Some space scientists believe manned missions are stunts that place astronauts in unnecessary danger. But the task for the human-factors scientists and psychologists at Nasa's Ames Research Center near San Francisco is to bring comfort to those places where space sceptics believe humans should not go.
They have to make the best of a bad job. Often a human-factors scientist's wish is an engineer's headache. The former wants more space for humans, for example, but the latter wants to minimise the volume of the spacecraft for fuel economy and structural reasons.
Nevertheless, an unhappy astronaut could wreck a mission as effectively as a mechanical fault. Writing in the Journal of the British Interplanetary Society, Yvonne Clearwater, of Nasa Ames, and Albert Harrison, a Nasa consultant and professor of psychology at the University of California at Davis, warn: 'Human factors are not simply a nuisance that can be ignored until take-off.'
Ms Clearwater and Professor Harrison permit themselves only mild equivocation on the merits of manned space exploration. Humans will go to Mars, they say, 'if for no other reason than the fact that they are able to do so'. They continue, in a defensive tone: 'Far from being a 'weak link' in the Mars mission, humans are the most valuable asset.'
There are some good reasons for putting men and women into space. Human creativity can occasionally interpret data and solve problems in ways that machines cannot; and there are examples of unmanned missions that have gone wrong when they could have been put right with the human touch.
But the unspoken justification for sending astronauts to Mars must be Nasa's dependence on the American taxpayer. Manned missions - and landings - make great television, ensuring the public's goodwill and underwriting future research.
George Bush's appointment of Daniel Goldin as Nasa administrator in April this year, and the President's call for a manned programme to establish a lunar outpost by 2010, have focused attention on distant horizons. The lunar base would be an important stepping-stone to Mars.
The emphasis of research at Nasa Ames has shifted, too: 'Most of us have moved on from space-station work, because it's such a political football, to lunar and Mars habitats,' says Ms Clearwater.
Nowhere is the human-factors conflict clearer than over the question of windows. 'In spacecraft,' says Professor Harrison, 'windows are points of weakness. But the occupants gain a lot.' On a Mars mission it may prove impossible to provide real windows. Astronauts may have to make do with high-definition television images relayed by an exterior camera.
The Mars mission accentuates concerns that affect any space environment, because of the duration of the voyage and the remoteness of the destination. One month into the journey, the astronauts will have seen Earth shrink to a dot. The increasing delay in signal transmission means that interactive video links with family and colleagues will soon cease. How will the astronauts react?
Ergonomic studies from oil rigs, submarines and Antarctic bases are of some use in creating a suitable physical environment for astronauts, but videotape records and debriefing interviews from past missions provide the most valuable information. Both the space station, intended to be built by the end of the decade, and the lunar base would provide additional data for conditions that are hard to simulate on Earth.
As if the design problems for space flight were not great enough, Nasa has determined that because of the likely international make-up of future mission personnel, spacecraft must be designed to fit 95 per cent of people, male and female, from all parts of the world. And in the weightless conditions of space people grow by several centimetres and change shape, with ballooning torsos and shrinking legs.
Psychological needs must be taken into account as well. Privacy is a major concern for long missions. 'Given the stresses, a place to retreat is of some value,' says Professor Harrison. 'Of course, one would like to create as much real space as possible. But one can create the impression by various design techniques that the spaces aren't quite as cramped as they are.'
Darker colours on the spacecraft's 'floor' provide a sense of up and down, important for orientation in weightless conditions. Pictures on the 'walls' are beneficial. Recreation is important, not only to maintain muscles that tend to deteriorate under weightlessness, but also to combat tedium. Conventional diversions will not be enough during the long months, however - escapism will be vital. According to Professor Harrison, the use of drugs and alcohol have been officially ruled out - they could impair clear thinking in a crisis. Moreover, it is felt that US taxpayers would not condone them.
More acceptable is the use of virtual reality apparatus to create fantasy environments. A video headset combined with an exercise bike would allow an astronaut to cycle through a virtual landscape, exploring highways and byways. Having slipped the surly bonds of Earth, it seems, an astronaut's greatest wish might be to slip right back again.
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