There was some material left over from the solar nebula once the Sun and the planets had formed. Some of this debris remains in our Solar System in the form of asteroids and comets.

Asteroids

Asteroids, which are sometimes called minor planets, are rocky bodies mostly found in the planetary region between Mars and Jupiter. This region is known as the asteroid belt, and it stretches from about 250 million km to about 600 million km from the Sun. The largest known asteroid is Ceres with a diameter of roughly 1000 km. Only around a dozen are more than 250 km across. Over 100 000 asteroids larger than one kilometre in diameter are known to exist, with more being discovered all the time.

We often hear of asteroids on the news, when near-Earth asteroids pass close enough to our planet to cause concern of a potential impact either now, or in the future. These near-Earth objects have highly elliptical orbits, which bring them into the inner Solar System, crossing the orbit of Mars and occasionally coming close to Earth.

Comets

Comets are often referred to as 'dirty snowballs', as they are made up of ice and dust. The ones we can see travel around the Sun in highly elliptical orbits taking from a few years to thousands of years to return to the inner Solar System. Typically comets are just a few kilometres across, which makes them very difficult to spot for most of their orbit. As they approach the Sun, however, solar radiation vaporizes the gases in the comet and the characteristic comet 'tail' is formed. The tail of a comet consists of two parts: a whiter part made of dust, which always points away from the Sun, and a blue part consisting of ionised gas. Comets are mainly found in two regions of the Solar System: the Kuiper belt, a region that extends from around the orbit of Pluto to about 500 AU from the Sun, and the Oort Cloud (from the Kuiper Belt to about 50 000 AU from the Sun).

The Rosetta mission will track the comet 67 P/Churyumov-Gerasimenko and deploy a lander onto its surface. On its 10-year journey to rendezvous with the comet, the spacecraft will hopefully pass by at least one asteroid.

Meteors

Occasionally small rocks or dust particles enter the Earth's atmosphere. The dust particles and small rocks burn up in the atmosphere leaving behind brief trails in the sky witnessed as meteors. It is estimated that more than 200 million kg of meteoritic material is swept up by the Earth each year, with around 10% reaching the ground.

Much of this material orbits the Sun in distinct streams, usually as debris from different comets. At various times throughout the year the Earth crosses these streams and for a few nights an observers can witness a meteor shower.

Table 1.3: Dates of Primary Meteor Showers

Sometimes larger fragments survive their passage through the atmosphere and impact the surface, where they become known as meteorites. Most impacting fragments are tiny and cause little or no damage. Historically, however, there have been several major impacts, which may be responsible for changes in climate and the mass extinction of species.

The image alongside shows the Barringer Meteor Crater in the United States. It was created around 50 000 years ago when a 50 m wide rock hit the ground at an estimated speed of 65 000 kilometres per hour. The resulting crater is 175 m deep, 1500 m wide and scattered debris in a 15 km radius.

Figure 1.1: Barringer Meteor Crater (credit: NASA)

	Planets and Moons
	Bodies of the Universe