SpaceShipOne, the first private manned spacecraft, is undergoing final preparations before its historic flight to the edge of space today.
The craft, built by aviation pioneer Burt Rutan, will be launched into the sky by its carrier, White Knight, then rocket upwards to 100km.
No private craft has ever been so high. In May, it reached 64km in a test flight, doubling its last best.
The attempt over California's Mojave Desert is due to launch at 6:30am.
Rutan and his team from his company, Scaled Composites, hope the flight will take them closer to winning the Ansari X-prize of US$10 million awarded to the first non-government, manned flight into space.
The as yet unnamed pilot will also rocket into the record books to become the first civilian to fly a spaceship out of Earth's atmosphere.
If the mission is a success, SpaceShipOne will attempt the X-prize at a later stage.
To beat 25 other teams to the X-prize, SpaceShipOne must reach 100km -- space's official boundary -- twice in two weeks with a crew of three.
When the X-prize is claimed, it could open up the skies to future tourist trips to the edge of space for those bored of the usual beach holiday.
Today's historic flight is being attempted early in the day because it is less likely to be very windy at that time and a lower Sun angle affords a better view.
High winds or cloudy skies could jeopardize the flight plans.
If the weather does behave, today's flight will see White Knight lift off from the runway in front of gathered crowds. SpaceShipOne will be carried by the craft to an altitude of 15km.
That should take an hour, after which SpaceShipOne will be unleashed into the skies.
It will glide very briefly before firing up its rocket for about 80 seconds. It will blast off to its target height of 100km in a vertical climb at Mach 3 (three times the speed of sound).
When it has reached its target altitude, the vehicle will change its wing configuration to allow for high drag, and will start to fall back towards Earth during which the pilot will be weightless.
At re-entry, the ship's special wing configuration will allow it to glide back down to Earth.