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Most Near
Earth Asteroids are believed to be main belt asteroids that were knocked out of
the belt by collisions with other asteroids and/or by the gravitational forces
of Jupiter. Some NEAs may actually be remnants of dead comets. The orbits of
near Earth asteroids are thought to be influenced by gravitational interactions
with the Sun or terrestrial planets or by collisions with other bodies.
What
is a Near Earth Object?
A Near Earth Object (NEO) is an asteroid or comet whose orbit brings it close to
the Earth .
 This includes an object that will come close to the Earth at some
point in its future orbital evolution. NEOs generally result from objects that
have experienced gravitational perturbations from nearby planets, moving them
into orbits that allow them to come near to the Earth.
What
is an Asteroid?
Asteroids
are small minor planets that orbit the Sun .
 The majority are in orbits that
confine them between Mars and Jupiter (the asteroid belt). Most asteroids are
composed of rock, but some have large metallic components such as iron and
nickel. Some asteroids are believed to be “burnt-out” comets that have lost
all their frozen gases. The largest known asteroid is Ceres, about 950
kilometres in diameter.
What
is a Comet?
Comets are celestial bodies in elliptical or parabolic orbits around the Sun.
Some originate from the Edgeworth-Kuiper Belt, beyond Neptune, and some from the
Oort cloud, a spherical shell comprised of millions of bodies at the edge of our
solar system. They are small, irregularly shaped bodies composed of a mixture of
rock, organic molecules and frozen gases. When in the proximity of the Sun, they
are usually characterised by a long tail which is composed of material vaporised
off the comet body.
Where
do NEOs come from?
Asteroids
are believed to be remnants from the formation of the inner planets, including
Earth. Gravitational perturbations from the massive planet Jupiter occasionally
kick asteroids out of the main asteroid belt and into orbits that bring them
close to Earth. Comets are probably the remnants from the formation of outer
planets. Short period comets (with periods less than 20 years) are thought to
originate from the Edgeworth-Kuiper Belt from where they are occasionally
perturbed by the gravity of the large outer planets. Long period comets are
perturbed from the Oort Cloud, a spherical shell of objects at the edge of the
solar system, probably by passing stars.
How
many NEOs are there?
It
has been estimated that there are roughly 150 million ten metre objects, some
300,000 one hundred metre objects, about 10,000 five hundred metre objects and
perhaps 1500 which are one kilometre or larger. There is in general an
exponential increase in the number of smaller objects. More research is required
to refine the numbers.
What
are Atens, Apollos and Amors?
Atens,
Apollos and Amors are subgroups of Near Earth
Asteroid. They are classed in
terms of their orbits.
Atens,
| Atens: |
Earth-crossing
with semi-major axes smaller than Earth's. |
| Apollos: |
Earth-crossing
with semi-major axes larger than Earth's (named after asteroid 1862
Apollo). |
| Amors: |
Earth-approaching
with semi-major axes exterior to Earth's but interior to Mars' (named
after asteroid 1221 Amor). |
Near Earth Asteroid
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