Google co-founder Sergey Brin has placed a $5 million deposit to secure a seat on a future spaceflight.

Space Adventures, the Vienna, Va.-based private space exploration company, on Wednesday announced that Brin has become a founding member of its Orbital Mission Explorers Circle, a program that provides preferential access to space mission seats or the opportunity to sell one's place on a mission.

"I am a big believer in the exploration and commercial development of the space frontier, and am looking forward to the possibility of going into space," Brin said in a statement. "Space Adventures helped open the space frontier to private citizens and thus pave the way for the personal spaceflight industry. The Orbital Mission Explorers Circle enables me to make an immediate investment while preserving the option to participate in a future spaceflight."

Google didn't immediately respond to a request for further comment. Google is the sponsor of the Google Lunar X-Prize, a $25 million award for landing an unmanned space vehicle on the moon.

Space Adventures has already helped wealthy clients, including Dennis Tito, Mark Shuttleworth, Greg Olsen, Anousheh Ansari, and Charles Simonyi, make brief sojourns into space. The cost of a ride into space has been estimated to be somewhere between $20 million and $40 million, though the contracts governing such arrangements have not been made public.

NASA, which has a close relationship with Google, also did not respond to a request for comment. In the past, NASA frowned on private space tourism, but lately the agency appears to have become more accepting of private-sector space initiatives.

Eric Anderson, president and CEO of Space Adventures, said in a statement that his company established the Orbital Mission Explorers Circle "to build a definitive consortium of future private space explorers who share a lifetime goal of orbital spaceflight or the investment therein." He said there are six Founding Explorer slots, one of which Brin now occupies.

The next private astronaut helped into orbit by Space Adventures is likely to be computer-game designer Richard Garriott, who is known for creating the Ultima series in the 1980s and 1990s. Garriott is currently training at the Yuri Gagarin Cosmonaut Training Center in Star City, Russia, for a scheduled Oct. 12 launch. Garriott is documenting his training on the Web site RichardInSpace.com.

In April, officials for Russian space agency Roscosmos said that by 2010 it expected to cease the practice of selling spare seats on Soyuz missions to the international space station to private astronauts. It may be that Russia, now flush with oil and natural gas revenue, no longer needs private funding to keep its space program running. Future private astronauts likely will have to pay the full cost of launching a dedicated private rocket rather than the fractional cost of a single seat.

The extent to which Russia's space industry will play a role in future private space exploration isn't clear. Russian news agency Interfax on Wednesday said that Roscosmos is not commenting on possible use of Soyuz for space tourism in 2011.

Space Adventures, however, on Wednesday said that it had reached an agreement with the Russian federal space agency to launch a private mission to the international space station in the second half of 2011.