Meet the asteroids

Are they gonna hurt us?

Will they obliterate the earth?

How are asteroids found?

Did asteroids deliver life?

What are they made of?

What about comets?

Asteroids in orbit

Spacy spin doctor
Scientists view asteroids as remnants of the "proto-planetary disk," the giant, rotating disk of dust and gas that formed the sun and the planets about 4.6 billion years ago. For whatever reason, when these large objects coalesced, some dregs remained. Formally known as asteroids, they mainly orbit between Mars and Jupiter.

gaspraIntriguingly, the asteroids serve to divide the solid, rocky planets -- Mercury, Venus, Earth and Mars -- from the gas planets, Jupiter, Saturn and so on.

If the origin of asteroids is clear, their nature is not. Are they solid rock, or agglomerations of rubble: rocks, sand, grit and dust? The answer to that question has been the "Holy Grail" of asteroid science, according to Alan Harris, an asteroid researcher at the Jet Propulsion Laboratory (JPL). "Until now, we've never had unequivocal knowledge that an asteroid must be glued together."

Results released by a JPL team this October show that at least one asteroid, poetically named 1998 KY26, is solid rock. This hunk of stone "can't be a pile of debris," according to JPL's Lance Benner, who reported radar data on the asteroid. "It must be monolithic and have tensile strength." His reasoning? Asteroids have very weak gravity, and a debris pile would self-destruct by centrifugal force if it rotated faster than every two hours or so. And this one turns a full revolution in less than 11 minutes.

All together now
Proof that at least one asteroid is solid rock strengthens their link to meteorites, which are also solid rock, and which presumably originated in asteroids. Until now, however, there's been no proof that asteroids have the same solid construction as meteorites.

Looks like a rock to me. Galileo images of the asteroids 951 Gaspra (upper right) 243 Ida and its small satellite, Dactyl (near right).
Courtesy of NASA.
idaBut how did the dust of the proto-planetary disk fuse into solid rock? Planets had enough radioactive decay and mass to make and retain the heat needed to accomplish this fusion. In the freezing asteroid belt, the heat may have originated in the rapid decay of radioactive isotopes like aluminum-26, during the first billion years of the solar system. Alternatively, it may have come from the interaction of the solar wind and planetary magnetism.

Studies of meteorites find that they originated in rocks that cooled at the rate to be expected for objects at least 100 kilometers in diameter. According to asteroid researcher Alan Harris of the JPL, this means the original asteroids were real hunks, and that their present smaller size is explained by continual collisions in orbit. (Indeed, several moons in the solar system may be left-over asteroids.)

Further evidence of heating comes from mineralogical evidence that some asteroids had liquid water near their surfaces early in their history. Today, temperatures deep in the asteroid belt are chilly -- say -260 degrees Celsius. But within the first million years of the solar system, explains Faith Vilas, a planetary scientist at Johnson Space Center, the asteroids might have been heated by radioactive decay of aluminum 26 they contained, or by an electromagnetic current induced by passing through strong magnetic fields and the charged solar wind. This heating was apparently enough to melt ice, without immediately evaporating the water.

Heard about those weirdo comets?

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The Why Files
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