## NEO dictionary

NEO Near-Earth Objects (NEOs) are asteroids and comets with perihelion distance of less than 1.3 AU. Near-Earth Comets (NECs) are further restricted to include only short-period comets (i.e orbital period less than 200 years). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are divided into groups (Aten, Apollo, Amor) according to their perihelion distance, aphelion distance and their semi-major axes.

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:

 Group Description Definition Atens Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten) a<1.0 AU, Q>0.983 AU Apollos Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo). a>1.0 AU, q<1.017 AU Amors Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor). a>1.0 AU, 1.017
where "q" is the perihelion distance, "Q" the aphelion distance and (a) their semi-major axes. Asteroids are also assigned a type based on their surface composition that is inferred from spectral observations. For small bodies it can be assumed that this does not differ too much from the internal composition, however it is most likely not the case for larger bodies. The most common types are:
• C-group dark carbonaceous objects
• S-type silicaceous (or "stony") objects
• X-group (e.g.metallic objects)

Last update: 14 May 2012