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
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,
Atens (about 6% of the three groups of NEAs) have aphelia Q > 0.983 AU and
semi-major axes a < 1 AU;
Apollos (about 65% of the NEAs) have perihelia between 0.983 < q < 1.017 AU, and
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
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
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 AU (astronomical unit) is the mean distance of the Earth from the Sun = 149,598,000 km.