Billions of rocky lumps, called asteroids or minor planets, circle the Sun between the orbits of Mars and Jupiter. They occupy a broad band around 180 million km (112 million miles) wide, known as the asteroid belt. Most of the METEORITES that bombard the Earth from space appear to be asteroid fragments. There is also a region of small bodies in the outer Solar System, beyond Neptune, called the KUIPER BELT.
Asteroid number 243, called Ida, is an elongated lump of rock 56 km (35 miles) long. Amazingly, this small rocky body has a moon 1.6 km (1 mile) across circling round it. Ida seems to be made up entirely of rock, but asteroids can also be made up mainly of metal, or a mixture of rock and metal.
Most asteroids circle the Sun within the asteroid belt, but others have orbits that take them out beyond the orbit of Saturn or in towards Earth. Asteroids that pass close to Earth are known as Near Earth Asteroids, but there is no imminent danger of collision. At its closest, asteroid Eros passes about 22 million km (14 million miles) from Earth.
Every day small space rocks rain down on Earth and reach the ground as meteorites. Most meteorites are made up of rock and are called stones. Others are mainly iron and are called irons. Stony-irons are a mixture of rock and iron. Most meteorites are tiny, but occasionally large ones tens or even hundreds of metres across hit the Earth.
Around 50,000 years ago an iron meteorite about 46 m (150 ft) across gouged out a huge crater in the Arizona Desert, in the USA. Called the Arizona Meteor Crater or Barringer Crater, it measures roughly 1,265 m (4,150 ft) across and 175 m (575 ft) deep. If a meteorite of similar size struck a city today, it would cause immense devastation.
From Neptune’s orbit and far beyond Pluto is a region known as the Kuiper Belt, in which small icy objects orbit the Sun. Even further out, at the very edge of the Solar System, lies a swarm of comets called the Oort Cloud. The material in both these areas is thought to be debris left over from the formation of the Solar System 4.6 billion years ago.
Several hundred Kuiper Belt Objects (KBOs), hundreds of kilometres across, have been found since 1992, but there are estimated to be tens of thousands in total. KBOs seem to be icy bodies very similar to comets, and short-period comets are believed to originate from the Kuiper Belt. Astronomers now believe that Pluto and its moon Charon are in fact large KBOs. Neptune’s moon Triton may also once have been a KBO, but was then captured by Neptune’s gravity.