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Getting a Feel for Braille.

In a plan both daring and daunting, NASA's Deep Space 1 flew past minor planet 9969 Braille on July 29th at 4:46 Universal Time. The ion-propelled spacecraft used an autonomous navigation system intended to maneuver it to within about 15 kilometers of the asteroid -- the closest interplanetary flyby ever attempted. Disaster was narrowly averted just 16 hours before the encounter when an emergency electronic shutdown occurred onboard. Marc Rayman (Jet Propulsion Laboratory), the mission's chief engineer, lauded his team's "heroic recovery" of the spacecraft during the crisis. Final commands were beamed up just 4 minutes before the spacecraft wheeled out of contact with Earth to close in on its target.

But hours later, as DS1 began to send back results, it became obvious that something had gone awry. Instead of seeing crisp images of the asteroid's surface, waiting scientists saw star-specked black sky. Flight controllers speculate that the asteroid proved too faint to be tracked by DS1's camera during the crucial final hour of the approach. But they don't yet know how close the spacecraft came to its target (the guess is 26 km) or where it was looking when it zipped by at 1511/42 km per second.

Scientists may have been disappointed by not getting close-ups of Braille, but they are thrilled with the spacecraft's high-quality spectra of the asteroid. A dozen near-infrared signatures, acquired 17 minutes after the problematic flyby, show that Braille is a close spectroscopic match for the large, basalt-covered asteroid 4 Vesta. "It's astounding and very surprising," exclaims Robert M. Nelson (JPL), who heads the science team. Vesta lies deep in the asteroid belt, and planetary astronomers have long wondered how basaltic meteorites (presumably blasted from Vesta's surface) manage to reach Earth. The similarity of Earth-approaching Braille to Vesta hints that the former may have been knocked from the latter -- or from an even larger precursor. Over time Braille then migrated into an orbit susceptible to the gravitational influence of Jupiter and was eventually flung in Earth's general direction. "A picture may be worth a thousand words," notes Nelson, "but in this case a spectrum is worth a thousand pictures."

That sentiment is especially true for July's encounter, since the spacecraft returned only two images showing any surface detail at all. As suspected, Braille is a very irregular, elongated body, 2.2 by 1.0 km in size. The vague outlines hint that Braille might be a loose assemblage of two or more large chunks. DS1's plasma detector, which worked flawlessly despite the pointing problems, recorded no unusual ion or electron populations in the asteroid's vicinity.

As the first undertaking of NASA's New Millennium program, DS1 was built primarily to test 12 cutting-edge technologies, including autonavigation and a propulsion system powered by xenon ions. All of those test objectives have been met, but the project team hopes to gain an extension of the mission beyond its planned conclusion on September 18th. Toward that end DS1's ion engine will be firing almost continuously throughout August, September, and October. The resulting gradual course change will allow flybys of the enigmatic comet-turned-asteroid 4015 Wilson-Harrington in January 2001 and of the effusive Comet 19P/Borrelly in nine months later.

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Publication:Sky & Telescope
Article Type:Brief Article
Date:Oct 1, 1999
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