On May 18, after a delay of more than three years, astronauts aboard the International Space Station (ISS) will take a soccer-ball-sized experimental satellite designed by MIT students, and place it in midair, somewhere in the middle of the station's interior. Then they'll watch as the satellite uses its built-in computer software to maintain its position and orientation, and later tries to find its way to small beacons attached to the station walls.

MIT students testing the SPHERES satellite prototypes aboard NASA's zero-gravity aircraft. In those tests, air turbulence posed some difficulties, as the plane was jostled around, but that won't be a problem in the upcoming space station tests. (Photo courtesy of NASA.)

The colorful satellites, called SPHERES, for Synchronized Position Hold Engage and Reorient Experimental Satellites, were first prototyped as part of an undergraduate MIT course in 1999 taught by aero-astro professor David Miller, who continues to run the project, and then refined as part of a graduate student project. The final flight hardware -- the actual SPHERES used on the ISS -- were built by Cambridge-based Payload Systems under the direction of MIT's Space Systems Lab, and have plastic shells that are each a different color to make it easier to keep track of them when they fly together.

[For images of these small satellites and their testing, click here.] 

The first satellite arrived on the space station last week, delivered by a Russian Progress resupply vessel. Two more are scheduled to be brought on the next two U.S. space shuttle missions (assuming these get off the ground). The satellites were ready to be delivered to the ISS back in 2003, just before the Columbia accident shut down the shuttle program.

Along with the satellites, the system involves a series of beacons, each about the size of a TV remote control, that will be attached to the space station walls at various points. These will emit ultrasound signals to provide a set of reference points, so the satellites can determine their exact positions and which way they are pointing. In actual free-flying satellites, these would be replaced by GPS signals.

This month's experiment is all about software: developing and testing systems for the operation and coordination of autonomous satellites and spacecraft in the future. "The relevance is in the algorithms," explains Jonathan How, a professor in MIT's aero and astro department who's assisting in the program. By operating inside the space station, with astronauts present to monitor activities - yet with real zero-gravity (or technically, microgravity) -- a lot of testing can be done at low cost, compared with using satellites in space. "It's a way of buying down the risk," says How.

The algorithms in one of these micro-satellites constantly monitor and compare the arrival time of the signals from the different beacons in order to compute the relative distances of each beacon, and thus derive the satellite's exact position. (The principle is the same as counting the interval between a lightning flash and its thunderclap to figure out the distance away of a storm.) The software must compute, ten times per second, the timing of each impulse, to derive what direction it's coming from, so that the satellite's reaction-control jets can be powered to bring the satellite to its intended position. Making sure the software doesn't produce unexpected oscillations or movements is part of the test's goals.