Space station loses orbit-boosting options

[davemania]

Several failures on the International Space Station in recent weeks have left the orbiting outpost with fewer altitude-boosting options. The failures leave the station slightly more vulnerable to being hit by a piece of space debris, although it is a remote possibility.

The station has to move to avoid space debris about once a year on average, explains Johnson Space Center spokeswoman Kylie Clem. It is done whenever there is a 1 in 10,000 chance of an object hitting the station or the area immediately around it - to within 37 metres.

The problems began on 19 April 2006, when the Russian Zvezda service module's main engines failed during a test. The failure may have been due to a sunshade cover that was not completely open, according to a station status report.

It was the first engine test since 2000, when Zvezda first docked to the fledgling station. The service module main engines are not planned to be used often because they cannot be replaced, unlike the Progress ships, which periodically dock to deliver station supplies.

Then on 4 May, the Progress ship docked to the station fired its engines for 6.5 minutes to boost the station's orbit by 2.7 kilometres. Such manoeuvres are periodically necessary because the station's orbit degrades over time.

Remote control

But after the thruster firing, the crew inside the station received an error message, saying that the station software was not properly communicating with the Progress hardware. Mission managers are still trying to sort out what is wrong and determine how to fix it. Until that happens, Progress firings controlled from the space station will no longer be an option. "It's a layer of redundancy that isn't there now," says Clem.

NASA says that Russian ground controllers could still command the thrusters to fire remotely. However, those commands can only be sent when the ISS is in contact with Russian ground stations and it is out of range for six out of its 16 orbits every day.

In the meantime, the Zvezda service module thrusters ? which are distinct from its now-problematic main engines ? could still be used to change the station's altitude. Such a manoeuvre would be necessary if flight controllers determined that a piece of debris was about to pass near the station.

The ISS could potentially be without a way to dodge fast-flying space debris if the service module thrusters fail during a period when the Progress thrusters were beyond Russia's ground control. But the chance of such a failure is remote because NASA typically detects potentially dangerous objects well in advance of a manoeuvre, Clem says.

http://www.newscientistspace.com/article/d...ng-options.html