNO) – that was captured by Neptune’s gravity. This is evidenced by its retrograde motion, meaning it orbits the planet in the opposite direction as its other satellites. It also has a number of Trojan Objects occupying its L4 and L5 Lagrange points. These “Neptune Trojans” can be said to be in a stable 1:1 orbital resonance with Neptune.
Seasonal Change:
Much like the other planets of the Solar System, Neptune’s axis is tilted towards the Sun’s ecliptic. In Neptune’s case, it is tilted 28.32° relative to its orbit (whereas Earth is tilted at 23.5°). Because of this, Neptune undergoes seasonal change during the course of a year because one of its hemispheres will be receiving more sunlight than the other. But in Neptune’s case, a single season lasts a whopping 40 years, making it very hard to witness a full cycle.
While much of the heat that powers Neptune’s atmosphere comes from an internal source (which is currently unknown), a study conducted by researchers from Wisconsin-Madison University and NASA’s Jet Propulsion Laboratory revealed that seasonal change is also driven by solar radiation. This consisted of examining images of Neptune taken by the Hubble Space Telescope between 1996 and 2002.
These images revealed that Neptune’s massive southern cloud bands were becoming steadily wider and brighter over the six year period – which coincided with the southern hemisphere beginning its 40-year summer. This growing cloud cover was attributed to increased solar heating, as it appeared to be concentrated in the southern hemisphere and was rather limited at the equator.
Images taken by Hubble, showing seasonal change in its southern hemisphere. Credit: NASA, L. Sromovsky, and P. Fry (University of Wisconsin-Madison)
Neptune remains a planet of mystery in many ways. And yet, ongoing observations of the planet have revealed some familiar and comforting patterns. For instance, while it’s composition is vastly different and its orbit puts it much farther away from the Sun than Earth, its axial tilt and orbital period still result in its hemispheres experiencing seasonal changes.
It’s good to know that no matter how far we venture out into the Solar System, and no matter how different things may seem, there are still some things that stay the same!
New Horizon's July 2015 flyby of Pluto captured this iconic image of the heart-shaped region called Tombaugh Regio. Credit:
NASA/JHUAPL/SwRI.
Discovered in 1930 by Clyde Tombaugh, Pluto was once thought to be the ninth and outermost planet of the Solar System. However, due to the formal definition adopted in 2006 at the 26th General Assembly of the International Astronomical Union (IAU), Pluto ceased being the ninth planet of the Solar System and has become alternately known as a “Dwarf Planet”, “Plutiod”, Trans-Neptunian Object (TNO) and Kuiper Belt Object (KBO).
Despite this change of designation, Pluto remains one of the most fascinating celestial bodies known to astronomers. In addition to having a very distant orbit around the Sun (and hence a very long orbital period) it also has the most eccentric orbit of any planet or minor planet in the Solar System. This makes for a rather long year on Pluto, which lasts the equivalent of 248 Earth years!
Orbital Period:
With an extreme eccentricity of 0.2488, Pluto’s distance from the Sun ranges from 4,436,820,000 km (2,756,912,133 mi) at perihelion to 7,375,930,000 km (4,583,190,418 mi) at aphelion. Meanwhile, it’s average distance (semi-major axis) from the Sun is 5,906,380,000 km (3,670,054,382 mi). Another way to look at it would be to say that it orbits the Sun at an average distance of 39.48 AU, ranging from 29.658 to 49.305 AU.
New Horizons trajectory and the orbits of Pluto and 2014 MU69.
At its closest, Pluto actually crosses Neptune’s orbit and gets closer to the Sun. This orbital pattern takes place once every 500 years, after which the two objects then return to their initial positions and the cycle repeats. Their orbits also place them in a 2:3 mean-motion resonance, which means that for every two orbits Pluto makes around the Sun, Neptune makes three.
The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years. The last time this cycle took place was between 1979 to 1999, when Neptune was farther from the Sun than Pluto. Pluto reached perihelion in this cycle – i.e. its closest point to the Sun – on September 5th, 1989. Since 1999, Pluto returned to a position beyond that of Neptune, where it will remain for the following 228 years – i.e. until the year 2227.
Sidereal and Solar Day:
Much like the other bodies in our Solar System, Pluto also rotates on its axis. The time it takes for it to complete a single rotation on its axis is known as a “Sidereal Day”, while the amount of time it takes for the Sun to reach the same point in the sky is known as a “Solar Day”. But due to Pluto’s very long orbital period, a sidereal day and a solar day on Pluto are about the same – 6.4 Earth days (or 6 days, 9 hours, and 36 minutes).
View from the surface of Pluto, showing its large moon Charon in the distance. Credit: New York Time
It is also worth noting that Pluto and Charon (its largest moon) are actually more akin to a binary system rather than a planet-moon system. This means that the two worlds orbit each other, and that Charon is tidally locked around Pluto. In other words, Charon takes 6 days and 9 hours to orbit around Pluto – the same amount of time it takes for a day on Pluto. This also means that Charon is always in the same place in the sky when seen from Pluto.
In short, a single day on Pluto lasts the equivalent of about six and a half Earth days. A year on Pluto, meanwhile, lasts the equivalent of 248 Earth years, or 90,560 Earth days! And for the entire year, the moon is hanging overhead and looming large in the sky. But factor in Pluto’s axial tilt, and you will come to see just how odd an average year on Pluto is.
