Asteroids
Rocky leftovers from the birth of the planets. Through binoculars the brightest look like ordinary stars, until you notice that one of them has moved.
What asteroids are
Asteroids are rocky and metallic bodies, from boulders to a few hundred kilometers across, that never assembled into a planet. Most orbit in the main belt between Mars and Jupiter, where Jupiter’s gravity kept them from coming together. Others share Jupiter’s orbit, and a smaller group, the near-Earth asteroids, crosses or comes close to Earth’s path. Well over a million have been cataloged.
The largest, Ceres, is about 940 kilometers across and round, so since 2006 it has been classed as a dwarf planet. Vesta is smaller but more reflective, and at a favorable opposition it is the only main-belt asteroid that can occasionally be glimpsed without optical aid from a very dark site.
How to spot one
Even through a large telescope an asteroid is a point of light, indistinguishable from a star at a glance. The giveaway is motion. Find the field with binoculars or a small telescope, sketch or photograph the stars around it, and look again the next night: the asteroid will have shifted while the stars stay put. Near opposition, when an asteroid is opposite the Sun in the sky, it is closest and brightest, and the change can be visible within an hour or two.
In the app, SkyMate’s Comets & Asteroids data pack covers about 15,000 orbits from the IAU Minor Planet Center, so it can point you to the brighter asteroids for your place and time. On the web we keep to the objects we can describe accurately, so there is no list of individual asteroids here yet.
Asteroids behind meteor showers
Most meteor showers come from comets, but two of the strongest come from rocky bodies: the Quadrantids (asteroid 2003 EH1) and the Geminids (asteroid 3200 Phaethon). Both may be extinct comets or rock fragments shedding debris as they pass near the Sun.