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June
1, 2006: Six years ago, MIT engineering Professor
David Miller showed the movie Star Wars to his students on
their first day of class. There's a scene Miller is particularly
fond of, the one where Luke Skywalker spars with a floating
battle droid. Miller stood up and pointed: "I want you
to build me some of those."
 So
they did. With support from the Department of Defense and
NASA, Miller's undergraduates built five working droids. And
now, one of them is onboard the International Space Station
(ISS).
Right:
MIT undergrads flight-test a prototype droid onboard NASA's
KC-135 reduced gravity aircraft. [More]
"It
only looks like a battle droid," laughs Miller. It's
actually a tiny satellite—the first of three NASA plans to
send to the ISS. Together, they'll navigate the corridors
of the space station, learning how to fly in formation.
Tiny satellites
are a hot new idea in space exploration: Instead of launching
one big, heavy satellite to do a job, why not launch lots
of little ones? They can orbit Earth in tandem, each doing
their own small part of the overall mission. If a solar flare
zaps one satellite—no problem. The rest can close ranks and
carry on. Launch costs are reduced, too, because tiny satellites
can hitch a ride inside larger payloads, getting to space
almost free of charge.
But
there's a problem: Flying in formation is trickier than it
sounds. Ask a crowd of people to line up single file, and
they'll be able to figure it out and do it rather easily.
Getting a group of orbiting satellites to do the same thing,
it turns out, is extremely hard.
"Suppose
you've got a cluster of satellites in orbit," says Miller,
"and one or two of them lose their place." Maybe
a solar flare temporarily scrambles their nav-computers, or
a thruster firing didn't work as expected. The whole cluster
finds itself out of whack. Correcting the problem requires
a complex set of 3-dimensional adjustments, coordinated among
all the satellites—perhaps dozens or hundreds of them. "We've
got to break this down into step-by-step, concrete instructions
that a computer can understand," Miller says.
And
that takes us back to the ISS:
Miller's
challenge to his undergraduate engineering class back in 1999
was to design a small, roughly spherical robot that could
float aboard the ISS and maneuver using compressed CO2 thrusters.
The project, called SPHERES (Synchronized Position Hold Engage
Re-orient Experimental Satellite), would serve as a testbed
for trying out experimental software to control clusters of
satellites. The robotic spheres provide a generic platform
consisting of sensors, thrusters, communications and a microprocessor;
scientists working on new software ideas can load their software
into that platform to see how well those ideas work. It's
a quick and relatively cheap way to test new theories on software
design.
 Possible
applications include NASA's return to the Moon (see the Vision
for Space Exploration). One way to build a moonship is
to assemble it piece by piece in Earth orbit. "Software
designed to control small satellites could just as well be
used to maneuver the pieces of a spaceship together,"
says Miller.
Right:
A CAD-model of a SPHERES satellite. [More]
The first
SPHERE arrived on the ISS in April tucked inside a Progress
supply rocket. (Remember, tiny satellites make good hitchhikers.)
Eventually two more SPHERES will join it, one later this year
when the space shuttle Discovery (STS-121) returns to the
station, and another carried to orbit by a future shuttle
mission.
How
will astronauts tell the three SPHERES apart? "They're
color coded," explains Miller. The one onboard now is
red; the second will be blue and the third yellow.
"Red"
is already busy. "We've commanded it to do a variety
of maneuvers—loops and turns, for instance. And we've tested
the robot's ability to solve problems." Astronauts tried
to trick Red by causing one of its thrusters to stick "on."
The robot diagnosed the fault, turned the thruster off, and
returned to station-keeping.
"Not
bad for one little droid," says Miller. "I can’t
wait to see what three of them can do."
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Authors: Dr. Tony
Phillips and Patrick L. Barry | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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