Fifty years after the first American astronaut rocketed into space, one commercial spaceflight company is hoping to push the envelope even further, with tourist trips around the moon. And now they plan to use a bigger spaceship.

The Virginia-based space tourism firm Space Adventures has brokered commercial rides to the International Space Station for the last 10 years under a partnership with Russia's Federal Space Agency, which provided the Soyuz spacecraft for the flights. The three-person Soyuz vehicle also forms the core of Space Adventures' trip for two around the moon at $150 million per passenger, but the U.S. company today (May 5) announced a new twist: an extra module to give customers more room during the lunar visit.

Space Adventures already has one customer signed on for the circumlunar joyride and is in contract negotiations with a second, which means the first flight could occur as soon as the end of 2015, said the company's chairman Eric Anderson. [Photos: The First Space Tourists]

"The mission, in my mind, will be another watershed event," Anderson said in a news briefing today. "It's remarkable that a private company will be able to work in the market and finance what is likely to be humanity's first return to the moon in what will, at that time, be 45 years."

A private moon shot

The circumlunar trip will first take customers aboard a three-seat Soyuz to the International Space Station, Anderson said. Two seats in the capsule will be occupied by private customers, and the third will be reserved for a Russian mission commander.

The passengers will spend 10 days visiting the space station, during which time a separate rocket with an upper stage engine and additional habitation module will be launched into low-Earth orbit.

This newly announced habitation module will almost double the room in the Soyuz, adding substantial volume to the otherwise cramped quarters of the Russian capsule, Space Adventures officials said.

The vehicle used for Space Adventures' circumlunar mission includes a Russian Soyuz spacecraft with an extra habitation module and upper stage engine. (Image credit: Space Adventures)

"It would be an extraordinarily comfortable trip to the moon and back," said Richard Garriott, vice chairman of Space Adventures, who himself flew on one of the company's commercial flights to the space station in 2008. "It's considerably larger than any that the Apollo era people had on their journeys to the moon and back." [Giant Leaps: Top Milestones of Human Spaceflight]

To complete the next phase of the trip, the Soyuz would undock from the station and rendezvous with the separate booster and module in orbit. The upper stage engine would then fire, sending the passengers on a 3 1/2-day voyage toward the moon.

After a flyaround that promises spectacular views of the moon and Earth, the spaceflyers will begin the journey home, which should also take 3 1/2 days. While the spacecraft will not land on the lunar surface, the passengers will be treated to an experience that, so far, has been limited to only a few dozen astronauts with NASA's Apollo lunar program. [Video: Flashback to America's First Spaceflight]

The price for Space Adventures' maiden journey to the moon is not set, but will likely cost each passenger between $120 million and $150 million, Anderson said. This figure depends on a number of factors that will be negotiated with the company's Russian and space station partners.

Aiming for the moon

The voyage will also require at least two months of training to help familiarize the customers with their vehicle and microgravity environment.

Space Adventures is the only company that has booked private flights to the International Space Station. The company has arranged eight multimillion-dollar trips for seven extremely wealthy clients since 2001 (one customer flew twice). [10 Years of Space Tourism]

Dennis Tito, a California-based multimillionaire, became the first-ever space tourist when he launched to the station in a Soyuz capsule on April 28, 2001. His groundbreaking trip ushered in a new era of private spaceflight and opened the market for private investment in such enterprises.

Now, Space Adventures is hoping to continue that legacy with its first circumlunar flight.

"The moon holds a special place in all of our hearts," Anderson said. "It's a symbol of the space future that humanity wishes for, a symbol of our curiosity, and something that we see every night. When the private moon mission launches, the eyes of the world will truly be upon those people, and it will truly be an extraordinary event."

You can follow SPACE.com Staff Writer Denise Chow on Twitter @denisechow. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.