NASA canceled a re-boost of the International Space Station scheduled for tomorrow (Feb. 4) and is investigating whether engine firings last month to put the orbital outpost into a higher orbit caused any structural damage that could possibly reduce the station’s useful life. On January 14 the service module (Zvezda) main engines on the station were fired for 2 minutes and 22.4 seconds. The engines cut off abruptly instead of gradually, causing higher than usual structural oscillations. The extra vibrations may have caused damage to the station that could affect its longevity, said NASA spokesman Kelly Humphries in an article in USA Today. “Anytime you impart a vibration to the station it has potential implications” for the station’s solar panels and the connections between the station’s parts, Humphries said.
The cancelled reboost was supposed to bring the station into a better position for the arrival of the Progress M-66 re-supply vehicle, which is scheduled to launch on February 9 and dock on February 13. According to NASA, the decision to cancel the reboost will not affect the Progress flight or the STS-119 shuttle mission, which is scheduled for launch on February 12.
Reportedly, after the re-boost abruptly ended the station’s solar arrays began swaying back and forth, and an interior camera showed views of equipment and cables flopping back and forth.
NASA’s on-orbit daily status reports stated on January 29 that “As of now, evaluation of the external video survey conducted over the last weekend and a review of subsystem data have not shown any off-nominal results” from the higher oscillations. The station was built with extra structural strength, Humphries said, and the current analysis is “just making sure we haven’t eaten into that margin.” Engineers also want to make sure the abrupt stop by the engines won’t occur again.
A Janaury 26 report said that engineers in Moscow “reported that the root cause of the observed strong structural oscillations was an error in parameter settings uploaded to the service module (SM) engine gimballing control system, which then caused a malfunction of a dynamic (frequency) control filter. Both the MCS (Motion Control System) and the filter itself are continuing to function properly. Corrective measures are underway.”
The mean altitude increase was for the January reboost was 5.36 km (2.89 nmi).
The rockets on the station are also used to move the station out of the path of any oncoming debris that could puncture its exterior shield. Humphries said he did not know whether such maneuvers would be performed given the current uncertainty over the rockets’ performance.
The station also needs another reboost in March so it can receive the Russian Soyuz TMA 14 spacecraft carrying two new residents and space tourist Charles Simonyi to the ISS.
Later in 2009, the crew size for the ISS is supposed to increase from three to six to allow more science activities, finally bringing the station to its full potential.
NASA is hoping no structural damage occured that could possibly shorten the life of the station. However, how long the station will be in operation is unclear. NASA has no firm plans to make use of the orbiting laboratory after 2015. Many of the other 13 nations that helped build and operate the outpost want to keep it going until at least 2020.