The six-member crew of the International Space Station took shelter in two Russian Soyuz spacecraft early Tuesday because of a predicted close approach by an unknown piece of space debris. Radar tracking indicated the debris could pass within about 820 feet of the space station at 8:08 a.m. EDT (GMT-4), but no impact was detected and the crew was told to resume normal operations.
"We are currently at TCA plus four minutes and you are clear to egress Soyuz," Lindgren radioed the crew from mission control in Houston at 8:12 a.m., four minutes after the time of closest approach.
"Copy that. Thank you," replied Expedition 28 flight engineer.
Safety procedures are put into effect when radar tracking indicates debris could pass within an imaginary box around the space station that takes into account tracking errors to provide a margin of safety. "Sheltering in place" aboard the Soyuz crew ferry craft is required when notification of a possible debris "conjunction" occurs too late to orchestrate a space station maneuver to get out of the way.
"We have a fourth update from (tracking) and the probabilities are still in the red threshold and we are still planning to have you shelter in place," a flight controller radioed from Houston a few minutes before 7:30 a.m. EDT (GMT-4). The time of closest approach is still 12:08 GMT (8:08 a.m. EDT)."
Station commander Andrey Borisenko, Alexander Samokutyaev and Ronald Garan took shelter aboard the Soyuz TMA-21 spacecraft docked to the Poisk module. Sergei Volkov, Michael Fossum and Furukawa sheltered aboard the Soyuz TMA-02M spacecraft docked to the Rassvet module.
The size and source of the debris were not immediately known.
Space debris is an ongoing concern for space station crews because of the extreme velocities of objects in low-Earth orbit -- about five miles per second.
"The way we do all of this is we get tracking data from Space Command on the objects that are a threat to the space station," space station flight director Ron Spencer said before a debris event in 2009. "Initially, we have a screening box, which is .75 kilometers radial miss, which would be up or down, by 25 kilometers in cross track, which would be left or right, by 25 kilometers down track, which is either in front or behind us.
"Space Command will alert us of any debris objects out there that are going to get that close to us. Then they increase tasking on those objects to try to get a better solution and decrease the uncertainty. And then we calculate a probability of collision on that, based on the data Space Command gives us, on the object and if the probability of collision is greater than 10 to the minus five, then we will begin to start looking at taking action."