Description: In addition to the Sun and planets, our Solar System contains numerous smaller bodies, notably asteroids and comets. The collision of an asteroid or comet with Earth can have disastrous consequences. So far, none of the objects we know is on a collision course with Earth, but there are a number of objects it makes sense to keep an eye on, nonetheless. Any asteroid or comet that, on its orbit, comes closer to the Sun than 1.3 times the Earth–Sun distance (in astronomical terms: 1.3 astronomical units) is called a Near-Earth Object (NEO). Most NEOs are Near-Earth Asteroids (NEAs).

A NEO is called a Potentially Hazardous Object (PHO) if it has the following properties: For one, it needs to come closer to Earth's orbit than 5% of the Earth–Sun distance (closer than 0.05 astronomical units). In addition such an object needs to have a certain minimum size – otherwise, it would pose no danger to Earth. Size is difficult to measure for smaller objects in the Solar System, so astronomers instead use a minimal value for such an object's absolute brightness – after all, the larger an object is, the brighter it is likely to be. To qualify as a PHO, an object needs to have an absolute magnitude of 22.0 or brighter (using the magnitude system as astronomy's standard way of measuring brightness). Most PHOs are Potentially Hazardous Asteroids (PHAs).

Related Terms:

• Asteroid
• Comet
• Solar System

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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