Asteroids, or minor planets, are small and often irregularly shaped
celestial bodies. The known majority of them orbit the Sun in the
so-called main asteroid belt, between the orbits of the planets Mars and
Jupiter. However, due to gravitational perturbations caused by the
planets, a continuous migration brings asteroids closer to Sun, thus
crossing the orbits of Mars, Earth, Venus and Mercury. An asteroid is
coined a Near Earth Asteroid (NEA) when its trajectory brings it within
1.3 AU [Astronomical Unit; for units, see below in section Glossary and
Units] from the Sun and hence within 0.3 AU of the Earth's orbit. A NEA
is said to be a Potentially Hazardous Asteroid (PHA) when its orbit
comes to within 0.05 AU (= 19.5 LD [Lunar Distance] = 7.5 million km) of
the Earth's orbit, the so-called Earth Minimum Orbit Intersection
Distance (MOID), and has an absolute
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