The spacecraft determined Eros is a mostly solid sample of rock left over from the solar system’s birth almost 4.6 billion years ago, when solid objects theoretically condensed from a swirling disk of gas and dust, then clumped together to form planets.

The asteroid’s solidity and primordial composition suggest Eros did not form when smaller pieces clumped together, but instead broke off an even bigger object during a huge collision as the solar system was born.

That means scientists are closer to showing that gas and dust in the earliest solar system were able to condense into objects large enough – about 60 miles (100 kilometers) wide – to gravitate toward each other to form planets, Zuber said. Eros is 20 miles (32 kilometers) long, so its parent body must have been almost big enough to have planet-forming potential, she said.

NEAR-Shoemaker confirmed that Eros – a member of the S-class, or most common type of asteroid – has the same mineral makeup as the most common meteorites, ordinary chondrites, which are believed to have been blasted off asteroids during collisions.

Researchers long suspected chondrites came from S-class asteroids, located mostly in the inner part of the Asteroid Belt between Mars and Jupiter. NEAR has helped astronomers definitively tie the meteorites’ chronology of solar system formation to their "return address" in the inner Asteroid Belt, Binzel said.

Global view of asteroid Eros from the NEAR-Shoemaker spacecraft at a distance of 125 miles (200 kilometers). The lowest (blue) region on Eros lies within the 6-mile-(10-kilometer-) wide saddle-like depression (top right).

"We are getting an idea of what was likely there, just beyond the orbit of Mars, just before the solar system formed," said Don Yeomans, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. "These are the same chunks that formed the Earth."

NEAR-Shoemaker’s X-ray spectrometer revealed the asteroid contains low levels of aluminum relative to silicon and magnesium, indicating it never re-melted or formed layers like Earth, and thus is unchanged since the solar system’s birth. Nevertheless, a spiral pattern of grooves encircling the asteroid resembles layering – a mystery not yet solved, Zuber said.

Zuber led a study in which NEAR-Shoemaker bounced a laser beam off Eros 8 million times to precisely measure distances, producing a three-dimensional topographic map of Eros, named for the Greek god of love.

The method determined the asteroid is 20.318 miles (32.697 kilometers) long – a bit different than earlier estimates. It is almost 7 miles (11 kilometers) tall and deep.

Lighter areas on the top- and bottom-left sides in the photo may be landslides in Psyche Crater on asteroid Eros. In sketch map, dashed lines show boundaries of possible landslides. Black squares are large blocks of rock.

The map and some of NEAR’s 103,300 photos showed Eros has a 3.4-mile- (5.5-kilometer-) wide impact crater provisionally named Psyche, for Eros’ beloved, and a 6-mile- (10-kilometer-) wide saddle-shaped depression named Himeros, after Eros’ primary attendant. The saddle is a low ridge connecting two peaks.

Himeros, which is less cratered than the rest of Eros, may be the surface where Eros broke off a larger object that was struck by yet another space rock, Zuber said. Or Himeros may just be the place where part of Eros was knocked off during a collision with a similar-size object.

Once thought to be a rubble pile held together only by gravity, Eros’ topography shows the asteroid was shaped both by gravity and by the material strength of its solid bedrock.

Evidence of gravity at work includes lighter-colored areas where rubble landslides moved down crater walls or other slopes; places where impacts pulverized rock, which fell to form crater rims; and bowl-shaped impact craters typical of planets and moons with gravity fields.