Developers hope that this inflatable pod could soon go to Mars. Credit: © NASA

An inflatable lifeboat could one day ferry stranded astronauts back to Earth, if a prototype's test flights are successful next month.

The re-entry vehicle weighs just 130 kilograms and is being developed to carry cargo back from the International Space Station (ISS). But its inventors believe that it could also let astronauts bail out of the space station, or deliver robots to the surface of Mars.

"We are developing an idea for lifeboat missions," says Stephan Walther of Return and Rescue Space Systems (RRSS), Bremen, Germany, which has built the craft with colleagues in Russia. But this may take more than five years and a huge investment, he says.

A Martian trip could happen sooner - Walther is hoping that European Space Agency will use the craft in their ExoMars mission, slated for 2009. Using inflatables could halve the mass of the descent vehicle, Walther says, allowing a greater payload.

The ISS needs a new cargo capsule to carry scientific samples and equipment for repair. The Russian Raduga capsule currently used is nearing the end of its life, and can only transport up to 150 kg at a time. The inflatable pods could carry about 200 kg.

Inflatable spacecraft are not a new idea - NASA developed one in the 1960s - but they have never seen active duty. RRSS's craft, which has so far taken US$2 million and six years to develop, has been tested twice before, in 2000 and 2002. In the last test, the pod failed to detach from its rocket; the craft has been redesigned to solve this problem, says Walther.

Blow up

The latest prototype will launch on a rocket from a Russian submarine in the Barents Sea off Murmansk. At about 200 kilometres up (the equivalent of a low-Earth orbit) the ship will detach and inflate, then spend 20 minutes or so falling to Earth, eventually landing, the team hopes, on Russian soil in Kamchatka.

The prototype craft will be launched by a rocket from a Russian submarine in the Barents Sea near Murmansk, Russia. Credit: © NASA

The demonstration vehicle is shaped like a shuttlecock, and is just over 3 metres across. It carries pressure sensors and other equipment to monitor its descent. It will inflate using tanks of nitrogen, but RRSS hope eventually to use the same chemical reaction used in car airbags, which generates nitrogen gas from a powder.

An inflatable heat shield will protect the ship and slow it down. A second, larger inflatable emerges from the rear of the craft to act as a parachute, reducing its speed to about 35 kilometres per hour before it hits the ground. The surface is made from a tough, flexible polymer coated with a paint that can withstand temperatures of around 900 °C. The exact composition of the paint is a closely guarded secret, says Joachim Thäter, an engineer at RRSS.

The lifeboat could not help astronauts escape a stricken space shuttle, as the shuttle's design would prevent it launching. It may, however, be incorporated into the next generation of vehicles to replace the shuttle, and is one of several possibilities being considered by NASA.