PHO A PHO is a Potentially Hazardous Object, an asteroid or comet with a MOID (Minimum Orbit Intersection Distance) of 0.05 AU or less and a diameter of ~ 150 m or more.

A.U. (Astronomical Unit) A unit of distance employed in astronomy usually, for expressing distances within the solar system. 1AU is the mean distance of the Earth from the Sun. The value of this length is 149,598,500 Km or 92,956,000 miles

MOID (Minimum Orbital Interseccion Distance) The MOID indicates the closest possible approach of two objects, and it is the minimum distance between the osculating orbits of the two objects. A large MOID between the Earth and an asteroid indicates that the asteroid will not collide with the Earth in the near future. On the other hand, asteroids with a small MOID could potentially collide with the Earth and are carefully tracked.

Perihelion/Aphelion The orbit of any light object (such as an asteroid, a comet or a planet) orbiting around the Sun has the shape of an ellipse (described by the three Keplero's laws). In this orbit, two important geometrical points can be found, the perihelion, the point where the object is nearest to the Sun, and the aphelion, the point of this orbit where the object is farthest.

MT – megaton The energy freed during an impact is usually measured by Megatons (MT). 1 MT is the energy of almost 100 Hiroshima bombs. To have an idea of the scale of energies: if a 2 meters compact body with a speed of 20 Km/s impacts the Earth, about 1 MT is released.

Asteroid classifications Two main types of classifications exist, one based on their orbital characteristics and a second based on their surface composition. Atens, Apollos and Amors are subgroups of Near-Earth asteroids (asteroids with perihelion distance q less than 1.3 AU) defined as follows:

• C-group dark carbonaceous objects
• S-type silicaceous (or "stony") objects
• X-group (e.g.metallic objects)