In the 18th century, observations made of all the known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn) led astronomers to the realization that there was a pattern in their orbits. Eventually, this led to the Titius–Bode law, which predicted the amount of space that naturally existed between each celestial body that orbited our Sun. In accordance with this law, astronomers noted that there appeared to be a discernible gap between the orbits of Mars and Jupiter.
Investigations into this gap eventually resulted in astronomers observing several bodies of various size. This led to the creation of the term “asteroid” (Greek for ‘star-like’ or ‘star-shaped’), as well as “Asteroid Belt”, once it became clear just how many there were. Through various methods, astronomers have since confirmed the existence of several million objects between the orbit of Mars and Jupiter. They have also determined, with a certain degree of accuracy, how far it is from our planet.
Structure and Composition:
The Asteroid Belt consists of several large bodies, coupled with millions of smaller size. The larger bodies, such as Ceres, Vesta, Pallas, and Hygiea, account for half of the belt’s total mass, with almost one-third accounted for by Ceres alone. Beyond that, over 200 asteroids that are larger than 100 km in diameter, and 0.7–1.7 million asteroids with a diameter of 1 km or more.
It total, the Asteroid Belt’s mass is estimated to be 2.8×1021 to 3.2×1021 kilograms – which is equivalent to about 4% of the Moon’s mass. While most asteroids are composed of rock, a small portion of them contain 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.
Despite the impressive number of objects contained within the belt, the Main Belt’s asteroids are also spread over a very large volume of space. As a result, the average distance between objects is roughly 965,600 km (600,000 miles), meaning that the Main Belt consists largely of empty space. 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.
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 gaps are similar to what is seen with Saturn’s and other gas giants’ systems of rings.
Orbit Around the Sun:
Located between Mars and Jupiter, the belt ranges in distance between 2.2 and 3.2 astronomical units (AU) from the Sun – 329 million to 478.7 million km (204.43 million to 297.45 million mi). It is also an estimated 1 AU thick (149.6 million km, or 93 million mi), meaning that it occupies the same amount of distance as what lies between the Earth to the Sun.
The distance of an asteroid from the Sun (its semi-major axis) depends upon its distribution into one of three different zones based on the Belt’s “Kirkwood Gaps”. Zone I lies between the 4:1 resonance and 3:1 resonance Kirkwood gaps, which are roughly 2.06 and 2.5 AUs (3 to 3.74 billion km; 1.86 to 2.3 billion mi) 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 (4.22 billion km; 2.6 mi) from the Sun. Zone III, the outermost section of the Belt, extends from the outer edge of Zone II to the 2:1 resonance gap, located some 3.28 AU (4.9 billion km; 3 billion mi) from the Sun.
Distance from Earth:
The distance between the Asteroid Belt and Earth varies considerably depending on where we measure to. Based on its average distance from the Sun, the distance between Earth and the closest edge of the Belt can be said to be between 1.2 to 2.2 AUs, or 179.5 and 329 million km (111.5 and 204.43 million mi). But of course, at any given time, part of the Asteroid Belt will be on the opposite side of the Sun relative to us as well.
From this vantage point, the distance between Earth and the Asteroid Belt ranges from 3.2 and 4.2 AU – 478.7 to 628.3 million km (297.45 to 390.4 million mi). To put that in perspective, the distance between Earth and the Asteroid Belt ranges from being slightly more than the distance between the Earth and the Sun (1 AU), to being the same as the distance between Earth and Jupiter (4.2 AU) when they are at their closest.
Naturally, any exploration or other kind of mission launched from Earth is going to take the shortest route, unless it is aiming for a specific asteroid. And even then, mission planners will time the launch to ensure that we are closest to the destination. Hence, we can safely use the estimates of 1.2 – 2.2 AU to gauge the distance between us and the Main Belt.
Even so, at its closest, getting to the Asteroid Belt would involve a bit of a hike! In short, it is approximately 179.5 million km (or 111.5 million mi) distant from us at any given time. As such, knowing just how much time and energy it would take to get their and back is going to come in handy if and when we begin mounting crewed missions to the Belt, not to mention the prospect of asteroid mining!
We continue our “Definitive Guide to Terraforming” series with a look at another body in our Solar System – the dwarf planet Ceres. Like many moons in the outer Solar System, Ceres is a world of ice and rock, and is the largest body in the Asteroid Belt. Humans beings could one day call it home, but could its surface also be made “Earth-like”?
