Spacecraft set for risky asteroid flyby
Artist's conception of Deep Space 1 encounter with asteroid 1992
July 28, 1999
Web posted at: 4:45 p.m. EDT (2045 GMT)
By Robin Lloyd
CNN Interactive Senior Writer
(CNN) -- A NASA spacecraft that already has been pushed through space for 1,800 hours by an advanced xenon gas propulsion system is set for a risky close encounter Thursday morning with an asteroid.
The Deep Space 1 spacecraft is scheduled at 12:46 a.m. to come within 15 kilometers (10 miles) of an asteroid called Braille.
Although spacecraft instruments are set to return photographs and compositional data, the encounter primarily is a test of Deep Space 1's autonomous navigation system, the last of 12 technologies the mission was designed to pioneer.
"This fly-by is a very high-risk space activity," said the mission's chief engineer, Marc Rayman. "The purpose of Deep Space 1 is to take that risk so other spacecraft don't have to."
Unlike a conventional spacecraft, which would be navigated on the basis of signals sent to and from ground controllers, Deep Space 1 is programmed to make its own left-right, up-down decisions on data from sensors that look at the stars to track position and guide course changes.
Other spacecraft have come close to asteroids but never nearly as close as Deep Space 1. The encounter will occur about 118 million miles from Earth.
Asteroid Braille, believed to be about one to five kilometers (.6 to 3 miles) across, will be the smallest body in the solar system ever studied up close by a spacecraft.
Asteroids are so poorly understood at this point that scientists cannot predict their exact locations, including that of Braille.
An analogous test on Earth, Rayman said, would be a scenario in which an automobile found its own way from Los Angeles to Washington, D.C., and arrived in a designated parking space, getting 300 miles per gallon (that's 10 times more efficient than normal).
Xenon ions propel the craft
Deep Space 1, launched October 1998 from Cape Canaveral, is propelled by heavy xenon gas particles, which are subjected to a charge and then move quickly through a chamber to propel the craft. Most NASA missions in the past have relied on chemical propellants, which are far heavier and more expensive.
Some problems arose when the xenon propulsion system was first activated, but they were worked out and the system has functioned well since then.
Although Deep Space 1 will be moving at 56,000 km/h (35,000 mph) during the encounter, the spacecraft will be moving more slowly than the asteroid. It is more accurate to say that the asteroid, one of thousands in the solar system, will zoom by the spacecraft.
Overall, the $152 million mission is designed to test and validate new technologies for further missions, making them smaller, less expensive and more capable of independent decision-making.
Like all asteroids, Braille, previously known as 1992 KD, is thought to be made of the primordial materials from which the solar system formed, said Deep Space 1's chief scientist Robert Nelson.
Miniaturized instruments packed inside the oil-drum-shaped spacecraft include a camera and spectrometer that will return black-and-white photos, images taken in infra-red wavelengths and data on the asteroid's surface composition.
Also, DS1 includes a device that will sample the material in the vicinity of the spacecraft to learn the distribution of ions and electrons surrounding it.
The first images of the asteroid are set for release Thursday and Friday. If NASA extends Deep Space 1 beyond its original September mission ending, it could fly by a comet in January 2001 and another comet later that year.
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