Scientists say the asteroid 433 Eros, now being orbited by a Maryland-built spacecraft, is not a loose pile of space rubble, but a solid, if fractured, piece of rock.
And that bit of information might come in handy some day.
"If we ever have to deflect an asteroid that's coming toward our Earth, we really need to know that, if we push it in the wrong way, it might split into two asteroids, both of which are still heading towards Earth," said Andrew F. Cheng, project scientist for the Near Earth Asteroid Rendezvous mission.
It's an unexpected finding, he said. For many scientists "it's almost impossible to imagine how it could not be a rubble pile."
The discovery is contained in one of four papers published today in the journal Science. They are the first to be published since the spacecraft began orbiting Eros in February.
The 20-mile-long asteroid is one of a class of "near-Earth" asteroids that cross Mars' orbit and approach Earth's. Eros is now 109 million miles away. NEAR was designed and built at the Johns Hopkins Applied Physics Laboratory and is controlled from the APL campus in Laurel. It was launched in February 1996.
In a companion paper in Science, investigators discuss their analysis of key elements in Eros surface soils, using data from NEAR's X-ray spectrometer. They conclude that the asteroid is a sample of the oldest solid materials in the solar system.
"The proportions of magnesium, silicon, aluminum and iron are identical to what we see in the most primitive meteorites, called ordinary chondrites, and also to what we see in the sun," Cheng said. That means Eros formed at the time of the birth of the solar system 4.5 billion years ago, consolidating from the cloud of dust and gas from which the sun and planets also formed. And Eros has scarcely changed since.
The NEAR paper presents the first direct evidence that ordinary chondrites - the most common meteorites that have fallen to Earth - are fragments chipped from asteroids in a class that includes Eros. They may even have come from Eros itself.
Cheng said the X-ray study also provides evidence that Eros was never part of a vanished planet, or an asteroid large enough to melt rock at its core - the kind thought to be the sources of the rarer nickel-iron meteorites.
A third study published today concludes from NEAR studies that Eros' interior has a uniform density, about equal to that of the Earth's crust. And that, said Cheng, may hold clues to its true origins.
"A lot of members on the team are convinced that Eros was once part of a larger body," he said, though not one big enough to have a molten core. Their ideas may get a public airing next month.
Future papers from the NEAR team may also detail evidence of geological change on Eros. "We're seeing a lot of indications of geological activity that no one would have expected on an asteroid," Cheng said.
The $216 million NEAR mission is scheduled to end next February, when controllers attempt to crash-land the spacecraft on the asteroid's surface. But the scientific work will continue.
"We have barely scratched the surface of this data set," Cheng said. "We will be looking at it for years."