WHO, WHAT, WHY? The Magazine answers...

It's all to do with air flow. On earth, in the West at least, your standard toilet is a water-flush affair, that takes waste and washes it down a pipe.

THE ANSWER Space toilets use air flow as water flushes have drawbacks in zero gravity Adult nappies are used on space walks and during take-off and landing

The lack of gravity on the shuttle and the space station mean a water-flush system is not an option. You don't need a particularly vivid imagination to see the potential problems.

Instead, on the shuttle, urine and faeces are carried away by rapid flow of air.

The unisex toilet resembles a conventional loo, but with straps over the feet and bars over the thighs to make sure that the astronauts don't drift off mid-go. The seat is designed so the astronaut's bottom can be perfectly flush to make a good seal.

The good news for fans of convenience is that, on the shuttle at least, urinating standing up is possible. A funnel-on-a-hose contraption is included so that astronauts - both male and female - can urinate standing up. Or sitting down if they prefer. They just attach it to the toilet using a pivoting bracket.

I would wait for 10 hours and then once back inside get someone to help me off with the suit and rush to the bathroom British astronaut Piers Sellers to Prince Philip

The system separates solid and liquid waste. Solids are compressed and remain on-board to be unloaded after landing. Liquids are released into space. Nasa hopes one day to recycle waste productively.

Researchers at the University of Guelph in Canada have said such recycling will be key to tackling any future mission to Mars in order to feed the astronauts.

The air used in the space shuttle's toilet system has to be filtered to get rid of the smell and bacteria before it is returned to the living area.

Incinerated waste

On the International Space Station, the fundamental principle is similar. The fan-powered air-flow toilet system stores waste. Urine is sucked up and stored in 20 litre containers which are dumped into the Progress resupply craft. The ship is dumped into the atmosphere where it burns up.

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For solid waste, a plastic bag covered in holes is placed inside the toilet. Air is sucked through the holes so everything ends up in the bag. The elasticised top closes and the bag is pushed into a metal container. A new bag is popped in for the next visitor. Again the waste heads off to Progress.

Space toilets have come a long way. In the book The Right Stuff and its film adaptation, an astronaut on an early mission feels the need to urinate during a massively delayed take-off. With no facilities provided - and no adult nappies, as used today during take-off and landing - he is eventually allowed to urinate in his suit, causing his sensors to go haywire.

And Prince Philip is among good company in wondering how astronauts attend to their bodily functions.

A spokesman for Nasa confirms it is a question much asked by children and journalists alike.