By Helen Briggs BBC News Online science reporter

The launch countdown has begun for the two men who plan to take the biggest manned balloon ever to the fringes of space.

British pilots Andy Elson and Colin Prescot are waiting for a clear day between now and mid-September.

When weather conditions are right, they will try to soar to the highest altitude ever recorded for a balloon.

We're going into the unknown

Andy Elson

The Met Office is providing special weather forecasts for the pair, who will take off from a ship near the north coast of Cornwall.

The adventurers hope to ascend beyond 130,000 feet (40 kilometres) into the Earth's stratosphere.

At peak altitude, the pilots will be able to see the curvature of the planet and will be floating in a virtually atmosphere-free environment.

Speaking at a press conference in London on Tuesday, Andy Elson said they were looking forward to a stunning view. But there would be some nerve-wracking moments along the way, he said.

"I'm going to be nervous," he told BBC News Online. "I mean, we're going into the unknown - how fast should we make the balloon climb, how much ballast should we drop, will we be going too quickly?

"The view's going to be fantastic," he added, "but at some point we have a finite limit to our life support and we have to make the decision to descend."

Longest 'space walk'

The pilots will sit on an open flight deck wearing space suits that have been designed with the help of the Russian engineers who make cosmonaut suits.

At the proposed altitude, the suits will have to withstand extreme pressure and temperatures as low as -73 Celsius.

"This is effectively the longest space walk in history because we have to rely on the spacesuits," co-pilot Colin Prescot told reporters.

The flight, if successful, will last for about 12 hours, and will end by splashing down into the Atlantic.

The balloon will be 395 metres (1,295 feet) tall - seven times the height of Nelson's Column - and should be visible up to 966 kilometres (600 miles) away.

The current height record for a balloon journey stands at 34,747 metres (114,000 feet).

Brian Jones is mission control director for the challenge, known as QinetiQ 1, after its science and technology business sponsor.

Mr Jones, himself a round-the-world balloonist, said it would be a grand adventure and "real Boy's Own stuff".

"Who can imagine sitting on an open platform at the edge of space, gazing down at the Earth below them; it's just extraordinary," he told BBC News Online.

The mission also has a scientific objective. Sensors on board will collect data about temperature, pressure and radiation in the atmosphere.

Cosmic clues

Dr Richard Crowther, space consultant at QinetiQ, said the journey was more important to scientists than the destination.

"We fly our sensors on board Concorde and on board the space shuttle, and they measure the radiation environment encountered by pilots and astronauts," he told BBC News Online.

He said they had no measurements for the region the QinetiQ 1 balloon would fly in.

Rockets that pass though on their way to space go through too quickly to gather data.

The balloon flight would "plug the data gap that exists between those two flight vehicles", he said.

"For us it is extremely important because it allows us to predict the radiation exposure for air crew, for flight systems on aircraft and also for passengers flying on polar routes," said Dr Crowther.