Near-Earth asteroid
An asteroid whose orbit brings it fairly close to Earth's path around the Sun.
A near-Earth asteroid (NEA) is one whose closest point to the Sun lies within about 1.3 times Earth's distance from the Sun. Most began in the main asteroid belt and were pushed inward by the gravity of Jupiter and other planets. Their new orbits don't last forever: over a few million years many hit a planet, fall into the Sun or are flung away.
Tens of thousands of NEAs are now known, from boulders a few metres across to the largest, Ganymed, which is roughly 35 kilometres wide. Astronomers track them for safety, but they are also prized targets for spacecraft, because some are easier to reach than the Moon.
The best example of an NEA turning into meteorites is 2008 TC3. Only a few metres across, it was spotted about 20 hours before it struck the atmosphere over Sudan on 7 October 2008. Students and scientists later searched the desert and recovered hundreds of fragments, now called Almahata Sitta. It was the first time an asteroid had been seen in space and then collected on the ground. Through a telescope an NEA looks like a faint star that moves noticeably against the background stars, sometimes within minutes.
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Level: Explorer Ages 10 and up