Impromptu landing on an asteroid

For the past year, a tiny spacecraft nearly 200 million miles from home has orbited a space rock the size of southern California's Catalina Island, giving earthlings their first close look at debris from the dawn of the solar system.

Now, as the craft's fuel gauge nudges "E" and time on NASA's deep-space communications network runs out, mission planners are racing to cement the craft's place in the history books.

On Monday, they will try to gently park the craft on potato-shaped Eros - performing the first-ever landing of a spacecraft on an asteroid.

Turning Eros into the craft's final resting place, instead of sending it off into space, caps a high point in what some planetary scientists call a renaissance for asteroid studies.

Climbing the social ladder

Since the first asteroid was discovered on Jan. 1, 1801, the objects once cast as the vermin of the solar system have risen to a place of respect.

Scientifically, asteroids have come to be recognized as key links to the solar system's past.

"We're seeing to the very beginnings of the solar system, when planets were being built," says Andrew Cheng, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory and the mission's project scientist.

Beyond the science, however, lies a practical need to understand these lonely rocks. Close-up studies of these objects could help scientists devise strategies to deflect or destroy asteroids that have Earth in their cross hairs.

"These are objects that in the past have caused some bad days for some species on the earth," says Edward Weiler, associate administrator for space science at the National Aeronautics and Space Administration (NASA).

Although an entry in the record books is a plus, researchers are more interested in the close-up views of Eros they hope to get as the craft closes in on the surface. Those views could yield clues to some last-minute mysteries Eros has thrown their way.

In the year that the so-called NEAR craft has been waltzing with Eros, it has provided an intimate portrait of what once was a tiny point of light in a telescope. The mission has returned more than 160,000 images. It's tickled the surface with more than 11 million laser shots to build detailed maps of the asteroid. In all, it's beamed back 10 times more data than originally planned - covering everything from the varying strength of its gravity and the chemical composition of its surface to its solid but cracked interior.

Scientists have only scratched the surface of those data. Even so, a scant seven months after NEAR first began orbiting the asteroid, the team thought it had Eros pretty well figured out - at least in broad terms, says Joseph Veverka, a Cornell University astronomer who heads the NEAR imaging team. Last September, they published findings in a set of papers in the journal Science.

An unexpected puzzle

But in October, the team dropped the spacecraft into an orbit just 4 miles above the rock.

"We began to see things we never expected, things that really puzzle us," Dr. Veverka says. Layers of fine particles appeared to have collected in craters, while more fine-grained material appears to have slumped down the sides of crater walls.

During the past few weeks, he continues, flight controllers have brought the craft to within 11,000 feet of the surface, allowing cameras to spot objects as small as 20 inches across. Boulders appear to have skirts of fine dust building around them.

"All of a sudden, we see these strange weathering processes going on where there's no atmosphere and no water," he says. "We've seen nothing like it in the solar system."

With a gravitation field so weak that at some spots Michael Jordan would launch himself into space attempting a layup, direct collisions with a crater or boulder from incoming meteoroids would merely blast the material off the surface, never to return. One explanation, for now at least, is that the dust gets rattled when a meteoroid hits somewhere else on the surface, says Peter Thomas, another Cornell astronomer working on the NEAR mission.

"When you hit an asteroid with something big enough to leave a half-kilometer crater, the whole asteroid shakes," he says. With such weak gravity, "you don't need a big seismic event to shake and settle the fine material."

Long-shot landing attempt

By bringing the craft to the surface in a controlled manner, the team hopes it will get fresh clues about the processes behind Eros's slowly shifting silt from a final batch of close-up photos.

If they get them.

"It's not like this craft is landing on a sphere," notes Cornell researcher Ann Harch, part of a team that has used NEAR images of Eros to build a computer model of the asteroid for the navigation team to use. "It's descending on a potato-shaped rock that is 22 miles long, and the rock has a large, saddle-shaped hole on one side. The rock continuously spins end over end. Geometry is forcing us to land there - where there is more motion than at the poles - so that NEAR's solar panels face the sun, its antenna points to Earth and its camera faces the asteroid."

"Other than that," she quips, "it's easy."

While the craft was never designed as a lander, mission director Robert Farquhar says it may be possible to pick up signals once it has touched down. "We hope to get a beacon signal from the surface," he says, acknowledging that it's a long shot.

Mission's long-term impact

The quest to snap final photos of seemingly esoteric dust piles and perhaps capture the radio farewell from a high-tech flea on the back of an asteroid will capture headlines for a few days.

But the NEAR mission will have a lasting impact on our understanding of the solar system, according to Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology.

Turning to a vintage file cabinet in his office, he spins a combination lock, slides open the drawer, and pulls out a piece of rock made of the same material as Eros. Much of what scientists have pieced together about the early solar system comes from studying meteorites that have landed on Earth, he says.

"Does Eros resemble any object in our meteorite collection?" he asks. "We had no idea until we went there. Eros has not yielded a lot of surprises. But the NEAR mission has been very important for providing ground truth for what we think we know about these little worlds. It gives us confidence that we can infer the properties of more than 20,000 objects in the solar system."

(c) Copyright 2001. The Christian Science Publishing Society