In the Solar System’s Main Asteroid Belt, there are literally millions of celestial bodies to be found. And while the majority of these range in size from tiny rocks to planetesimals, there are also a handful of bodies that contain a significant percentage of the mass of the entire Asteroid Belt. Of these, the dwarf planet Ceres is the largest, constituting of about a third of the mass of the belt and being the sixth-largest body in the inner Solar System by mass and volume.
In addition to its size, Ceres is the only body in the Asteroid Belt that has achieved hydrostatic equilibrium – a state where an object becomes rounded by the force of its own gravity. On top of all that, it is believed that this dwarf planet has an interior ocean, one which contains about one-tenth of all the water found in the Earth’s oceans. For this reason, the idea of colonizing Ceres someday has some appeal, as well as terraforming.
Under the 3 year, $67 million contract award, Aerojet Rocketdyne will develop the engineering development unit for an Advanced Electric Propulsion System (AEPS) with the potential for follow on flight units.
NASA hopes that the work will result in a 10 fold increase in “spaceflight transportation fuel efficiency compared to current chemical propulsion technology and more than double thrust capability compared to current electric propulsion systems.”
The SEP effort is based in part on NASA’s exploratory work on Hall ion thrusters which trap electrons in a magnetic field and uses them to ionize and accelerate the onboard xenon gas propellant to produce thrust much more efficiently than chemical thrusters.
The solar electric propulsion (SEP) system technology will afford benefits both to America’s commercial space and scientific space exploration capabilities.
For NASA, the SEP technology can be applied for expeditions to deep space such as NASA’s planned Asteroid Robotic Redirect Mission (ARRM) to snatch a boulder from the surface of an asteroid and return it to cislunar space during the 2020s, as well as to carry out the agency’s ambitious plans to send humans on a ‘Journey to Mars’ during the 2030s.
“High power SEP is a perfect example of NASA developing cross cutting technologies to enable both human and robotic deep space missions. Basically it enables high efficiency and better gas mileage,” said Steve Jurczyk, associate administrator of NASA’s Space Technology Mission Directorate (STMD) in Washington, at a media briefing.
“The advantage here is the higher power and the higher thrust.”
“Our plan right now is to flight test the higher power solar electric propulsion that Aerojet Rocketdyne will develop for us on the Asteroid Redirect Robotic Mission (ARRM), which is going to go out to an asteroid with a robotic system, grab a boulder off of an asteroid, and bring it back to a lunar orbit.”
ARRM would launch around 2020 or 2021. Astronauts would blast off several years later in NASA’s Orion crew capsule in 2025 after the robotic probes travels back to lunar orbit.
For industry, electric propulsion is used increasingly to maneuver thrusters in Earth orbiting commercial satellites for station keeping in place of fuel.
“Through this contract, NASA will be developing advanced electric propulsion elements for initial spaceflight applications, which will pave the way for an advanced solar electric propulsion demonstration mission by the end of the decade,” says Jurczyk.
“Development of this technology will advance our future in-space transportation capability for a variety of NASA deep space human and robotic exploration missions, as well as private commercial space missions.”
“This is also a critical capability for enabling human missions to Mars, with respect to delivering cargo to the surface to Mars that will allow people to live and work there on the surface. Also for combined chemical and SEP systems on a spacecraft to propel humans to Mars,” elaborated Jurczyk at the briefing.
“Another application is round trip robotic science missions to Mars to bring back samples – such as a Mars Sample Return (MSR) mission.”
The starting point is NASA’s development and technology readiness testing of a prototype 13-kilowatt Hall thruster and power processing unit at NASA’s Glenn Research Center in Cleveland.
Under the contract award Aerojet Rocketdyne aims to carry out the industrial development of “high-power solar electric propulsion into a flight-qualified system.”
They will develop, build, test and deliver “an integrated electric propulsion system consisting of a thruster, power processing unit (PPU), low-pressure xenon flow controller, and electrical harness,” as an engineering development unit.
This engineering development unit serves as the basis for producing commercial flight units.
If successful, NASA has an option to purchase up to four integrated flight units for actual space missions. Engineers from NASA Glenn and the Jet Propulsion Laboratory (JPL) will provide technical support.
“We could string together four of these engine units to get approximately 50 kilowatts of electrical propulsion capability and with that we can do significant orbital transfer operations. That then becomes the next step in deep space exploration operations that we are trying to do,” said Bryan Smith, director of the Space Flight Systems Directorate at NASA’s Glenn Research Center in Cleveland, at the media briefing.
