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NASA will attempt what has been described as a 鈥渞isky鈥� fix for the Hubble Space Telescope after several weeks of troubleshooting following an unexpected shutdown.

Spacecraft tend to use tried-and-tested technology and some of 贬耻产产濒别鈥檚 computer systems date back to the 1970s. The telescope鈥檚 payload computer is a custom-designed NASA Standard Spacecraft Computer-1 developed in 1974. This machine stopped communicating with the telescope鈥檚 main computer and caused a 鈥渟afe mode鈥� shutdown on 13th June.

Since then, NASA engineers have been conducting tests and switching between main systems and their redundant backups. What was initially suspected to be a memory problem with the payload computer is now thought to be a symptom of a power supply failure or an issue with the command unit that is the heart of the telescope鈥檚 control system.

Engineers are only able to issue commands via radio link as the telescope orbits about 500 kilometres above Earth. Now that the Space Shuttle has been decommissioned, there is no way to replace broken components of the telescope.

The payload computer, power supply and command unit sit within a device known as the Science Instrument Command and Data Handling (SI C&DH) unit, which controls and synchronises all the experiments on board and communicates with Earth. One of the two original SI C&DH units fitted to Hubble 鈥� the 鈥淎 side鈥�, as NASA calls it 鈥� failed in 2008 and was replaced during a Space Shuttle mission. Since then, Hubble has been running on the original backup unit 鈥� the 鈥淏 side鈥�.

at NASA says that engineers have been switching in bits of the A side replacement to work out which part of the B side has failed, but they haven鈥檛 found the culprit. Now they will switch in many more components of the A side systems simultaneously in an attempt to finally divert around the broken component. It is a 鈥渞isky鈥� move, he says, because the current A side has never been turned on in space.

鈥淭he only things we can try are things that can be commanded. You can鈥檛 actually put your hands on and change hardware or take a voltage, so that does make it very challenging,鈥� says Hertz.

There are long delays between each attempted fix because engineers have to go through each plan with a fine-toothed comb to check that no upgrade or change over 贬耻产产濒别鈥檚 decades of operation will cause problems. When they have tested the plan on an exact duplicate of Hubble on the ground, it must be approved by NASA management before it can be tried for real.

Hubble was launched in 1990 at a cost of $4.7 billion and has led to a series of discoveries that helped determine the rate of expansion of the universe. More than 15,000 scientific papers have been published based on Hubble data.

Famously, once in orbit, it was discovered that the telescope鈥檚 mirror had been ground into the wrong shape, resulting in distortion. A Space Shuttle servicing mission three years after the telescope was launched resolved the issue.

By 1999, four of 贬耻产产濒别鈥檚 six gyroscopes had failed, leaving it unable to perform any science. Another Space Shuttle mission replaced all six gyroscopes and put it back into service.

Hertz says he is confident that NASA can fix this latest problem too, but admits that there are no guarantees. Even if successful, it will only be a matter of time until Hubble goes offline for good. It is hoped that it might continue to function for at least another five years and perhaps as many as 15 years, allowing a great deal more science to be done.

鈥淓ventually everything breaks. The second one of some redundant system will fail, whether that鈥檚 the last gyro or a second power unit or second computer,鈥� says Hertz. 鈥淭hat鈥檚 just random. It鈥檚 like which light bulb in your house is going to burn out first.鈥