The ambitious timetable is entirely contingent on NASA's budget, but it raised the possibility that the U.S. may be able to reduce a projected five-year gap in manned space flights following the shuttle's retirement last month — and ease reliance on Russia to ferry U.S. astronauts and cargo to the $100 billion space laboratory.

"This is the quickest way to close the gap and get U.S. crews flying again," said John Elbon, vice president and program manager of commercial crew programs for Boeing. "It's an affordable approach that will then leave NASA funding to develop capabilities for exploration beyond low-Earth orbit."

The effort by the NASA contractor to create the first commercial spacecraft system rated safe for astronauts represents a significant step toward fulfilling President Barack Obama's vision for future space exploration.

Obama ended development of the post-shuttle manned spacecraft known as Constellation to return astronauts to the moon in favor of using resources on deep space exploration and leaving commercial craft to carry out service missions to the space station.

Boeing and United Launch Alliance made their announcement in a conference call Thursday, just two weeks after a final shuttle flight by Atlantis ended the program.

"We believe Boeing's selection of the Atlas V rocket reflects another positive step in the development of a commercial crew transportation system," said NASA spokesman Grey Hautaluoma. "This brings Boeing and United Launch Alliance one step closer to developing one that NASA potentially could use one day to fly astronauts to and from the International Space Station."

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Obama envisions landing astronauts on asteroids by 2025 and orbiting Mars by 2035.

Partnering Boeing's crew capsule with United Launch Alliance's Atlas V rocket will be the most affordable way to service the space station "so there's funding left over in NASA's budget to invest in vehicles that can take us beyond low-Earth orbit to (gravity-neutral) Lagrange Points, asteroids, perhaps back to the moon, the moons of Mars, Mars - all those kinds of exciting adventures," Elbon said.

By choosing the Atlas V rocket to carry the Boeing crew space transportation capsule into orbit, Boeing opted for a powerful workhorse that has had 26 successful missions delivering payloads and satellites into orbit since 2002.

The Atlas V enjoys the federal government's highest safety and reliability rating, certified to carry multibillion-dollar satellite systems and payloads with nuclear fuel into orbit. The Atlas V and its immediate predecessors have scored 97 successful launches.

Orbit by 2015?

George Sowers, a ULA vice president, said Atlas' routine use of Russian rocket engines would have no impact on gaining NASA's certification to carry humans into low-Earth orbit.

The Russian-built RD-180 engine has been used without incident on 32 Atlas launches. "Our Russian partners continue to perform very well," Sowers said. "We have no plans to replace them at this time."

The officials outlined three tests designed to enable two Boeing test pilots to fly into orbit to the space station in the final three months of 2015. The tests include a launch pad test, an Atlas V going into orbit and a rocket ascent with an abort by the crew capsule.

"All this is going to bring down the cost and make human spaceflight more affordable," Sen. Bill Nelson, D-Fla., chairman of the Senate subcommittee with jurisdiction over NASA, said of Boeing's announcement.

Congress has yet to enact Obama's request for $18.7 billion for the current fiscal year. NASA is seeking $3.9 billion for human exploration, including $850 million for commercial spaceflight and $289 million for exploration research and development such as next-generation propulsion systems for deep space exploration.

stewart.powell@chron.com