The Maryland-built NEAR spacecraft is circling the asteroid Eros at an altitude of as little as 12 miles - its closest approach since arriving at the ancient space rock in February.
"This is the payoff time," said NEAR project scientist Andrew Cheng of the Johns Hopkins University's Applied Physics Lab in Laurel.
Thousands of photos sent back by NEAR have revealed the 21-mile-long asteroid to be a bleak and battered place, cratered by meteorites and strewn with jagged boulders the size of buildings.
Long ridges and chains of craters or sinkholes suggest an underlying geology formed when Eros was part of a larger object that broke apart, perhaps billions of years ago.
And data from NEAR's spectroscopic experiments have shown Eros to be a relic of stony minerals little changed since they condensed from the original cloud of dust and gas that formed the solar system 4.5 billion years ago.
"The spacecraft has been doing extremely well," said Cheng. "We have taken five times to 10 times as much data as we originally planned."
His team has been "awestruck" by Eros, Cheng said. And in the light of recent NASA failures, "it's good to have a success every now and then," he said. "I feel very fortunate to have participated in a very exciting, successful mission."
NEAR (for Near Earth Asteroid Rendezvous) was designed and built at APL and launched in February 1996. After missing their first approach to Eros in December 1998, APL controllers steered NEAR toward a second attempt a year later.
It finally began orbiting the asteroid on Valentine's Day and is now about 72 million miles from Earth, low in the southeastern sky at midnight.
The $216 million NEAR mission has had its failures and worries.
On May 13, the spacecraft's Near Infrared Spectrometer - used to identify and map surface minerals - began drawing too much electricity, so engineers shut it down.
The instrument had already mapped 60 percent of the asteroid's surface. "We gathered everything and more than we expected from the northern hemisphere," said Joseph Veverka, the Cornell University astronomer who leads the infrared spectrometry team.
NEAR is also nearly out of gas, with barely 3 percent of the maneuvering fuel it carried at launch. "We're critically low on fuel," said Cheng.
The problem stems from the loss of 66 pounds of fuel - nearly half NEAR's total - during an engine misfire in 1998.
"We're still able to fly a mission that meets all the objectives," Cheng said, "but there will be nothing left over."
Mission planners have been forced to minimize the time NEAR spends in low orbits, where staying aligned with Eros, the sun and Earth consumes more fuel. That's a disappointment to scientists mapping Eros' geology, who want NEAR's X-ray and gamma ray spectrometers as close to the surface as possible.
NEAR has photographed the entire asteroid. The last pieces of the photo-mosaic were added June 27 when Eros' seven-month "autumn" season began, and sunlight fell across its southern region for the first time since NEAR arrived.
Scientists hope NEAR's low orbits this week will resolve several questions, including the nature and origins of Eros' boulders. From higher altitudes, Cheng said, "some of them look like they're rocks, some of them look like they're clods of dirt."
Scientists also want to know how small the boulders get. "Is it flat and sandy with blocks here and there or, as you get lower, is it rougher and rougher?" Cheng asked. That would make a difference to anyone trying to land.
Scientists also want a closer look at Eros' "regolith" - the blanket of soil ground up by eons of meteor impacts. In high-altitude photos it seems to drift, burying some features and revealing others.
NEAR will remain in its current low orbit until Monday. Scientists at APL are using the time to make precise measurements of NEAR's irregular loops around the peanut-shaped asteroid. Its path will reveal the shape of Eros' gravitational field and its internal structure.
NEAR will swoop low again in October for a one-time flyover just 3 miles above the surface. At that altitude, Cheng said, "almost anything is worth looking at."
NEAR's camera should be able to see objects the size of a gym bag.
Then, after two more months of photography and map-making from 60 to 120 miles above the asteroid, NEAR might close in for a dramatic finale in February.
"We're now starting to determine what it would take to land on the asteroid," Cheng said.
NEAR has no landing gear. But with little to lose, NEAR team members want to try a soft touchdown. "What I've been saying is that we want to do this to acquire data from the surface at a minimum altitude," Cheng said.
If they pull it off, the APL team would be the first ever to land a spacecraft on an asteroid, beating a consortium of NASA and Japanese scientists working to land a rover on another asteroid in 2005.
In NEAR's favor are its slow orbital speed - less than 7 mph - and Eros' feeble gravity. A grown man there would weigh less than 2 ounces. With the right approach, and a lot of luck, NEAR scientists think they might touch down without wrecking the spacecraft.
But to be able to transmit to Earth any data gathered during the landing, NEAR would have to take off again immediately and point its antenna at Earth.
"We're trying to figure out how to do it, what are the risks," Cheng said. A decision is expected from NASA by the end of August.
For NEAR photos and movies, go to http://near.jhuapl.edu