Sciency Words is a special series here on Planet Pailly celebrating the rich and colorful world of science and science-related terminology. Today, we’re looking at the term:
RUBBLE PILE
“Rubble pile” is not a formal part of any asteroid classification system, but it appears so often in scientific literature about asteroids that it deserves special attention. A rubble pile asteroid is really multiple asteroids tenuously held together by their own gravity.
Rubble piles probably form in the aftermath of asteroid collisions. When two asteroids smash into each other, gravity starts pulling the resulting debris back together again. A rubble pile can then form in as little as a few hours.
Since they’re not single, solid objects but conglomerations of multiple objects, rubble piles tend to have empty spaces inside them. This lowers the overall density of the asteroid, which helps astronomers distinguish rubble piles from regular “monolithic” asteroids.
Rubble piles have a minimum rotation period of approximately two hours. If they start spinning faster than that, they’re liable to fling themselves apart. However, astronomers have observed some rubble piles rotating faster than they should be able to, suggesting that additional forces besides gravity may help hold them together.
Exerting even a tiny force on a rubble pile could cause the thing to break apart. You can’t easily land on a rubble pile’s surface for mining purposes, and deflecting a rubble pile from a collision course with Earth would be tricky. As a result, rubble piles could pose many challenges to humanity in the future.
Links
Potentially Dangerous Asteroid is Actually a Pile of Rubble from Space.com.
Cohesive Forces Prevent Rotational Breakup of Rubble-Pile Asteroid (29075) 1950 DA from Nature.
Rubble-Pile Asteroid from The Worlds of David Darling.
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Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.
Planet Pailly 
The fast-spinning “rubble piles” sound like asteroids that have been hollowed out as habitats!
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On the other hand, extracting material from a rubble pile may be a lot easier, if we can just get to it.
One of the ways in which an asteroid’s course could be changed would be by having a spacecraft maintain a position near the asteroid, firing thrusters as necessary to maintain its distance. Over time, the spacecraft’s gravity would alter the asteroid’s orbit. Of course, for this to work, months or years might be required, so it wouldn’t work for a discovery of an relatively imminent collision.
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