Balloon pilots head for altitude record with 25-mile ascent

Two British pilots will embark on a record-breaking attempt today to fly the world's biggest manned helium balloon from St Ives, Cornwall, to the edge of space, in a spectacle that could be visible as far away as London.

The attempt on the 40-year-old altitude record for a crewed balloon will require the two pilots, Colin Prescot and Andy Elson, to wear pressurised space suits as they seek to surpass the 113,740ft (21 miles or 34.6km) mark set on 4 May 1961 as part of the US space programme.

The attempt carries serious dangers, besides extreme cold, low pressures and high-energy radiation. Strong winds could shred the balloon while returning to Earth - or more precisely, the Atlantic. When the American pilots Malcolm Ross and Victor Prather set their record in 1961, all went well until the balloon ditched in the Gulf of Mexico. Prather drowned when his space suit filled with water.

The 1,270ft (381m) balloon, sponsored by the British science and research company QinetiQ, was due to be launched from a ship moored about 10 miles off St Ives between 6am and 8am today. Yesterday, Mr Prescot, 53, from Stockbridge, Hampshire, said he was "really, really pleased we are about to get off. It has been a long time coming, but it has been worth it."

Mr Prescot and Mr Elson, 49, from Wells, Somerset, are both commercial balloon pilots and have already set the world endurance record for any aircraft in the Earth's atmosphere, when they flew from Spain to the Pacific and splashed down off the coast of Japan in 17 days, 18 hours and 25 minutes, as part of a round-the-world attempt.

They planned to breathe pure oxygen for several hours before the launch. Three hours are needed to inflate the balloon - which, despite being made from super-thin polythene, weighs 1.7 tons, and once inflated is as tall as the Empire State Building.

Mr Prescot said the last 15 minutes of the launch operation would involve unwinding the balloon and attaching the open gondola in which they would be travelling. "All that has to be a pretty slick operation," he said. A crucial part of the hi-tech operation will involve using rolls of sticky tape to repair tears in the balloon's fabric as it unwinds.

The conditions on their open platform will resemble those on Mars, with temperatures dropping as low as minus 70C (minus 94F) amid levels of radiation four times higher than they would receive on a commercial aircraft. Douglas Millard, a spokesman for QinetiQ, said: "If they were to lose pressure, there could be problems. If they weren't in space suits, their blood would literally boil."

At their target altitude, the pilots will be floating in an almost atmosphere-free environment and will be able to see the curvature of the Earth. They will, though, be a long way below the typical orbiting altitude of spacecraft or satellites. The International Space Station orbits at about 240 miles; and 200 miles is the lowest altitude feasible for a satellite. The team hopes to reach a maximum of 132,000ft.

QinetiQ 1 will be mounting several experiments, with sensors on board to collect data about temperature and pressure, which will help scientists to plan hypersonic flight vehicles of the future.

The duo have been waiting since July for a suitable weather window, after bad weather prevented an attempt last year. They could not eat during the flight, and carried just two-thirds of a litre of water each, accessed through tubes in their helmets. A Royal Navy vessel was standing by to pick them up from the Atlantic.