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Braille

Sky & Telescope, News Notes, October 1999, p. 28:

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 DS l 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 DS 1'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 15 ½ 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. DS 1'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, DS 1 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 DS 1'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 nine months later.


Best regards,

Bernd

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