Radiation in space is bad for the health of astronauts. But no-one is certain of the best ways to protect interplanetary space crews from it.

It comes from two main sources - explosions on the Sun called solar flares and from cosmic rays which are actually energetic atomic particles from deep space.

The problem is not new. As early as 1952, Dr Wernher von Braun and other space pioneers suggested using lunar soil to protect a manned lunar expedition from space radiation.

But how much is enough? And what do you use for protection on the journey out and back?

Dr Jim Adams of the Naval Research Laboratory in Washington said: "Although space radiation has been measured extensively since the 1950s. Its intensity changes, and our knowledge of how it reacts with materials still lags in many areas."

One of the problems with radiation shielding is that a little can be worse than none. With high energy radiation the shielding itself produces showers of secondary radiation, like splinters from a wall hit by a bullet.

Scientists discussed the problem of radiation shielding at conference held in Alabama.

Scientists will soon start radiation tests using simulated Mars soil based on findings obtained about the Martian surface by the pathfinder lander than touched down last year.

Designs for spacecraft to carry men to Mars, a three-year round trip, include a 'storm shelter.' This is a heavily shielded part of the spacecraft where the crew can hide from radiation from solar flares.