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July 6, 2007: Picture this: Two robots hang suspended in space, nose to nose. One reaches out a crooked silver arm and begins to minister to the needs of the other. Fuel is exchanged, a battery is replaced; servicing complete, the two silently drift apart.

These robots, named ASTRO and NextSat, are real and they are in Earth orbit now.

Right: An artist's concept of ASTRO, the Autonomous Space Transport Robotic Operations service vehicle (left), and NextSat, the Next-generation serviceable satellite (right). Click on the image to view a

On March 8, 2007, an Atlas V rocket boosted the pair into space. Their mission: to demonstrate autonomous on-orbit satellite servicing, a technology crucial to future space exploration. The Defense Advanced Research Projects Agency (DARPA) manages the project, which is called Orbital Express.

ASTRO and NextSat look more like ordinary satellites than high-tech robots, but they are far from ordinary. ASTRO, in particular, seems to have a mind of its own. It can approach NextSat and dock with it. ASTRO has its own arm for reaching, grappling and servicing—tasks once reserved for the hands of living astronauts. NextSat plays a less glamorous but no less essential role as it races around Earth offering itself to ASTRO for whatever tests ground controllers command.

This is all new, and indeed ground controllers are proceeding cautiously to see what ASTRO can actually do.

The first on-orbit test took place in April. The two satellites remained safely docked together as ASTRO's mechanical arm grappled NextSat, moving it into a variety of positions and attitudes to calibrate rendezvous and capture sensors. ASTRO also transferred fuel and a battery to NextSat. Score: A+.

Above: NextSat, photographed by ASTRO as the pair flew in formation on May 5, 2007. [

The next big test occurred on May 5th. ASTRO and NextSat completely undocked and flew perfectly in formation for about 90 minutes. The distance between the two during this maneuver was about 10 meters. ASTRO then approached and rejoined NextSat, conducting the first autonomous rendezvous and docking in the history of the American space program! This test also included an autonomous fuel transfer.

The milestone was made possible by ASTRO's Advanced Video Guidance Sensor—AVGS for short—developed at the Marshall Space Flight Center. It is one of the key technologies that gives ASTRO "a mind of its own."

ASTRO needed all the intelligence it could muster in mid-May when something unexpected happened. An ASTRO flight computer glitch caused a docking test to abort at 10 meters, before the vehicles re-mated. Over the next few days, ASTRO and NextSat drifted more than 6 kilometers (almost 4 miles!) apart. On May 19th, at about 150 meters—greater than any distance ground tested for Orbital Express—AVGS locked on and began to track NextSat. Disaster averted.

NextSat, moments before docking with ASTRO on April 17, 2007. [Larger image]

"AVGS was very helpful in getting the two spacecraft back together," commented Fred Kennedy, the program manager at DARPA. "Our mission operations team spent long days diagnosing sensor and navigation anomalies, and was finally able to manually reposition ASTRO within a kilometer of NextSat. It was then a matter of returning guidance control to ASTRO, which performed a series of autonomous maneuvers to get us within AVGS's fully operational range so the two spacecraft could re-mate."

This unplanned test may have been the most valuable of all, showing that ASTRO and NextSat can deal with the unexpected, and perform beyond their theoretical boundaries.

The mission is now drawing to a close after establishing several firsts in US space history. In addition to the first US autonomous rendezvous and docking, ASTRO and NextSat also demonstrated the first fully autonomous fly-around and docking, plus an exciting free-flyer capture of NextSat using ASTRO's robotic arm.

It all goes to show that automated rendezvous and servicing may be a realistic option for future space missions. Indeed, technologies proven by Orbital Express could revolutionize the way space is explored, making it possible within the next decade to refuel and repair space vehicles without the touch of a human hand. This, in turn, frees humans for jobs that only humans can do.

It's a partnership: ASTRO and NextSat, humans and machines, into the void together.

*The Boeing Company of Huntington Beach, California, is the prime contractor for the Orbital Express Mission.

Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA