LOS ANGELES — Brian Emmett’s childhood fantasy came true when he won a free trip to outer space. He was crushed when he had to cancel his reservation because of Uncle Sam.

Mr. Emmett won his ticket to the heavens in a 2005 sweepstakes by Oracle Corp., in which he answered a series of online questions on Java computer code. He became an instant celebrity, giving press interviews and appearing on stage at Oracle’s trade show.

For the self-described space buff, who has attended space camp and watched shuttle launches from Kennedy Space Center, it seemed like a chance to become an astronaut on a dime.

Then reality struck. After some number-crunching, Mr. Emmett realized that he would have to report the $138,000 galactic joy ride as income and owe $25,000 in taxes. Unwilling to sink into debt, the 31-year-old software consultant from the San Francisco Bay area gave up his seat.

“There was definitely a period of mourning. I was totally crestfallen,” Mr. Emmett said. “Everything you had hoped for as a kid sort of evaporates in front of you.”

With commercial spaceships still under development, it’s uncertain when the infant space tourism industry will actually get off the ground. Still, ultra-rich thrill-seekers are already plunking down big — though refundable — deposits to experience a few minutes of weightlessness 60 miles above Earth. And in recent years, space tourism companies have teamed with major corporations to stage contests with future suborbital spaceflights as the grand prize.

The partnerships have interstellar hype, but as Mr. Emmett found out, they can get mired in that most earthbound hassle: taxes.

“From a consumer perspective … I’d be wary,” the director of the University of Southern California’s Marshall Center for Technology Commercialization, Kathleen Allen, said. “I’d check to see the fine print.”

Since the Internal Revenue Service requires winnings from lottery drawings, TV game shows, and other contests to be reported as taxable income, tax experts contend there’s no such thing as a free spaceflight. Some contest sponsors provide a check to cover taxes, but that income is also taxable.

“I don’t see how an average person can swing that kind of tax payment. It’s a big, big bite,” a contributing editor for J.K. Lasser’s annual tax guide, tax attorney Donna LeValley, said.

To reduce the financial burden, winners can argue that they don’t owe any taxes until their flight lifts off. Another option is working out an installment plan to pay taxes over time, Greg Jenner of the American Bar Association said.

The IRS declined to comment, saying it does not talk about individual matters.

Despite Mr. Emmett’s cancellation, Oracle said its contest was a success. The software giant is in the process of naming his replacement and still has two other winners on board from Asia and Europe.

That spaceflight will be provided by Space Adventures Ltd., the same company that brokers deals for trips on Russian rockets to the orbiting international space station for a reported $20 million a customer.

The company’s chief executive, Eric Anderson, insists that contests are the best way for most people to get into space. He said Space Adventures has given away about 20 reservations through competitions and that the majority of winners are satisfied.

Space contest rules vary widely but generally require winners to undergo astronaut training before the trip and sign a waiver freeing the sponsors from any liability if there’s an accident.

Microsoft Corp. is the latest company to dangle a free space ride. This month, it launched an elaborate online puzzle game as part of its promotional campaign for its new Vista PC operating system. The grand prize winner — to be named this week — gets a seat with Rocketplane Ltd., which is building a souped-up Lear jet that it hopes will ferry passengers to space in late 2009. The $50,000 check that comes with the prize, which is valued at $253,500, should cover the winner’s taxes, the group marketing manager for Vista, Brian Marr, said.

Back on Earth, Mr. Emmett said he has no regrets about turning down his trip and doesn’t blame anyone.

“I was, however briefly, a potential astronaut,” he wrote last fall in a Web log entry titled “Clipped Wings.”