BioEd Online

All seems normal on the International Space Station today, as the space shuttle Atlantis prepares to return to Earth, leaving the usual allotment of astronauts behind to mind the station. But engineers are still working hard to understand the cause of a computer glitch that for some 48 hours or so put the station in what was arguably the greatest peril it has ever faced.

All six Russian navigational computers responsible for maintaining the station's position or 'attitude' in the sky inexplicably crashed on 7 June, around the time that the station's new solar panel was being attached by shuttle astronauts. The computers refused to restart until the next day, when Russian engineers at ground-control tracked the problem to a set of power switches designed to protect the computers from surges in the station's electrical grid. The astronauts bypassed the switches and the station appeared to be fully operational as of Monday 11 June.

But what caused the switches to fail in the first place? That's the question now facing NASA and Russian engineers, according to Brandi Dean, a spokesperson at the Johnson Space Center in Houston, Texas. Dean says that preliminary analysis shows that electrical 'noise' on the station's power grid may have been responsible for the failure.

Static build-up

The source of noise is still under review, but one possibility is static charge build-up on the station itself or the new solar panel.

The layer of atmosphere in which the station lives is filled with free-wheeling charged particles, including electrons. These can build up on the surface of the station, creating a net voltage. "Charges build up on spacecraft all the time," says James Oberg, a consultant and former NASA mission-control chief now based in Houston, Texas. The International Space Station has a pair of special devices that are designed to 'ground' it by sending a small current of xenon atoms into space.

But the new solar panel may have been vulnerable, especially if it was ungrounded and in the shadow of the station, according to Keith Holbert, an electrical engineer at Arizona State University in Tempe, who specializes in spacecraft charging. Sunlight can kick charged particles off the surface of a spacecraft. In the shade, charge could have built up over the multi-day mission to attach the panels.

Dean, however, declined to comment on whether static charge was indeed the culprit. More time is needed to understand how the addition of the new array affected the spacecraft's systems, she says.

Abandon ship

When the computers were down, the station relied on the thrusters of the docked space shuttle to perform emergency adjustments to its orbit. Had the problem persisted past the point when the shuttle had to return home, then astronauts would have been forced to abandon ship and leave the station unmanned for the first time since it was staffed in 2000.

Like an ocean-going ship, the ISS requires constant maintenance — leaving it uncrewed for a length of time could prove disastrous. Studies have shown a 50% chance that the station would be destroyed within a year if there were no crew on board to handle problems such as a micrometeorite puncture.

The station previously faced the possibility of disaster when its oxygen-generating equipment shut down for unknown reasons in 2004. Investigations have previously revealed weaknesses in the station's hardware and software, including bugs in the computer code that runs its operations.