How would you shape up for take-off?

Space? It's exactly the place to go if you enjoy feeling sick, constipated and bloated, with a thumping headache to boot. And if you're prone to anaemia, asthma, high blood pressure, osteoporosis or claustrophobia, your personal space flight might prove a touch troubling.

Then again, the views are unbeatable. "Absolutely glorious," says Helen Sharman, the first British astronaut, who spent eight days in orbit in May 1991. "At the height of the Shuttle or Mir you can't see all of Earth at once, but you can see, say, western Europe, and after a while you can pick things out - you say `That's Madagascar, you can tell because of the river systems.'

"And the colours are superb. They're more intense than anything I've ever seen."

But those plus points have to be weighed against the physical negatives. Anybody who seriously wanted a flight in space would have to pass a thorough medical test to assess their fitness - external, internal and mental.

The first problem that the body has to face is the strain of lift-off. Most rocket systems nowadays would only impose a load of 3G - making you feel that your weight had tripled. You can experience that on many funfair rides, though only briefly. On a space flight, the strain would last up to a minute. Anyone liable to a heart attack would have it at this point.

But soon afterwards, the spacecraft would reach orbit - which is where the fun begins. In the weightless environment, the balance system in the ears becomes confused about which way is up - rather like being very drunk. The results are often similar: "About half of all astronauts are spacesick," says Sharman, who was one of the other, lucky half. "It last for a few days, but then you get used to it."

The next effect of weightlessness is that fluid migrates from the legs (where gravity usually keeps it) to the chest and head. "You start looking wonderfully young - no wrinkles, nice thin legs - but then every cell in your head gets pushed full of fluid," says Sharman. "If you have any tendency to asthma it could inhibit your breathing." While this is going on, your digestive system would be struggling to do its work in the absence of the usual downward pull. For those without well-trained colons, constipation is a likely bedfellow.

But after the bloating comes the thinning down. Within a couple of days, the body recognises that it is retaining too much fluid, and begins excreting up to two litres a day - but that also includes useful minerals such as calcium (from the bones) and potassium (useful for regulating blood and nerve efficiency). And meanwhile the muscles, without the usual downward pull, lose tone: anyone who is more than a few per cent over the average weight for their height would not be able to stand without assistance on returning to Earth.

However, many people might think those are small, and temporary, penalties to pay for a view that takes you closer to the rest of the Universe. On the Moon - a trip that would take a few days each way - you would be able to look at Neil Armstrong's footprints, pose by the spring-loaded flag, perhaps hitch a ride on the Moon buggy. And how could you put a price on seeing the Earth rise over the Moon's horizon?