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Mysterious forces hold threatening asteroid together

Paul Sutherland, Feature writer
Aug 17, 2014, 13:36 UTC

Sen—With those lesser bodies of the Solar System—asteroids and comets—becoming targets of special interest to astronomers, space scientists and prospective miners, there is great interest in learning more about how they are constructed.

Latest surprising finding is that an asteroid that comes close enough to Earth to be an impact threat is held together, not by gravity, but by forces never observed before on such an object.

The asteroid is known as 1950 DA, after the year in which it was discovered. However, after being observed at that time for 17 days, it was lost from view and on rediscovered more than half a century later on 31 December 2000.

Asteroid 1950 DA, which is about 1.1 km wide, hit the headlines when it was found that it posed the highest known threat of hitting the Earth—though still only odds of one in 300—when it crosses our orbit on 16 March 2880. Radar observations were made in March 2001, by telescopes at Goldstone, California, and Arecibo, in Puerto Rico, while the steroid was 7.8 million km from Earth. 

Optical observations showed that the asteroid spins once every 2.1 hours, the second fastest rate ever observed for an object of its size. That is so rapid that an astronaut would be unable to stand on its surface at the equator without being thrown off into space.


Three radar images of asteroid 1950 DA obtained by the Arecibo radio telescope in Puerto Rico on 4 March, 2001. Image credit: NASA/JPL

Effectively, this means that 1950 DA  experiences negative gravity. So what is holding it together? New studies by researchers at the University of Tennessee, Knoxville, show that the answer is what is known as van der Waals forces. These are cohesive forces that are said also to explain how water molecules combine, and how a species of lizard called geckos clings to sheer surfaces, including glass.

Previously, asteroid research has shown that they are loose piles of rubble held together by gravity and friction. That led the researchers to wonder how it was being kept from flying apart, considering its rate of spin.

The team, Ben Rozitis, Eric MacLennan and Joshua Emery, examined thermal images and orbital drift to calculate thermal inertia and bulk density. These allowed them to observe the action of cohesive forces in an environment with little gravity.

Rozitis explained in a statement: “We found that 1950 DA is rotating faster than the breakup limit for its density. 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.”

Although such cohesive forces had been predicted to be present in small asteroids, no definitive evidence has previously been observed. The findings, reported in the latest issue of the journal Nature, will be useful when it comes to dealing with a threatening, incoming asteroid, say the team.

Rozitis said: “Following the February 2013 asteroid impact in Chelyabinsk, Russia, there is renewed interest in figuring out how to deal with the potential hazard of an asteroid impact. Understanding what holds these asteroids together can inform strategies to guard against future impacts.”

They also speculate that an asteroid called P/2013 R3, which was observed to disintegrate in 2013 and 2014 by the Hubble Space Telescope, may have been similarly held together but been destroyed in a collision with another object.

The researchers’ findings will also be of interest to European space scientists preparing for the landing of Rosetta’s piggyback probe Philae on the surface of Comet 67P/Churyumov-Gerasimenko in November.