August 18, 2014 | David F. Coppedge

Fluffballs in Space Shouldn't Exist

A rubble-pile asteroid has such low density it should have disintegrated, but it’s doing just fine.

A near-earth asteroid named 1950 DA is barely holding itself together, astronomers have found.  In Science Magazine, Eric Hand asks “Why hasn’t this asteroid disintegrated?”

Planetary scientists have found an asteroid spinning too fast for its own good. The object, known as 1950 DA, whips around every 2.1 hours, which means that rocks on its surface should fly off into space. So apart from gravity, some other sticky force—identified in a new study—must help to hold the asteroid together.

Nature gives an even more graphic description:

Our logical concepts for how asteroids should behave have taken another knock, as evidenced in a paper by Rozitis et al. on page 174 of this issue. The researchers establish that a kilometre-sized, near-Earth asteroid known as (29075) 1950 DA is covered with sandy regolith (the surface covering of an asteroid) and spins so fast — one revolution every 2.12 hours — that gravity alone cannot hold this material to its surface. This places the asteroid in a surreal state in which an astronaut could easily scoop up a sample from its surface, yet would have to hold on to the asteroid to avoid being flung off.

Scientists’ best explanation is that atomic forces called van der Waal’s forces are providing the edge over gravity alone, otherwise this body should be too flimsy to exist.  These are the same atomic forces thought to allow geckos to stick to walls and ceilings.  Stephen Lowry discusses this force on The Conversation.

Nature explains why this is significant: “Although this image of fairy-castle asteroids is entertaining, the implications of these measurements are far-reaching…. ”

The evident stability of such a strange body as 1950 DA exposes our ignorance of how the geophysics of asteroids works in the microgravity regime, with its current state being difficult to reconcile with classical views of how rubble-pile bodies form from catastrophically disrupted parent bodies. Although Rozitis et al. lay out a plausible story for the current state of 1950 DA, the development of a complete theory of microgravity geophysics could have significant consequences, beyond this single case, for our evolving understanding of asteroids and the Solar System.

This finding will also impact plans to disrupt near-earth asteroids that some day find themselves with our planet in its cross-hairs.  Astrobiology Magazine says that trying to disrupt a large rubble-pile asteroid might create several dangerous bodies out of one.  These small, sticky asteroids might be the most dangerous, New Scientist warns.  Lowry thinks that disrupting one might allow centrifugal forces to overtake the van der Waals force (which only acts over small distances), resulting in complete disintegration of the body.  Those are questions for governments to worry about if one is ever discovered on an impact trajectory.  The question for now is: how long can these bodies exist in a tenuous balance of forces?

This is a worthy class of objects to study for possible age determinations: how long can a body like this exist before disrupting by encountering other bodies?  This one seems in a delicate balance.  One would think after very long it would disintegrate, with all the solar and planetary disturbances at work.  How many rubble-pile asteroids are there?  Can dust come together like this?  Many questions need new thinking, since the standard story was caught unprepared.

 

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