Seasonal Change:
It has been estimated that for someone standing on the surface of Pluto, the Sun would appear about 1,000 times dimmer than it appears from Earth. So while the Sun would still be the brightest object in the sky, it would look more like a very bright star that a big yellow disk. But despite being very far from the Sun at any given time, Pluto’s eccentric orbit still results in some considerable seasonal variations.
On the whole, the surface temperature of Pluto does not change much. It’s surface temperatures are estimated to range from a low of 33 K (-240 °C; -400 °F ) to a high of 55 K (-218 °C; -360°F) – averaging at around 44 K (-229 °C; -380 °F). However, the amount of sunlight each side receives during the course of a year is vastly different.
Compared to most of the planets and their moons, the Pluto-Charon system is oriented perpendicular to its orbit. Much like Uranus, Pluto’s very high axial tilt (122 degrees) essentially means that it is orbiting on its side relative to its orbital plane. This means that at a solstice, one-quarter of Pluto’s surface experiences continuous daylight while the other experiences continuous darkness.
This is similar to what happens in the Arctic Circle, where the summer solstice is characterized by perpetual sunlight (i.e. the “Midnight Sun”) and the winter solstice by perpetual night (“Arctic Darkness”). But on Pluto, these phenomena affect nearly the entire planet, and the seasons last for close to a century.
Even if it is no longer considered a planet (though this could still change) Pluto still has some very fascinating quarks, all of which are just as worthy of study as those of the other eight planets. And the time it takes to complete a full year on Pluto, and all the seasonal changes it goes through, certainly rank among the top ten!
For more information, be sure to check out NASA’s Solar System Exploration page on Pluto, and the New Horizon’s mission page for information on Pluto’s seasons.
Before we really get started on today’s episode, I’d like to share a bunch of really cool pictures created by my friend Kevin Gill. Kevin’s a computer programmer, 3-D animator and works on climate science data for NASA.
And in his spare time, he uses his skills to help him imagine what the Universe could look like. For example, he’s mapped out what a future terraformed Mars might look like based on elevation maps, or rendered moons disturbing Saturn’s rings with their gravity.
Earth’s Rings over San Bernadino. Credit: Kevin Gill (CC BY-SA 2.0)
But one of my favorite sets of images that Kevin did were these. What would it look like if Earth had rings? Kevin and his wife went to a few cool locations, took some landscape pictures, and then Kevin did the calculations for what it would look like if Earth had a set of rings like Saturn.
And let me tell you, Earth would be so much better. At least you’d think so, but actually, it might also suck.
Last time I checked, we don’t have rings like this. In fact, we don’t have any rings at all.
Why not? Considering the fact that Saturn, Jupiter, Uranus and Neptune all have rings, don’t we deserve at least something?
Did we ever have rings in the past, or will we in the future? What’s it going to take for us to join the ring club? Short answer, an apocalypse.
Before we get into the inevitable discussion of death and devastation, let’s talk a bit about rings.
A lovely view of Saturn and its rings as seen by the Cassini spacecraft on Aug. 12, 2009. Credit: NASA/JPL-Caltech/Space Science Institute.
Saturn is the big showboat, with its fancy rings. They’re made of water ice, with chunks as big as a mountain, or as small as a piece of sand. Astronomers have been arguing about where they came from and how old they are, but the current consensus – sort of – is that the rings are almost as ancient as Saturn itself: billions of years old. And yet, some process is weathering the rings, grinding the particles so they appear much younger.
Jupiter’s rings. Image Credit: University of Maryland
Jupiter’s rings are much fainter, and we didn’t even know about them until 1979, when the Voyager spacecraft made their flybys. The rings seem to be created by dust blown off into space by impacts on the planet’s moons.
Hey, we’ve got a moon, that’s a sign.
Uranus imaged by Voyager 2 in 1986. Credit: NASA
The rings around Uranus are bigger and more complex than Jupiter’s rings, but not as substantial as Saturn’s. They’re much younger, perhaps only 600 million years old, and appear to have been caused by two moons crashing into each other, long ago.
Again, another sign. We still have the potential for stuff to crash around us.
The labeled ring arcs of Neptune as seen in newly processed data. The image spans 26 exposures combined into a equivalent 95 minute exposure, and the ring trace and an image of the occulted planet Neptune is added for reference. Credit: M. Showalter/SETI Institute
The rings around Neptune are far dustier than any of the other ring systems, and much younger than the Solar System. And like the rings around Uranus, they were probably formed when two or more of its moons collided together.
Now what about our own prospects for rings?
The problem with icy rings is that the Earth orbits too closely to the Sun. There’s a specific point in the Solar System known as the “frost line” or “snow line”. This is the point in the Solar System where deposits of ice could have survived for long periods of time. Any closer and the radiation from the Sun sublimates the ice away.
This point is actually located about 5 astronomical units away from the Sun, in the asteroid belt. Mars is much closer, so it’s very dry, while Jupiter is beyond the frost line, and its moons have plenty of water ice.
The Earth is a mere 1 AU from the Sun. That’s the very definition of an astronomical unit, which means it’s well within the frost line. The Earth itself can maintain water because the planet’s magnetosphere acts like a shield against the solar wind. But the Moon is bone dry (except for the permanently shadowed craters at its poles).
And if there was an icy ring system around the Earth, the solar wind would have blasted it away long ago.
Instead, let’s look at another kind of ring we can have. One made of rock and dust, containing death and sorrow, from a pulverized asteroid or moon. In fact, billions of years ago, we definitely had a ring when a Mars-sized planet crashed into the Earth and spewed out a massive ring of debris. This debris collected together into the Moon we know today. That impact turned the Earth’s surface inside out. It was all volcanoes, everywhere, all the time.
