What is a near-Earth asteroid?
A near-Earth asteroid is one whose orbit brings it within about 1.3 times Earth's distance from the Sun, so it can pass relatively close to our planet. More than 40,000 are now known.
Most asteroids stay in the main belt between Mars and Jupiter, but some have been nudged onto orbits that reach into the inner Solar System. Astronomers call an asteroid near-Earth if its closest approach to the Sun is less than 1.3 astronomical units, where one astronomical unit is the average Earth–Sun distance of about 150 million kilometres. Together with near-Earth comets they make up the near-Earth objects, or NEOs.
These bodies do not stay near Earth for ever. Their orbits are chaotic over millions of years, and many eventually hit a planet, plunge into the Sun or are flung out of the inner Solar System. The population is constantly topped up from the main belt, where collisions and slow orbital drift feed fragments into zones where Jupiter's gravity stretches their orbits. That same pipeline delivers most meteorites.
Survey telescopes have found more than 40,000 near-Earth asteroids, and the tally passed that mark in 2025. They include the sample-return targets Ryugu and Bennu, the double asteroid Didymos and its moon Dimorphos, struck by NASA's DART spacecraft in 2022, and Apophis, a body several hundred metres across that will pass within about 32,000 kilometres of Earth's surface in April 2029 without hitting us. A subset is labelled potentially hazardous if larger than about 140 metres and with an orbit passing within about 7.5 million kilometres of Earth's orbit.
Finding them matters for planetary defence. Most of the largest near-Earth asteroids, a kilometre or more across, have now been found, and none is on a collision course for at least the next century. Smaller ones remain under-counted, which is why new survey telescopes are being built. Occasionally a tiny asteroid is spotted just hours before entering the atmosphere, letting scientists predict where meteorites will fall.