What is a near-Earth object, and how many are there?

A near-Earth object, or NEO, is a small body, an asteroid or a comet, whose orbit around the Sun brings it close to Earth's neighbourhood. The formal rule is that its closest approach to the Sun is less than 1.3 times the Earth–Sun distance. As of the end of 2024, 37,378 NEOs had been discovered: 37,255 asteroids and 123 comets.

The word "near" is about the orbit, not the moment. An object counts as a NEO even when it is far away at the moment, because its path is one that can come close. If its orbit can bring it within about 7.5 million kilometres of Earth's orbit and it is roughly 140 metres or bigger, it is called a potentially hazardous object. That label describes a possibility of coming close, not a prediction of a hit.

Where do they come from? Near-Earth asteroids last only a few million years in these orbits before they are thrown out of the Solar System or destroyed in a collision. The accepted explanation is that main-belt asteroids are pushed inward through orbital resonances with Jupiter, so the supply is constantly refilled.

Finding them is a long project. The first near-Earth asteroid, 433 Eros, was found in 1898. In 1998 the US Congress told NASA to find 90% of the asteroids over 1 km across within a decade. The share known rose from about 20% in 1998 to 93% in 2011, so the goal was met three years late.

Smaller ones are harder. In December 2023 only an estimated 38% of NEOs 140 metres or larger had been found. A 2020 estimate said objects of that size hit Earth only about once in 30,000 years.