ISS Computer Problems NEW YORK -- For the International Space Station, it's a case of live by the sword, die by the sword -- or rather, the computer.
NASA's orbital outpost uses computers to run many of its day-to-day functions, which is why the
failure of the station's three main computers earlier this week was something of a crisis. "We rely on computers to control much, much more of the items on the station than we have in any previous spacecraft," said NASA spokesman Greg Lange at the Johnson Space Center. These computers have replaced the familiar rows of buttons and switches, directly wired to individual systems, that are so prevalent in the space shuttle and other piloted spacecraft.
Network architecture
The ISS actually employs a small army of computers organized in three different levels, or tiers.
The highest, designated Tier 1 -- the "generals" in the army -- consists of three command-and-control computers with 386 processors located in the Destiny laboratory module (which controls the U.S. elements of the station).
These three, in turn, communicate with several Tier 2 computers, each of which controls a specific system aboard the station -- communications, attitude control and so on.
Lowest on the totem pole are the Tier 3 computers, smaller units that run individual devices such as experiments. Computers in all three tiers are linked together via "junction boxes" called buses.
To operate individual systems, the astronauts aboard the ISS use laptops tied into the Tier 1 command-and-control computers that do anything from opening a docking latch to shutting a coolant valve to turning on an experiment.
Mission Control can also issue commands from Earth via the station's telemetry radio, but both the ISS crew and mission controllers lost this ability when all three command-and-control units mysteriously stopped working earlier this week.
"There are problems with the computers accessing the hard drives," Lange said. "They can't pull in data they request from the hard drives, and for some reason it's causing the computers to lock up."
Software problem?
One of the command-and-control computers apparently suffered a hard drive failure; the shutdown of the other two computers is still unexplained.
"We think it's a software problem," adds Lange, who notes that several new software packages were loaded into the Tier 1 computers a few days before the shutdown occurred.
The additions included software for controlling the station's new
Canadarm 2 robotic manipulator arm, as well as other new components delivered by the STS-100 crew aboard the shuttle Endeavour. Mission controllers are now reviewing the added software to look for bugs that may have caused the problem.Fortunately, because the Tier 2 and Tier 3 computers
kept running, many basic functions on the station -- such as the life-support systems -- continued operating. "Those lower-tier computers kept running in the same mode they'd been commanded to [before the shutdown]," says Lange.
However, the failure did make it impossible to send new commands to these lower-tier units. As a result, a planned maneuver to use the shuttle's engines to boost the station's orbit had to be delayed, Lange says, "because we couldn't tell the station's control system to turn off, to let the shuttle control system do the re-boost."
Backup procedures in place
While the failure of all three Tier 1 computers came as a surprise to the astronauts and ground controllers, it was a possibility that station designers had anticipated.
When the shutdown took place, the Tier 2 computers sprang into action as they had been programmed to do in such an emergency and activated a backup control system called Mighty Mouse -- named for the cartoon character whose battle cry is "here I come to save the day!"
Mighty Mouse lived up to its name: It turned the command-and-control computers off, then on again, rebooting them. Subsequently, on Thursday, one of the three disabled machines came back on line.
But mission controllers, anxious to have a backup computer, instructed ISS Expedition Two crew member Susan Helms to remove a hard drive from one of the Tier 2 computers and install it in one of the failed Tier 1 machines.
And as a hedge against future problems, NASA
tried to dispatch a spare hard drive to the station via the Soyuz taxi crew that includes American millionaire Dennis Tito. Unfortunately, the drive was not ready in time for the Soyuz launch and remained behind.A further backup for controlling the station exists in the computers aboard the Russian Zvezda module. However, these computers can only receive commands via Russian ground stations. But the ISS passes over these stations infrequently, making communications more limited.
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