Nasa's new route to space

Space: the final frontier. But can you really go there in a balloon? Nasa's scientists reckon so, and they're working on a project to make it happen. The US space agency plans to build a balloon almost 100m in diameter which would be able to lift more than a tonne of instruments almost to the edge of space - 35 km (22 miles) up - and stay aloft for 100 days. That would be enough to circle the globe five times, given favourable winds.

The Ultra Long Duration Balloon project is now underway, and Jack Tueller, the chief scientist, reckons it will be able to carry more weight and fly longer than any previous scientific balloon.

But Nasa's project does not include a crew. Instead it will conduct astronomy experiments, at a cost far lower than using even its cheapest rocket launcher, the Pegasus, which costs about $15m per launch.

By contrast, the balloon would cost about $1m, and if it can be recovered could substantially cut the average cost of a mission. However, because it cannot fly as high as a rocket, its payloads, such as satellites, would stay aloft for shorter times.

Nasa plans to make the balloon from a combination of a high-tech fabric and an aluminized polymer material, such as Mylar. The balloon would be partially inflated with helium on the ground, pumping in just enough to make it ascend. (It was at this stage earlier this month that Mr Branson's balloon escaped its moorings in Morocco and flew unattached to Algeria.)

As the balloon rises, the air pressure declines, while the helium would expand until the balloon becomes spherical, about 98m in diameter. Its altitude could be controlled by a combination of gas venting and heating from the sun.

Nasa plans a test flight next summer. Various proposals for experiments are being considered.