asteroids

For future we can expect: Hayabusa (MUSES-C) to asteroid Itokawa, Rosetta to Comet Churyumov-Gerasmenko, Deep Impact to Comet Tempel 1, and Dawn to orbit asteroids Vesta and Ceres. 243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathilde (left) on 1997 June 27 returning many images. NEAR (now renamed "NEAR-Shoemaker") entered orbit around 433 Eros (right) in January 1999 and returned a wealth of images and data. At the end of its mission it actually landed on Eros. The largest asteroid by far is 1 Ceres. It is 933 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known asteroids are less than 340 km across. There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Saturn, Uranus and Neptune are all more similar to asteroids than to the larger moons. (The composite image at the top of this page shows Ida, Gaspra, Deimos and Phobos approximately to scale.) Asteroids are classified into a number of types according to their spectra (and hence
their chemical composition) and albedo: C-type, includes more than 75% of known asteroids:

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