All systems on NASA's Stardust spacecraft performed successfully when tested in a flyby of asteroid Annefrank on Friday, heightening anticipation for Stardust's encounter with its primary target, comet Wild 2, 14 months from now.
As a bonus, Stardust discovered that Annefrank is about twice the size anticipated, but with a dimmer surface. The dimmer surface increased the challenge of sighting the object as the spacecraft approached.
The Annefrank flyby offered a unique opportunity to thoroughly test all the operations planned for the encounter at Wild 2, where Stardust will collect samples from the cloud of dust around the comet for return to Earth.
"We performed a full dress rehearsal with the cometary dust collector deployed, the spacecraft poised in its flyby attitude and with all science instruments on," said Stardust's principal investigator, Prof. Donald Brownlee of the University of Washington, Seattle.
The spacecraft passed within about 3,300 kilometers (2,050 miles) of Annefrank at 04:50 Nov. 2, Universal Time (8:50 p.m. Nov. 1, Pacific time). Engineers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and at Lockheed Martin Space Systems- Astronautics, Denver, Colo., ran the operation through the hours before and after the closest approach.
The approach geometry was more difficult than it will be at Wild 2 because of the angles toward the Sun and Earth relative to the angle toward the asteroid. "The spacecraft performed every command perfectly," said Allan Cheuvront, Stardust spacecraft systems engineer at Lockheed.
"Its performance under these severe conditions was outstanding."
"It was a challenge for the navigation camera to see Annefrank during approach," said JPL's Ray Newburn, lead scientist for the camera. The predicted brightness of Annefrank was near the lower limit of what the camera can detect. Engineers tried techniques such as taking multiple long exposures and adding them together, said Dr. T. S. Mike Wang, optical navigation specialist at JPL. "Annefrank was not cooperating," Wang said. "It was just too dim."
The spacecraft still had not sighted Annefrank 12 hours before the closest approach, but the flyby had been designed to be successful without needing to see the target until 20 minutes before the encounter.
"The flyby distance was chosen as one far enough away so there would be no risk to the spacecraft, but close enough for Annefrank to be in camera view at the start of the encounter sequence," said Ed Hirst, mission design manager at JPL. When Annefrank could not be seen during earlier approach, the flight team concluded the asteroid might be dimmer than anticipated, so it sent up commands for the camera to use longer exposures during the early stages of the encounter.
"NASA's Deep Space Network gave us highest priority, allowing us to play back earlier images that were not received well, as well as sending our encounter commands up," said Robert Ryan, mission manager at JPL.
Stardust executed the encounter commands without missing a beat -- no surprise to Joe Vellinga, flight system manager at Lockheed. "We have built up over three years of flight experience a tremendous amount of confidence for our spacecraft to perform such operations routinely," he said.
The main function tested during the flyby was a flight computer program that took control of the spacecraft to keep the camera view locked on Annefrank during a 25-minute period around its closest encounter.
Dr. Shyam Bhasharan developed it based on software that NASA's Deep Space 1 spacecraft used for successfully tracking a comet nucleus during a flyby of comet Borrelly last year. Lockheed's David Gingerich, a flight software specialist who tested the tracking software, said, "Its performance was executed just like the coach drew it on the blackboard."
The encounter images show Annefrank to be irregularly shaped, cratered, and about 8 kilometers (5 miles) in diameter. Stardust's dust instruments were on for the encounter, although no dust was anticipated. Scientists are still checking data to see if, by chance, the instruments may have seen a dust particle.
"The dust flux measurement instrument ran for at least 27 minutes and performed all expected functions," said Dr. Tom Economou of the University of Chicago, lead scientist for that instrument. The lead scientist for Stardust's interstellar dust analyzer, Dr. Jochen Kissel of Max Planck Institute, Garching, Germany, said the Annefrank experience will enable him to put the German instrument into an even better configuration at Wild 2.
"Performing such flight testing before the primary encounter is a critical part of reducing risks and significantly increasing the probability of success at the primary target, Wild 2" said JPL's Thomas Duxbury, Stardust project manager. "We learned at lot that will improve our operations at Wild 2 based upon the lessons learned at Annefrank, but the bottom line is that if Annefrank had been Wild 2, we would have succeeded."
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Annefrank Flyby A Success
Seattle - Nov 4, 2002
A group of scientists, including UW astronomy professor Donald Brownlee, spent last Friday and Saturday at the NASA Jet Propulsion Laboratory in Pasadena, Calif., to coordinate an outer-space mission more than seven years in planning. At a critical moment around 8:50 p.m. on Friday afternoon, the Stardust spacecraft passed within 3,000 kilometers of an asteroid called 5535 Annefrank, sending photographs and other data back to Earth.
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