Near-Earth Asteroids: Zoom Astronomy

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Zoom Astronomy
ASTEROIDS

Introduction	Near-Earth Asteroids (NEA)	Asteroids and Dinosaurs	List of Some Asteroids	Composition	Activities, Web Links

Near-Earth Asteroids

NEAR-EARTH ASTEROIDS

Eros, Asteroid #433, is an elongated Near-Earth Asteroid which is 21 by 8 by 8 miles (33 by 13 by 13 kilometers). The density of Eros is 2.4 grams per cubic centimeter, roughly the same as the density of Earth's crust.

Asteroids whose orbits bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are called Near-Earth Asteroids (NEA) or Earth-Approaching asteroids. These asteroids probably came from the main asteroid belt, but were jolted from the belt by collisions or by interactions with other objects' gravitational fields (primarily Jupiter).

About 250 NEAs have been found so far, but many, many more exist. The largest known NEA is 1036 Ganymede, with a diameter of 25.5 miles (41 kilometers). According to astronomers there are at least 1,000 NEA's whose diameter is greater than 0.6 miles (1 kilometer) and which could do catastrophic damage to the Earth. Even smaller NEA's could cause substantial destruction if they were to collide with the Earth.

There are three types of NEA's:

Amors (named for asteroid Amor, #1221): Asteroids which orbit between the orbits of Earth and Mars. Eros (#433) is an Amor.
Apollos (named for asteroid Apollo, #1862): Asteroids which cross Earth's orbit and have a period longer than 1 year. They have semimajor axes greater than 1 astronomical unit (AU), and perihelion distances less than 1.017 AU. Geographos is an Apollo.
Atens (named for asteroid Aten, #2062): Atens are asteroids that are always closer to the Sun than the Earth is; they have a period shorter than 1 year (the semi-major axis is smaller than Earth's). Ra-Shalom is an Aten.

Cruithne is an asteroid about 3 miles (5 kilometers) in diameter that is co-orbital with the Earth, which means that it shares roughly the same orbit as the Earth. It is a Near-Earth asteroid (NEA 3753). From the Earth, it appears to have a horseshoe-shaped orbit; it gets close to the Earth, then it moves away again. Its orbit is highly inclined to Earth's orbit. At its closest approach (which happens every 100,000 years), Cruithne comes to within 10 million miles (15 million km) of Earth (40 times the distance from the Earth to the Moon). Cruithne was named for the first Celtic tribal group that settled in the British Isles. Cruithne was discovered on October 10, 1986 by D. Waldron at Siding Spring Observatory, Coonabarabran, Australia.

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