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NEO stands for Near Earth Object. These are bodies in space in our Solar System whose orbits approach the path of our planet to within 0.3 Astronomical Units (one Astronomical Unit, or 1 AU, is the distance between the Sun and the Earth). Potentially Hazardous Objects or PHOs approach the Earth to within 0.025 AU which is nearly 20 times the distance from the Earth to the Moon.
There are two types of NEO. Asteroids, which are objects made mainly out of carbon-rich materials, silicate rock and metal, and comets, which consist mostly of ice and dust. Near Earth Objects sometimes collide with the Earth and can cause significant damage.
Most NEOs are small, only a few metres in size, however, some objects that approach the Earth are more than 40 km in diameter. Around 50,000 fragments of NEOs fall on Earth as meteorites each year but are too small to cause much damage. Forty thousand tonnes of dust, much from NEOs, also lands on Earth every year. This is nearly 100 billion particles.
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