Sciency Words: Rubble Pile

Today’s post is part of a special series that first appeared on Planet Pailly. Every Thursday, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s scientific term is:

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.


Potentially Dangerous Asteroid is Actually a Pile of Rubble from

Cohesive Forces Prevent Rotational Breakup of Rubble-Pile Asteroid (29075) 1950 DA from Nature.

Rubble-Pile Asteroid from The Worlds of David Darling.

Today’s post is part of asteroid belt month for the 2015 Mission to the Solar System. Click here for more about this series.

Article by James Pailly. Check out James’ blog for more great science articles.

  • Kirov

    The last part about deflecting incoming piles reminded me of my high school physics teacher, who had a thing against the Deep Impact movie. Blowing up a comet doesn’t get rid of any of its kinetic energy, so he would always point out that instead of getting one giant explosion, you get all that energy turned into heat as the pieces burn up in the atmosphere. Most everyone still dies, just in a slower and more painful way as the Earth becomes temporarily inhospitable and species die off.

    • John H Reiher Jr

      Yeah, the difference from being hit by a single large caliber bullet and a shotgun blast of double ought shot. Still dead.

      I thing a gravity tug would work on a rubble pile, we just need to know about it ahead of time.

      • Kirov

        Really interesting! I’d never heard of the idea before, and after some quick research, I don’t see why this isn’t our first option against NEOs. It seems simple enough, its just a matter of having the fuel to get the job done.

  • Covertwalrus

    Interestingly, the first depiction of a “rubble pile” asteroid was in a Scrooge McDuck adventure from the 1940s; Scrooge decides to fund a mission to the Asteroid belt in search of minerals, and drags his long-suffering nephew and grand-nephews along. Donald does an EVA on a promising asteroid, and when stepping on it, travels right through the small racks clinging together that compose it due to his inertia from pushing away from the ship.
    Artist and writer Carl Barks has an asteroid named after him – no idea if it’s a “rubble pile” though.