How Asteroids Differ from Planets and Other Bodies
With the exception of Ceres, asteroids are not massive enough for gravity to pull them into spheres, hence the fact they are irregularly shaped like 243 Ida depicted in this photo. Remarkably, however, this particular asteroid has a tiny one-mile-wide moon named Dactyl!.
Asteroids form a broad subcategory of minor planets, but they are most often associated with the Asteroid Belt spanning a huge area of space between Mars and Jupiter. By their simplest definition, asteroids refer to celestial bodies that are too small to be described as dwarf planets or planets. With the sole exception of Ceres, none of the asteroids are massive enough to reach hydrostatic equilibrium, meaning that their gravity is too weak to pull them into spherical shapes.
Ceres is by far the largest asteroid to the extent it’s perhaps better described as a dwarf planet. With a diameter of 587 miles (945 km), it accounts for about a third of the entire collective mass of all the millions of objects in the Asteroid Belt. Owing to its relatively enormous size, it was the first asteroid discovered, by Italian astronomy Giuseppe Piazzi in 1801. Since then, countless more have been found, shedding profound insights into the formation of our solar system.
The smallest asteroids are as little as three feet (1 metre) across. Any object smaller than this is typically referred to as a meteoroid or, when it interacts with our atmosphere, as a meteor. Asteroids should not be confused with comets, which are icy bodies that produce a stream of water vapour and other gasses when they are at their closest to the Sun. However, comets, despite being far fewer in number, have also played an important role in the evolution of the solar system.