Moons, Asteroids and Comets

Moons, Asteroids and Comets

Moons are large, round, rocky objects that travel around planets. Asteroids are smaller, lumpy rocks that orbit the Sun. Comets orbit the Sun too, but on much longer paths.

Earth has one moon, simply called 'the Moon'. It is the Earth's only natural satellite. Held in place by our planet's gravity, the Moon orbits Earth once every 27.3 days at an average distance of 384,400 kilometres (238,855 miles). The Moon is the fifth-biggest moon in the Solar System at 3,474 kilometres (2,158 miles) wide. Most planets have more moons Mars has two small ones while Saturn has over 60. The biggest moon is Jupiter's Ganymede, which is 5,268 kilometres (3,273 miles) wide. The second biggest moon is Saturn's Titan.

Asteroids

Asteroids are space rocks that orbit around the Sun. They are sometimes called minor planets. Most asteroids travel around the Sun in a gap between Mars and Jupiter, called the Asteroid Zone or Asteroid Belt. Thousands more asteroids orbit between other planets, including over 1,000 Near-Earth Asteroids (NEAs). In the future, one of these NEAs may crash into the Earth, but the risk is very low.

Comets

Comets orbit the Sun in a long, lopsided way and are usually travelling in deep space. As the comet nears the Sun and warms, its central core of dust and ice lets off gases and particles that reflect sunlight. This forms a fuzzy, bright 'coma' around its core. Comets also appear to have a glowing tail, which does not actually trail behind but instead points away from the Sun. After circling the Sun, the comet heads back into deep space, cools and again becomes dark.

Fascinating facts
Deep-space unmanned craft are gradually uncovering comet secrets.
• 2004: US spacecraft Stardust travelled near comet Wild 2, collected its dust and gases, and delivered samples back to Earth in 2006.
• 2005: Comet Tempel l's nucleus was deliberately hit by a fridge-sized part of US spacecraft Deep Impact, which took photographs of the smash.
• 2014: Europe's Rosetta craft will land on comet Churyumov - Gerasimenko and put a small robot lander onto its surface. Rosetta started its mission in 2004.