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Major Incident Occurs On ISS

February 4, 2009

NASA has spent two weeks searching for the cause behind turbulence felt on the international space station during a routine orbital adjustment.

According to experts in Houston and Moscow, the ISS shook back and forth for two minutes during a routine adjustment last month.  They are still searching for the cause.

Experts caution that it is too early to determine how much damage occurred during the incident, but in a worst-case scenario the ISS could have structural damage that could affect the station’s power-generating system.

The incident has delayed a rocket burn scheduled for Wednesday intended to prepare the ISS to receive visiting spacecraft, and has left the dates of the visiting spacecraft missions in doubt.

The original rocket command sequence that occurred on Jan. 14 involved two thrust engines on the station’s Russian-built Zvezda service module.  The adjustment was intended to raise the station’s orbit.

Previously, these firings had gently adjusted the ISS.  Other reboosts performed by thrusters aboard Progress cargo ships have been so gentle that crews have been able to sleep through them.

During the Jan. 14 adjustment, something went drastically wrong.  A camera mounted on the interior of the ISS captured images of equipment moving back and forth, and the station’s solar power wings swayed vigorously.

Researchers believe a periodic force, such as the buildup of the station’s resonant frequencies could have caused the shaking.  Resonant frequencies have been known to destroy structures, most notably the 1940 collapse of the Tacoma Narrows Bridge.

The incident was not reported until 10 days after its occurrence, and was described as “higher-than-usual structural oscillations” in a NASA station status report.

The report also said that the next reboost would be put on hold until the incident had been reviewed in full.

A Russian report, issued two days later, determined that the turbulence was caused by “an error in the parameter settings” in the engine control system.

The Russian engines, which are hinge-mounted, began swinging the engine up and down searching for the best thrusting position, but continually overshot and reversed direction.  The motion had a period dangerously close to the station’s resonant frequency.

One source from NASA told msnbc.com that the solar wings swung to a degree “five times greater than allowed.” 

According to another source, the extent of the damage caused by the incident is still being accessed. 

To major questions still need to be addressed.  First, researchers must analyze data from stress sensors to see how bad overloads were.  Secondly, they need to determine how the inaccurate Russian commands for the burn were approved.

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