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Leftovers from Planet Building: Asteroids

Color image of potato-shaped asteroid.
The asteroid Gaspra as seen by the Galileo spacecraft en route to Jupiter. (Image Credit: NASA)

When we build things, there are often building materials left over. Asteroids are bits of building material remaining from the formation of our solar system (for more about solar system formation, check out the YSS topic Birth of Worlds). During the solar system's formation, bits of dust and rock bumped into each other, sometimes sticking together -- accreting -- and sometimes scattering. But even after the planets formed, there remained residual materials -- asteroids.

Asteroids can have families!
Some asteroids have moons, some are pairs
orbiting each other and still others appear to
be rubble piles loosely held together by gravity.
Color image of asteroid Ida and its small moon Dactyl. Inset shows a larger view of Dactyl.

Today, most asteroids orbit the sun farther than Mars but closer than Jupiter -- a region popularly known as the asteroid belt. Though thousands of asteroids reside in the main belt, this region is not densely populated -- all of the asteroids together only make about 5% the mass of the Moon. So asteroids seldom collide, but when they do they tend to break up instead of accreting. While many asteroids likely formed in the main belt region, some may have formed elsewhere. New models of how the solar system evolved suggest that particular asteroids, like surplus nails flung into a scrap pile outside a new home, may have been flung into their current orbits by gravitational tugs of growing planets as they migrated to new orbits early in the history of the solar system.

Like the scraps from a building project, the leftovers of planetary formation come in many sizes and shapes. Most asteroids are irregularly shaped and all have craters from impacts with other asteroids. However, the largest asteroid, Ceres, has sufficient gravity to become nearly spherical, making it also a dwarf planet! Vesta, another large asteroid, has evidence of ancient lava flows on its surface. The Dawn mission has finished its study of Vesta and is on its way to examine Ceres, comparing these very different massive asteroids. Find out more about these different types of asteroids in this topic's Background.

Asteroids are an incredible scientific resource -- as scraps of the original building material of the solar system, they tell us about our own origins. Scientists are studying pieces of asteroids that have fallen to Earth's surface -- meteorites -- to learn more. The YSS topic Impacts! investigates these near Earth objects.

In this topic, the Year of the Solar System focuses on these rocky remains. Asteroid Resources will help you bring Asteroids: Leftovers from Planet Building to your programs!

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Last Updated: 12 Sep 2014