One day after NASA brought the shuttle Discovery back from low Earth orbit, a private company plans to announce a more audacious venture, a tourist trip around the Moon.

Space Adventures, a company based in Arlington, Va., has already sent two tourists into orbit. Today, it is to unveil an agreement with Russian space officials to send two passengers on a voyage lasting 10 to 21 days, depending partly on its itinerary and whether it includes the International Space Station.

A roundtrip ticket will cost $100 million.

The space-faring tourists will travel with a Russian pilot. They will steer clear of the greater technical challenge of landing on the Moon, instead circling it and returning to Earth.

Eric Anderson, the chief executive of Space Adventures, said he believed the trip could be accomplished as early as 2008. Mr. Anderson said he had already received expressions of interest from a few potential clients.

The Soyuz vehicle to be used does not have the power to reach the Moon on its own, so the Russians have devised a plan to send up a booster. The Soyuz would dock with the booster, either in low Earth orbit or at the International Space Station.

The booster would take the passengers the rest of the way. The price of the two tickets, Mr. Anderson said, would pay for the costs of the Moon shot. His company’s demographic research, he said, suggests that 500 to 1,000 people in the world can afford to do this.

“It’s the same number of people who could afford to buy a $100 million yacht,” Mr. Anderson added. Two people who have already paid Space Adventures to go into orbit, at a reported $20 million apiece, applauded the new initiative though they said they were not sure they would try the Moon orbit.

Dennis Tito, a financier who in 2001 became the first space tourist, said that he found the idea fascinating but added that he doubted he would make such a trip. Having just turned 65, and with the Moon orbit at least a few years away, he said he might be too old for the rigors of the voyage.

“I would be considering it if I were younger, and I had that kind of money to spare,” Mr. Tito said.

Another Space Adventures client, Greg Olsen, who made millions in the sale of his camera technology company, Sensors Unlimited, is preparing to visit the space station for several days in October. Of the Moon trip, he said, “It’s certainly intriguing, and it’s something I’d like to do.”

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