Hard experience has taught Russia and the United States not to take space dockings and undockings of the robot freighter Progress lightly.
In 100 missions over 30 years, the freighters have always succeeded in docking with the space station, but sometimes requiring dramatic second or even third attempts or with assistance of cosmonauts during emergency spacewalks.
During one undocking and redocking test in 1997, an off-course Progress smashed into the hull of the Mir space station, creating a crack and very nearly killing the three men on board.
Such near-calamities explain the extreme caution with which Russian space planners have approached a mechanical docking snag on their Progress M-58 supply ship, now docked with the international space station. It must undock smoothly in early April to clear the way for the arrival of a new crew aboard a Soyuz spacecraft.
A spacewalk Thursday morning is planned to ensure a smooth undocking six weeks from now. Russian cosmonaut Mikhail Tyurin and American spacewalk record-holder Michael Lopez-Alegria will take a collection of U.S. and Russian tools to the back end of the station and try to clear the snag.
Jammed antenna wouldn't budge
"It really is unknown what the vehicle will do if it separates (with the snag in place)," said Richard Labrode, U.S. space station lead flight director. "That's the main concern."
Labrode said that while the snag most likely would break loose safely if the undocking had to be attempted, it's still a risk no one is willing to take.
The snag occurred when the supply drone arrived at the international space station in October. A guidance antenna on a forward-facing swing-arm did not retract as planned before contact. The antenna came in contact with the station next to the docking port, lodging under a handrail installed there for crew access.
During a scheduled spacewalk the following month, Tyurin and Lopez-Alegria tried to manually tug the antenna loose, but couldn’t budge it. Mission Control in Moscow decided to try again on another spacewalk using better tools.
Ground teams have gone over the procedure back on Earth and sent up videos of how it should go the second time around.
The equipment includes a new set of Russian-built helmet lights sent up on a recent supply flight, and an array of NASA tools and crewman stabilization fasteners. It does not include any television cameras to show the work area. The spacewalkers will be out of sight of all external video equipment and will be in contact with Earth by voice only.
The antenna unit is mounted on four aluminum struts that swivel. The struts are similar to hollow aluminum tent poles, about 1½ inches in diameter and with the actual skin thickness about a tenth of an inch.
First, a mallet and chisel
One of the arms is attached to a drive motor, which appears to be frozen. It also has a latch that holds it steady but was supposed to be driven open by the first motion of the retraction process.
The spacewalkers' first trick will be to use a mallet and chisel to try to pop that latch open, allowing the strut to be swung back, back-driving against some resistance from the motor. Photographs from the failed attempt last time suggest that the latch may already be open, but the Russians aren't sure and want to do the simplest method first.
The second trick will be to take a U.S. "general purpose cutter" and slice through the motorized strut entirely. The tool, sent up on a shuttle flight in December, looks like a standard two-armed bolt cutter but with a force-multiplying ratchet assembly that converts multiple arm closings (about a dozen strokes will be required) into a small but powerful jaw closing.
If that cut succeeds — and the crew has been warned that the resulting strut ends will likely be quite sharp and thus hazardous to their spacesuits — the unit can be swung back on the remaining three legs. It would then be lashed into place with straps the crew will have in their toolkit.
Swinging it back will require the gimbaled dish antenna — now lodged under the station's handrail — to be flexed enough to clear the handrail as the unit rotates away. If it cannot be worked past the obstruction, more severe measures will be needed.
One of the techniques should work
If the unit can't swing freely, or if somehow all the strut hinges are jammed, the last trick is to cut all four struts and the connecting umbilical cable, and just throw the unit away. It would become just another piece of station-generated debris that, because of the station's relatively low altitude, would decay and burn up within a year or two.
These methods are straightforward and the task should be simple. One of the techniques, or a variation on them, likely will work.
Besides Mission Control having no televised views of the work site, the spacewalkers will be in contact with Moscow only for a few minutes every hour and a half orbit. Voice communication will be relayed continuously through U.S. satellites, but telemetry from their spacesuits (temperature, power, remaining supplies, etc.) will be readable only while over Russia.
Planners had considered the use of U.S. spacesuits, which would have provided continuous data and video as well as voice links. But that option was rejected largely because Tyurin was considered not adequately trained on the U.S. equipment.