It's getting harder to find good help these days. So these space engineers built their own
- By Michael Behar
- Air & Space magazine, July 2005
(Page 3 of 5)
The 3,600-pound Dextre robot is toward the back, where it’s suspended from a block-and-tackle rig that allows engineers to evaluate its performance in simulated zero gravity. “Repairing Hubble is such a noble mission, crossing the boundary of science and reaching into the public interest,” gushes Cooper, who stubbornly maintains that a robotic servicing mission would have been a viable option. “It’s not just some pie-in-the-sky design idea. Dextre really exists. And it has already been built and tested for the ISS.”
In Congressional hearings last Frebruary, members of the House Committee on Science questioned Cooper about the risks outlined in the NRC report—things like the time lag in robotically executing commands from Earth, and the feasibility of latching onto Hubble, which lacks a docking interface. Cooper pointed out that before a Hubble servicing flight would take place, other space missions would solve these technical challenges. Two such missions were launched this spring—NASA’s Demonstration of Autonomous Rendezvous Technology (DART) and the Air Force-sponsored XSS-11. Both were designed to prove that one spacecraft could meet up with another in orbit and work in close proximity, safely, with no human supervision.
“By the time we launch [Dextre],” Cooper argued in his testimony, “there’s going to be nothing left for us to do but actually go up there and do the mission, because everything we could possibly think of will have been covered by that point.” But DART had only mixed success, getting close to its target but then actually bumping into it.
Last summer, when robot rescue was still a possibility, astronauts and technicians at Goddard practiced maneuvers with Dextre on a life-size replica of the Hubble. The tests even simulated a two-second delay in Dextre’s response to human commands radioed from Earth. Cooper plays me a video from the Goddard shakedown that shows Dextre opening and closing panel doors on the Hubble mockup, twisting bolts, and yanking out power cords. Before Dextre can begin each task, it must switch the tool affixed to its “end-effector.” The device is similar to a dentist’s multi-tool driver: Dextre can swap out the heads—one for turning a screw, another for clamping onto a cable, a third for rotating a knob or opening a latch.
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.