Credit: Kevin Gill (CC BY-SA 2.0)
It’s also possible we had a second moon in the ancient past, which collided with our current Moon. That would have generated an all new ring of material for millions of years until it was recaptured by the Moon, kicked out of orbit, or fell down onto the Earth.
It’s that “fell down onto Earth” part that’s apocalyptic. As mountains of ring material entered the Earth’s atmosphere, it would increase the temperature, baking and boiling away any life that couldn’t burrow deep underground.
It’s kind of like the book Seveneves, which you should totally read if you haven’t already. It talks about what we would see if the Moon broke apart into a ring, and the terrible terrible thing that happens next.
Earth’s Rings from New Hampshire. Credit: Kevin Gill (CC BY-SA 2.0)
If Earth did get a set of rings, they’d be pretty, but they’d also be a huge pain for astronomers. As you saw in Kevin’s original pictures, the rings take up a huge chunk of the sky for most observers. The farther north or south you go, the more dramatically the rings will ruin your view. Only if you were right at the equator, you’d have a thin line, which would be borderline acceptable.
Furthermore, the rings themselves would be incredibly reflective, and completely ruin the whole concept of dark skies. You know how the Moon sucks for astronomy? Rings would be way way worse.
Finally, rings would interfere with our ability to launch spacecraft and maintain satellites. It depends on how far they extend, but we wouldn’t be able to have any satellites in that region or cross the ring plane. Oh, and that fiery death apocalypse I mentioned earlier.
We know that the Moon is drifting away from the Earth right now thanks to the conservation of angular momentum. But in the distant future, billions of years from now, there might be a scenario that turns everything around.
The Sun’s habitable zone in its red giant phase. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab
As you know, when it runs out of fuel in its core, the Sun is going to bloat up as a red giant, consuming Mercury and Venus. Scientists are on the fence about Earth. Some think that Earth will be fine. The Sun will blast off its outer layers, but not actually envelop Earth. Others think that at the Sun’s largest point, we’ll be orbiting within the outer atmosphere of the Sun. Ouch, that’s hot.
The orbiting Moon will experience drag as it goes around the Earth, slowing down its orbital velocity, and causing it to spiral inward. Once it reaches the Roche Limit of the Earth, about 9,500 km, our planet’s gravity will tear the Moon apart into a ring. The chunks in the ring will also experience drag in the solar atmosphere and continue to spiral inward until they crash into the planet.
The Moon tearing apart to become a ring around Earth. Credit: Universe Sandbox ²
That would be considered a very bad day, if it wasn’t for the fact that we were already living inside the atmosphere of the Sun. No amount of terraforming will fix that.
Sadly, the Earth doesn’t have rings like Saturn, and it probably never did. It might have had rings of rock and dust for periods, but they weren’t that majestic to look at. In fact, seeing rings around the planet would mean we’d lost a moon, and our planet was about go through a period of bombardment. I’ll pass.
Okay, so this article is Colonizing the Outer Solar System, and is actually part 2 of our team up with Fraser Cain of Universe Today, who looked at colonizing the inner solar system. You might want jump over there now and watch that part first, if you are coming in from having seen part 1, welcome, it is great having you here.
Without further ado let us get started. There is no official demarcation between the inner and outer solar system but for today we will be beginning the outer solar system at the Asteroid Belt.
Artist concept of the asteroid belt. Credit: NASA
The Asteroid Belt is always of interest to us for colonization. We have talked about mining them before if you want the details on that but for today I’ll just remind everyone that there are very rich in metals, including precious metals like gold and platinum, and that provides all the motivation we need to colonize them. We have a lot of places to cover so we won’t repeat the details on that today.
You cannot terraform asteroids the way you could Venus or Mars so that you could walk around on them like Earth, but in every respect they have a lot going for them as a candidate. They’ve got plenty for rock and metal for construction, they have lots of the basic organic elements, and they even have some water. They also get a decent amount of sunlight, less than Mars let alone Earth, but still enough for use as a power source and to grow plants.
But they don’t have much gravity, which – pardon the pun – has its ups and downs. There just isn’t much mass in the Belt. The entire thing has only a small fraction of the mass of our moon, and over half of that is in the four biggest asteroids, essentially dwarf planets in their own right. The remainder is scattered over millions of asteroids. Even the biggest, Ceres, is only about 1% of 1% of Earth’s mass, has a surface gravity of 3% Earth-normal, and an escape velocity low enough most model rockets could get into orbit. And again, it is the biggest, most you could get away from by jumping hard and if you dropped an object on one it might take a few minutes to land.
Don’t blink… an artist’s conception of an asteroid blocking out a distant star. Image credit: NASA.
You can still terraform one though, by definition too. The gentleman who coined the term, science fiction author Jack Williamson, who also coined the term genetic engineering, used it for a smaller asteroid just a few kilometers across, so any definition of terraforming has to include tiny asteroids too.
Of course in that story it’s like a small planet because they had artificial gravity, we don’t, if we want to fake gravity without having mass we need to spin stuff around. So if we want to terraform an asteroid we need to hollow it out and fill it with air and spin it around.
Of course you do not actually hollow out the asteroid and spin it, asteroids are loose balls of gravel and most would fly apart given any noticeable spin. Instead you would hollow it out and set a cylinder spinning inside it. Sort of like how a good thermos has an outside container and inside one with a layer of vacuum in between, we would spin the inner cylinder.
