Space Topics: Asteroids and Comets
Ceres, the first asteroid to be discovered, is by far the largest and most massive asteroid in the main asteroid belt. Its size and mass are sufficient to give it a spherical shape, much like a planet. It is so huge in comparison with the other asteroids that its mass is equal to more than one-third of the estimated total mass of all cataloged asteroids. At about 930 kilometers (578 miles) in diameter, it is similar in size to Saturn’s moons Tethys and Dione.
Ceres has never been visited by a spacecraft, but it has been observed by the Hubble Space Telescope. The Hubble observations suggest that Ceres, like Earth, has internal layering, with denser materials toward its center. NASA's Dawn mission will launch in 2007 for an August 2015 rendezvous with Ceres. Read more »
The second asteroid ever discovered, Pallas is probably the second largest main belt asteroid in terms of volume but the third largest in terms of mass. Pallas and Vesta are very similar in size, and they have jockeyed back and forth for the title of "second largest asteroid" over time. Pallas is slightly irregular in shape and seems to have a similar composition to C-type asteroids. Its orbit is centrally located in the main belt although it is unusually eccentric and inclined for an asteroid of its size. Pallas has been observed occulting stars. Observations during one occultation in 1980 seemed to indicate that the asteroid possessed a moon 175 kilometers in diameter, but this has not been confirmed.
No spaecraft has visited Pallas. Because its orbit is inclined 35 degrees to the ecliptic, it is difficult to design a spacecraft trajectory to reach it.
Vesta was the fourth asteroid to be discovered, and is either the second or third largest (it competes for that honor with Pallas). Hubble Space Telescope images of Vesta have revealed its surface to be surprisingly diverse. Vesta appears to have had a geologic history similar to the terrestrial planets. It may have a nickel-iron core and a basalt crust formed from the partial melting of a mantle of silicate rocks. At some point in its past, Vesta suffered a huge impact which left a crater as large as the asteroid itself. The crater is so deep that it exposes materials from deep in Vesta’s mantle. If this is true, a visit to Vesta could yield important information that would help to understand the mantles of the terrestrial planets, like Earth and Mars. The impact undoubtedly sent chunks of Vesta flying on their own paths through the solar system. Many smaller asteroids accompany Vesta on similar orbital paths and have spectral similarities to Vesta. Some meteorites found on Earth have been linked to Vesta based upon these spectral characteristics.
No spacecraft has yet visited Vesta, but it has been observed by the Hubble Space Telescope. NASA's Dawn mission is planned to enter orbit around the asteroid in October 2011. Read more »
Eros is the second largest known Near Earth Asteroid (the largest is 1036 Ganymed). As an Amor group asteroid, its orbital path poses no danger to Earth. It is an S-type asteroid, similar in composition to most main belt asteroids.Eros was the target of the Near Earth Asteroid Rendezvous (NEAR) mission in 2000 and 2001. NEAR’s photos revealed it to be a peanut-shaped body strewn with house-sized boulders. Puzzlingly, 40 percent of Eros’ surface lacks craters less than 0.5 meters in diameter. The unusually smooth area matches physical models of the areas of Eros that would have trembled as a result of the impact that created one of the large craters on the asteroid’s surface. The correlation suggests that Eros is not a rubble pile, but is a physically coherent body that is capable of transmitting seismic waves through its interior.