If clear skies break over southern California tomorrow, a B-52 bomber will take off from Edwards air force base and head out over the Pacific.
Slung beneath its right wing will be an experimental pilotless plane designed to break records by flying at ten times the speed of sound.
The 3.5m-long prototype, known to NASA as the X-43A, will hitch a ride on the bomber to 12,000m before an on-board rocket takes over, boosting it to 34,000 metres. The rocket will then fall away, leaving the plane free to fly -- powered by a revolutionary engine, the scramjet.
Shortly after 9.30pm GMT, the X-43A will accelerate, if all goes smoothly, to 11,500kph, before its engine switches off and it descends to plunge into the ocean some 850 miles from shore.
The test flight is the culmination of an eight-year, US$250m effort to prove that hypersonic flight is possible, and with it the dream of planes capable of flying from London to Sydney in two hours. But no matter how well Monday's flight goes, the X-43A will be shelved, a casualty of deep cuts made by the Bush administration to fund the president's vision of sending humans to Mars.
For NASA, the X-43A is a tale of mixed fortunes. A previous attempt in 2001 ended in failure when the rocket carrying it veered dangerously off course, forcing engineers to trigger its self-destruct mechanism. In March, a second attempt was claimed as a resounding success when the plane reached 8,000kph.
Tomorrow's flight will use the last of the X-43As ever built. The plane is powered by the exotic scramjet engine. Unlike conventional jet engines that use turbines to compress air before it is mixed with fuel, scramjets are designed so that air moving into the engine is compressed simply because the plane is going so fast.
Getting scramjets to work has been beset with problems. Air moves so fast through the engine that additives are needed to ensure that the hydrogen fuel it is mixed with ignites within milliseconds. And at such speeds, even a thin atmosphere makes the plane's body heat up dramatically, forcing engineers to build them from heat-resistant materials. Tomorrow, the wings of the X-43A are expected to reach temperatures of 2,000?C.
"It's a high risk project, but I'd say we're more than 50/50 confident of success," said Keith Henry of NASA's Langley research centre.
During the test flight, the X-43A's scramjet engine will fire for 11 seconds, after which NASA scientists will take readings from onboard sensors telling them how fast it is going and how it reacts to different manoeuvres.
NASA sees scramjets as making space flight cheaper and safer. "Part of the excitement about coming back to Earth in the space shuttle is it's like a brick with wings. Everything has to work exactly right, because you don't have anywhere near the flexibility of an aeroplane. A scramjet space launcher could give you that flexibility," said Mr Henry.
For now though, NASA will have to take a back seat. The US military is continuing its research into hypersonic engines with one eye on developing ultra-fast missiles, bombers and interceptors.