Gravitational Riddle

I N the James Bond film Moonraker, Bond is testing the centrifuge in an astronaut‑training facility. The order is given: ``See that some harm comes to Mr Bond'', and the machine is made to hurl him round at nearly six G‑forces, more than a human being can tolerate. This is an extreme example of artificial gravity, gravity created by centrifugal force, that Nasa and other organisations are desperate to build into a ship carrying people to Mars. For a weightless journey to the Moon that lasts a few days causes no problem to the heath of its astronauts, but one to Mars that lasts many months will cause catastrophic loss of calcium in their bones. "We'll be taking our own air, food, heat and light,'' says a medical specialist. ``So why not take gravity along with us as well?''

The trouble is that nobody yet knows how to do this in a spaceship at minimum cost. It would certainly work if one built a ring‑shaped spacecraft as large as the London Eye and made it spin. There would be gravity of any desired strength in the outer compartments (according to the velocity of spin), and almost zero gravity at the centre.   But this would be expensive; and how very embarrassing it would be if, having built such a huge contraption, it was later discovered that the same effect could have been achieved with a rotating chamber no more than 60 feet wide! This is an engineering mystery to which no one knows the answer. Will the rotating chamber work, or do we need a vast rotating system more than eight times its size? It is not merely a question of how best to travel to Mars. Its answer will tell us how to make long journeys safely to anywhere in space. And so it is a matter that affects our whole future. Yet none of the usual scientific channels of enquiry, no learned paper, no appeals to the mathematics of Newton or Einstein, give any clue to which is the correct answer. And when we turn to the best science fiction, which sometimes solve riddles that have baffled academics, we find ourselves being offered both solutions without any guidance as to which is the correct one. Even the film 2001: A Space Odyssey is ambiguous on the point. Early on the film we see the giant wheel-shaped space station which is surely at least 500 feet across, and designed this way to provide artificial gravity. Then, later in the story, when the astronauts are on their way to Jupiter, they get their gravity from a rotating floor and ceiling. Why this change? If the great wheel wasn't necessary, what was it doing there in the first place? (It might have been there to make good cinema.) In 2010 an attempt will be made to solve this puzzle by experiment. The privately designed and funded spacecraft Mars Gravity Biosatellite will fly for a month spinning mice inside a small centrifuge to recreate conditions in the low gravity of Mars.