At the end of the year, shuttle astronauts are expected to undertake the most complex series of space walks to date, when they attempt to fix the Hubble space telescope. Already they have started a new training programme aimed at refining the difficult art of the space walk. Beyond Hubble, the US space agency, Nasa, is preparing for the immensely more difficult space walks that will be needed to build its Space Station Freedom (if the US government can find the estimated dollars 30bn it will cost).
Walking in space - or extravehicular activity (EVA), as Nasa calls it - is fraught with difficulties. The astronauts are placing themselves in a vacuum with only their space suits as protection. A tiny puncture in the protective clothing - from a micro- meteorite the size of a pebble, for instance - would cause a space walker to explode. Lack of gravity makes manoeuvring themselves and other objects difficult and hazardous, and temperatures outside the spacecraft can range from 100C below freezing to 100C or more above. On top of this, the air supply and battery power available severely limit the time they can spend outside.
At the start of the year, shuttle astronauts began a new programme of research into EVA which Nasa scientists say will help them to develop the 'third phase' of space walking. James Hartsfield, a spokesman at Nasa's Johnson Space Centre, says the first phase in the shuttle programme was to prepare for emergency space walks, such as going outside to shut the cargo-bay doors by hand if the automatic mechanism failed. The next phase was space walks essential to the success of the mission: for instance, to retrieve a satellite by hand. The third phase is to carry out complex tasks involving two or more people that can be done by no other means, such as mending the Hubble telescope or building the space station from prefabricated structures.
Astronauts taking part in a series of test walks which began this year will repeat the procedure on their return to Earth, in the large water tanks where they train for weightlessness, Mr Hartsfield says. 'Within a week after the flight they'll go through the same procedures they performed on the flight. It's fresh on the mind what the exact differences are between that training environment and the real thing.' The aim is for experienced astronauts to pass on what they know by describing these differences in minute detail.
'We've noticed there is a difference between the very best imitation of space walking here on Earth - underwater in a swimming-pool where you are neutrally buoyant - and being in space. Sometimes the drag of the water can be a hindrance or a help. In the tank you still have the sense of up and down - your inner ear still works. Even though you are neutrally buoyant, gravity is still working. In space, it's not.'
One of the most difficult aspects of a space walk is coming to terms with weightlessness, especially when handling massive objects that on Earth would weigh a few hundred kilograms. 'In space everything has no weight. So even a five-ton object is as light as a feather. You would think it would be very easy to move it around and do your job. But the problem in space is that for every action there is an equal and opposite reaction,' Mr Hartsfield says. 'So just a simple thing like turning a bolt causes problems. On Earth, gravity keeps you in place. In space, you'll turn just as easily as the bolt. It makes it extremely difficult, because you have to make sure you are restrained - and in the correct way.'
Kathryn Thornton, one of the shuttle astronauts who will take part in the mission planned for this December to repair the Hubble telescope, recalls the difficulties she experienced on previous space walks. 'It takes much longer to work up there than it does down here, and one of the reasons is that you have to be very careful in controlling your body attitude. Here, if I was moving hand over hand in the swimming- pool and wanted to stop, I'd stop, because the drag of the water keeps my rear end behind my front end. Up there, if you stop, you end up with your feet going over your head.'
In space, Dr Thornton explains, linear momentum gets converted into angular motion and 'you end up with your feet somewhere where you don't want them to be.' This is why space walkers now attach their feet, whenever they can, to foot restraints on the shuttle which allow them to move their hands freely.
Dr Thornton flew on the shuttle mission which, in May 1992, ended in high drama more than 200 miles above the Earth when the crew tried to capture an Intelsat communications satellite that had failed to reach its correct orbit. Three other members of the crew spent more than eight hours trying to seize the four- ton satellite. After grabbing it by hand, they fitted a special capture bar to its base, in order to attach it to the shuttle and fit a booster rocket.
For Nasa's supporters, it was proof that it is necessary to have humans ready to walk in space: 'What we saw is the reason why robots can never replace men and women in space,' Robert Walker, a US Congressman, said after the Intelsat success. 'Robots can only do that which we tell them they can do before they leave Earth.'
Although the shuttle has a robot arm, which is manipulated from a control console in the spacecraft's cockpit, its usefulness is fairly limited. The arm - which has 'shoulder', 'elbow', and 'wrist' joints like a human arm - gives astronauts outside the spacecraft extra manoeuvrability. They can even attach a foot restraint to the end of the robot arm and be carried near to the place where they are working by a colleague in the cockpit.
For all the sophistication of the robot arm, however, it lacks the sensitivity of the human equivalent, especially in the delicate movements of the hand. It is the hand, however, that is one of the first parts of the body to suffer during a working EVA. This is because wearing a pressurised suit is like being inside an inflated balloon. As astronauts clench their fists when using tools, they are effectively pushing against the pressure of their suits.
'It's like squeezing on a balloon. Your hand becomes very tired,' says James Hartsfield. Nasa has recently added a hinge running across the palm of the spacesuit's gloves to deflect this pressure, and so help prevent muscle fatigue in the hand.
For Kathryn Thornton and other space-walking astronauts of the future, such technical improvements will help when they are working on complex instruments 300 miles above ground, and travelling at about 17,000mph. She likens the experience to working upside down on a car while wearing ski mittens; most tasks can be done, but they take time. 'We want to leave the satellite in a much better condition than we found it in,' says Dr Thornton, 'and to do that we have to do everything very carefully and slowly.'-
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