Potentially Dangerous Asteroid Turns Out to Be a Rubble Pile

An asteroid on NASA's list of potential impact threats to the Earth is actually a pile of loosely connected rubble held together by forces weaker than the weight of a penny, scientists say.

The discovery could be vital if humanity ever has to destroy a space rock before it hits Earth.

Astronomers investigated near-Earth asteroid 1950 DA, which is about four-fifths of a mile wide (1.3 kilometers). This asteroid is currently given a 1-in-4,000 chance of impacting the Earth in 2880. [Potentially Dangerous Asteroids Near Earth (Photos)]

A study in 2003 suggested that if asteroid 1950 DA smashed into the Atlantic Ocean, the resulting blast could pack as much energy as a 60,000-megaton explosion, causing tsunami waves at least 200 feet high (60 meters high) to crash against the East Coast.

Unexpectedly, the scientists found 1950 DA is a porous rubble pile, about half of which is empty space. They also discovered that the asteroid is spinning faster than the forces of gravity or friction would allow it to remain in one piece.

Scientists have previously suggested that cohesive forces help keep such rubble-pile asteroids from spinning apart. One possibility is van der Waals forces — weak, short-range electric forces that can attract particles together.

This work could inform future strategies to prevent asteroids from impacting Earth.

"With such an asteroid, you want to avoid interacting with it directly to prevent it breaking up," Ben Rozitis, an astronomer at the University of Tennessee at Knoxville, told "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 and his colleagues Eric MacLennan and Joshua Emery describe their research in Thursday's issue of the journal Nature.

— Charles Q. Choi,

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