NEW YORK — Future thrill-seekers will ride a sleek spacecraft berthed under a massive, twin-boom mothership to the fringe of space in a design unveiled Jan. 23 by Virgin Galactic.

The SpaceShipTwo spacecraft and its WhiteKnightTwo carrier will begin initial tests this summer to shakedown the novel spaceflight system designed by aerospace pioneer Burt Rutan and his firm Scaled Composites.

“2008 really will be the year of the spaceship,” said Sir Richard Branson, British entrepreneur and founder of the Virgin Group who unveiled a 1/16th-scale model of the new spacecraft here at the American Museum of Natural History. “We’re truly excited about our new system and what our new system will be able to do.”

Based on Rutan’s SpaceShipOne, a piloted and reusable spacecraft that won the $10 million Ansari X Prize for suborbital spaceflight in 2004, SpaceShipTwo is an air-launched vehicle designed to carry six passengers and two pilots to suborbital space and back.

But unlike SpaceShipOne, which launched from beneath its single-cabin WhiteKnight carrier, the new craft will drop from a twin-cabin high-altitude jet that can double as a space tourist training craft. WhiteKnightTwo carries four engines and a wingspan of about 42 meters, rivaling a B-29 bomber, and is built to handle unmanned rockets capable of launching small satellites into orbit, Virgin Galactic officials said.

Virgin Galactic is offering tickets aboard SpaceShipTwospaceliners for an initial price of about $200,000, though Branson said the cost is expected to drop after the first five years of operations. The space tourism firm plans to eventually launch flights out of a terminal at New Mexico’s Spaceport America, with additional trips through the aurora borealis to be staged from Kiruna, Sweden.

“It’s fantastic,” said British advertising executive Trevor Beattie, one of the some 100 Virgin Galactic ticket holders on hand for the unveiling. “I want to go now … with each milestone, it’s getting closer and closer.”

To date, Virgin Galactic has about 200 assured passengers for future flights, $30 million in deposits and about 85,000 registrations from customers interested in flying aboard SpaceShipTwo.

Rutan, whose Mojave, Calif.-based Scaled Composites has completed 60 percent of the first SpaceShipTwo, said his firm is building at least five of the suborbital vehicles – and two WhiteKnightTwo carriers – for Virgin Galactic.

“This is not a small program by any stretch of the imagination,” said Rutan, adding that his firm hopes to build at least 40 SpaceShipTwos and 15 carrier craft over the next 12 years.

Each spacecraft is designed to fly twice a day, with their WhiteKnightTwo carriers capable of up to four daily launches, Rutan said. Over 12 years, more than 100,000 people could fly to suborbital space aboard the vehicles, he added.

A roomy flight

Virgin Galactic passengers like Beattie and others already have undergone centrifuge tests to sample the experience launch and re-entry, which can exert forces of up to six times the Earth’s gravity on the human body.

Will Whitehorn, Virgin Galactic’s chief executive officer, said each SpaceShipTwo passenger will be equipped with a pressure suit as a safety precaution, be free to move about a roomy cabin equivalent in size to a Gulfstream aircraft and peer at the Earth through wide 46-centimeter windows during the several minutes of weightlessness offered on each spaceflight.

“Because clearly, if you’re going to go into space, you’re going to want to see the view,” Whitehorn said.

SpaceShipTwo’s cabin is much larger than the three-person capsule used on SpaceShipOne, and each of the two WhiteKnightTwo carrier craft cabins are identical to that of the spacecraft to make it a useful training tool, he said.

Family members of passengers or other space tourists can watch a SpaceShipTwo launch from inside a WhiteKnightTwo cabin, each of which sits just 7.6 meters from the center-mounted spaceship.

While the initial round of tests is slated for sometime this summer and the first spaceflights pegged for 2009, Whitehorn stressed that safety is paramount.

“We’re in a race with nobody, apart from a race with safety,” Whitehorn said.

Rutan said he is targeting a safety factor akin to that of the earlier airliners of the 1920s, which still should be 100 times better than the safety of the manned spacecraft used by large governments today.

“Don’t believe anyone who tells you that the safety level of new spacecraft is as safe as a modern airliner,” Rutan said.

The development and testing plan for SpaceShipTwo and its carrier craft has been slowed by an accidental fatal blast that killed three Scaled Composites workers last July at the Mojave Air and Space Port. On Jan. 18, California state occupation and safety inspectors cited Scaled Composites for failing to provide adequate training for workers and fined the firm more than $25,000.

Rutan said his firm has been working with state inspectors and officials to enhance worker safety, but the actual cause of the blast during a rocket oxidizer flow test still was unknown. SpaceShipTwo’s rocket engine will not be finalized until the source of the explosion is pinned down, he said.

Patricia Grace Smith, the Federal Aviation Administration’s outgoing associate administrator for commercial space transportation, lauded the commitment of Virgin Galactic and Scaled Composites to safety after SpaceShipTwo’s unveiling.

“It is the entrepreneurial spirit that will take this country forward,” Smith said. “This is going to catch like a wild fire we have never seen.”