A Plucky Spacecraft Explores a Distant Asteroid

By RICHARD MONASTERSKY

Stuck on a tiny potato-shaped world, 196 million miles from Earth,

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a pioneering robotic spacecraft is phoning home. But the line is about to go dead.

The money for the project has run out. The mission control office is shutting down. And the radio antennas that pick up signals from the lonely space traveler will soon turn their attention elsewhere, severing the link between the probe and Earth.

So ends the extraordinary mission of a plucky probe that defied the odds by landing safely on an asteroid, surprising even the engineers and scientists who pulled off this unprecedented feat of long-distance piloting. "This was successful I think beyond our highest expectations," said Jay Bergstralh, the acting director of the solar system exploration program at the National Aeronautics and Space Administration. He beamed like a proud father at a news conference on February 14, 48 hours after the landing of the spacecraft, called the Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission.

The story of the cheap and hastily constructed NEAR Shoemaker probe has given the space agency something to crow about as it tries to avert budget cuts and to recover from two failed Mars missions in 1999 that together cost well over $200 million. The mission has also provided enough data to keep gaggles of graduate students busy for years to come as they study the first up-close measurements of an asteroid. Scientists are keen to investigate these hunks of space flotsam because they served as the building blocks for Earth and the other rocky planets 4.5 billion years ago.

Before landing, the NEAR Shoemaker craft had spent a year circling the oblong 21-mile-long asteroid named 433 Eros, snapping pictures and taking readings with five other scientific instruments. By the beginning of February, the probe had collected 10 times the amount of data expected during the mission and was running low on fuel, even as the allotted funds for the project were about to dry up.

So with little to lose, Robert W. Farquhar, the mission director, and his team decided to try a landing, improbable as it seemed. He estimated NEAR Shoemaker had less than a 1 percent chance of surviving the impact and being able to communicate with Earth.

Such a landing was not supposed to happen. The team at the Johns Hopkins University Applied Physics Laboratory, which supervised the mission, had designed the spacecraft as an orbiter, so it had no landing gear. Making matters worse, NEAR Shoemaker would have to settle on a rapidly spinning asteroid with precious little gravity -- only about 0.1 percent of its strength on Earth. What gravity Eros has varies considerably from place to place across its surface because the asteroid has an irregular shape, like a curved yam with a big bite taken out of the middle.

Minutes before the attempted maneuver, the crowd of scientists, engineers, journalists, and onlookers in an auditorium at the Johns Hopkins laboratory in Laurel, Md., grew quiet. Then cheers and applause erupted when a slightly stunned Mr. Farquhar announced from the mission control room, "I'm happy to report that the NEAR spacecraft has touched down on the surface of Eros. We are still getting some signal so evidently it is still transmitting from the surface itself."

Although probes have landed on planets and the moon before, "this is the first time that any spacecraft has landed on a small body," such as an asteroid or comet, said Mr. Farquhar.

The NEAR Shoemaker team initially reported that the spacecraft bounced after it first touched down, traveling back into space before coming to rest on the dust-strewn surface of Eros. But subsequent analysis of data transmitted by the craft indicated otherwise. "If it bounced, it was not enough to be detected ... It was probably wobbling around -- teetering if you like -- on its solar panels for several seconds before it settled down," said Andrew F. Cheng, the project scientist.

The spacecraft has a boxy construction with four solar panels extending from the top like the rotors of a helicopter. Mission controllers had slowed the descent of the probe through a series of four separate engine burns that brought its vertical speed down to a little under four miles per hour -- about the pace of a person taking a brisk stroll.

"You've heard that this was a controlled crash," said Mr. Farquhar at the February 14 news conference, chastising the news media for their descriptions of the descent. "This was not a crash. This was a soft landing. Maybe the softest of all time," he said, relative to touchdowns on the moon and Mars.

NASA officials regard the Eros landing as the crowning achievement in an already successful mission. "This is the beauty of the space program," said an obviously pleased Dan Goldin, NASA's administrator. Mr. Goldin has touted the NEAR Shoemaker probe as a symbol of his call for "Faster, Better, Cheaper" space missions. The project was the first planetary mission planned by a center independent of NASA, and it cost a total of $223 million, far less than the billion-dollar price tag on previous planetary missions.

Edward Weiler, NASA's associate administrator for space science, called the landing a test for future missions. "It means we've got some practice now on how to operate near the surface of small bodies with almost no gravitation. So hopefully some day in the future, maybe in the next decade, we'll be able to send a mission to a comet, which has primordial material from the beginning of the solar system. We'd like to go to a comet and bring some of that material back to Earth to understand our origins here on Earth."

There is another, less academic, reason NASA wants some practice with such landings. That skill could prove critical if astronomers ever spy an asteroid or comet heading toward Earth. The planet's history is littered with evidence that cataclysmic impacts sometimes occur, decimating the unlucky species that happen to be alive at the time. One way to avert a future catastrophe would be to land on the incoming body and divert its course -- perhaps by vaporizing part of it -- so that the threat sailed safely past Earth.

Even though the official plan did not include such a risky landing maneuver, Mr. Farquhar had long entertained the possibility of ending the mission that way if he reached his primary goal of getting the spacecraft into orbit around Eros -- a difficult maneuver in its own right.

