No book about planets would be complete without a discussion of asteroids, because these are the most common objects to hit planets in the inner Solar System (where asteroid impacts are about ten times more common than comet impacts). In addition, the largest asteroid, Ceres, is offi cially classifi ed as a dwarf planet.
Shapes, sizes, and compositions
Ceres is the goal for NASA's Dawn spacecraft, which will spend fi ve months orbiting it in 2015, having already spent the year beginning in July 2011 at Vesta, the second most massive asteroid.
A few smaller asteroids have already been visited by spacecraft, providing images ( Figure 25 ) that confi rm their irregular shapes. Visualize a pock-marked potato scaled up to any size between tens of metres and a few hundred kilometres, and you should have a serviceable mental image of a typical asteroid. Telescopically observed periodic variations in asteroids'brightness show that mostly they take only a few hours to rotate. Generally, rotation is at right angles to their length, so they rotate like sausages twirled on a cocktail stick.
About 1 asteroid in 50 probably has its own satellite, and it was lucky that Ida, the second asteroid to be visited by a spacecraft when Galileo fl ew past in 1993, turned out to be one of these. That was the fi rst confi rmed discovery of an asteroid satellite, but subsequently many more have been found using advanced telescopic techniques, such as adaptive optics to compensate for the shimmering of the Earth’s atmosphere. Asteroid satellites range from the comparatively tiny up to sizes similar to the main asteroid. In fact, the asteroid named Antiope appears to consist of two mutually orbiting bodies of indistinguishable 110-kilometre size, whose centres are only about 170 kilometres apart. So far,there are two asteroids known to possess two small satellites each.Some asteroid satellites may be fragments from a collision, and others may be captured objects. Neither case is readily explicable,because it is hard to end up with objects orbiting rather than fl ying apart.