The Asteroid Belt, Solar System

• Discovered: Early 19th century
• Distance from the Sun: Varies between 387 million km and 489 million km

Asteroids are irregular fragments of rock left over from the formation of the Solar System 4.6 billion years ago. Millions of asteroids are thought to orbit the Sun and are largely concentrated in a belt, 180 million km wide, between the orbits of Mars and Jupiter.

The astronomer William Herschel first used the word asteroid (Greek for star-like) to describe these celestial objects. They range in size from less than 1 km across to the largest known asteroid, Ceres, which is 940 km in diameter.

Asteroid impacts

The asteroid belt is relatively stable but occasionally gravity from a larger body, such as a planet, pulls one of them out of orbit. Stray asteroids have hit Earth in the past. Many scientists believe one impact, around 65 million years ago in an area now known as Mexico, was responsible for a sudden change in climate and the extinction of dinosaurs.

The chances of a similar event are, thankfully, slim but ground-based telescopes are monitoring Near Earth Asteroids (NEAs) – defined as those whose orbits are able to approach or cross the orbit of Earth. BNSC is involved through the European Space Agency (ESA).

Telescopes on Earth track NEAs and detect new asteroids in case any of them are in an orbit that might collide with our planet. NASA’s Galileo spacecraft was the first to observe an asteroid close up in the early 1990s and in 2000 the Near Earth Asteroid Rendezvous (NEAR) mission orbited the asteroid Eros for a year before the first ever landing on an asteroid on 12 February 2001.

The European Rosetta mission, launched in 2004, will also make two fly-bys of asteroids before intercepting, orbiting and dropping a lander onto a comet in 2014.