Two centuries ago, Sicilian monk Guiseppe Piazzi discovered the first asteroid. It was named Ceres after the Roman goddess of harvest. Soon many more joined this group of asteroids, which lie between the orbits of Mars and Jupiter. Collectively these asteroids are known as the asteroid belt.
Not all asteroids in the solar system lie in the asteroid belt. Some are clustered together and have different orbits. It is easy to confuse these groups. Following is a concise guide to the different groups.
Amor objects: Asteroids that cross the orbit of Mars but does not reach Earth’s orbit
Apollo objects: Apollo objects cross the Earth’s orbit, thus Apollo-Amor objects have the potential to collide with an inner planet. Apollo objects are not the same as Near Earth Objects (NEOs). NEOs include asteroids, comets, meteroids which are in orbits closer to the Earth.
Trojan asteroids: Two groups of asteroids share the orbit with Jupiter 60°ahead and behind the planet. These points are the Langrangian points where the gravities of Sun and Jupiter combine to trap smaller objects into orbit.
Hirayama families: Japanese astronomer Kiyotsugu Hirayama discovered that some asteroids share similar orbits with same eccentricity and inclination. This suggested that they originated from a single body, which fragmented into smaller asteroids. Modern observations have confirmed this theory since asteroids of one family have similar spectroscopic characteristics.