In seeking approval to land, the team argued that the descent would allow the probe to obtain extremely close-up images of Eros's surface that would surpass all others in detail. During its yearlong orbit, the spacecraft skimmed as close as 1.25 miles above the asteroid, but most of the time it kept a safer distance.

The researchers hoped to get 40 new images during the descent; the craft surpassed that goal by collecting 67 pictures of Eros, the last captured at a height just under 400 feet, says Joseph F. Veverka, a professor of astronomy and planetary sciences at Cornell University and the leader of the camera team. Such proximity to the surface allowed the camera to obtain clear images of pebbles smaller than a dime, which will help scientists understand how other impacts have shaped Eros.

"We really have seen Eros in great detail," said Mr. Veverka. "That has allowed us to answer a few of the questions that we had. But having answered these questions, these images have presented us with actual mysteries that we'll be scratching our heads [over] for years to come."

During the main mission, Mr. Veverka and his colleagues were surprised to find that Eros lacked small fresh craters created by impacts with miniature bits of space debris. The team suspected that some process might be obscuring those craters, and the new images appear to confirm that hypothesis, said Mr. Veverka. Now scientists must try to figure out how the regolith -- the gritty dust and rock that covers Eros -- manages to fill in the craters.

On Earth, wind and water transport sediments from high points to low ones. But Eros lacks water, and there is no air to create wind. The NEAR Shoemaker team suspects that impacts with other asteroids may shake Eros, causing the regolith to slip downhill into the craters. Other processes may also play a role, said Mr. Cheng.

Another mystery emerged from the last image captured by NEAR Shoemaker before it touched down. That picture shows two regions where layers of regolith have collapsed, a pattern that looks almost as if a hand were stuck into a cake.

"It looks as if you extended your hand and made a depression about an inch deep," said Mr. Veverka. "There is some process depositing [the regolith] in layers, and then something comes along and makes the layers collapse. What is going on is a tremendous puzzle."

The spacecraft succeeded in sending back only about 75 percent of the final image before it hit the surface and the antenna pointed away from Earth momentarily, said Mr. Veverka. "The rest of that image, which may contain the explanation to this mystery, was beamed into space, probably in the direction of Alpha Centauri," one of the nearest stars to the Sun. He joked that the residents of that star system will receive the data in 4.3 years, but "for us, it will remain a mystery."

The surprisingly successful landing left the NEAR Shoemaker team with another problem: What should the spacecraft do as it sits on the surface of an alien world? The scientists hadn't considered that question because they had assumed that the craft would crash or lose contact.

The camera could not take any more images because it was not designed to focus at such a close range. Instead, the scientists decided to collect additional data with the spacecraft's gamma-ray spectrometer, which probes the elemental ingredients that make up Eros. Readings taken from the surface could improve the accuracy of the instrument by up to 10 times, said Jacob Trombka of NASA's Goddard Space Flight Center in Greenbelt, Md.

"It is a tremendous capability," said Mr. Trombka, chuckling like a giddy child opening his bag of Halloween loot. "There is such a difference between orbital measurements and surface measurements."

Such data are important, he said, because they will help scientists match asteroids in space with the different classes of meteorites that have fallen to Earth. Meteorites are thought to be chunks broken off of asteroids, and planetary scientists have tentatively linked certain types of meteorites with various groups of asteroids. But researchers have never before had a close-up look at an asteroid to test their theories.

The information will also provide a family portrait of Earth's ancestors -- the giant boulders that crashed together to form our planet in the infancy of the solar system.

"We believe this is a very primordial material," said Mr. Trombka, "and that's the exciting thing about it. These are the touchstones from which the building blocks of our planetary system evolved."

As of last week, the gamma-ray spectrometer started sending back usable data, but it will take weeks to analyze it.

Not everyone was happy with the team's decision to collect additional data from that one instrument. Norman Ness, a professor at the Bartol Research Institute at the University of Delaware, wanted the NEAR Shoemaker team to take more magnetic measurements instead. A magnetometer on the craft failed to detect any magnetic field from Eros while in orbit, but the device might be able to pick up a magnetic field from the surface, if one exists.

Mr. Cheng countered that the magnetic measurements were not a viable option. Given NASA's decision to extend the mission only until the last week of February, there was not time for the probe to send back enough meaningful data, he says.

To stay in contact with NEAR Shoemaker, the team must be granted time on the Deep-Space Network (D.S.N.), a series of giant antennas used for many different missions. NASA has no plans to provide any additional time to the NEAR Shoemaker team, nor is there any more money allotted to run the spacecraft after February.

So within a few days, the probe will be sending out signals toward deaf ears. Those unheard transmissions will be the vehicle's swan song, as Eros's orbit will soon plunge the landing site into an extended period of darkness and the solar-powered craft will go silent.

Perhaps not forever, though. Mr. Farquhar, for one, harbors some faint hope of staying in touch with the robotic explorer. "What I'd like to try to do really is to talk NASA into giving us some D.S.N. time a couple of years from now and let's try to contact [NEAR Shoemaker] again when it comes back into sunlight. Just to see if we can still contact it. That would be kind of interesting."

And as history has shown, Mr. Farquhar's dreams are often well-grounded, even if they are out of this world.

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