United We Orbit
It's a story of spacecraft meets spacecraft.
- By James E. Oberg
- Air & Space magazine, January 1997
(Page 4 of 5)
Before mission STS-71, the astronauts “flew” over 200 approaches in a variety of simulators. Docking with Mir requires a very slow closing speed—barely more than an inch per second during the final approach. It also demands great precision. The docking rings have to be parallel within two degrees in each axis, and the targets have to be aligned within three inches of each other. The astronauts have various tools to help them measure the alignment. A metal “stand-off cross” extends on a rod above and parallel to a black painted cross on the Mir target. If the crosses appear in TV views to line up perfectly, the pilot knows he’s on track. The TV cameras also have grid markings to make it easier for the astronauts to check their alignment.
One concern had been the disorienting view caused by the camera’s being at a distance from the pilot’s eyeballs. “You’re not looking at the real world,” explains Precourt. “It’s not like landing an airplane with a view straight out the front windshield.” It’s more like closing your eyes, holding your hands out, and trying to touch your fingertips, he says. But even though it took some getting used to during training, it turned out not to be a problem.
Gibson and Precourt, as well as every docking crew after them, learned in the simulators to hit the marks every time, even when jets and instruments and computers failed. On the STS-71 docking, the angular errors were measured in tenths of degrees, almost too small to be noticed. The arrival time was nearly perfect too: They were only two seconds off.
Experience has shown that on-time arrival doesn’t matter all that much. “I always argued against getting hung up on the docking time as if it were critical,” says Kevin Chilton. “I wanted to dock a minute later or a minute early just to show it’s not important.” He ended up docking “pretty much on time” anyway.
In fact, so far every docking has been a model of precision. “When you think about it,” says Precourt, “it’s pretty amazing that you’d have two vehicles flying in space that are subject to bending and moving, yet the relative position of the docking ports can be precisely known when we arrive.”
With at least five more shuttle-Mir missions planned, and with dockings to the international space station scheduled to begin in 1998, orbital docking is finally becoming, if not routine, then at least no cause for great anxiety. Engineers working on the space station have come up with a few modifications to the shuttle-Mir design but not many. They plan to fine-tune the orbiter’s damping mechanism to further reduce the energy transferred to the station at contact. The station also will have a few of the old-style probe-drogue ports, since a variety of Russian, American, European, and Japanese vehicles will have to dock with it.
Dockings have now taken place with four different configurations of the shuttle and Mir (approaching the Russian station, with all its protruding solar arrays, modules, and vehicles, is “like docking with a porcupine,” says STS-79 commander Bill Readdy). The STS-74 crew brought up a new docking module to attach to Mir last year, which provides greater flexibility and places the docking interface at a distance from the main station. This addition, plus the station’s different configuration and greater mass, may account for the fact that Mir crews are now feeling less of a jolt than Dezhurov and his companions experienced. Readdy says that when Atlantis pulled up to the docking port last September, Shannon Lucid and her cosmonaut crewmates hardly felt a thing.
The STS-74 astronauts even came up with a soundtrack to accompany all the slow, graceful maneuvers in space. A Strauss waltz had already been appropriated by Stanley Kubrick, and besides, it evoked Vienna, not Moscow. So Ken Cameron and his Atlantis crew went with Tchaikovsky’s “Swan Lake” for their final approach and docking.