You wouldn’t have to work hard to hollow out an asteroid either, most aren’t big enough to have sufficient gravity and pressure to crush an empty beer can even at their center. So you can pull matter out from them very easily and shore up the sides with very thin metal walls or even ice. Or just have your cylinder set inside a second non-spinning outer skin or superstructure, like your washer or dryer.
You can then conduct your mining from the inside, shielded from space. You could ever pressurize that hollowed out area if your spinning living area was inside its own superstructure. No gravity, but warmth and air, and you could get away with just a little spin without tearing it apart, maybe enough for plants to grow to normally.
It should be noted that you can potentially colonize even the gas giants themselves, even though our focus today is mostly on their moons. That requires a lot more effort and technology then the sorts of colonies we are discussing today, Fraser and I decided to keep things near-future and fairly low tech, though he actually did an article on colonizing Jupiter itself last year that was my main source material back before got to talking and decided to do a video together.
Jupiter with Io and Ganymede taken by amateur astronomer Damian Peach. Credit: NASA / Damian Peach
Hydrogen is plentiful on Jupiter itself and floating refineries or ships that fly down to scoop it up might be quite useful, but again today we are more interested in its moons. The biggest problem with colonizing the moons of Jupiter is all the radiation the planet gives off.
Europa is best known as a place where the surface is covered with ice but beneath it is thought to be a vast subsurface ocean. It is the sixth largest moon coming right behind our own at number five and is one of the original four moons Galileo discovered back in 1610, almost two centuries before we even discovered Uranus, so it has always been a source of interest. However as we have discovered more planets and moons we have come to believe quite a few of them might also have subsurface oceans too.
Now what is neat about them is that water, liquid water, always leaves the door open to the possibility of life already existing there. We still know so little about how life originally evolved and what conditions permit that to occur that we cannot rule out places like Europa already having their own plants and animals swimming around under that ice.
They probably do not and obviously we wouldn’t want to colonize them, beyond research bases, if they did, but if they do not they become excellent places to colonize. You could have submarine cities in such places floating around in the sea or those buried in the surface ice layer, well shielded from radiation and debris. The water also geysers up to the surface in some places so you can start off near those, you don’t have to drill down through kilometers of ice on day one.
Water, and hydrogen, are also quite uncommon in the inner solar system so having access to a place like Europa where the escape velocity is only about a fifth of our own is quite handy for export. Now as we move on to talk about moons a lot it is important to note that when I say something has a fifth of the escape velocity of Earth that doesn’t mean it is fives time easier to get off of. Energy rises with the square of velocity so if you need to go five times faster you need to spend 5-squared or 25 times more energy, and even more if that place has tons of air creating friction and drag, atmospheres are hard to claw your way up through though they make landing easier too. But even ignoring air friction you can move 25 liters of water off of Europa for every liter you could export from Earth and even it is a very high in gravity compared to most moons and comets. Plus we probably don’t want to export lots of water, or anything else, off of Earth anyway.
Artist’s concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA
We should start by noting two things. First, the Asteroid Belt is not the only place you find asteroids, Jupiter’s Trojan Asteroids are nearly as numerous, and every planet, including Earth, has an equivalent to Jupiter’s Trojan Asteroids at its own Lagrange Points with the Sun. Though just as Jupiter dwarfs all the other planets so to does its collection of Lagrangian objects. They can quite big too, the largest 624 Hektor, is 400 km across, and has a size and shape similar to Pennsylvania.
And as these asteroids are at stable Lagrange Points, they orbit with Jupiter but always ahead and behind it, making transit to and from Jupiter much easier and making good waypoints.
Before we go out any further in the solar system we should probably address how you get the energy to stay alive. Mars is already quite cold compared to Earth, and the Asteroids and Jupiter even more so, but with thick insulation and some mirrors to bounce light in you can do fairly decently. Indeed, sunlight out by Jupiter is already down to just 4% of what Earth gets, meaning at Jovian distances it is about 50 W/m²
That might not sound like much but it is actually almost a third of what average illumination is on Earth, when you factor in atmospheric reflection, cloudy days, nighttime, and higher, colder latitudes. It is also a good deal brighter than the inside of most well-lit buildings, and is enough for decently robust photosynthesis to grow food. Especially with supplemental light from mirrors or LED growth lamps.
But once you get out to Saturn and further that becomes increasingly impractical and a serious issue, because while food growth does not show up on your electric bill it is what we use virtually all our energy for. Closer in to the sun we can use solar panels for power and we do not need any power to grow food. As we get further out we cannot use solar and we need to heat or cold habitats and supply lighting for food, so we need a lot more power even as our main source dries up.
So what are our options? Well the first is simple, build bigger mirrors. A mirror can be quite large and paper thin after all. Alternatively we can build those mirrors far away, closer to the sun, and and either focus them on the place we want illuminated or send an energy beam, microwaves perhaps or lasers, out to the destination to supply energy.
We also have the option of using fission, if we can find enough Uranium or Thorium. There is not a lot of either in the solar system, in the area of about one part per billion, but that does amount to hundreds of trillions of tons, and it should only take a few thousand tons a year to supply Earth’s entire electric grid. So we would be looking at millions of years worth of energy supply.
Of course fusion is even better, particularly since hydrogen becomes much more abundant as you get further from the Sun. We do not have fusion yet, but it is a technology we can plan around probably having inside our lifetimes, and while uranium and thorium might be counted in parts per billion, hydrogen is more plentiful than every other element combines, especially once you get far from the Sun and Inner Solar System.
