On the video, the tasks seem simple enough. And Cooper reminds me that once Dextre is joined with Hubble in orbit, there will be no rush to finish the job: “One of the nice things about a robotic servicing mission is it doesn’t matter how long it takes. There are no astronauts to be fed, no shuttle landing schedule. Dextre could take days and weeks if it wants, trying different things over and over again—all of which enhances its probability of success.”
Of course, here inside NASA’s lab, Dextre and the Hubble replica are in a controlled setting. Nobody knows exactly what would happen if they’d been drifting side by side at 17,500 mph, hundreds of miles from Earth. The NRC panel worried that the robot could easily be thrown by unexpected glitches, like connecting pins that turned out to be bent instead of straight.
Kathy Thornton, a former shuttle astronaut, performed the first repairs on Hubble during a 1993 mission. She points out that most of the panels, latches, doors, and connectors on the telescope were designed for humans. “All those interfaces that were made for people to use would be more difficult for robots,” says Thornton, who left NASA in 1996 to teach engineering at the University of Virginia in Charlottesville. “Some of the connectors would be very hard to change, and not many of the end effectors [on the robots] are made to capture things when they start floating around.” Thornton says that a robotic servicing mission would have been “a great engineering exercise” but that it could have been more likely to damage Hubble than an astronaut repair mission.
Even a relatively simple teleoperated docking can end in disaster, something I witness firsthand inside a rectangular lab at MDRobotics known as the Bowling Alley. Engineers George Bailak and Andrew Allen are trying to develop a remotely operated spacecraft that can dock with a variety of satellites. In the center of the lab are two granite platforms positioned side by side. The engineers have placed a 1,500-pound satellite replica on one and their 260-pound robotic coupler on the other. They both rest on circular pads called precision air bearings. When high-pressure nitrogen is pumped through the bearings, the spacecraft begin to float like pucks on an air hockey table (this is Canada, remember), gliding a few millimeters above the pads.
After a pre-programmed computer sequence initiates the docking procedure, short bursts of pressurized air begin to slowly propel the coupler toward the satellite. A barbed hook on the end of the coupler is supposed to snag the inner lip of the satellite’s thruster cone. The two craft barely touch when the satellite suddenly swings sideways. A second try produces a similar result. Only on the third attempt—with Bailak and Allen physically nudging each craft to maintain the proper alignment—does the docking succeed. Later, I try the same procedure on a simulator. After a promising start, I crash the docking craft into the virtual satellite, tearing off half its solar panels and sending it into a death spiral.
Bailak and Allen brush off my cosmic train wreck as a minor hiccup along the robotic servicing learning curve. Besides, they say, autonomous rendezvous and docking are precisely the problems that projects like DART and XSS-11 are meant to address. DART’s program manager, Jim Snoddy, calls his mission “a prototype of many things to come,” including the orbital assembly of spacecraft bound for the moon and Mars. “When we start putting more pieces in space, we’re going to have to start putting them together,” he says.
A major hurdle will be developing a robotic arm that can adjust its sensitivity as it pushes and pulls on connectors and cables. Ideally, a robot would react to physical resistance much as a person does—by turning a screw more slowly and carefully, for instance, if it felt the threads beginning to strip.
Gerd Hirzinger, director of the Institute of Robotics and Mechatronics, part of the German space agency, DLR, hopes to overcome this problem with a remotely operated mechanical arm called ROKVISS (Robotic Component Verification on ISS). ROKVISS is a double-jointed arm about two feet long, with a self-contained power supply and a finger-length stylus tool. A Russian resupply craft delivered ROKVISS to the space station in December. It was mounted to the outer wall of the Russian Zvezda module, where it could be operated from a ground station located about 15 miles from Hirzinger’s lab outside Munich.
In March, ROKVISS completed its first set of maneuvers, “proving that the concept of torque-controlled joints” is mature enough to work in space, says Hirzinger. The joints, he adds, are “similar to human muscles—you can them make stiff or soft.” The ROKVISS arm also incorporates a stereo camera. According to Hirzinger, once the robot makes contact with a contoured shape, the arm can either maintain an even pressure anywhere along the object, or apply “high-fidelity force feedback” to vary the pressure. Imagine hand-sanding an intricately carved wooden table leg: If you don’t adjust your pressure to accommodate the leg’s curves, the finish will become uneven, smoother where it’s convex but still rough along the concave surfaces. Hirzinger plans to continue testing ROKVISS aboard the station for a year. “If the joints turn out to work perfectly in space,” he says, “then we’ll immediately start building a seven-degrees-of-freedom free-flying robot.”
Eventually, the follow-on system would be used to demonstrate in-orbit satellite servicing.
Meanwhile, in Houston, Rob Ambrose’s group has built a mobile platform for Robonaut. Funded by NASA and the Pentagon’s Defense Advanced Research Projects Agency, Robonaut has simmered along since 1996 as a low-priority technology development effort. Despite its popularity with the press and its Hollywood good looks, the humanoid robot has never been called for an assignment in space.