What is a
near-Earth object?

A near-Earth object is any natural body with an orbit that brings its perihelion - its closest point to the Sun - within 1.3 AU. One AU (astronomical unit) is 150 million km. Earth's orbit sits at roughly 1 AU, so a NEO at 1.3 AU can, at various points in its orbit, cross or approach Earth's path.

The 1.3 AU threshold is not arbitrary. Beyond that distance, solar gravity weakens enough that objects rarely come near Earth's orbit on human timescales. Inside it, gravitational interactions with Earth, Venus, and Mars can gradually shift orbits over thousands to millions of years.

The vast majority of NEOs are asteroids - rocky and metallic bodies left over from the formation of the solar system. A small fraction are comets: icy bodies from the outer solar system that have been nudged into shorter orbits. Around 100 known comets qualify as NEOs.

A subset of NEOs carries an additional label: potentially hazardous asteroid (PHA). To qualify, an object must meet two conditions simultaneously. Its orbit must pass within 0.05 AU of Earth's orbit - about 7.5 million km. And its estimated diameter must be 140 metres or larger.

The 140-metre threshold is grounded in physics. Below that size, most of the object burns up or breaks apart in the atmosphere before reaching the surface with enough energy to cause widespread damage. Above it, a ground impact releases energy comparable to a large nuclear weapon or greater.

Being a PHA does not mean an object will hit Earth. It means the object is large enough to matter and close enough, orbitally, that the scenario cannot be ruled out over centuries without sustained monitoring. NASA's CNEOS publishes impact probability tables for all PHAs. Currently, no known PHA has a meaningful probability of impact in the next century.

The primary method is repeated wide-field imaging. Survey telescopes photograph the same patch of sky several times in one night. Software compares the frames and flags anything that has moved - that movement is the signature of a nearby solar system body.

The main ground-based programmes are Catalina Sky Survey in Arizona, Pan-STARRS in Hawaii, and the ATLAS network operating from multiple sites. Together they account for most new NEO discoveries each year.

Once a candidate is flagged, follow-up observers confirm the detection over the next few nights. The positions go to the International Astronomical Union's Minor Planet Center, which computes a preliminary orbit. From there, CNEOS assesses whether the object is a NEO and checks it for impact probability.

NASA's target is to catalogue all NEOs larger than 140 metres - a goal set by the 2005 George E. Brown Jr. Near-Earth Object Survey Act. The Nancy Grace Roman Space Telescope, expected to launch later this decade, is designed to substantially complete that census.