“We hope to buy four of these units for the ARRM mission.”
What were some of NASA’s research and development (R&D) activities and further plans for Aerojet Rocketdyne?
“NASA is driving out the technology itself for feasibility. So we produced a developmental device to operate at these levels,” Smith told Universe Today during the briefing.
“Other key characteristics we were looking for is the ability to do magnetic shielding. The purpose was to allow for a long life thruster operation. We investigated attributes like thermal problems and balancing the erosion mechanisms in developmental units. So we were looking for things to get longer life and feasibility in developmental units.”
“Once we were comfortable with the feasibility in developmental units, we are now transferring the information, technology and knowhow into what is a production article, in this contract.”
Solar electric ion propulsion is already being used in NASA’s hugely successful Dawn asteroid orbiter mission.
Dawn was launched in 2007. It orbited and surveyed Vesta in 2011 and 2012 and then traveled outward to Ceres.
Dawn arrived at dwarf planet Ceres in March 2015 and is currently conducting breakthrough science at its lowest planned science mapping orbit.
A key part of the Journey to Mars, NASA will be sending cargo missions to the Red Planet to pave the way for human expeditions with the Orion crew module and Space Launch System.
Aerojet Rocketdyne states that “Solar Electric Propulsion (SEP) systems have demonstrated the ability to reduce the mission cost for NASA Human Exploration cargo missions by more than 50 percent through the use of existing flight-proven SEP systems.”
“Using a SEP tug for cargo delivery, combined with NASA’s Space Launch System and the Orion crew module, provides an affordable path for deep space exploration,” said Aerojet Rocketdyne Vice President, Space and Launch Systems, Julie Van Kleeck.
Another near term application of high power solar electric propulsion could be for NASA’s proposed Mars 2022 telecom orbiter, said Smith at the media briefing.
Other NASA technology work in progress includes development of more efficient, advanced solar array systems to generate the additional power required for the larger electric thrusters.
Orbital ATK was part of the development effort and already used some of its technology development in the ultraflex solar arrays on the recent Cygnus cargo ships delivering supplies to the ISS.
Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.
Now in orbit for just over a year at dwarf planet Ceres, NASA’s Dawn spacecraft continues to astound us with new discoveries gleaned from spectral and imagery data captured at ever decreasing orbits as well as since the probe arrived last December at the lowest altitude it will ever reach during the mission.
Mission scientists have just released marvelous new images of Haulani and Oxo craters revealing landslides and mysterious slumps at several of the mysterious bright craters on Ceres – the largest asteroid in the main Asteroid Belt between Mars and Jupiter.
The newly released image of oddly shaped Haulani crater above, shows the crater in enhanced color and reveals evidence of landslides emanating from its crater rim.
“Rays of bluish ejected material are prominent in this image. The color blue in such views has been associated with young features on Ceres,” according to the Dawn science team.
“Enhanced color allows scientists to gain insight into materials and how they relate to surface morphology.”
Look at the image closely and you’ll see its actually polygonal in nature – meaning it resembles a shape made of straight lines – unlike most craters in our solar system which are nearly circular.
”The straight edges of some Cerean craters, including Haulani, result from pre-existing stress patterns and faults beneath the surface,” says the science team.
Haulani Crater has a diameter of 21 miles (34 kilometers) and apparently was formed by an impacting object relatively recently in geologic time and is also one of the brightest areas on Ceres.
“Haulani perfectly displays the properties we would expect from a fresh impact into the surface of Ceres. The crater floor is largely free of impacts, and it contrasts sharply in color from older parts of the surface,” said Martin Hoffmann, co-investigator on the Dawn framing camera team, based at the Max Planck Institute for Solar System Research, Göttingen, Germany, in a statement.
The enhanced color image was created from data gathered at Dawn’s High Altitude Mapping Orbit (HAMO), while orbiting at an altitude of 915 miles (1,470 kilometers) from Ceres.
Data from Dawn’s VIR instrument shows that Haulani’s surface is comprised of different materials than its surroundings.
“False-color images of Haulani show that material excavated by an impact is different than the general surface composition of Ceres. The diversity of materials implies either that there is a mixed layer underneath, or that the impact itself changed the properties of the materials,” said Maria Cristina de Sanctis, the VIR instrument lead scientist, based at the National Institute of Astrophysics, Rome.
