With NASA's space shuttle fleet now permanently grounded, aerospace giant Boeing is aiming to fly astronauts to the International Space Station aboard a new private spaceship as early as 2015 or 2016, company officials say.

Boeing's CST-100 capsule (short for Commercial Space Transportation-100) is being designed to ferry astronauts to and from the space station and other destinations in low-Earth orbit. The spacecraft will initially launch from Florida atop United Launch Alliance's Atlas 5 rocket, but the company is not ruling out other booster options in the future, officials have said.

The capsule is being designed as part of a NASA program that supports the development of a new fleet of commercially built spaceships to fill the gap made by the retirement of the shuttle program.

"It's been an interesting last couple of years for us," Roger Krone, president of Boeing's network and space systems, told reporters this month. "I think many people in the industry associate Boeing with the shuttle program and the International Space Station. [This is] kind of a chance for us to rethink what our space strategy is."

Boeing is one of several competitors, including SpaceX and Sierra Nevada, who are engaged in a private space race to build new manned space taxis. Boeing is aiming to have the CST-100 ready to launch the first crew in 2015, but this is heavily dependent on the amount of funding received by NASA, said John Elbon, vice president and general manager of Boeing's space exploration division. [Photos: Boeing's Space Capsule CST-100]

"We could launch as early as 2015, depending on funding, but the way the budget is laid out, it most likely will be 2016," Elbon said.

Over the last two years, NASA's Commercial Crew Development program has divided $320 million among four American spacecraft builders: SpaceX, Blue Origin, Sierra Nevada and Boeing. So far, the agency has awarded Boeing with approximately $120 million for the company's work on the CST-100 capsule.

But funding for the program has been an ongoing challenge.

In 2012, Congress gave just $406 million for commercial crew development in 2012, which was less than half of the $850 million originally requested by NASA.

The bleak budget environment has already delayed NASA's first planned launch aboard a commercial spacecraft by two years, and the agency will likely face more financial constraints going forward.

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Boeing's Private Space Taxi to Take Flight by 2016