Near Earth Asteroids

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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.

Near Earth Object Picture

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.

asteroid belt photo

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.

Comets Picture

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: 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).


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