WASHINGTON — On the 10th floor of a Tysons Corner, Va., high-rise populated mostly by law firms and software companies is an office where travel agents book multimillion-dollar jaunts to outer space.

From quiet cubicles, they’ve launched clients on high-priced excursions into weightlessness and blackness, defying gravity and redefining travel destinations. If it weren’t for the spacesuits hanging a little eerily in the lobby, a visitor might not guess what’s going on.

Launching tourists into space doesn’t seem the least bit absurd to Eric Anderson, 32, founder and chief executive of Space Adventures. When billionaire Charles Simonyi blasted off April 7 in a Russian rocket from a launch pad in Kazakhstan, it was Space Adventures that booked the trip. As a guest on the international space station, Simonyi is now orbiting Earth 16 times a day at 17,500 mph while conducting experiments, taking photographs the planet and blogging. He is due home Friday.

Anderson, earthbound in Virginia, sees this journey — like the trips of the four other tourists who preceded Simonyi into space, all arranged by his company — as a milestone in the development of space tourism. It was a concept that evoked a fair amount of skepticism when Anderson started his company nine years ago with $500,000 from a few investors.

“We’ve finally proven that a market exists,” he said last week. “We’ve totally recalibrated the interest level in spaceflight.”

Anderson said his company has taken in about $150 million in ticket sales, a large chunk of which is paid to the Russian space agency for the use of its facilities and spacecraft.

While Anderson was one of the first entrepreneurs in the field, he is getting some deep-pockets competition: Robert Bigelow, founder of the Budget Suites hotel chain, is developing inflatable space stations and expects to launch vessels designed for people by 2010. PayPal founder Elon Musk started the rocket company SpaceX to build vehicles to carry cargo and people to the space station. Microsoft co-founder Paul Allen has sponsored the development of a manned spacecraft. Richard Branson’s Virgin Galactic space-tourism company is building vehicles for suborbital flights.

The Russian space agency, which needs to supplement its limited government funding, sells the extra seat on its Soyuz spacecraft, which travels to the space station about twice a year. Competing with other countries and space agencies that don’t have spacecraft, Anderson has been bidding on and buying some of those seats since the late 1990s, and he has purchased the seat for 2008 and 2009.

Simonyi and his four predecessors paid $20 million to $25 million each to climb aboard the Soyuz spacecraft at the Baikonur Cosmodrome in Kazakhstan.

Space Adventure soon plans to offer suborbital flights in a vehicle it is building with a Russian aircraft company — 90-minute trips to the edge of the atmosphere, including up to five minutes of weightlessness and the chance to gaze at the Earth’s curvature. Each trip would cost $102,000, Anderson said, and could be launched from either of the two spaceports that Space Adventures is spending $265 million to build in Singapore and the United Arab Emirates. Anderson said 200 people have made reservations for suborbital trips, putting up $3 million in deposits.

Anderson hopes one of those flights may also be his own inaugural venture into space. As a child growing up in Colorado, his dream was to become an astronaut. But in high school, his eyesight worsened, erasing his hopes. He studied aerospace engineering at the University of Virginia and won an internship with NASA. After school he worked for an aerospace software company but soon realized that “what I was really passionate about was getting people into space.”

Space Adventures largely targets clients by the size of their wallets. Anderson’s crew — advised by a lineup of 10 former astronauts, including Buzz Aldrin of the first lunar landing in 1969 — first does research to identify wealthy people who might be interested. They may have expressed interest in space travel in media interviews, written about rocket dreams in a personal blog, or given money to space-oriented ventures. People who have had other extreme adventures, such as climbing Mount Everest, are also candidates. Working from that roster, Anderson’s crew organizes presentations around the world, and then recruits.

Simonyi won a contest to become his native Hungary’s junior astronaut and has followed the space industry ever since. As the lead developer of Word and Excel software, who reportedly came out of Microsoft with a fortune of about $1 billion, Simonyi saw the price of a space trip as merely an afterthought.

“Spaceflight is a unique, once-in-a-lifetime opportunity,” Simonyi wrote in an e-mail from space. “The opportunity presented itself and I decided to take it.”

Howard McCurdy, a professor of public administration at American University who studies international space policy, likens the Space Adventures business model to that of the early years of air travel, when only rich thrill-seekers took the ride.

“People did it as a lark, and no one thought there was any industrial potential, and now we’re flying to Cancun for spring break,” he said. “People will be flying to space in 50-person rockets in 50 years.”