So it is much better power source, an effectively unlimited one except on time scales of billions and trillion of years. Still, if we do not have it, we still have other options. Bigger mirrors, beaming energy outwards from closer to the Sun, and classic fission of Uranium and Thorium. Access to fusion is not absolutely necessary but if you have it you can unlock the outer solar system because you have your energy supply, a cheap and abundant fuel supply, and much faster and cheaper spaceships.
Of course hydrogen, plain old vanilla hydrogen with one proton, like the sun uses for fusion, is harder to fuse than deuterium and may be a lot longer developing, we also have fusion using Helium-3 which has some advantages over hydrogen, so that is worth keeping in mind as well as we proceed outward.
Since NASA’s Cassini spacecraft arrived at Saturn, the planet’s appearance has changed greatly. This view shows Saturn’s northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Image credit: NASA/JPL-Caltech/Space Science Institute.
Okay, let’s move on to Saturn, and again our focus is on its moons more than the planet itself. The biggest of those an the most interesting for colonization is Titan.
Titan is aptly named, this titanic moon contains more mass than than all of Saturn’s sixty or so other moons and by an entire order of magnitude at that. It is massive enough to hold an atmosphere, and one where the surface pressure is 45% higher than here on Earth. Even though Titan is much smaller than Earth, its atmosphere is about 20% more massive than our own. It’s almost all nitrogen too, even more than our own atmosphere, so while you would need a breather mask to supply oxygen and it is also super-cold, so you’d need a thick insulated suit, it doesn’t have to be a pressure suit like it would on Mars or almost anyplace else.
There’s no oxygen in the atmosphere, what little isn’t nitrogen is mostly methane and hydrogen, but there is plenty of oxygen in the ice on Titan which is quite abundant. So it has everything we need for life except energy and gravity. At 14% of earth normal it is probably too low for people to comfortably and safely adapt to, but we’ve already discussed ways of dealing with that. It is low enough that you could probably flap your arms and fly, if you had wing attached.
On the left is TALISE (Titan Lake In-situ Sampling Propelled Explorer), the ESA proposal. This would have it’s own propulsion, in the form of paddlewheels. Credit: bisbos.com
It needs some source of energy though, and we discussed that. Obviously if you’ve got fusion you have all the hydrogen you need, but Titan is one of those places we would probably want to colonize early on if we could, it is something you need a lot of to terraform other places, and is also rich in a lot of the others things we want. So we often think of it as a low-tech colony since it is one we would want early on.
In an scenario like that it is very easy to imagine a lot of local transit between Titan and its smaller neighboring moons, which are more rocky and might be easier to dig fissile materials like Uranium and Thorium out of. You might have a dozen or so small outposts on neighboring moons mining fissile materials and other metals and a big central hub on Titan they delivered that too which also exported Nitrogen to other colonies in the solar system.
Moving back and forth between moons is pretty easy, especially since things landing on Titan can aerobrake quite easily, whereas Titan itself has a pretty strong gravity well and thick atmosphere to climb out of but is a good candidate for a space elevator, since it requires nothing more sophisticated than a Lunar Elevator on our own moon and has an abundant supply of the materials needed to make Zylon for instance, a material strong enough to make an elevator there and which we can mass manufacture right now.
Titan might be the largest and most useful of Saturn’s moons, but again it isn’t the only one and not all of the other are just rocks for mining. At last count it has over sixty and many of them quite large. One of those, Enceladus, Saturn’s sixth largest moon, is a lot like Jupiter’s Moon Europa, in that we believe it has a large and thick subsurface ocean. So just like Europa it is an interesting candidate for Colonization. So Titan might be the hub for Saturn but it wouldn’t be the only significant place to colonize.
While Saturn is best known for its amazing rings, they tend to be overlooked in colonization. Now those rings are almost all ice and in total mass about a quarter as much as Enceladus, which again is Saturn’s Sixth largest moon, which is itself not even a thousandth of the Mass of Titan.
In spite of that the rings are not a bad place to set up shop. Being mostly water, they are abundant in hydrogen for fusion fuel and have little mass individually makes them as easy to approach or leave as an asteroid. Just big icebergs in space really, and there are many moonlets in the rings that can be as large as half a kilometer across. So you can burrow down inside one for protection from radiation and impacts and possibly mine smaller ones for their ice to be brought to places where water is not abundant.
In total those rings, which are all frozen water, only mass about 2% of Earth’s oceans, and about as much as the entire Antarctic sheet. So it is a lot of fresh water that is very easy to access and move elsewhere, and ice mines in the rings of Saturn might be quite useful and make good homes. Living inside an iceball might not sound appealing but it is better than it sounds like and we will discuss that more when we reach the Kupier Belt.
Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons
But first we still have two more planets to look at, Uranus and Neptune.
Uranus, and Neptune, are sometimes known as Ice Giants instead of Gas Giants because it has a lot more water. It also has more ammonia and methane and all three get called ices in this context because they make up most of the solid matter when you get this far out in the solar system.
While Jupiter is over a thousand times the mass of Earth, Uranus weighs in at about 15 times the Earth and has only about double the escape velocity of Earth itself, the least of any of the gas giants, and it’s strange rotation, and its strange tilt contributes to it having much less wind than other giants. Additionally the gravity is just a little less than Earth’s in the atmosphere so we have the option for floating habitats again, though it would be a lot more like a submarine than a hot air balloon.
