Ongoing analysis of the trajectory of a piece of space junk that was believed to pose a possible threat to the International Space Station showed the debris would not pass close enough to the lab complex to force the crew to seek refuge in their Soyuz lifeboats, a NASA official said late Friday.
An agency spokesman said the station's six-member crew would be awakened early, at 10 p.m. EST as planned, but the astronauts would be told to go back to bed and not to press ahead with a tentative plan to shelter in place aboard the station's Soyuz ferry craft.
Earlier Friday, NASA flight controllers predicted the debris, of unknown origin, could pass within about six-tenths of a mile of the space station at 10:48 p.m., toward the end of the crew's normal sleep period. During the evening planning conference Friday afternoon, the astronauts were told to plan on getting up early so they could make their way to the Soyuz lifeboats by around 10:30 p.m. if necessary.
"The ballistics are saying they are looking at conjunction with space debris," Russian mission control radioed. "As you know, this is something we are prepared for. In the past, we have performed avoidance maneuvers, but this time maneuvering away from the path of the debris is not an option.
"Because we cannot perform avoidance maneuver, you will have to ingress Soyuz vehicles. Both Soyuz crews should be in their vehicles. This is what we have. We are going to work on the ballistics data to get greater precision, but right now we are in the red box. The probability of collision is non zero."
NASA flight controllers told the astronauts the tracking data was uncertain and that engineers did not yet have confidence in the trajectory projections. Pending additional analysis later in the afternoon, the crew was told to play it safe and plan on boarding the Soyuz lifeboats after shutting internal hatches in the U.S. segment of the lab complex.
After additional analysis, however, flight controllers concluded the unidentified debris would not pose a threat to the station, according to a NASA spokesman.
Last March, the station's three-man crew - Mike Fincke, Yury Lonchakov and Sandra Magnus - faced a similar situation and briefly took refuge in the lab's single Soyuz ferry craft when another piece of debris from an old rocket motor made a close approach.
There are more than 18,000 pieces of space junk in low-Earth orbit the size of a baseball and larger. U.S. Strategic Command prioritizes radar tracking to protect manned spacecraft first, followed by high-priority military and civilian payloads.
NASA monitors an imaginary volume around the space station roughly the shape of a pizza box measuring 0.466 miles thick and 15.5 miles square.
"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 station Flight Director Ron Spencer said in September.
"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. Then we calculate a probability of collision based on the data Space Command gives us."
Spencer said NASA has two levels of concern.
"We have two thresholds, yellow and red," he wrote in an email exchange. "The yellow is 1-in-100,000 and the red is 1-in-10,000. We will not take any action if it is below the yellow threshold. If it between the yellow and red, we will only take action if it is easy to do so without impacting the mission. For a red threshold violation we will take action in most cases."
Updated at 6:45 p.m. EST: NASA officials say analysis shows the space debris in question poses no serious threat to the International Space Station.