Asteroids are rocky objects which orbit the Sun in our Solar System, but are too small to be considered planets. They are in fact, commonly known as Minor Planets due to their size. The majority of the asteroids in our Solar System orbit the Sun in what we call the "Asteroid Belt." This is located between Mars and Jupiter as can be seen in the diagram above from the NASA Lunar and Planetary Institute. Some other asteroids are found in Jupiter's orbit at Lagrangian points points in Jupiter's orbit where they have a stable orbit. These asteroids are called the Trojan asteroids.
However, there are some asteroids that have left this region after being influenced by the gravitational forces exerted by the planets in the Solar System, and are on paths which bring them near to the Earth. If they approach the Earth at a distance of less than 1.3 AU (1 AU is the distance between the Earth and the Sun), then they are considered to be a Near Earth Object, or NEO. Although the chances of such asteroids hitting the Earth are very slim, our planet has been struck in the past. An example of an asteroid impact creating a crater on Earth is shown in the image of the Barringer Meteor Crater in Arizona.
Asteroids come in different shapes and sizes, ranging from about 1000km across, to a few cm. They are thought to be the leftovers of the formation of the Solar system which didn’t quite make it into a planet, but remained in orbit around our Sun. Most of our knowledge of asteroids comes from our study of meteorites found on Earth. An asteroid which is on a collision path with the Earth is called a meteroid. If the meteoroid then enters the Earth’s atmosphere, as it burns up due to the speed of its entry, it will leave a streak of light in the sky, and it becomes known as a meteor (or a shooting star!). What is left of the meteor, when it hits the Earth, is known as a meteorite.
One of the largest asteroids, and also the brightest, is Vesta. Discovered in 1807, this asteroid is 530km in diameter. The smallest asteroid known to date is approx. 3-6m in diameter. It passed very close-by to the Earth in 2003 - in fact, it was the closest approach by an asteroid that didn’t hit the Earth that has ever been documented, at a distance of 88,000km. This is less than a quarter of the distance to the Moon, and therefore, much less than 1.3AU, earning it NEO status. The largest asteroid in the asteroid belt used to be Ceres until it was given "dwarf planet" status in 2006.
- edited from text provided by Dr. Sarah Roberts