This is only the first phase, however, of a planned series of tests over years, involving three of the satellites, which the researchers will attempt to fly in tight formation inside the ISS. Each of the mini-satellites will control its position and orientation through a combination of internal gyroscopes and puffs of carbon dioxide through reaction jets.

Other groups, including ones at Stanford and NASA's Marshall Space Flight Center, have done tests of formation flying and automated docking, using flat air-tables on the ground and therefore allowing only two-dimensional movements, or underwater with the complication of water's viscosity, or using blimps that allow for 3-D movement, but with the problem of air turbulence. And all these environments are obviously different from those faced by actual satellites.

The MIT group's initial tests on NASA's zero-gravity airplane (colorfully named the "vomit comet") in 2000 and 2001, and now on the space station, are the only U.S. tests of such multiple-satellite systems so far performed in a true weightless environment. Japan conducted a successful multiple-satellite test several years ago, though, and NASA attempted a single-satellite docking with a military satellite in 2005, but that single-shot test ended with a collision.

"On the International Space Station, we can afford to be more aggressive in what we test [than is possible with a real satellite]," says Simon Nolet, an MIT graduate student who's been working on the project. "If we fly out of control, the astronauts can just grab it and start again. That makes it possible to test algorithms we would be scared to fly in a real satellite."

The research has potential applications for both civilian and military space missions, which is why both NASA and DARPA have provided funding. In the near term, the software developed in these tests could lead to fully automated docking satellites, which could be crucial for such missions as taking samples on Mars, where a return spacecraft will have to be able to rendezvous in Mars orbit with a sample-carrying craft sent up from the surface. Such docking capabilities could also be used for the in-orbit assembly of large rockets, for example, in a manned Mars mission, which are too big to be launched from Earth in one piece.

The most ambitious application would be for projects such as NASA's Terrestrial Planet Finder. This is to be a constellation of separate spacecraft, each carrying a telescope, whose light beams will be focused together into a central optical detector -- providing the resolution equivalent to a single huge telescope. This would make it possible for the first time to get direct images of Earth-sized planets around other stars, and even perform spectrographic measurements that could detect any signs of life. But doing so requires an incredible degree of precision in the alignment and pointing of the separate craft -- a feat that has to be thoroughly tested before the investment in such a multibillion-dollar system.

"We don't want it to just work," says How, "we want it to work robustly."