A calm, bright morning on the Cornish coast and the sky gives no telltale signs of the clouds high in the stratosphere that have delayed the record attempt for a day.
By Helen Briggs
BBC News Online science reporter, in St Ives
Preparations are continuing on the launch ship, the RV Triton, which lingers in the bay off St Ives, on the southwest coast of England.
Waiting: QinetiQ 1's launch ship sits in Carbis Bay, St Ives
On board is balloon adventurer Don Cameron, the man charged with deciding whether the flight, if successful, has broken the manned world altitude record.
He says Britons are leading the world in ballooning and smashing this record would complete a big four.
"The British have three out of the four - it would be nice to have the fourth," he told BBC News Online.
"It certainly is a very important record and it's one that's very hard to do."
British balloon pilots already hold the three big records in ballooning - distance, duration and shortest time around the world via the Northern Hemisphere.
The altitude record is a particular challenge because the balloon has to be very large and very light to reach the 40 km target height.
High winds pose the biggest threat to the delicate balloon during inflation and the flight itself.
Co-pilots Colin Prescot and Andy Elson are exposed to the elements and rely on artificial life support systems built into their space suits.
Don Cameron, who runs the world's largest balloon manufacturing company, says the attempt is "completely at the limits of balloon technology".
Cameron: His job to see the rules are followed
As an observer on behalf of the British Balloon and Airship Club, he will be making sure the data exist to certify any record set.
To break the world altitude record for a manned balloon, the QinetiQ balloon must exceed by 3% the record set by two US pilots in 1961 - 113,740 feet (34,668 metres).
The balloon has to be deemed under control throughout the flight, and both pilots must survive for at least 24 hours after the landing.
The height of the balloon during the flight will be monitored by several means. A radar station in Wales will track its height and position and a GPS transmitter on the balloon will send data back to the ground.
Special codes have been released to the balloon team by the GPS operators, the US military, to enable it to gather accurate data high above the Earth.
The UK's Brian Jones flew around the Northern Hemisphere with Swiss pilot Bernard Piccard
The traditional method - measuring air pressure around the balloon - will also be employed but the results are uncertain because the pressure will be so low.
Don Cameron will be making sure that everything possible is done to get all the data required.
"If there was any mistake, it would be absolutely terrible," he says.
The team on the Triton is now putting the finishing touches to the equipment and carrying out final checks.
The next crucial deadline is this evening when the pilots will meet mission controller Brian Jones to assess the latest meteorological data.
If the weather holds, the giant balloon will be unfurled and inflated ready for a take-off at first light on Wednesday.
HISTORIC BALLOON FLIGHTS
Pilots have constantly pushed the latest technology to fly their balloons ever higher and further
The record bid, scrutinised by Cameron, follows the rules set down by the Federation Aeronautique Internationale (FAI), the world's air sports federation.
It is the FAI that will eventually ratify any new record.