On July 16, 2011, NASA's Dawn spacecraft, with its ion propulsion system, completed a flight of 2.8 billion km (1.7 billion miles) to go into orbit around the asteroid belt's second most massive object, Vesta. On March 6th, Dawn will go into orbit around the most massive asteroid belt object, the dwarf planet, Ceres, becoming the first spacecraft ever to orbit two solar system bodies. These small bodies can tell us much about the processes that influenced the formation and evolution of our solar system over four billion years ago. What are we learning about these two very different solar system bodies?
Launched in January 2006, NASA's New Horizons spacecraft is set to give us the first closeup look at the dwarf planet, Pluto, its fledgling atmosphere and its five known moons (Charon, Nix, Hydra, Styx, and Kerberos) flying just 13,695 km above its surface on July 14, 2015.
Comets, asteroids, dwarf planets, minor planets, Plutoids, or Trans Neptunian Objects (TNOs); what ever you call them, these pint sized members of our solar system make up the class of objects known as "Small Bodies". These objects orbit the sun similar to planets but do not have enough mass to sweep out debris from their orbits. Most are the size of small rocks (< 1m in diameter) but some have diameters of hundreds of meters and in a few cases, hundreds of kilometers with asteroids (or minor/dwarf planets) tending to be larger than comets on average. Because of their relatively low mass, most asteroids and comets have irregular shapes. They just don't have enough gravity to smoosh the ice and rock they are made of into a spherical shape (think of making a round snowball with your hands).
Their different compositions are thought to have been driven long ago by the sun! in the chaotic early solar system, 4-4.5 billion years ago, objects that formed inside the frost line, that is, inside the distance from the sun that would allow water to condense and remain solid, generally had little ice and lots of rock. These are the asteroids. Objects that formed outside the frost line where water could condense and become solid had much more ice (water, CO2, CO, methane, etc.) and less rock and metals.
The frost line in our solar system lies somewhere between the orbits of Mars and Jupiter. Here millions of asteroids orbit the sun. Most are very small but an estimated 750,000 have diameters greater than 1 km and perhaps 200 have diameters greater than 100 km; the largest being Ceres with a diameter of 940 km. It is interesting to note that asteroids orbiting on the outer edges of the asteroid belt contain large quantities of ices (or "volatile" elements). Those orbiting on the inner part of the belt are generally devoid of ices and are made almost entirely of what are called refractory elements (rocks and metals). Because they have no weather or plate tectonics to modify their surfaces like the Earth, asteroid surfaces are very old and can tell us much about the conditions present during the formation of the solar system.
Comets exist in two realms. Short period comets with orbital periods less than 200 years orbit just outside the orbit of Neptune in a region called the "Kuiper Belt" (after its discoverer, Gerard Kuiper). Far outside the solar system, more than a light year away, long period comets orbit in the "Oort Cloud" (named after Jan Oort). Although there have been no direct observations of the Oort cloud (its trillions of miles away!), it is theorized to exist by observing the orbits of long period comets (like Comet Hale-Bopp) with orbital periods of thousands of years. We see these comets only when they venture into the inner solar system in their highly elliptical orbits. They are believed to have formed much closer to the sun but been thrown far out to the outer reaches of the sun's gravitational influence by the gravity of Jupiter or Saturn. At these great distances from the sun, the sun's gravity is very weak and galactic tidal forces have significant effect on cometary orbits causing them occasionally to fall toward the sun. The gravitational fields of nearby stars is thought to be another perturbing force on Oort cloud comets.
Far from the sun, comets, whether Kuiper Belt or Oort Cloud in origin remain frozen solid making them difficult to observe. As a comet approaches the sun, the sun's radiation causes ices in the comet to sublime (vaporize) and stream outward from the sun as ionized gasses and the dust that was trapped in the ice. These two streams (gas and dust) form separate, distinct tails making the comet much easier to observe.
Some sunspots are cool enough that water vapor can form at a temperature of 1,550 °C.