Galaxy guru

With its glorious images of distant stars and colliding galaxies, not to mention the huge amount of ground-breaking science it has unearthed, the Hubble Space Telescope has changed forever how we view the depths of space. But even global icons eventually run out of steam.

For a number of years, the strains of life in the bleak extremities of space have taken their toll on the 'people's telescope'. While many believe that Hubble's time has come and that it should be allowed to 'de-orbit' gracefully,

administrator Michael Griffin has surprisingly granted the venerable old astronomer one last stay of execution.

Hubble's fifth servicing mission, the confusingly titled Servicing Mission Four (SM4) after missions 3A and 3B in 1999 and 2002 respectively, nearly didn't happen. The US Senate had initially baulked at funding an expensive 'patch-up' job to keep Hubble functioning for another 18 months, particularly at a time when large amounts of money were required to re-equip the shuttle fleet for the Return to Space programme after the Columbia disaster.

It was a close call and Martin Barstow, head of

department of physics and astronomy and an

representative on key Hubble committees, is delighted at the decision.

For 10 years Barstow has used Hubble as an astronomer, particularly in his work to further scientific understanding of white dwarf stars. For the past two years he has also had a more involved role in the telescope's future as the UK's only representative on the Space Telescope User's Committee, a NASA advisory body that helps decide on Hubble's future scientific programmes.

This committee played an important part in helping NASA reach its decision to service its ageing stalwart one last time. The $900m (£475m) servicing mission planned for mid-2008 is a reprieve for a piece of equipment that has done more than any other to bring the excitement of space to a worldwide audience.

Although primarily an astronomer, Barstow also has extensive experience with space flight hardware. He has worked with NASA on numerous occasions helping develop instrumentation to be flown on board satellites. He is well aware of the potential difficulties of the mission.

'This will be the most complex servicing mission ever run,' Barstow told

the day after it was announced the mission was to go ahead.

'There are two elements to the servicing mission — replacing or fixing things that are broken but also taking up new technology and instrumentation that will make the telescope even more powerful.'

As well as substituting the six failing gyroscopes on board with a fresh set, SM4 will replace the telescope's two degrading battery modules, which are the originals from its launch in 1990. Two of its three Fine Guidance Sensors are also failing, so a new unit will be installed to replace both of them. Together the sensors and the gyroscopes keep Hubble stable in space. Stainless steel 'blankets' will also be installed on the telescope's exterior for extra thermal protection.

However, according to Barstow, the trickiest aspect of the mission is likely to be the replacement of the Space Telescope Imaging Spectograph (STIS). This was the most important piece of equipment onboard Hubble and produced much of the telescope's most awe-inspiring science, but has not been working since August 2004. To repair it, a spacewalk of unprecedented complexity must be undertaken.

'This will be quite novel as no-one has ever done this kind of repair on Hubble before,' said Barstow. 'Usually what you do is take out the old instrument — it's a bit like disconnecting a bit of your hi-fi — and replace the entire unit. But they haven't done that in this case as there isn't a spare spectograph. There is no time to build one. The only option is to fix the electronics, which will be a real challenge as it was never designed to be played around with in orbit.'

To replace a key electronics board on the STIS, an astronaut — encumbered by a pressurised spacesuit and wearing large padded gloves — will have to carefully remove the instrument's 111 individual screws.

NASA has revealed that it will get the astronauts to watch the pit-changing techniques employed by engineers at the US NASCAR motor-racing circuits to see what they can learn.

Barstow said in previous meetings at NASA's

near Washington, that engineers had developed a robot that could replace the electronics. 'We were working on an assumption that the only way we could repair Hubble was with a robotic mission, but watching the robot remove the screws was like watching paint dry — it was such a slow process,' he said. The decision was finally made to use humans, if it ever went ahead.

'It was a combination of cost and difficulty that brought humans back into the mission,' he added. 'Humans are so flexible and every Hubble servicing mission has had an unexpected element to it so far. It's like if you have a car that you don't drive for 20 years and then try to check it out inside, there will always be an issue. It is very hard to programme that unpredictability into a robotic model — it gets too complex and stretches its capability.'

To aid the astronauts on the five or six planned spacewalks, specially developed tools will have to be used. Wearing thick gloves makes even holding a screwdriver difficult, so all of the 20 tools that may be needed will have to be designed so they are easy to manipulate and locate.

'You can't just stick them on your belt,' said Barstow. 'There will probably be specially-designed racks. There are all sorts of logistical problems involved in missions like this.'

As well as the running repairs, the astronauts will attempt to fit the Hubble with two new instruments: the Wide Field Camera 3 that will help probe the galaxy for evidence of elusive 'dark matter' and the Cosmic Origins Spectograph, which will be the most sensitive ultraviolet spectrograph ever flown on Hubble. However, whether this exciting kit makes it on board will depend on the many variables that affect such complicated missions. Spacewalks might have to be shortened or some of the more complex tasks could overrun.

While Hubble is a joint NASA and ESA mission, the decision on whether it should be serviced is one that NASA had to take unilaterally because it decides whether or not the shuttles can be used.

The decision was not taken lightly. Hubble lies in a different orbit to the

(ISS), and whereas a problem with the shuttle on an ISS mission would mean the astronauts could take refuge on board the ISS until they could be rescued, Hubble provides no such safety net. NASA plans to keep a second shuttle on permanent standby, ready to be launched on a rescue mission if a problem is detected with the original craft.

This is the first time the agency has ever had to prepare two spacecraft and crew simultaneously and it is fraught with complications and logistical issues, said Barstow.

The final component that will be added to Hubble will herald its demise a few years from now. The Soft Capture Mechanism is a device that will allow the Shuttle's successor, Orion (formerly the CEV), to grab hold of the telescope and gently nudge and usher Hubble back into a safe, controlled re-entry through the Earth's atmosphere and its ultimate fate in one of the world's oceans.

It is an ending that those bound up with the telescope's fate — like Barstow and his colleagues — accept, and plans are under way for Hubble's ultimate successor: the James Webb Space Telescope.

'We are at the limit of what Hubble is going to be capable of,' said Barstow. 'The science you can do is dependent on two things — the size of the mirror and the efficiency of the instruments you use. Now the instruments are as efficient as we can make them and so we need bigger mirrors.'

He added: 'This servicing mission will carry us almost through until the James Webb can be launched. It will be a sad day when Hubble finally goes, but all satellite missions ultimately fail, it's the name of the game.

'What we have to remember is that Hubble is the only space mission that has been repairable while in space — that's quite an achievement.'