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From The "JPL Universe"
August 6, 1999

Successful DS1 flyby sheds light on asteroid mystery

By JOHN G. WATSON

NASA's Deep Space 1 experimental spacecraft successfully flew approximately 26 kilometers (16 miles) above the surface of asteroid 9969 Braille at 9:46 p.m. Pacific time Wednesday, July 28 (04:46 Universal Time July 29), using a sophisticated new space autopilot system, exceeding 100 percent of the mission's objectives. An exultant operations team looked on as preliminary data returned to the Deep Space 1 operations control area in historic Building 230, indicating that the AutoNav autopilot system skillfully flew the spacecraft to a face-to-face closeup with asteroid Braille.

This was by far the closest flyby of an asteroid ever attempted and yet one more accomplishment in JPL's long history of encounters with previously unexplored solar system bodies.

"This is a dramatic finale to an amazingly successful mission," said Dr. Marc Rayman, chief mission engineer and deputy mission manager. "The encounter with Braille has allowed us to complete the testing and validation of 12 new technologies and to return some exciting science as a bonus.

"JPL can be proud that one of its missions has now visited the smallest solar system body ever targeted."

Ten minutes after the flyby, when the spacecraft signals reached Earth after a 10-minute journey, the team burst into spontaneous applause at the news that the spacecraft was turning back to face the asteroid, a clear signal that the flyby had succeeded.

Making the encounter all the more memorable-and serving as a testimonial to the team's quick ability to think on its feet-was the fact that the spacecraft experienced a "safing" event earlier in the day, starting at about 5 a.m. PDT on July 28 and ending at about 11 a.m. PDT. A small software glitch, now fully diagnosed, was detected by Deep Space 1's fault-detection software, which triggered a protective program that causes several events: the spacecraft halts non-critical activity, orients its solar panels toward the Sun, points light- and heat-sensitive instruments away from the Sun and reverts to its low-gain antenna while awaiting new commands.

"This has been by far the most challenging and dramatic day on the project," said Rayman. "The last 16 hours before the flyby were really, really exciting. We had the safing event, we recovered from it and we managed to squeeze in a trajectory correction maneuver to update Deep Space 1's flight path.

"The entire operations team deserves great praise for its heroic response to an event that otherwise would have eliminated any chance of conducting the encounter," he added.

Science results were downlinked in a series of telemetry sessions through Friday morning, July 30. The results from an infrared sensor within the spacecraft's integrated spectrometer and imaging instrument proved to be startlingly significant, for they showed nearly matching compositional fingerprints between near-Earth Braille and Vesta, a much larger asteroid in the main asteroid belt between Mars and Jupiter.

"This clear link between Vesta and Braille is an important finding," said Dr. Laurence Soderblom, from the U.S. Geological Survey, team leader for Deep Space 1 experiments using the spacecraft's integrated spectrometer and imaging instrument.

Thus the novel spacecraft not only passed a technology milestone by flying itself past Braille, but also it now has scientists pondering a deep-space "family tree" mystery.

Scientists are wrestling with a thorny question: is Braille a chip off the old block, Vesta, or are the two siblings that originated elsewhere, perhaps thrown off a larger body long since destroyed?

The scientists made their finding from three sets of data collected by the spacecraft's infrared spectrometer. Called spectra, a form of data resulting from the instrument breaking light into component colors much like a prism does (usually displayed as graphs), the data sets cover different parts of the asteroid and were taken just after closest approach.

Braille's longest side is now estimated at 1.3 miles (2.2 kilometers) and its shortest side appears to be 1 kilometer (0.6 miles). This elongated asteroid was expected to be irregular, and two black-and-white photographs taken approximately 15 minutes after closest encounter have helped to confirm this.

By contrast, Vesta, discovered in 1807, has a diameter of about 500 kilometers (310 miles). The fourth asteroid ever discovered, Vesta shares with Braille a high visual reflectivity, or albedo. In fact, Vesta is the most reflective of the main-belt asteroids.

Apart from flyby findings, project scientists have determined that Braille is one of the asteroids that drift in and out of Earth's orbit over eons and that it will return to Earth's vicinity within a few thousand years.

Diagnosis of an apparent target-tracking problem that impacted visible imaging (the taking of black-and-white photos) during the flyby continues. Preliminary results suggest that a combination of the asteroid's highly irregular shape, its orientation relative to the Sun and the camera's response under these unusual conditions are responsible.

Launched Oct. 24, 1998, Deep Space 1 is the first mission under NASA's New Millennium Program, which tests new technologies for future space and Earth-observing missions. The technologies that have been tested on Deep Space 1 will help make future science spacecraft smaller, less expensive, more autonomous and capable of more independent decision-making so that they rely less on tracking and intervention by ground controllers.

Of the 12 new technologies on board, all but the spacecraft's autonomous navigation system had been completely tested since launch. With the asteroid encounter, AutoNav finished its last 5 percent of testing. Science return was a bonus for this technology validation mission.

A Deep Space 1 asteroid flyby press kit, along with mission status reports from launch to the present, is available online at

http://www.jpl.nasa.gov/ds1news

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