An asteroid considered a possible impact threat to Earth is in reality just a big pile of rubble loosely held together not just by gravity but also by forces so weak they couldn't lift a penny, astronomers say.
The asteroid named 1950 DA rotates once every 2.1 hours, so fast the outward centrifugal forces should be greater than the gravitational attraction of its surface rocks and should be flung into space, they say.
So what's holding the almost mile-wide asteroid together?
Researchers say they suspect it involves van der Waals forces, weak short-range forces that arise from the attraction of molecules with slightly different electrical charges.
If so, that would explain how 1950 DA, a porous rubble pile that's almost half empty space, manages to hold itself together despite its rapid spin.
"We found that 1950 DA is rotating faster than the breakup limit for its density," says Ben Rozitis, an astronomer 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 finding, published in the journal Nature, could help in forming a strategy to deal with a similar asteroid that might present an impact risk to the Earth, the researchers say.
"With such an asteroid, you want to avoid interacting with it directly to prevent it breaking up," Rozitis said.
That could simply create an entire swarm of smaller space rocks heading for an impact, the researchers suggest.
"An alternative is to use a 'gravity tractor,' or a heavy spacecraft placed near the asteroid, which uses the force of gravity to pull the asteroid off course," Rozitis said.
In 2002, 1950 DA caught the public's attention when astronomers said it had a one-in-300 possibility of hitting the Earth in the year 2880, although that likelihood has since been recalculated to about one in 19,800.
Rozitis and his colleagues calculated the mass of the asteroid at 2.1 trillion kilograms.
Given its high rate of spin, particularly at its equator, rocks ought to be flying from its surface without some additional force -- like the van der Waals force -- holding them on.
The smooth surface of the asteroid even suggests that may have happened, and that any rocks larger than around 2 inches across may have already been flung away, the researchers say.
There can be no doubt that asteroids spinning as fast as 1950 DA are fragile and possible accompanied by a cloud of rocks and pebbles already spun off, they say, suggesting that if NASA goes ahead with plans to send future human exploration missions to asteroids, specimens like 1950 DA might not be the best candidates to start with.
No astronaut would feel comfortable standing on such a body, they said.
"With such tenuous cohesive forces holding one of these asteroids together, a very small impulse may result in a complete disruption," says Rozitis.
