Two pilots ran through their dress rehearsal on Thursday ahead of their attempt to take a helium balloon to the edge of space.
The two pilots were put through their paces in Boscombe Down
Andy Elson, from Somerset, and Colin Prescot, from Hampshire, were subjected to temperatures of nearly -50C (-48.2F) in a special MoD environmental chamber at Boscombe Down, Wiltshire.
The plan was to see how their pressurised spacesuits and life support systems performed under realistic conditions.
The pilots are hoping to rise 25 miles above the Earth to break the current altitude record held by the Americans.
The launch date depends on the weather, but the current plan is to take off from a ship off the north Cornwall coast some time between late July and September.
Mr Elson, 48, from Wells, said: "This cold test was the final piece of the jigsaw before
"It's cold, but it doesn't really feel like -44C. We're protected by our suits and I've got about five pairs of underwear on."
As well as mimicking the cold, a bank of solar lights was set up at Boscombe Down to reproduce the effects of any intermittent exposure to the sun during the flight.
Mr Prescot, 51, from Stockbridge, added: "We're very happy. It's going to be amazing to see the earth's curvature.
"It's very exciting and is a real privilege to be able to do something only a few people in history have ever witnessed."
1,270 ft tall
The pilots will be unable to eat during the estimated 12-hour flight and will carry just two-thirds of a litre of water each, accessed through tubes in their helmets.
Project sponsor QinetiQ has
advised the Glastonbury-based team on a diet which will help energy levels and
reduce production of bodily waste.
Three days before the launch, the pilots will follow a low-residue, high-carbohydrate diet, including white bread, white rice and pasta.
Once airborne, the 1,270 ft tall balloon (the height of the Empire State Building) will be able to be seen for a radius of some 300 miles, across the UK and France.