What Are They?
Asteroids are rocky objects primarily found in the asteroid belt, a region of the solar system that lies more than 2 ½ times as far from the Sun as Earth does, between the orbits of Mars and Jupiter. These objects are sometimes called minor planets or planetoids. They are likely the leftovers from the early formation of the solar system and their composition may shed light on what the early solar system was like. They probably formed from the protoplanetary disk that surrounded the Sun but never had enough mass to form into the roughly spherical shape required to be considered a planet. Despite thousands having been discovered, their total collective mass is still far less than the mass of the Earth.
Asteroids are meteoroids' big cousins: they range in size from 1 km to larger than 100,000 km where meteoroids are generally smaller than 1 meter in diameter, but they are made from the same rocky or metallic materials.
A subset of these objects, the Trojan asteroids co-orbit the Sun with Jupiter and are in gravitationally stable points between the two, called Lagrange points. It has been estimated that there are as many Trojans as there are asteroids in the asteroid belt.
Asteroids' Encounters with Earth
Asteroids that fall into the category of Near Earth Objects have orbits that bring them in close proximity with our planet, and a number of notable impact sites have been attributed to asteroids. Most famously, the Chicxulub crater under the Yucatán Peninsula in Mexico is the result of an asteroid impact that might have wiped out the dinosaurs. Another asteroid known as Tunguska didn't impact Earth, but exploded a few miles above the Podkamennaya Tunguska River on June 30,1908. A similar explosion made headline news in 2013, when the near-Earth asteroid dubbed Chelyabinsk exploded in air blast that left 1,500 Russians seeking medical attention.