Like Venus, Uranus has very long days, at least in terms of places receiving continual sunlight, the poles get 42 years of perpetual sunlight then 42 of darkness. Sunlight being a relative term, the light is quite minimal especially inside the atmosphere. The low wind in many places makes it a good spot for gas extraction, such as Helium-3, and it’s a good planet to try to scoop gas from or even have permanent installations.
Now Uranus has a large collection of moons as well, useful and colonizable like the other moons we have looked at, but otherwise unremarkable beyond being named for characters from Shakespeare, rather than the more common mythological names. None have atmospheres though there is a possibility Oberon or Titania might have subsurface oceans.
Neptune makes for a brief entry, it is very similar to Uranus except it has the characteristically high winds of gas giants that Uranus’s skewed poles mitigate, meaning it has no advantages over Uranus and the disadvantages of high wind speeds everywhere and being even further from the Sun. It too has moons and one of them, Triton, is thought to have subsurface oceans as well. Triton also presumably has a good amount of nitrogen inside it since it often erupts geysers of nitrogen from its surface.
Neptune’s largest moon Triton photographed on August 25, 1989 by Voyager 2. Credit: NASA
Triton is one of the largest moons in the solar system, coming in seventh just after our Moon, number 5, and Europa at number 6. Meaning that were it not a moon it would probably qualify as a Dwarf Planet and it is often thought Pluto might be an escaped moon Neptune. So Triton might be one that didn’t escape, or didn’t avoid getting captured. In fact there are an awful lot of bodies in this general size range and composition wandering about in the outer regions of our solar system as we get out into the Kuiper Belt.
Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA
The Kuiper Belt is one of those things that has a claim on the somewhat arbitrary and hazy boundary marking the edge of the Solar System. It extends from out past Neptune to beyond Pluto and contains a good deal more mass than the asteroid Belt. It is where a lot of our comets come from and while there is plenty of rocks out there they tend to be covered in ice. In other words it is like our asteroid belt only there’s more of it and the one thing the belt is not very abundant in, water and hydrogen in general, is quite abundant out there. So if you have a power source life fusion they can be easily terraformed and are just as attractive as a source of minerals as the various asteroids and moons closer in.
Discovered in 2005, Makemake, a Kuiper Belt Object (KBO) has . Credit: NASA
We mentioned the idea of living inside hollowed out asteroids earlier and you can use the same trick for comets. Indeed you could shape them to be much bigger if you like, since they would be hollow and ice isn’t hard to move and shape especially in zero gravity. Same trick as before, you place a spinning cylinder inside it. Not all the objects entirely ice and indeed your average comet is more a frozen ball of mud then ice with rocky cores. We think a lot of near Earth Asteroids are just leftover comets. So they are probably pretty good homes if you have fusion, lots of fuel and raw materials for both life and construction.
This is probably your cheapest interstellar spacecraft too, in terms of effort anyway. People often talk about re-directing comets to Mars to bring it air and water, but you can just as easily re-direct it out of the solar system entirely. Comets tend to have highly eccentric orbits, so if you capture one when it is near the Sun you can accelerate it then, actually benefiting from the Oberth Effect, and drive it out of the solar system into deep space. If you have a fusion power source to live inside one then you also have an interstellar spaceship drive, so you just carve yourself a small colony inside the comet and head out into deep space.
You’ve got supplies that will last you many centuries at least, even if it were home to tens of thousand of people, and while we think of smaller asteroids and comets as tiny, that’s just in comparison to planets. These things tend to be the size of mountain so there is plenty of living space and a kilometer of dirty ice between you and space makes a great shield against even the kinds of radiation and collisions you can experience at relativistic speeds.
Artists’ impression of the Kuiper belt and Oort cloud. Credit: NASA/JPL
Now the Oort Cloud is much like the Kupier Belt but begins even further out and extends out probably an entire light year or more. We don’t have a firm idea of its exact dimensions or mass, but the current notion is that it has at least several Earth’s worth of mass, mostly in various icy bodies. These will be quite numerous, estimates usually assumes at least trillion icy bodies a kilometer across or bigger, and even more smaller ones. However the volume of space is so large that those kilometer wide bodies might each be a around a billion kilometers distant from neighbors, or about a light hour. So it is spread out quite thinly, and even the inner edge is about 10 light days away.
That means that from a practical standpoint there is no source of power out there, the sun is simply too diffuse for even massive collections of mirrors and solar panels to be of use. It also means light-speed messages home or to neighbors are quite delayed. So in terms of communication it is a lot more like pre-modern times in sparsely settled lands where talking with your nearest neighbors might require an hour long walk over to their farm, and any news from the big cities might take months to percolate out to you.
There’s probably uranium and thorium out there to be found, maybe a decent amount of it, so fission as a power source is not ruled out. If you have fusion instead though each of these kilometer wide icy bodies is like a giant tank of gasoline, and as with the Kupier Belt, ice makes a nice shield against impacts and radiation.
And while there might be trillions of kilometer wide chunks of ice out there, and many more smaller bodies, you would have quite a few larger ones too. There are almost certainly tons of planets in the Pluto size-range out these, and maybe even larger ones. Even after the Oort cloud you would still have a lot of these deep space rogue planets which could bridge the gap to another solar system’s Oort Cloud. So if you have fusion you have no shortage of energy, and could colonize trillions of these bodies. There probably is a decent amount of rock and metal out there too, but that could be your major import/export option shipping home ice and shipping out metals.
That’s the edge of the Solar System so that’s the end of this article. If you haven’t already read the other half, colonizing the inner Solar System, head on over now.
