The glitch that cut off communications during Tuesday’s spacewalk and allowed the international space station to drift off course was the result of anomalies that engineers have been trying to resolve for months, MSNBC.com has learned. And while a special contingency procedure was developed, it failed to work as planned, NASA sources said on condition of anonymity.
Various explanations have been given for Tuesday's glitch: Some news accounts indicated that the station’s stabilizing gyroscopes were overloaded by the crew’s strenuous work replacing equipment at the Russian end of the station. In Moscow, spokesmen for Russia's space agency blamed American stabilizers and the rough handling of the station by the two men in orbit, NASA astronaut Mike Fincke and Russian cosmonaut Gennady Padalka.
But according to internal NASA documents obtained by MSNBC.com, a U.S.-Russian team determined last month that the station was likely to be forced out of proper orientation by a feature of the Russian spacesuits the men were wearing. Similar drifts had been seen on earlier spacewalks, and will be again, the team concluded.
To keep the spacewalkers cool during their exertions, the Russian-made Orlan suits use a standard technique that involves the evaporation of water ice in the backpacks. This process results in a weak spray of water molecules from a small port in the packs. Such technology has been used in various forms on spacewalks for decades without any problem.
But in recent years, spacewalkers have ventured farther and farther out from the international space station's center of mass. The result is that the weak spray actually can exert a significant "push" on the station, just as children can push down their side of a playground seesaw more easily if they sit farther back on the board.
Space engineers have estimated that each suit puts out a force of about one newton — about as much force as you feel when an apple presses down on your hand. For decades, such a force was too tiny to take note of.
But during a spacewalk last February, ground controllers were startled to see a slow flexing of the entire station, with a period of 30 to 40 minutes. The last two pulses reached about three-quarters of the way to overloading the control gyroscopes. This would have automatically activated rocket thrusters to restore the station’s orientation.
Analysis of the station’s motion during the most recent spacewalk in early July showed similar peaks, though not quite as severe. Some of the wavering could be attributed to slow flexing of long solar arrays, but there were other effects whose severity depended on whether the spacewalkers were near the station’s center of mass, or far from it. Another force seemed related to gas leaking from the open air lock and bouncing off nearby solar panels.
The mathematics were compelling — a force of only a few newtons would have been enough to induce such motions.
Space engineers dubbed the effect “phantom torque,” and last month they realized that the problem had to be solved before this week's spacewalk could proceed. That was because the normal process of reacting to a torque that exceeds the control capability of the gyroscopes is to switch on rocket thrusters. But this spacewalk required the work to be done at the same end of the station as those thrusters. If the thrusters fired while anybody was nearby, it could be fatal.
At a safety briefing early in July, guidance engineers debated what to do about the mysterious force.
“There is a high probability that a disturbance will be seen” during the spacewalk, the engineers warned — correctly, as it turned out. They also said there was “a possibility that the disturbance could result in loss of attitude control within a few hours.” And that was exactly what happened.
The engineers expressed “little confidence” that procedural workarounds could be put into place soon enough to mitigate the disturbances. This was true whether the force was caused by air leaking from the Russian air lock, or by water spraying out the backpacks of the Russian spacesuits.
Consequently, Mission Control operators developed a procedure for use if the gyroscopes became overloaded. NASA officials were unable to provide MSNBC.com with a written copy of the procedure, but it was described in interviews.
With the gyroscopes “saturated” and the thrusters inhibited for the safety of the nearby spacewalkers, the station would very slowly drift out of its orientation relative to the sun. Since this would reduce the amount of electricity generated by the solar power panels, the station would automatically begin a “power load shed” process to shut down non-essential equipment.
At the same time, according to the contingency plan, the crew would be radioed to evacuate the end of the Russian module where the thrusters were located, and wait in a safe place while the thrusters were activated to restore the station's proper orientation. Once the thrusters were disarmed again, the men would return to the work area.
As expected, the “phantom torque” cropped up during Tuesday's spacewalk, especially when the two men were working side-by-side at the very far end of the station. Only two hours through a planned six-hour spacewalk, the disturbances grew too strong for the gyroscopes to counteract.
The “load shed” procedure kicked in, automated by the station’s control computers. With only a two-man station crew, there was no third crew member inside to override or modify the procedure — and it turned out it needed to be modified.
Speaking on condition of anonymity, space workers said one of the pieces of equipment shut off by the computer was the radio transmitter that was supposed to be used to warn the crew away from the station's thruster section. NASA spokeswoman Kylie Moritz in Houston confirmed that the transmitter’s loss was “unexpected” and that “it happened in the load shed.”
Backup to a backup
In such a situation, the Russian space program's tracking network can serve as a backup system. Unfortunately, the tracking sites are located only within Russia, and are available for no more than 10 to 12 minutes every hour and a half.
This time, the spacewalkers were lucky, since the communications disruption occurred about half an hour before the next pass over Russia. Under less fortunate circumstances, the gap could have lasted an hour or more, during which the station would have drifted farther and farther off course.
Once the Russians radioed a warning to the crew, Padalka and Fincke hustled 50 feet (15 meters) along the Russian module and hunkered down out of the exhaust plumes of the rocket thrusters.
NASA officials did not have information on how far out of alignment the station had drifted during this period, but it was quickly realigned and the spacewalk activities resumed.
Padalka and Fincke have one more spacewalk to make before they return to Earth in mid-October. Because that outing will also require Russian spacesuits and will involve more work near one end of the station, the “phantom force” can be expected to return. But with experience, the space station team should be able to hone its avoidance strategies and countermeasures —and keep one more phantom at bay.