|Near Earth Object Program||
Near-Earth Objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood. Composed mostly of water ice with embedded dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system between the orbits of Mars and Jupiter.
Near-Earth Objects (NEOs)
The scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the solar system formation process some 4.6 billion years ago. The giant outer planets (Jupiter, Saturn, Uranus, and Neptune) formed from an agglomeration of billions of comets and the left over bits and pieces from this formation process are the comets we see today. Likewise, today's asteroids are the bits and pieces left over from the initial agglomeration of the inner planets that include Mercury, Venus, Earth, and Mars.
As the primitive, leftover building blocks of the solar system formation process, comets and asteroids offer clues to the chemical mixture from which the planets formed some 4.6 billion years ago. If we wish to know the composition of the primordial mixture from which the planets formed, then we must determine the chemical constituents of the leftover debris from this formation process - the comets and asteroids.
In terms of orbital elements, NEOs are asteroids and comets with perihelion distance q less than 1.3 AU. Near-Earth Comets (NECs) are further restricted to include only short-period comets (i.e orbital period P 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 (q), aphelion distance (Q) and their semi-major axes (a).
||q<1.3 AU, P<200 years
||Earth-crossing NEAs with semi-major axes smaller than Earth's
(named after asteroid 2062 Aten).
||a<1.0 AU, Q>0.983 AU
||Earth-crossing NEAs with semi-major axes larger than Earth's
(named after asteroid 1862 Apollo).
||a>1.0 AU, q<1.017 AU
||Earth-approaching NEAs with orbits exterior to Earth's but interior
to Mars' (named after asteroid 1221 Amor).
||a>1.0 AU, 1.017<q<1.3 AU
||Potentially Hazardous Asteriods: NEAs whose
Minimum Orbit Intersection Distance (MOID)
with the Earth is 0.05 AU or less and whose absolute magnitude (H)
is 22.0 or brighter.
||MOID<=0.05 AU, H<=22.0
What is a PHA?
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid's potential to make threatening close approaches to the Earth. Specifically, all asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs. In other words, asteroids that can't get any closer to the Earth (i.e. MOID) than 0.05 AU (roughly 7,480,000 km or 4,650,000 mi) or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%) are not considered PHAs.
There are currently 586 known PHAs.
This "potential" to make close Earth approaches does not mean a PHA will impact the Earth. It only means there is a possibility for such a threat. By monitoring these PHAs and updating their orbits as new observations become available, we can better predict the close-approach statistics and thus their Earth-impact threat.
For more information about this topic please visit: http://neo.jpl.nasa.gov/neo/
Excerpt from the Near Earth Object Program