Since mid-December, Dawn has been orbiting Ceres in its Low Altitude Mapping Orbit (LAMO), at a distance of 240 miles (385 kilometers) from Ceres, resulting in the most stunning images ever of the dwarf planet.
By way of comparison the much higher resolution image of Haulani crater below, is a mosaic of views assembled from multiple images taken from LAMO at less than a third of the HAMO image distance – at only 240 miles (385 kilometers) above Ceres.
Dawn has also been busy imaging Oxo Crater, which despite its small size of merely 6-mile-wide (10-kilometer-wide) actually counts as a “hidden treasure” on Ceres – because it’s the second-brightest feature on Ceres!
Only the mysterious bright region comprising a multitude of spots inside Occator Crater shine more brightly on Ceres.
Most importantly, Oxo Crater is the only place on Ceres where Dawn has detected water at the surface so far. Via VIR, Dawn data indicate that the water exists either in the form of ice or hydrated minerals. Scientists speculate that the water was exposed either during a landslide or an impact.
“Little Oxo may be poised to make a big contribution to understanding the upper crust of Ceres,” said Chris Russell, principal investigator of the mission, based at the University of California, Los Angeles.
The signatures of minerals detected on the floor of Oxo crater appears to be different from the rest of Ceres.
Furthermore Oxo is “also unique because of the relatively large “slump” in its crater rim, where a mass of material has dropped below the surface.”
Dawn is Earth’s first probe in human history to explore any dwarf planet, the first to explore Ceres up close and the first to orbit two celestial bodies.
The asteroid Vesta was Dawn’s first orbital target where it conducted extensive observations of the bizarre world for over a year in 2011 and 2012.
The mission is expected to last until at least later into 2016, and possibly longer, depending upon fuel reserves.
Dawn will remain at its current altitude at LAMO for the rest of its mission, and indefinitely afterward, even when no further communications are possible.
Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.
NASA’s Dawn spacecraft has unveiled a new patch of intriguing bright features in the most recent series of striking close-up images taken just after the probe reached the lowest altitude it will ever reach during the mission.
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.
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.
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.
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.
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).
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.
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.
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.
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 Hayabusamissions 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.
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.
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.
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).
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.
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.
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.
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.
There’s a big white spot on Ceres and we don’t know what it is. We’ve known about the white spot since the Hubble Space Telescope first captured images of it in 2003 and 2004, and in subsequent images taken by Hubble, the spot remains visible. Now, in images released yesterday from the Dawn spacecraft, currently on approach to Ceres, the spot remains. In the animated image, below, the spot almost seems to glint in the sunlight.
What is it?
One of the most anticipated aspects the Dawn spacecraft being in orbit around Ceres HAS to be finding out what this spot is. It could be ice, it could be a cryovolcano or geysers, or it could be something else. But we do know fairly certain that it is a real feature and not an image artifact, since it shows up in most of the recent Hubble images and now the Dawn images.
Planetary scientists have long suspected that water ice may be buried under Cere’s crust. A few things point to subsurface ice: the density of Ceres is less than that of the Earth’s crust, and because the surface bears spectral evidence of water-bearing minerals. Scientists estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres’ water, unlike Earth’s, would be in the form of water ice and located in the mantle, which wraps around the asteroid’s solid core.
And then last year, the Herschel space telescope discovered water vapor around Ceres, and the vapor could be emanating from water plumes — much like those that are on Saturn’s moon Enceladus – or it could be from cryovolcanism from geysers or icy volcanoes. Without huge a planet or satellite nearby tugging on it, the mechanism for how Ceres is active is also intriguing.
Some scientists also think Ceres may have an ocean and possibly an atmosphere.
As we discussed in our article yesterday, with all that water potentially at Ceres, could it theoretically host microbial life? Some scientists have hinted that Ceres and other icy bodies could be a possible source for life on Earth, another intriguing proposition.
Yesterday, I asked Dawn scientist Paul Schenk what other factors would have to be present in order for microbial life to have arisen on Ceres.
“The presence of carbon molecules is often regarded as necessary for life,” he replied, “and we think we see that on the surface spectroscopically in the form of carbonates and clays. So, I think the questions will be, whether there is actually liquid water of any kind, whether the carbon compounds are just a surface coating or in the interior, and whether Ceres has ever been warm. If those are true then some sort of prebiotic or biotic activity is in play.”
