NASA Pins the Tail of a Comet to Capture a Bit of Its Stardust
In a feat akin to hitting one high-speed bullet with another, NASA’s Stardust spacecraft flashed through the tail of a comet Friday morning, collecting tiny grains of primordial matter in an effort to better understand the origins of the solar system and perhaps even how life began on Earth.
After traveling nearly 2.5 billion miles in five years, the 16-foot-long probe passed within 143 miles of the nucleus of comet Wild-2 at approximately 11:22 a.m., snapping 72 pictures of the 4-mile-wide snowball of rock and ice. It collected a thumbnail-sized parcel of stardust that it is set to carry home to a Utah landing in two years.
During its eight-minute plunge through the tail of the comet, Stardust encountered what scientists called a nightmare of millions of dust particles striking the craft at speeds of 14,000 mph. Only a pair of massive carbon and ceramic barriers called Whipple shields prevented the probe from being sandpapered into oblivion.
“Life is tremendously good,” project manager Tom Duxbury of NASA’s Jet Propulsion Laboratory said a few minutes after the craft emerged from the comet’s tail unscathed. “We’ve flown through the worst of it and are still in contact with our spacecraft.”
Controllers at JPL in Pasadena began receiving data from the probe shortly after it left the comet’s coma, or tail, but researchers said it would require 30 hours to download everything acquired during the encounter.
The first picture of Wild-2 showed a surprisingly spherical body pocked with massive sinkholes produced when ice and snow exposed to sunlight evaporated from the surface, a process called sublimation.
Researchers had predicted a more oblong shape because the comet is not large enough for gravity to pull it into a compact sphere. “I’m going to have to eat some crow about that,” Duxbury said.
The massive pockmarks were also a surprise. Previous missions have taken pictures of comets Borrelly and Halley, but “none showed features like this,” said astrophysicist Don Brownlee of the University of Washington.
The crater in the lower right-hand side of the picture “is one heck of a hole,” Brownlee said. “You could put 30 to 50 Rose Bowls inside that crater.... We are seeing processes we have never seen before on a primitive body, processes we didn’t even know existed.”
The picture also shows at least five distinct jets where the comet is venting gases. With more time, researchers hope to match those jets up with specific features on the surface of the comet.
“The camera did us all proud today,” said JPL’s Ray Newburn, lead scientist for the camera.
Controllers also received some surprising initial data from an instrument that monitored the number and speed of particles the spacecraft encountered. The team had expected to see a steadily increasing number of particles until Stardust reached the center of the coma and then decreasing numbers as it exited.
Instead, the craft passed through several jets of particles, including two unexpectedly large ones. Had the team known about the existence of such large jets, they would probably have tried to avoid them for fear of damaging the spacecraft, Brownlee said.
“Fortunately, we came through all right,” he added.
Observers hailed Stardust’s survival. “We’ve learned never to sneer at anything in space as being easy,” said Louis D. Friedman, executive director of the Pasadena-based Planetary Society. “So far, it’s a great engineering triumph -- we hope it will be a great scientific triumph too.”
Comets are the oldest, most primitive bodies in the solar system, preserving the materials that were present in the nebula that formed the sun and its planets 4.6 billion years ago.
They are often called dirty snowballs because they are made up of dust, ice and complex carbon-containing molecules similar to those that are the signature of life.
Comet 81P/Wild-2, named after its discoverer Paul Wild (pronounced “vilt”), provided a unique opportunity for astronomers because it has spent most of its life beyond the orbit of Jupiter and thus has lost little of its original matter through outgassing.
On Sept. 10, 1974, however, it passed close to Jupiter and was shifted into its current orbit, which carries it between the orbits of Jupiter and Mars.
“That’s a phenomenal bonus,” Brownlee said. “This is one hell of a comet.”
The heart of the probe is a tennis-racket-shaped frame that holds cells of aerogel, an astoundingly light material that is 99.8% air and 0.2% silicon dioxide, the same material in glass and sand. Foamy like a sponge, the aerogel is capable of slowing and capturing dust particles streaming from the comet at up to six times the speed of a high-powered rifle bullet without significantly degrading them. (It is also a very good insulating material used on the Mars rovers scheduled to land this month.)
On the five-year trip to Wild-2, one side of the collection device was pointed forward to trap interstellar dust grains blown into the solar system. Days before the cometary encounter, the device was flipped 180 degrees to provide a fresh face for the comet’s dust.
Today, Stardust is expected to safely tuck the collector away in its clamshell-shaped carrying case, where it will remain until after its fiery reentry into Earth’s atmosphere in January 2006.
The capsule is designed to parachute into the U.S. Air Force’s Utah Test and Training Range. If it survives the descent, its sample will be recovered by a helicopter and sent to the clean rooms at the Johnson Space Center in Houston for analysis.
Bruce C. Murray, professor of planetary science at Caltech and a former director of JPL, called the successful flyby “extremely important,” because if particles are successfully transported back to Earth, they will be pristine specimens.
“It will carry us back in time more completely than meteorites that come down to Earth and are contaminated by the Earth’s atmosphere,” he said.
The successful flyby was major vindication for JPL’s Peter Tsou, deputy principal investigator of the Stardust mission.
“It’s quite a crescendo,” said an ecstatic Tsou, who worked unsuccessfully from 1981 to 1994 to convince the U.S., European and Japanese space agencies to complete the mission. NASA finally agreed in 1994 to his 13th proposal.
