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.

Color image of asteroid Ida and its small moon Dactyl. Inset shows a larger view of Dactyl.
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.
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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.

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.

Artist's concept showing spacecraft orbiting an asteroid.
This illustration depicts the Dawn Mission in orbit around the large asteroid Vesta. (Image Credit: McREL)


There are several different types of asteroids, identifiable through their composition.

Silicon-rich (S-type) asteroids dominate the inner part of the asteroid belt, closest to the Sun. These asteroids are composed of rocky materials-iron- and magnesium silicates-plus small amounts of metallic iron. About 17 percent of known asteroids are S-type.

M-type (metallic) asteroids that condensed in the middle of the main belt are predominantly metallic iron and nickel, although some of them may have some anhydrous (without water) silicates, hydrated clay minerals, magnetite, and sulfides.

More than 75 percent of known asteroids are C-type (carbonaceous) asteroids, which inhabit the main belt's outer regions. These asteroids are usually composed of organic compounds and hydrated minerals. They are rich in carbon, either in elemental form or in organic matter, such as aliphatic and aromatic hydrocarbons. Any metals in these asteroids are found as silicates, oxides, or sulfides rather than in the free form. C-type asteroids that formed beyond the Frost line were able to accumulate ices and hydrated minerals.

Additional websites with information about asteroids, at a variety of levels:

This information is written for a public audience, including older children: Comets vs. Asteroids Fact Sheet (PDF, 1.3 MB)

We recommend you start with Solar System Exploration: Asteroids. This page contains information written at an adult level.

Another starting resource is "The Main Asteroid Belt" by Carolyn Crow.

For information about planetary migration and altering positions of asteroids, check out this press release written by the Southwest Research Institute.

Some asteroids have been compared to enormous rubble piles. For more information on "rubble pile" asteroids, check out Killer Asteroids. Loosely Bound Piles of Trouble.

There are a variety of asteroid articles in Planetary Science Research Discoveries; this educational bulletin is written at a high level for those already familiar with fundamentals of geology:

Discover technological breakthroughs that revolutionized the study of asteroids:

See also the related Year of the Solar System topics: Comets: Small Bodies / Big Impacts and Collisions and Craters in the Solar System: Impacts.


Featured Missions

Artist's illustration of Dawn spacecraft.
Artist's illustration of Dawn spacecraft.
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NASA missions to asteroids are uncovering critical evidence about conditions in our early solar system and the make-up of the objects that formed the building blocks of our solar system.


Dawn arrived at asteroid Vesta in July 2011. Dawn is designed to study the conditions and processes of the solar system's earliest time by investigating in detail two of the largest asteroids remaining intact since their formations. The orbiter will visit both the asteroid Vesta and dwarf planet Ceres, two main asteroid belt worlds that followed very differently evolutionary paths.