And we’ll soon find out more about this intriguing dwarf planet.
As the deputy principal investigator for Dawn, Carol Raymond said following the Herschel water vapor discovery, “We’ve got a spacecraft on the way to Ceres, so we don’t have to wait long before getting more context on this intriguing result, right from the source itself.”
NASA says that Dawn’s images will surpass Hubble’s resolution at the next imaging opportunity, which will be at the end of January.
The spacecraft arrives at Ceres on March 6, when it will be captured into orbit. The images will continue to improve as the spacecraft spirals closer to the surface during its 16-month study of the dwarf planet. Dawn will eventually be about 1,000 times closer to Ceres than it was for the images released yesterday and therefore will provide 1,000 times as much detail. Dawn at Ceres is primarily a mapping mission, so it will map the geology and chemistry of the surface in high resolution.
It should reveal the processes that drive the outgassing activity, and it should reveal how much water this dwarf planet holds.
And it should reveal the mystery of that white spot.
Vesta is one of the largest asteroids in the Solar System. Comprising 9% of the mass in the Asteroid Belt, it is second in size only to the dwarf-planet Ceres. And now, thanks to data obtained by NASA’s Dawn spacecraft, Vesta’s surface has been mapped out in unprecedented detail.
These high-resolution geological maps reveal the variety of Vesta’s surface features and provide a window into the asteroid’s history.
“The geologic mapping campaign at Vesta took about two-and-a-half years to complete, and the resulting maps enabled us to recognize a geologic timescale of Vesta for comparison to other planets,” said David Williams of Arizona State University.
Geological mapping is a technique used to derive the geologic history of a planetary object from detailed analysis of surface morphology, topography, color and brightness information. The team found that Vesta’s geological history is characterized by a sequence of large impact events, primarily by the Veneneia and Rheasilvia impacts in Vesta’s early history and the Marcia impact in its late history.
The geologic mapping of Vesta was made possible by the Dawn spacecraft’s framing camera, which was provided by the Max Planck Institute for Solar System Research of the German Max Planck Society and the German Aerospace Center. This camera takes panchromatic images and seven bands of color-filtered images, which are used to create topographic models of the surface that aid in the geologic interpretation.
A team of 14 scientists mapped the surface of Vesta using Dawn data. The study was led by three NASA-funded participating scientists: Williams; R. Aileen Yingst of the Planetary Science Institute; and W. Brent Garry of the NASA Goddard Spaceflight Center.
The brown colored sections of the map represent the oldest, most heavily cratered surface. Purple colors in the north and light blue represent terrains modified by the Veneneia and Rheasilvia impacts, respectively. Light purples and dark blue colors below the equator represent the interior of the Rheasilvia and Veneneia basins. Greens and yellows represent relatively young landslides or other downhill movement and crater impact materials, respectively.
The map indicates the prominence of impact events – such as the Veneneia, Rheasilvia and Marcia impacts, respectively – in shaping the asteroid’s surface. It also indicates that the oldest crust on Vesta pre-dates the earliest Veneneia impact. The relative timescale is supplemented by model-based absolute ages from two different approaches that apply crater statistics to date the surface.
“This mapping was crucial for getting a better understanding of Vesta’s geological history, as well as providing context for the compositional information that we received from other instruments on the spacecraft: the visible and infrared (VIR) mapping spectrometer and the gamma-ray and neutron detector (GRaND),” said Carol Raymond, Dawn’s deputy principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, California.
The objective of NASA’s Dawn mission is to characterize the two most massive objects in the main asteroid belt between Mars and Jupiter – Vesta and the dwarf planet Ceres.
Asteroids like Vesta are remnants of the formation of the solar system, giving scientists a peek at its early history. They can also harbor molecules that are the building blocks of life and reveal clues about the origins of life on Earth. Hence why scientists are eager to learn more about its secrets.
The Dawn spacecraft was launched in September of 2007 and orbited Vesta between July 2011 and September 2012. Using ion propulsion in spiraling trajectories to travel from Earth to Vesta, Dawn will orbit Vesta and then continue on to orbit the dwarf planet Ceres by April 2015.
The high resolution maps were included with a series of 11 scientific papers published this week in a special issue of the journal Icarus. The Dawn spacecraft is currently on its way to Ceres, the largest object in the asteroid belt, and will arrive at Ceres in March 2015.