June 20, 2007
(PLANETQUEST) -- Three basketball-size "spheres" recently flew in tight formation in the International Space Station, achieving a major milestone along the path to developing NASA's Terrestrial Planet Finder Interferometer.
Under the direction of David Miller, a professor at Massachusetts Institute of Technology, Cambridge, the MIT Space Systems Laboratory created the Spheres (Synchronized Position, Hold, Engage and Reorient Experimental Satellites) program as a testbed for developing technologies needed for Terrestrial Planet Finder and other missions. The program employs nano-satellites which can sense each other's location to within 2 to 3 millimeters (about a 10th of an inch) and maintain their relative positions to within about 2 centimeters (about three-quarters of an inch). Each one controls its motion with a bank of 12 low-velocity gas thrusters.
Two experiments in April 2007 marked the first time that three of the roughly spherical, 18-sided test vehicles in the Spheres program flew in formation. In the first experiment, they formed an equilateral triangle and maintained their relative positions as they revolved around their common center of mass. In the second experiment, the axis about which they rotated was made to slowly precess, like the motion of a spinning top before it topples over. This maneuver was designed to simulate re-pointing the multi-spacecraft TPF interferometer system from one target star to another.
"While there are testbeds on the ground that can do planar motions (motions all in one plane)," Miller said, "precessing that spinning array is really a three-dimensional problem, and that's where having a facility on space station is unique."
The precession maneuver was attempted in the first of the two April experiments, but two of the satellites collided with each other. At their low masses and slow speeds, no damage was done. "The spheres cannot get going fast enough that they pose any impact threat," Miller said. "The jets of gas that come out of those thrusters, it could hit you right in the eye and it's not going to do any damage. The sphere itself can only get going up to a certain speed. It just can't hurt anything and it can't outrun an astronaut."
"Obviously we wouldn't do this with any real satellite," said Dr. Alvar Saenz-Otero, the program's lead scientist. "But given that we are Spheres, we can risk it. We learn a lot more by risking than by always being safe."
Other innovations tested in the April experiments included docking with a tumbling target and a "lost in space" maneuver. "The idea is to get up on orbit, and your launch vehicle releases two or more satellites," Miller said. "When it releases them, they're still sort of half-asleep. They may be tumbling a little bit, things like that. At some point, they need to start rotating around and figuring out where each other is, then start pointing at each other, then start controlling their relative separation so they don't collide. How do you start lost in space and end up not so lost in space?"
The Spheres program began in 1999 when, on the first day of an undergraduate class at MIT, Miller showed his students a clip from "Star Wars." "There's a scene where Luke Skywalker is practicing his light-saber skills with this little laser-shooting droid," Miller recalled. "I said 'I want you to build three of these, and I want them to be able to fly on station or shuttle.'"
The undergraduate class designed a prototype for the devices, and grad students worked on the design and construction of the actual "spheres" in collaboration with a local company called Payload Systems Inc.
MIT grad students have played a critical role in each of the six flight programs Spheres has conducted. "My grad students, when they graduate they know what it takes to put hardware in space," Miller said. "The most exciting part of it is just watching that if you give students the opportunity, they can do pretty amazing things."
"I think it's very cool that we can try all these things that nobody else would even dare to think about doing in space," Saenz-Otero said. "Space is not the final frontier, it's actually part of your lab."
Written by Bob Silberg/PlanetQuest
