Physics

Bizarre Asteroid Has “Negative Gravity”

August 14, 2014 | by Stephen Luntz

Photo credit: JSA. Asteroid Itokawa was discovered to be a rubble pile, rather than a solid object. Now astronomers think they have a better understanding of how such objects hold together

Asteroid 1950 DA appears impossible at first glance. It spins so rapidly no one could work out why it hadn’t pulled itself apart long ago. Now the mystery has a solution, and it seems the asteroid is using a similar trick to geckos climbing glass walls, with implications for how to tackle objects that might threaten the planet.

Many small asteroids are not so much solid rock as loose piles of rubble. Their gravity is very weak, but in the absence of other forces it can be enough to keep the asteroid together. However, 1950 DA is spinning rapidly. 

“We found that 1950 DA is rotating faster than the breakup limit for its density," said Dr Ben Rozitis at the University of Tennessee. "So if just gravity were holding this rubble pile together, as is generally assumed, it would fly apart. Therefore, interparticle cohesive forces must be holding it together."

The spin is rapid enough that, near the equator, an object on the asteroid’s surface would experience “negative gravity” – the acceleration from its turning is stronger than the tiny gravitational pull it can produce and frictional forces can supplement. So how does it hold together? 

The answer, Rozitis has claimed in Nature, lies in van der Waals forces between grains of material. These forces, including those between two dipoles, determine whether substances can be dissolved in oil or water and are used by geckos to climb sheer surfaces. The forces exist because many molecules have a slight negative charge at one end and positive charge at the other. When opposite charges align particles are drawn to their neighbors.

“Cohesive forces in the form of small van der Waals forces between constituent grains have recently been predicted to be important for small rubble piles (ten kilometres across or less)” the paper notes, and at strengths similar to those observed between grains on the moon this could be enough to keep 1950 DA together.

Such a loosely held-together object should be far easier to destroy, if it was on a collision course for Earth, than something more solid. "With such tenuous cohesive forces holding one of these asteroids together, a very small impulse may result in a complete disruption," says Rozitis.

Where previously nuclear weapons were proposed as the answer to Earth-threatening asteroids, this work suggests, it might instead be possible to find a way to disrupt the van der Waals forces in less dramatic fashion, something which will no doubt disappoint Hollywood greatly. Indeed there is speculation that asteroid P/2013 R3 recently came apart from an encounter with a rather small object.

The Rosetta mission could help confirm or refute this theory.