The super-rich lovebirds went without a rumoured pre-launch wedding engagement, but Stewart could hardly contain her excitement as Simonyi left the space training center on Saturday for a ride to the launch pad.

Stewart screamed and clapped along with about 50 other friends and well-wishers as Simonyi boarded a bus with Russian cosmonauts Fyodor Yurchikhin and Oleg Kotov, then ran to the bus and banged her hands on the window.

"Charles! Charles! Charles!" she cried, beaming at her beau.

The American homemaking guru will watch the blast-off, scheduled for 23:31 p.m. (1731 GMT), from a platform about a kilometre (half a mile) away.

Simonyi is due to dock at the International Space Station (ISS) on Monday, where he will spend 10 days conducting experiements, recording his impressions, and -- likely with advice from Stewart -- preparing a gourmet dinner for his hosts.

The US-Hungarian programmer, who helped design Microsoft's benchmark Word and Excel programs, paid 25 million dollars (19 million euros) for the trip.

"Dr. Simonyi is very important for us because I believe his flight proves this wasn't just a one-off.... It's now a business," said Eric Andersen, CEO of Space Adventures, the company that organized Simonyi's space tour and four previous ones.

Andersen said Space Adventures had a buyer for a 2007 flight who would go public within a few months, and that the company would soon expand its offerings to include a 100-million-dollar trip around the moon and a 100,000-dollar budget option: five minutes of sub-orbital space flight.

Simonyi spokeswoman Susan Hutchinson said the billionaire programmer felt "very comfortable about the flight."

"He is very pleased with all the training process.... He's going into it very confident."

He is scheduled to return to Earth on April 20 together with the current ISS team -- Russia's Mikhail Tyurin and American Miguel Lopez-Alegria -- while the two Russian cosmonauts stay on for a 190-day shift in orbit.

Though other family members will be watching the lift-off at Stewart's side, Simonyi's mother was too worried to come witness it herself, Hutchinson said.

"She has typical ordinary fears for her son," she said.

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