Near-Earth Objects (NEOs) are asteroids, comet nuclei, and meteoroids. A subclass of NEOs are Potentially Hazardous Objects (PHOs) that are larger than about 50 m in diameter and that intersect the capture cross section of the Earth as it orbits the Sun. PHOs could collide with the Earth with devastating consequences. Among PHOs are Near-Earth Asteroids (NEAs) with three types of orbits defined by the perihelion, q, and aphelion, Q, distances of the Earth (0.983 AU and 1.017 AU, respectively)(1):

  1. Atens (about 6% of the three groups of NEAs) have aphelia Q > 0.983 AU and semi-major axes a < 1 AU;
  2. Apollos (about 65% of the NEAs) have perihelia between 0.983 < q < 1.017 AU, and
  3. Earth-crossing Amors (about 29% of the NEAs) have perihelia 1.017 < q < 1.3 AU.

Atens orbit the Sun mostly in a region interior to the Earth's orbit, but have eccentricities sufficiently large to allow them to cross Earth's orbit near aphelion. The Amors may traverse the Earth's capture cross section since their orbits evolve over tens of thousands of years as a result of long-range planetary perturbations.

Comets are classified according to their orbital period, P, around the Sun. Comets with P < 200 years are called short-period comets and those with P > 200 years are long-period comets. Some long-period comets have periods of about 1 million years. Since they were not recorded in the distant past, their reappearance cannot be predicted.

Techniques to mitigate a collision of an asteroid or a comet nucleus with Earth require detailed knowledge of geophysical and geological properties of the object. In particular, we must gather data on these objects including their mass, internal mass distribution, moments of inertia, material strengths, internal structure, and the relationship of these global properties to their observed surface properties. Global material strength and structure are best determined from artificially activated seismology experiments and from multifrequency radio tomography. Other important properties of NEOs include their shapes and spin states. These can usually be measured by more conventional schemes including ground-based radar observations and observations of lightcurves.

[1]1 AU (astronomical unit) is the mean distance of the Earth from the Sun = 149,598,000 km.

Southwest Research Institute: Space Instrumentation Division To report any problems, email: Walter Huebner
Last Updated: 3/12/2009