It was a fitting end to a 2 billion-mile odyssey. For five years, a spacecraft named NEAR has pursued a distant asteroid known as 433 Eros. Last Feb. 14, the amorous robot finally caught up with its quarry and slid into a delicate orbit around the tumbling, 21-mile-long space rock, beginning a year of scientific studies.
With its fuel nearly exhausted and its mission scheduled to end on Valentine's Day, 2001, NEAR's controllers decided it should go out in style -- rather than simply being turned off in orbit. So on Feb. 12, NEAR, which stands for Near Earth Asteroid Rendezvous, began a series of braking firings of its engines for what NASA called a "controlled descent." This brought it from an orbit 21 miles high to a slow-motion crash landing on the nearly gravity-free surface of the hourglass-shaped asteroid.
End of the Ride: This sequence shows NEAR's last images as it approached the surface of EROS
Courtesy of NASA/APL
On the way down, NEAR sent back its last snapshots of Eros' cratered and boulder-strewn surface. It's final image, taken from an altitude of just over 300 feet and covering just 20 square feet, clearly shows rocks just four inches across.
Then the cameras went out of focus and, drifting down like a feather at about four miles an hour, NEAR kissed the surface of Eros at 3:07 p.m. ET. It bounced about 300 feet back aloft before settling into its final resting place, a puzzling saddle-shaped depression named Himeros. "I'm happy to say the spacecraft is safely on the surface of Eros," announced Mission Director Robert Farquhar, as cheers and congratulations filled the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., which built the spacecraft and manages the mission for NASA.
Surprisingly, NEAR, which is about the size of a Volkswagen and weighs in at about 1,100 pounds on Earth, survived the impact. But gravity on Eros is only one one-thousandth of that on Earth -- an average person would weigh only an ounce or two. Soon after the touchdown, NASA's Deep Space Network began receiving signals from a radio beacon on the spacecraft. Its fragile solar panels are apparently still intact. "NEAR wasn't designed for landing, and such a maneuver has never been attempted before," says Farquhar. "But the risk was worth taking -- this was a bonus."
Touchdown: The arrow marks NEAR's landing spot on asteroid 433 Eros
Courtesy of NASA/APL
Indeed, although the stranded spacecraft's instruments are useless on the surface of Eros, NEAR leaves behind an incomparable legacy. Until NEAR, virtually all that was known about these ancient, orbiting rocks in the distant reaches of the solar system was inferred from meteorites that had been altered by a flaming ride through earth's atmosphere. Some of the oldest rocks known -- asteroids are believed to date to the birth of the solar system 4.6 billion years ago.
So far, astronomers have discovered more than 7,000 asteroids, and several hundred more are identified each year. Most of them are clustered in a vast, doughnut-shaped belt between the orbits of Mars and Jupiter, though others stray within the orbit of Mercury and well into the outer solar system. Scientists estimate that the total mass of all asteroids is equivalent to a solid body roughly 900 miles across -- less than half the moon's diameter.
Eros itself is known as a "near-Earth asteroid." These lonely wanderers are closely watched because their paths come close enough to Earth that they pose a threat of a deadly impact, such as the cataclysmic collision that may have caused the mass extinction of dinosaurs and much of life on this planet 65 million years ago. Scientists estimate that Eros has a small chance -- single-percentage digits at most -- of hitting Earth within the next 100 million years or so.
NEAR, a bargain-basement spacecraft constructed as part of NASA's low-budget Discovery Program, soared into space on Feb. 17, 1996. The galactic voyager, which looks less like Cupid and more like a tin can with solar panels for wings, journeyed on a circuitous course that took it far out beyond the orbit of Mars. There, it waltzed past another homely rock called Mathilde in June, 1997, and sent back what were until then the closest images ever obtained of an asteroid. Afterward, it swung back to within 400 miles of home so that Earth's gravity would fling it like a slingshot on a course to Eros.
But NEAR missed its planned blind date with Eros on Jan. 23, 1999. A malfunction caused the spacecraft's engine to shut down during the braking maneuver that would bring it close to the asteroid. Instead, it flew past its date, taking some 8,000 pictures. Then, the suitor matched its trajectory to Eros and tagged along, inching toward the asteroid at barely 18 miles an hour. On Feb. 14, 2000, it was finally inserted into orbit around the asteroid.
For the past year, NEAR has been hard at work, dipping ever closer to the surface, making X-ray and gamma-ray measurements of Eros' composition and mapping every foot of its surface, an area about twice the size of Manhattan. Now, with it's mission over, NEAR has far exceeded astronomers' expectations, collecting 10 times more data than originally planned and taking 80 pictures during its final descent.
All told, the data include details culled from more than 11 million laser pulses; radar and laser information on Eros' weak gravity and solid but cracked interior; X-ray, gamma-ray, and infrared readings on its composition and spectral properties; and about 160,000 images covering all of its battered, dusty terrain.
Eros is an S-type, or stony, asteroid, a type that appears to be up to 50% metal, mostly iron. Meteorites of this kind rarely fall to Earth -- so-called C-type, or carbon-based, meteorites are much more susceptible to our planet's pull. And the data from NEAR seem to confirm a lot of what astrogeologists expected to find. "We now know that Eros is a solid body of uniform composition, made of material probably older than the Earth," says Andrew Cheng, APL project scientist for NEAR.
Yet the spacecraft also turned up some surprises. Boulders seem to be disintegrating on the surface by some unknown mechanism, and fine material seems to fill craters, creating flat surfaces. When NEAR started sending back images that could show objects about a yard across, "we noticed strange processes we haven't seen on the moon or anywhere else," says Joseph Veverka, NEAR imaging team leader from Cornell University in Ithaca, N.Y. "It's like another door has opened."
Veverka hopes that the images from NEAR's final descent may offer some clues. He predicts that scientists will be mining the data for years. In addition, the mission may provide invaluable experience in landing future spacecraft on small, distant bodies in the solar system.
Meanwhile, a memorial service of sorts was planned for NEAR, in the form of what NASA calls a "post-mission briefing" with team leaders at noon on Valentine's Day. Afterward, NASA's communication with NEAR will cease. And 196 million miles away, the emissary from Earth and the asteroid are finally joined. With NEAR snuggled in, Eros will ride away in its long orbit through the cold wastes of the outer solar system. So, happy Valentine's Day, NEAR -- and Godspeed.
NEAR Mission home page
Background information on asteroids
Near Earth Asteroid Tracking page from Jet Propulsion Laboratory
By Alan Hall in New York
Edited by Douglas Harbrecht