This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
In 1655, astronomer Christiaan Huygens became the first person to observe the beautiful ring system that surrounds Saturn. And while they are certainly the most spectacular, astronomers have since discovered that all the gas and ice giants of the Solar System (i.e. Jupiter, Saturn, Uranus and Neptune) have their own system of rings.
These systems have remained a source of fascination for astronomers, largely because their origins are still something of a mystery. But thanks to a recent study by researchers from the Tokyo Institute of Technology and Kobe University, the origins of these rings may be solved. According to their study, the rings are pieces of Dwarf Planets that got torn off in passing, which were then ripped to pieces!
The Kuiper Belt was named in honor of Dutch-American astronomer Gerard Kuiper, who postulated a reservoir of icy bodies beyond Neptune. Credit: JHUAPL
First, they considered the diversity of the various ring systems in our Solar System. For instance, Saturn’s rings are massive (about 100,000 trillion kg!) and composed overwhelmingly (90-95%) of water ice. In contrast, the much less massive rings of Uranus and Neptune are composed of darker material, and are believed to have higher percentages of rocky material in them.
To shed some light on this, the team looked to the Nice Model – a theory of Solar System formation that states that the gas giant migrated to their present location during the Late Heavy Bombardment. This period took place between 4 and 3.8 billion years ago, and was characterized by a disproportionately high number of asteroids from Trans-Neptunian space striking planets in the Inner Solar System.
They then considered other recent models of Solar System formation which postulate that the giant planets experienced close encounters with Pluto-sized objects during this time. From this, they developed the theory that the rings could be the result of some of these objects getting trapped and ripped apart by the gas giants’ gravity. To test this theory, they performed a number of computer simulations to see what would happen in these instances.
As Ryuki Hyodo – a researcher at the Department of Planetology, Kobe University, and the lead author on the paper – told Universe Today via email:
“We performed two simulations. First, using SPH (Smoothed-particle hydrodynamics) simulations, we investigated tidal disruption of Pluto-sized objects during the close encounters with giant planets and calculated the amount of fragments that are captured around giant planets. We found enough mass/fragments to explain current rings is captured. Then, we performed the longer-term evolution of the captured mass/fragments by using N-body simulations. We found that the captured fragments can collide each other with destruction and form thin equatorial circular rings around giant planets.”
A composite image of Uranus in two infrared bands, showing the planet and its ring system. Credit: W. M. Keck Observatory (Marcos van Dam)
The results of these simulation were consistent with the mass of the ring systems observed around Saturn and Uranus. This included the inner regular satellites of both planets – which would have also been the product of the past encounters with KBOs. It also accounted for the differences in the rings’ composition, showing how the planet’s Roche limits can influence what kind of material can be effectively captured.
This study is especially significant because it offers verifiable evidence for one of the enduring mysteries of our Solar System. And as Hyodo points out, it could come in mighty handy when it comes time to examine extra-solar planetary systems as well.
“Our theory suggested that, in the past, we had two possible epochs to form rings,” he said. “One is during the planet accretion phase and the other is during the Late heavy bombardment. Also, our model is naturally applicable to other planetary systems. So, our theory predicts that exoplanets also have massive rings around them.”
In the meantime, some might find the idea that ring systems are the corpses of Dwarf Planets troublesome. But I think we can all agree, a Soylent Green allusion might be just a bit over the top!
In the distant past, the orbit of 67P/Churyumov-Gerasimenko extended far beyond Neptune into the refrigerated Kuiper Belt. Interactions with the gravitational giant Jupiter altered the comet's orbit over time, dragging it into the inner Solar System. Credit: Western University, Canada
In the distant past, the orbit of 67P/Churyumov-Gerasimenko extended far beyond Neptune into the refrigerated Kuiper Belt. Interactions with the gravitational giant Jupiter altered the comet’s orbit over time, dragging it into the inner Solar System. Credit: Western University, Canada
Rosetta’s Comet hails from a cold, dark place. Using statistical analysis and scientific computing, astronomers at Western University in Canada have charted a path that most likely pinpoints comet 67P/Churyumov-Gerasimenko’s long-ago home in the far reaches of the Kuiper Belt, a vast region beyond Neptune home to icy asteroids and comets.
According to the new research, Rosetta’s Comet is relative newcomer to the inner parts of our Solar System, having only arrived about 10,000 years ago. Prior to that, it spent the last 4.5 billion years in cold storage in a rough-and-tumble region of the Kuiper Belt called the scattered disk.
The Kuiper Belt was named in honor of Dutch-American astronomer Gerard Kuiper, who postulated a reservoir of icy bodies beyond Neptune. The first Kuiper Belt object was discovered in 1992. We now know of more than a thousand objects there, and it’s estimated it’s home to more than 100,000 asteroids and comets there over 62 miles (100 km) across. Credit: JHUAPL
In the Solar System’s youth, asteroids that strayed too close to Neptune were scattered by the encounter into the wild blue yonder of the disk. Their orbits still bear the scars of those long-ago encounters: they’re often highly-elongated (shaped like a cigar) and tilted willy-nilly from the ecliptic plane up to 40°. Because their orbits can take them hundreds of Earth-Sun distances into the deeps of space, scattered disk objects are among the coldest places in the Solar System with surface temperatures around 50° above absolute zero. Ices that glommed together to form 67P at its birth are little changed today. Primordial stuff.
