How do you catch a speeding, tumbling, aging satellite? Very carefully.

Levity aside, the question is central to the plan scientists at DARPA have for catching up to and grabbing old satellites in an effort to ultimately refurbish and reuse the systems.

Scientists at DARPA say there are some 1,300 satellites worth over $300B sitting out in Earth's geostationary orbit (GEO) that could be retrofitted or harvested for new communications roles and it designed a program called Phoenix which it says would use a squadron "satlets" and a larger tender craft to grab out-of-commission satellites and retrofit or retrieve them for parts or reuse.

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DARPA recently concluded some of the most critical design tests of the Phoenix program - designing the algorithms that would help these satlets approach and tumble in sequence with the system they are trying to catch.

This summer participants in the DARPA's Zero Robotics Autonomous Space Capture Challenge designed and demonstrated algorithms to control the bowling-ball-sized programmable SPHERES (Synchronized Position, Hold, Engage, and Reorient Experimental Satellites) satellites aboard the International Space Station (ISS). The Massachusetts Institute of Technology Space Systems Laboratory developed the three SPHERES satellites and they have been onboard the ISS since 2006 to provide DARPA, NASA, and other researchers with a system that could help those agencies test technologies for use in formation flight and autonomous docking, rendezvous and reconfiguration algorithms, MIT stated.

In the Zero Robotics challenge, three finalist teams emerged from a series of four, one-week qualifying rounds: "y0b0tics!" (Montclair, NJ); "The Catcher in the Skye" (Sparta, NJ); and "Nitro" (Eagleville, PA). Then in June the teams gathered at the Massachusetts Institute of Technology to watch via video link as their algorithms were tested on board the ISS, DARPA said. The algorithms were applied across three situations in which the SPHERES satellite simulated an active spacecraft approaching an object tumbling through space. In each scenario, at least one of the teams was able to approach the tumbling target and remain synchronized within the predefined capture region, DARPA said.

Phoenix spacecraft will need to maneuver into position and synchronize with a tumbling object such that tools can be extended to remove or attach necessary parts, DARPA said.

The latest competition on the ISS helped identify key attributes in how to optimize fuel use and time to match an object's random tumble in space and be able to approach and dock with it safely," said Dave Barnhart, DARPA program manager. "The control procedures that were developed for the Zero Robotics Challenge will certainly benefit the Phoenix program, but they also potentially have much wider implications for space-based technologies. Our efforts can help to reduce the risks and costs of future complex satellite-to-satellite interactions in space to lower the barrier of entry for future space operations and missions."

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When DARPA first talked about the Phoenix program in October, it acknowledged the difficulties facing any Phoenix developers.

"Satellites in GEO are not designed to be disassembled or repaired, so it's not a matter of simply removing some nuts and bolts," Barnhart said. "This requires new remote imaging and robotics technology and special tools to grip, cut, and modify complex systems, since existing joints are usually molded or welded. Another challenge is developing new remote operating procedures to hold two parts together so a third robotic 'hand' can join them with a third part, such as a fastener, all in zero gravity. For a person operating such robotics, the complexity is similar to trying to assemble via remote control multiple Legos at the same time while looking through a telescope."

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