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.
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".
Planetary Nebula NGC 2818, Hubble Space Telescope
Month in Space
See the fireworks of a planetary nebula, a fresh crater on Mars and other cosmic highlights from January.
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…"