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Vigorous vibrations caught on video during orbital reboost last month

HOUSTON – A faulty rocket command sequence aboard the international space station caused the 300-ton structure to shake back and forth vigorously for two minutes last month, during what was supposed to be a routine, gentle orbital adjustment. Space experts in Houston and Moscow have spent the last two weeks searching for the cause of the shaking and doing a damage assessment.

Under the worst-case scenario, such vibrations could rattle the station so much over the long term that the structure might begin to crack and leak. One of the solar arrays might bend out of position, affecting the station’s power-generating system. Experts cautioned that it was too early to determine how likely or unlikely these scenarios might be.

Partly as a result of last month’s anomaly, a second rocket burn originally scheduled for Wednesday has been canceled. Since the maneuvers had been intended to line up the station’s path to receive future visiting spacecraft, the dates of those missions are now in doubt. A Russian Soyuz visit in late March already has been postponed a day.

The original rocket firing on Jan. 14 involved two 440-pound (200-kilogram) thrust engines attached to the station’s Russian-built Zvezda service module at the trailing end of the station. The engines fired for 142 seconds under autopilot control to raise the station’s orbit slightly. The purpose was to shift the station’s ground track into a proper path to facilitate rendezvous with future visiting spacecraft.

In the past, such firings had given the station a gentle push, enough to send free-floating objects adrift toward the cabin’s back end in 30 or 40 seconds. Reboosts also have been performed with smaller thrusters aboard Progress cargo ships, as well as with the small maneuvering jets on docked space shuttles. Those firings are usually so gentle that crews don’t notice – they even can sleep through them.

But during the Jan. 14 firing, something went seriously awry. The station’s solar power wings began swaying back and forth alarmingly. More dramatically, an interior camera captured views of wall-mounted equipment and cables flopping back and forth to a two-second beat, as the camera itself swayed on its mounting bracket.

Buildup of gyrations

It was quickly apparent that some periodic force had excited the space station’s structure at one of its resonant frequencies, leading to a buildup of gyrations rather than a damping down. As with the traditional “soldiers marching across a bridge” story, and the all-too-real Tacoma Narrows bridge collapse in 1940, resonance buildup in a large structure can quickly lead to serious consequences.

Kelly Humphries, a spokesman at the Johnson Space Center in Houston, told msnbc.com in an e-mail that the space station’s structure “has been cleared for tomorrow’s burn, but the burn has been canceled.” Other sources told msnbc.com that the clearance was granted only for use of the much gentler small engines on a docked Progress. Use of the larger engines, and their autopilot control software, remains suspended. The sources spoke on condition of anonymity because they were not authorized to discuss the limitation publicly.

Humphries said that the latest burn cancellation was a good idea for other reasons. “This will help us conserve propellant and set up better for the 2JA mission [a shuttle flight scheduled for launch on May 15], where we were trying to deal with too much altitude,” he wrote.

These are real benefits, sources told msnbc.com, but they would never have caused the cancellation of the burn if the original overshaking hadn’t occurred.

Delayed reports

The shaking was not discussed publicly when it happened, but 10 days afterward, the anomaly was described as “higher-than-usual structural oscillations” in one of NASA’s routine station status reports. The report said the next reboost would be put on “temporary hold until results … have been reviewed in depth”.

Two days later, another status report said the Russians determined that the shaking was brought on after “an error in parameter settings” was uploaded to the engine control system for the Zvezda module.

The Russian engines are hinge-mounted to allow a small variation in thrust pointing direction, and somehow the autopilot began swinging the engine up and down seeking the “sweet spot” for thrusting, but always overshooting and repeatedly reversing direction. By unfortunate coincidence, that back-and-forth swinging had a period close to the station’s resonant frequency. So, like a child pushing periodically on the back of her swing-mounted playmate, the swings became wilder and wilder.

How bad was the shaking?

The thrashing of the long solar array wings reached a degree “five times greater than allowed,” one source told msnbc.com in an e-mail. Another source said the swaying was the “biggest I’ve ever seen… Outside the certified limits of ISS structure…”

The extent of the damage or structural weakening induced by the shaking is “very much still in work,” a third source said in an e-mail. The attach points of the long booms, and the docking interfaces between pressurized modules, are particularly susceptible to accumulated flex-induced fatigue. In the latter case, loss of pressure integrity is a potential consequence.

This source said two “million-dollar questions” had to be answered soon. First, data gathered from accelerometers and stress sensors on the structure must be analyzed to see how bad the overloads were. Second, why were the Russian commands for the automated rocket burn approved without adequate simulation and testing?

A review of such issues has been going on for the past couple of weeks without any breaks, sources said.

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