Asteroids, now known as small solar system bodies, are the bits and pieces left over after the formation of the inner planets, including the Earth.
There are millions of these objects, and most are located in the main belt, between the orbits of Mars and Jupiter. In the past, asteroids have collided with the Earth, significantly modifying our planet’s biosphere, and will continue to do so in the future. They are also the source of most meteorites that have struck our planet’s surface.
An asteroid is formally referred to as a number in brackets followed by a name, or if newly discovered then the year of discovery, two letters and, if need be, further digits, until a formal name is agreed upon, which is selected once the orbit is determined.
Though they are rarely visible with the unaided eye, many asteroids are visible through binoculars or small telescopes.
As the name “asteroid” suggests, they appear starlike in a telescope. One way to distinguish an asteroid from a star is by its motion, typically 30 arcseconds per hour relative to the background stars. Asteroids are usually much closer to us at opposition than at other times, and are therefore brighter and easier to see.
The irregular shape and rough surfaces of asteroids are evident in the detectable variation of reflected sunlight as seen from Earth. This variation, caused mainly by rotation, is usually repeated after several hours. Thus rotational periods are found to be mostly between 5 and 8 hours, although a few have rotational periods in excess of 24 hours.
Further information is available from the Shallow Sky Section.