An NEO is any asteroid or comet whose orbit around the Sun brings it close to Earth’s orbit. The official definition of ‘close’ in this situation is 45 million kilometres. That’s a huge distance by everyday standards, but in astronomical terms it is a mere whisker! Asteroids and comets are the left over building blocks from when the planets formed, 4.5 billion years ago (that’s 4,500,000,000!). Asteroids are made of rocky materials similar to those of the four inner planets; Mercury, Venus, Earth and Mars. Most asteroids orbit the Sun in a well defined region between the orbits of Mars and Jupiter, known as the Asteroid Belt.

Unlike asteroid, comets are mostly made of water ice and frozen gases. They also reside at much greater distances from the Sun, in the cold and dark outer reaches of our Solar System. The further an object is from the Sun, the longer it takes to complete one orbit. Comets that take less than 200 years to orbit the Sun are known as short-period, whilst those taking longer are known as long-period. Some long-period comets can take many thousands of years to orbit the Sun.

When asteroids and comets are nudged by the gravity of massive object like planets, their orbits can be changed causing them to leave these well defined regions and head inwards towards the Earth’s neighbourhood. There are far more near-Earth asteroids than comets.

Because most asteroids and comets have not experienced the same processes that take place on planets, they have remained largely unaltered since their formation. For this reason, they act like time capsules that can reveal a lot about the conditions present when the Solar System was first born. Small pieces of asteroids sometimes survive a fiery fall through the Earth’s atmosphere and land on the ground. Such rocks earn the title of ‘meteorite’, and are highly prized by scientists studying the origins of the Solar System.