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July
6, 2007: Picture this: Two robots hang suspended
in space, nose to nose. One reaches out a crooked silver arm
and begins to minister to the needs of the other. Fuel is
exchanged, a battery is replaced; servicing complete, the
two silently drift apart.
 These
robots, named ASTRO and NextSat, are real and they are in
Earth orbit now.
Right:
An artist's concept of ASTRO, the Autonomous Space Transport
Robotic Operations service vehicle (left), and NextSat, the
Next-generation serviceable satellite (right). Click on the
image to view a computer
animation.
On
March 8, 2007, an Atlas V rocket boosted the pair into space.
Their mission: to demonstrate autonomous on-orbit satellite
servicing, a technology crucial to future space exploration.
The Defense Advanced Research Projects Agency (DARPA) manages
the project, which is called Orbital Express.
ASTRO
and NextSat look more like ordinary satellites than high-tech
robots, but they are far from ordinary. ASTRO, in particular,
seems to have a mind of its own. It can approach NextSat and
dock with it. ASTRO has its own arm for reaching, grappling
and servicing—tasks once reserved for the hands of living
astronauts. NextSat plays a less glamorous but no less essential
role as it races around Earth offering itself to ASTRO for
whatever tests ground controllers command.
This
is all new, and indeed ground controllers are proceeding cautiously
to see what ASTRO can actually do.
The
first on-orbit test took place in April. The two satellites
remained safely docked together as ASTRO's mechanical arm
grappled NextSat, moving it into a variety of positions and
attitudes to calibrate rendezvous and capture sensors. ASTRO
also transferred fuel and a battery to NextSat. Score: A+.
Above:
NextSat, photographed by ASTRO as the pair flew in formation
on May 5, 2007. [Larger
image]
The
next big test occurred on May 5th. ASTRO and NextSat completely
undocked and flew perfectly in formation for about 90 minutes.
The distance between the two during this maneuver was about
10 meters. ASTRO then approached and rejoined NextSat, conducting
the first autonomous rendezvous and docking in the history
of the American space program! This test also included an
autonomous fuel transfer.
The
milestone was made possible by ASTRO's Advanced Video Guidance
Sensor—AVGS for short—developed at the Marshall Space Flight
Center. It is one of the key technologies that gives ASTRO
"a mind of its own."
ASTRO
needed all the intelligence it could muster in mid-May when
something unexpected happened. An ASTRO flight computer glitch
caused a docking test to abort at 10 meters, before the vehicles
re-mated. Over the next few days, ASTRO and NextSat drifted
more than 6 kilometers (almost 4 miles!) apart. On May 19th,
at about 150 meters—greater than any distance ground tested
for Orbital Express—AVGS locked on and began to track NextSat.
Disaster averted.
 Right:
NextSat, moments before docking with ASTRO on April 17, 2007.
[Larger
image]
"AVGS
was very helpful in getting the two spacecraft back together,"
commented Fred Kennedy, the program manager at DARPA. "Our
mission operations team spent long days diagnosing sensor
and navigation anomalies, and was finally able to manually
reposition ASTRO within a kilometer of NextSat. It was then
a matter of returning guidance control to ASTRO, which performed
a series of autonomous maneuvers to get us within AVGS's fully
operational range so the two spacecraft could re-mate."
This
unplanned test may have been the most valuable of all, showing
that ASTRO and NextSat can deal with the unexpected, and perform
beyond their theoretical boundaries.
The
mission is now drawing to a close after establishing several
firsts in US space history. In addition to the first US autonomous
rendezvous and docking, ASTRO and NextSat also demonstrated
the first fully autonomous fly-around and docking, plus an
exciting free-flyer capture of NextSat using ASTRO's robotic
arm.
It
all goes to show that automated rendezvous and servicing may
be a realistic option for future space missions. Indeed, technologies
proven by Orbital Express could revolutionize the way space
is explored, making it possible within the next decade to
refuel and repair space vehicles without the touch of a human
hand. This, in turn, frees humans for jobs that only humans
can do.
It's
a partnership: ASTRO and NextSat, humans and machines, into
the void together.
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*The
Boeing Company of Huntington Beach, California, is the prime
contractor for the Orbital Express Mission.
Author: Dauna Coulter | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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