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Asteroids: Overview
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Black and white image of asteroid Eros.
This picture of Eros, the first of an asteroid taken from an orbiting spacecraft, is a mosaic of four images obtained by NASA's NEAR mission immediately after the spacecraft's insertion into orbit.

Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-earth objects.

10 Need-to-Know Things About Asteroids:

  1. If all of the asteroids were combined into a ball, they would still be much smaller than Earth's moon. If the sun was as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest object in the main asteroid belt) would be as small as a sesame seed.
    Link to Dawn Mission
    Dawn: First to orbit an asteroid
  2. Most Asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.
  3. Days and years vary by asteroid. A day on asteroid Ida, for example, takes only 4.6 hours (the time it takes to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid's time) in 4.8 Earth years.
  4. Asteroids are solid, rocky and irregular bodies.
  5. Asteroids do not have atmospheres.
  6. More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.
  7. One asteroid, named Chariklo, is known to have two dense and narrow rings.
  8. More than 10 spacecraft have explored asteroids. NEAR Shoemaker even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta).
  9. Asteroids cannot support life as we know it.
  10. Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi) and the closest dwarf planet to the sun, encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.

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Just the Facts
Discovered in 1801, Ceres was the first asteroid discovered. It has since been reclassified as a dwarf planet because it is more like a planet that its neighbors in the main asteroid belt.

About once a year, an automobile-sized asteroid hits Earth's atmosphere, creates an impressive fireball, and burns up before reaching the surface.

Asteroids that populate the main asteroid belt between Mars and Jupiter, and pose no threat to Earth, can be as big as 940 km (about 583 miles) across.
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"I study what meteor showers and meteorites teach us about comets and asteroids, the origin of the Zodiacal cloud, and our own past and future." Read More...
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Last Updated: 13 May 2014