Spacewalk thruster incident alarms NASA

Enter the thrusters
The spacewalk ended successfully after 5 hours and 28 minutes. The planned equipment had been deployed, and in an important discovery, the crewmen had noted some strange "goo" around the dump ports from Russian life support equipment that had been mysteriously malfunctioning in recent months.

But those who followed the spacewalk live on NASA TV, or later watched the tape, noticed some other strange events. Less than an hour into the spacewalk, the gyroscopes that keep the space station oriented had become overloaded and needed assistance.

“The Russian thrusters are now back in control of the space station,” the NASA commentator announced. “The crew is taking a pause to allow the thrusters to reestablish control” against what he called “a slight deviance” that was “nothing significant.”

“Now I can see the thruster firing,” one of the spacewalkers commented a few minutes later, as reported by an interpreter (both men were speaking Russian). Speaking to the other spacewalker, he continued: “It is very interesting to watch the thrusters firing right behind your back.”

Over the remaining hours of the spacewalk, while the men worked at various locations on the outside of the space station, the thrusters came on twice more. Television views would show the men in one area, and brief white thruster plumes appearing on another section of the module. But no audio discussion over the NASA TV channel gave any indication of any concern.

NASA’s internal status report, which is not distributed to the public, later contained a cryptic paragraph on thruster activity during the spacewalk: “Attitude control momentum again was observed to build up in the US [Control Moment Gyros] from reacting to external torques,” the report stated. This “required control authority transfer to [Russia’s Service Module] thrusters to permit gyro desaturation.” After a brief period, “control then returned to the CMGs”.

That is, the station was, as in normal, kept in position by spinning up or slowing down heavy gyroscopes on the U.S. section. But when something "torqued" (or forced to turn) the station to a degree beyond the ability of the gyroscopes to handle, or forced the gyros to spin at dangerously high rates, rocket thrusters on the Russian side had to be turned on to relieve the load on the gyroscopes.

NASA’s official public report made no mention of the orientation control issue or of the Russian thruster firings near the spacewalkers.

Where were the astronauts?
However, within hours of the apparently uneventful completion of the spacewalk, the halls at NASA’s Johnson Space Center were abuzz with rumors about a serious contingency that had come up.

“You might want to ask about turning on a Russian thruster when the crew is in the keep-out zone,” one contact e-mailed me, “and the U.S. flight director is telling [the Russian] not to because they are [too close].”

Subsequent private inquiries obtained several different -– and sometimes conflicting –- accounts of the incident. Geyer, who spoke with MSNBC.com only after the first version of this story was published, attributed the different versions to varying interpretations of what was shown in the TV images of the mission. While some at NASA thought it looked like the crew was in the "keep-out" zone, their exact location was unclear. Geyer denied reports that there had been a dispute between the U.S. and Russian officials as to whether to fire the thrusters and said that officials had followed correct safety protocols.

Sources explained that the Russian thrusters that control the station’s orientation are in two sets. For pitching and yawing (turning the station up-down and side-to-side), the thrusters of the docked Progress supply ship are wired into the station’s autopilot. But for rolling the station along its long axis, the Progress thrusters are too weak, so the main thrusters along the back rim of the Russian service module are used. Since rotation control involves the thruster pushing largely parallel to the station’s rim, the thruster plume sweeps out a large area near the station skin.

“Yes, a major close-call incident occurred,” one source claimed. While installing one experiment, he continued, the crew had to work close to a thruster. By prior agreement, when crewmen were in this zone, the Russian thrusters were supposed to remain disarmed. But if the thrusters were urgently needed, the crew was to be instructed to remain outside a "keep-out zone" until they could again be disarmed.

During a brief period when television views had become available, Houston controllers thought they saw the crew move to a worksite within such a zone. They then heard Moscow announce that the thrusters would be armed to respond to another gyro overload.

According to multiple sources, Hassman, the NASA flight director, told his Moscow counterpart that the crew was too close to the thruster. Moscow disagreed and told Hassman that the crew was safely at another worksite. But the NASA team could see on television that this wasn’t the case. It looked to some of those there that since the Russians  had assumed that no television would be available for the spacewalk, they were not even looking at the NASA video feed.

Witnesses delicately described “an exchange” between the two flight directors in Houston and Moscow, but the net result was that the Russian ground controllers did not tell the spacewalkers to leave the forbidden area, nor did they disarm the thrusters for more than ten minutes, until the overloaded gyros had been "desaturated". During this period, say sources, the crew reported thrusters firing near them.

"The video gives the appearance that the crew was in a worksite that had a keep-out zone, before the thrusters were inhibited," Geyer confirmed, noting it was up to the special investigation team to find out exactly where each spacewalker was.

But "there was never a disagreement [with Moscow] as to when to inhibit the thrusters," Geyer said, rejecting tales of a dispute.

He also rejected any suggestion that NASA officials had ignored the potential risks involved.

"Because of the possibility of contamination," Geyer said, "the team looked at all the flight rules and reference books for that contingency." They found that ground tests had shown that a splash of hazardous fuel residue on a spacesuit would quickly evaporate in the vacuum of space.

"An hour [of vacuum exposure] is sufficient to get way below any concern with toxicity," he said. Much more time than that was available before the two spacewalkers had to return to the station.

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