Landslides indicate the asteroid is "not just a stagnant body sitting out there," said Zuber, who knew of no previous observations of landslides on an asteroid. She said the slides likely were triggered when other objects hit Eros.

Evidence that Eros is mostly consolidated rock comes from steep slopes. Four percent of the slopes exceed 30 degrees – the angle of repose needed for loose rock to slide – so they must be solid rock.

Some impact craters on Eros have angular rather than circular shapes and have subdued rims – signs that incoming objects, moving slowly in low gravity, dented hard rock rather than rubble, Zuber said.

Tracking the spacecraft as it was tugged by Eros’ gravity, the NEAR radio-science team calculated the asteroid’s mass as 14,742 trillion pounds (6,687 trillion kilograms).

"It’s about one-billionth the mass of the Earth," said Yeomans, the team leader. "A 200-pound (90-kilogram) person on Earth would weigh about an ounce (28 grams) on Eros. It wouldn’t take an Olympian to leap completely off this thing."

Yeomans’ calculations reinforce the existence of surface rubble. He found rotational forces on Eros are inadequate to overcome gravity and fling rock and dust into space. Learning Eros is mostly solid is important in case it or a similar asteroid ever is on a collision course with Earth and nuclear bombs are used to deflect it, Zuber said.

"Knowing what it’s made of and how tightly it is held together would aid significantly in planning how best to deflect it," she said. "You don’t want to break the thing into more pieces that would hit Earth."

NASA has approved ending NEAR’s mission next February by attempting a soft landing on Eros, said Mike Buckley, spokesman for John Hopkins University’s Applied Physics Laboratory, which runs the mission. The effort is a test for future spacecraft that would scoop samples from asteroids.