Watch how Rosetta’s Comet’s orbit has evolved since the comet’s formation
There are two basic comet groups. Most comets reside in the cavernous Oort Cloud, a roughly spherical-shaped region of space between 10,000 and 100,000 AU (astronomical unit = one Earth-Sun distance) from the Sun. The other major group, the Jupiter-family comets, owes its allegiance to the powerful gravity of the giant planet Jupiter. These comets race around the Sun with periods of less than 20 years. It’s thought they originate from collisions betwixt rocky-icy asteroids in the Kuiper Belt.
Fragments flung from the collisions are perturbed by Neptune into long, cigar-shaped orbits that bring them near Jupiter, which ropes them like calves with its insatiable gravity and re-settles them into short-period orbits.
Comet 67P/Churyumov-Gerasimenko is a Jupiter-family comet. Its 6.5 year journey around the Sun takes it from just beyond the orbit of Jupiter at its most distant to between the orbits of Earth and Mars at its closest. Credit: ESA with labels by the author
Mattia Galiazzo and solar system expert Paul Wiegert, both at Western University, showed that in transit, Rosetta’s Comet likely spent millions of years in the scattered disk at about twice the distance of Neptune. The fact that it’s now a Jupiter family comet hints of a possible long-ago collision followed by gravitational interactions with Neptune and Jupiter before finally becoming an inner Solar System homebody going around the Sun every 6.45 years.
By such long paths do we arrive at our present circumstances.
Artist's concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA
The Solar System is filled with what are known as Trojan Asteroids – objects that share the orbit of a planet or larger moon. Whereas the best-known Trojans orbit with Jupiter (over 6000), there are also well-known Trojans orbiting within Saturn’s systems of moons, around Earth, Mars, Uranus, and even Neptune.
Until recently, Neptune was thought to have 12 Trojans. But thanks to a new study by an international team of astronomers – led by Hsing-Wen Lin of the National Central University in Taiwan – five new Neptune Trojans (NTs) have been identified. In addition, the new discoveries raise some interesting questions about where Neptune’s Trojans may come from.
The PS1 telescope at dawn, with the mountain of Mauna Kea visible in the distance. Credit: pan-starrs.ifa.hawaii.edu
The team used data obtained by the PS-1 survey, which ran from 2010 to 2014 and utilized the first Pan-STARR telescope on Mount Haleakala, Hawaii. From this, they observed seven Trojan asteroids around Neptune, five of which were previously undiscovered. Four of the TNs were observed orbiting within Neptune’s L4 point, and one within its L5 point.
The newly detected objects have sizes ranging from 100 to 200 kilometers in diameter, and in the case of the L4 Trojans, the team concluded from the stability of their orbits that they were likely primordial in origin. Meanwhile, the lone L5 Trojan was more unstable than the other four, which led them to hypothesize that it was a recent addition.
As Professor Lin explained to Universe Today via email:
“The 2 of the 4 currently known L5 Neptune Trojans, included the one L5 we found in this work, are dynamically unstable and should be temporary captured into Trojan cloud. On the other hand, the known L4 Neptune Trojans are all stable. Does that mean the L5 has higher faction of temporary captured Trojans? It could be, but we need more evidence.”
In addition, the results of their simulation survey showed that the newly-discovered NT’s had unexpected orbital inclinations. In previous surveys, NTs typically had high inclinations of over 20 degrees. However, in the PS1 survey, only one of the newly discovered NTs did, whereas the others had average inclinations of about 10 degrees.
Animation showing the path of six of Neptune’s L4 trojans in a rotating frame with a period equal to Neptune’s orbital period.. Credit: Tony Dunn/Wikipedia Commons
From this, said Lin, they derived two possible explanations:
“The L4 “Trojan Cloud” is wide in orbital inclination space. If it is not as wide as we thought before, the two observational results are statistically possible to generate from the same intrinsic inclination distribution. The previous study suggested >11 degrees width of inclination, and most likely is ~20 degrees. Our study suggested that it should be 7 to 27 degrees, and the most likely is ~ 10 degrees.”
“[Or], the previous surveys were used larger aperture telescopes and detected fainter NT than we found in PS1. If the fainter (smaller) NTs have wider inclination distribution than the larger ones, which means the smaller NTs are dynamically “hotter” than the larger NTs, the disagreement can be explained.”
According to Lin, this difference is significant because the inclination distribution of NTs is related to their formation mechanism and environment. Those that have low orbital inclinations could have formed at Neptune’s Lagrange Points and eventually grew large enough to become Trojans asteroids.
Illustration of the Sun-Earth Lagrange Points. Credit: NASA
On the other hand, wide inclinations would serve as an indication that the Trojans were captured into the Lagrange Points, most likely during Neptune’s planetary migration when it was still young. And as for those that have wide inclinations, the degree to which they are inclined could indicate how and where they would have been captured.
“If the width is ~ 10 degrees,” he said, “the Trojans can be captured from a thin (dynamically cold) planetesimal disk. On the other hand, if the Trojan cloud is very wide (~ 20 degrees), they have to be captured from a thick (dynamically hot) disk. Therefore, the inclination distribution give us an idea of how early Solar system looks like.”
In the meantime, Li and his research team hope to use the Pan-STARR facility to observe more NTs and hundreds of other Centaurs, Trans-Neptunian Objects (TNOs) and other distant Solar System objects. In time, they hope that further analysis of other Trojans will shed light on whether there truly are two families of Neptune Trojans.
This was all made possible thanks to the PS1 survey. Unlike most of the deep surveys, which are only ale to observe small areas of the sky, the PS1 is able to monitor the whole visible sky in the Northern Hemisphere, and with considerable depth. Because of this, it is expected to help astronomers spot objects that could teach us a great deal about the history of the early Solar System.
