GO into any park on a summer's day and you will seem them - the frisbee fanatics. Why throwing a saucer-shaped piece of plastic around should be so popular might seem hard to imagine. But frisbee-throwing's a bit like surfing. You have to get it right, or it won't work.

Just as a skilfully piloted surf board will catch and ride a wave, so a spinning frisbee thrown the right way catches the air and flies straight and true. Misjudge the flick of your wrist just slightly, and the disc will wobble precariously before flipping over and falling to earth.

Embarrassingly for scientists, until recently no-one has been able to give a good explanation for why it is so hard to throw a frisbee straight. But now a series of experiments in Britain and the United States have started to shed light on the frisbee phenomenon.

Why should researchers working at the cutting edge of aeronautics and engineering bother to investigate frisbees? The reason is that the toys share a lot of common ground with space technology. No-one's invented a flying saucer (yet), but space craft landing on other planets are deliberately made to spin like a frisbee to provide stability during their descent.

This is far easier than monitoring every aspect of their motion and firing thrusters to correct the slightest wobble.

Britain's Beagle 2 Mars probe was sent into a spin as it left its mothership and prepared to land on the Red Planet. Although the craft was ultimately lost, the separation and ''spin-out'' manoeuvre worked perfectly and Beagle 2 followed precisely the right trajectory for its descent.

Dr Ralph Lorenz, from the Lunar and Planetary Laboratory at the University of Arizona in Tucson, outlined the physics of frisbees in New Scientist magazine. He revealed that frisbees have been around for more than 130 years. The name is derived from William Frisbie, a 19th century American baker from Connecticut whose pie tins, turned upside down, were renowned fliers.

What makes frisbees so good at flying is their shape, a flat disc with a deep, downward-curving lip all round. As the frisbee flies forward, the flow of air around the lip makes it behave like a short, stubby wing. Air passing over the top of the disc moves faster than the air below it, lowering pressure on the top and creating aerodynamic lift.

But, there are complications. The front of the disc experiences more lift than the back, which makes its flight unstable. Unlike aeroplanes, which have a tail to keep them steady, frisbees have a tendency to flip. However they possess one advantage aircraft lack - they spin around their vertical access.

Dr Lorenz's work arose as a direct result of the need to find out more about what happens to a spinning space probe as it enters a planet's atmosphere.

Dr Lorenz equipped a frisbee with a miniature equivalent of an aircraft's black box flight recorder. It included instruments to measure forces on the disc, and its orientation relative to the Earth's magnetic field and the Sun.

Test flights showed that wobble causes a lot of extra drag, which increases with the square of the angle of attack.

In other words, knowing the average angle of attack alone is not enough to work out the average drag on the disc. You have to know the extent of the wobble as well if you want to profile alien atmospheres accurately.

l Harry Mead is on holiday