WASHINGTON -- Ecstatic scientists getting their first good look at Asteroid 433 Eros from NASAs Near Earth Asteroid Rendezvous (NEAR) spacecraft said Thursday that the cosmic rock is a chunk that broke off from a larger, planet-sized body.

New images from the probe are serving up tantalizing glimpses of the ancient asteroid, which is dotted with house-sized boulders, grooves and craters.

"Im stunned speechless," said Mark Robinson, a NEAR scientist from Northwestern University, Evanston, Illinois. He and other team members presented a preliminary review of their findings, just three few days after the probe swung into orbit around Eros.

"These boulders came from depth, from the asteroid, so they give us a look at the inside of this body," Robinson said.

Where there aren't boulders, there are craters galore. "Almost all of the asteroid is saturated in impact craters," he said.

But there's also a mystery to Eros, which is now about 160 million miles (257 million kilometers) from Earth. In the middle of a huge valley on the asteroid, there are no craters at all. That means the surface there is somehow younger than the rest of the asteroid for reasons that are still unclear to scientists.

Andrew Cheng, NEAR's lead scientist at the Applied Physics Laboratory (APL) in Laurel, Maryland, said there were tantalizing hints that Eros has a layered structure that appears to extend throughout the object.

It's "as if this asteroid were made up of layers of plywood sheets," he said. Such layering points to Eros being a fragment from a planet-sized body that broke up ages ago.

Another NEAR scientist, Donald Yeomans of NASAs Jet Propulsion Laboratory in Pasadena, California, said early data indicate that Eros is no rubble pile. Rather, it is a far more solid object than asteroid Mathilde, the space rock that NEAR flew by in December 1998 on its way to Eros.

Finding out just how solid Eros is, however, will have to wait.

"As the mission goes on, well know more and more about, not only the mass and the bulk density of Eros, but how that mass is distributed," Yeomans said.

Over the next year, NEAR will drop to lower and lower orbits, scanning the mini-world with different classes of instruments. It now is circling Eros at an altitude ranging from 200 to 280 miles (320 to 450 kilometers) and cruising along at a mere 2 m.p.h. (3.2 kilometers per hour) -- "a slow walk," as Yeomans put it.

"As we learn more about the asteroid and its gravity field, well feel comfortable about bringing it down closer and closer to Eros," said Andrew Santo, NEAR mission engineer for APL where the spacecraft was built. "The body is so small, the maneuvers dont take much fuel."

Not needing to use loads of fuel is good news. The probes propellant already is nearing empty. Keeping an eye on its fuel consumption is critical, perhaps making it feasible to gently land the probe at mission's end in February 2001.

When NEAR was launched in February 1996, it had 700 pounds (318 kilograms) of fuel aboard. But during the spacecrafts first attempt to loop around Eros in late 1998, the spacecraft went haywire, aborting the orbit try. It mysteriously fired its thrusters thousands of times, slurping up large quantities of precious fuel. So easing the craft ever closer around the asteroid is a navigational nail-biter.

"We dont have a lot of fuel," Santo said. "Because Eros is so small and light, anytime you get an error in a burn...that could be devastating. Soits a challenge. You are always on the edge. The lower we go, the more tenuous it will become. Thats why were starting at such a high orbit, then get closer and closer as we get more courageous."