Objects in space tend to spin — and spin in a way that’s totally different from the way they spin on earth. Understanding how objects are spinning, where their centers of mass are, and how their mass is distributed is crucial to any number of actual or potential space missions, from cleaning up debris in the geosynchronous orbit favored by communications satellites to landing a demolition crew on a comet.

In a forthcoming issue of the Journal of Field Robotics, MIT researchers will describe a new algorithm for gauging the rotation of objects in zero gravity using only visual information. And at the International Conference on Intelligent Robots and Systems this month, they will report the results of a set of experiments in which they tested the algorithm aboard the International Space Station.

On all but one measure, their algorithm was very accurate, even when it ran in real time on the microprocessor of a single, volleyball-size experimental satellite. On the remaining measure, which indicates the distribution of the object’s mass, the algorithm didn’t fare quite as well when running in real time — although its estimate may still be adequate for many purposes. But it was much more accurate when it had slightly longer to run on a more powerful computer.

Space trash

“There are satellites that are basically dead, that are in the ‘geostationary graveyard,’ a few hundred kilometers from the normal geostationary orbit,” says Alvar Saenz-Otero, a principal research scientist in MIT’s Department of Aeronautics and Astronautics. “With over 6,000 satellites operating in space right now, people are thinking about recycling. Can we get to that satellite, observe how it’s spinning, and learn its dynamic behavior so that we can dock to it?”

Moreover, “there’s a lot of space trash these days,” Saenz-Otero adds. “There are thousands of pieces of broken satellites in space. If you were to send a supermassive spacecraft up there, yes, you could collect all of those, but it would cost lots of money. But if you send a small spacecraft, and you try to dock to a small, tumbling thing, you also are going to start tumbling. So you need to observe that thing that you know nothing about so you can grab it and control it.”

Joining Saenz-Otero on the paper are lead author Brent Tweddle, who was an MIT graduate student in aeronautics and astronautics when the work was done and is now at NASA’s Jet Propulsion Laboratory; his fellow grad student Tim Setterfield; AeroAstro Professor David Miller; and John Leonard, a professor of mechanical and ocean engineering.

The researchers tested their algorithm using two small satellites deployed to the space station through MIT’s SPHERES project, which envisions that herds of coordinated satellites the size of volleyballs would assist human crews on future space missions. One SPHERES satellite spun in place while another photographed it with a stereo camera.