Asteroid eludes camera but yields secrets
Deep Space 1 managed to capture this low-quality image of asteroid Braille
By Robin Lloyd
CNN Interactive Senior Writer
August 3, 1999
Web posted at: 5:16 p.m. EDT (2116 GMT)
(CNN) -- A corkscrew-shaped asteroid that dodged a NASA spacecraft camera last week could crash into Earth four millennia from now and cause catastrophic damage, scientists said Tuesday.
Data returned from Deep Space 1's encounter with the asteroid Braille showed that its orbit will shift over time such that it could collide with Earth, said Laurence Soderblom, a U.S. Geological Survey researcher who heads up the camera team that failed to photograph the asteroid.
"It'll be driven in to a point in about 4,000 years where its oscillations will be centered on the Earth's orbit, increasing the probability that it may have near approaches to the Earth and could become the Y6K problem," Soderblom said, chuckling at his joke about the millennium computer bug.
Asteroid scientist Eileen Ryan, with New Mexico Highlands University, said the asteroid is big enough that it would make it through the Earth's atmosphere before burning up and crashing to Earth.
"Something this size, two kilometers (across), would really be a catastrophic impact on the Earth and could cause global consequences," she said. "It's tiny, but luckily we have 4,000 years to worry about it."
Soderblom and Ryan made their comments at a news conference broadcast from NASA's Jet Propulsion Laboratory. JPL oversees the $152 million Deep Space 1 project, set to conclude its primary mission in September.
After a nine-month journey, Deep Space 1 swooped on Thursday within 16 miles of Braille, flying only twice as high as a commercial jet would over Earth but traveling twice as fast as the space shuttle, said chief mission engineer Marc Rayman.
"The accuracy we achieved would be like kicking a soccer ball on the Earth and scoring a goal on the moon," Rayman said.
The mission was a test of 12 advanced, risky technologies, 11 of which functioned quite well. The spacecraft is pushed along by an ion propulsion system that achieves thrusts 10 times more powerful than conventional chemical propellants.
The failure came when Deep Space 1 tried to rely on its autonomous navigation system within hours of its encounter with Braille to locate the asteroid, correct the craft's course to pull in close and then photograph it.
The camera doubled as a navigation eye in the final hours before the encounter, Rayman said, but it lost track of Braille 70 minutes before the encounter because the asteroid was more dim than expected due to its odd shape. The camera was not sensitive enough to pick up its faint reflection.
"It's like driving your car down a dark country road," he said. "If you only glimpse your surroundings, you can't expect to reach your destination based on that information."
The mission returned some infrared and spectral data on the asteroid but not all that was hoped for.
Fifteen minutes after the encounter, the camera detected the asteroid, probably because it had a better reverse angle for reflecting sunlight, Rayman said.
"Right after the spacecraft flew past the asteroid, it switched back to its normal tracking mode and turned the spacecraft in very complex turn, and the pointing was dead on and it collected both pictures and spectra," he said.
Up to six hours before the encounter, flight engineers worked frantically to rescue the spacecraft after it unexpectedly put itself in a "safe" mode. Engineers performed a "heroic" reconfiguring of DS1's software that made possible the data that were collected later, Rayman said.
An instrument designed to detect ions near the asteroid detected none, said project scientist Robert Nelson, who compared the effort to smelling a bowling ball from 50 yards.
Artist conception of the Deep Space 1 spacecraft approaching asteroid Braille
Spectroscopic data from the encounter show that the asteroid's composition is remarkably close to that of a huge, well-known asteroid called Vesta, as well as some meteorites that have fallen to Earth and been studied in laboratories, the USGS's Soderblom said.
Vesta, thought to be the second largest identified asteroid, measures 500 km (310 miles) across -- about the size of Arizona -- and is one in a minority of asteroids with a core and crust. Most asteroids are just a jumble of minerals and rocks.
Asteroid scientist Ryan and Soderblom think Braille was knocked from Vesta but have yet to learn how the smaller rock came to have an orbit that approaches Earth while Vesta remains out in the asteroid belt between Mars and Jupiter.
Images taken of Braille 13,000 to 14,000 km from the spacecraft were merged to reveal a fuzzy picture of the asteroid and engineers translated that image into a corkscrew shape.
"This is really an irregular shaped object and led to the hypothetical view of a twisted balloon you'd get at the county fair," Soderblom said.
Scientists think Braille is about 1 km wide and rotates slower on its own axis than most asteroids -- once every nine days.
"This is very exciting for understand very basic things about our solar system formation and evolution," Ryan said. "Collisions are a very common process in our solar system. This is like a giant laboratory impact model."
Flight engineers turned on Deep Space 1's ion propulsion engine Friday to push it toward comet Wilson-Harrington, which it will fly by in January 2001, continuing on past comet Borrelly several months later. NASA has yet to allocate money to pay for mission operators to oversee that fly-by and ensure that data are collected.
Wilson-Harrington intrigues scientists because it resembled a comet when first discovered in 1949 but now looks more like a dry asteroid than the a "dirty snowball."
Rayman said the two upcoming targets are brighter and better understood by scientists so it will be easier for the spacecraft's camera to detect them.
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NASA Jet Propulsion Laboratory
Deep Space 1
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