WASHINGTON -- The asteroid Eros is a 20-mile-long world with boulders the size of mansions, rocks layered like plywood and mysterious bright splotches on a terrain of mostly dull, moon-like grays and tans.
And there are early signs that Eros may have once been part of a "lost planet."
Pictures of Eros, snapped by the Near Earth Asteroid Rendezvous spacecraft during the first three days of orbital visit, have raised stacks of questions for scientists, who say that's just the way they like it.
"Work is just starting, but it's already clear that Eros is much more exciting and geologically diverse than we had expected," said Andrew Cheng, NEAR project scientist at the Johns Hopkins Applied Physics Laboratory.
APL scientists and engineers built the NEAR spacecraft and are managing the $216 million mission for NASA.
Eros is now 162 million miles from Earth, close to the orbit of Mars. NEAR arrived there on Monday after a four-year flight. The spacecraft is orbiting Eros once every 27 days at an altitude of 240 miles. If NEAR remains functional during the next few months, controllers will carefully move it to within just a few miles of the surface.
As it gets closer, NEAR and its instruments will reveal Eros' shape, internal structure, mineral composition and its origins with increasing precision.
The stark vistas received this week -- each one a first -- have enthralled and excited scientists and kept them working day and night.
"I was kind of stunned speechless by the beauty of this asteroidal landscape," said Mark Robinson of Northwestern University, a member of NEAR's imaging team. "We can see Eros is not a boring object. There is a lot of neat geology going on."
So far, scientists have determined that Eros is an S-type asteroid, made primarily of silicon-rich rocky material. It is about as dense as the Earth's crust, but twice as dense as the asteroid Mathilde -- photographed by NEAR during a quick 1997 flyby.
Other asteroids, made primarily of iron and nickel, are far denser than Eros.
Scientists aren't sure why asteroids show such varied minerology. But it suggests that at least some of them were once part of much larger objects -- bodies smaller than Earth's moon but big enough to cause their interior rocks and minerals to melt and separate like oil and water.
A subsequent breakup of those "failed planets" would explain why some asteroids, like Eros, are rocky, and others are metallic.
NEAR scientists want to examine all of Eros' surface, and whatever they can see of its interior.
They want to focus their instruments on the "mansion-sized" boulders they see scattered across Eros' surface. The boulders are believed to have been wrenched from Eros' interior by meteorite impacts, and they could reveal whether the asteroid's interior is any different from its surface.
They also want a closer look at the odd ridges and grooves that run along the asteroid's length. "It seems to extend globally through the asteroid, like a sheet of plywood," Cheng said. "That would most likely come about if Eros were once part of a larger body."
On the other hand, he said, they could be parallel fractures caused by impacts. When NEAR gets closer, spectral analyses of light reflecting from the rocks eventually could reveal whether the ridges and grooves are chemically any different from the rest of the asteroid.
Scientists also are intrigued by several bright patches that appear scattered across Eros' surface like spilled flour. At least one of those bright spots is 25 percent brighter than the surrounding rock and seems likely to be chemically different.
Another curiosity on Eros is an oddly smooth region that stands in sharp contrast to the rest of the asteroid, which is heavily cratered by meteor impacts. "We don't really know how to interpret that at this time," Robinson said.
Because craters accumulate over time, scientists believe heavily cratered surfaces reveal "ancient" surfaces perhaps billions of years old.
Something, Robinson said -- a landslide perhaps -- must have smoothed and resurfaced a patch of Eros.