NASA is turning to private space companies to plug a worrisome five-year gap in its ability to boost astronauts into orbit and return them safely to Earth.

The gap runs from the end of next year, when the three remaining space shuttles are supposed to be retired, until 2015, the earliest that NASA's slow-moving replacement system, called Constellation, will be ready to do the job.

After the shuttle' retirement, the U.S. will have to rely on Russia to ferry Americans to and from the International Space Station , where six crew members from five nations are now circling the Earth.

This situation frustrates and embarrasses NASA and annoys lawmakers, who fear citizens will wonder why the U.S. has to depend on its old Cold War rival for rides into space.

As a result, two private companies are being asked first to demonstrate their ability to deliver cargo — food, water, equipment and supplies — to the space station starting in 2011. Commercial launches of human crews, a much riskier operation, would come no sooner than 2012, if at all.

There will be "a significant gap" in our ability to get cargo and people into orbit, Michael Suffredini , ISS program manager, told a high-level "Review of U.S. Human Space Flight Plans Committee" last week.

The committee, headed by Norman Augustine , the retired chief executive of the space-military contractor Lockheed Martin , was created by the Obama administration to evaluate NASA's plan to send humans to the moon and eventually to Mars. It's supposed to make its recommendations — to continue, alter or scrap the project — by the end of August.

"We believe we can eliminate the gap," Elon Musk , chief executive of Space Exploration Technologies — SpaceX for short — of Hawthorne, Calif. , told the Augustine committee. "We can provide housekeeping and logistical services," he said. "We'll let NASA concentrate on the Moon and beyond."

SpaceX and its chief rival, Orbital Sciences Corp. of Dulles, Va. , received preliminary NASA awards totaling $500 million in December. SpaceX's contract calls for it to make 12 cargo flights starting in March. Orbital is supposed to launch eight cargo flights, beginning in March 2011 . The contract's total cost will be $3.5 billion .

"Cargo doesn't seem glamorous, but we need it to keep America flying in space," Frank Culbertson , Orbital's senior vice president for human space flight, told the committee.

Skeptics are concerned about the ability of private companies to meet NASA's safety and reliability standards. SpaceX's Falcon 1 rocket failed three times before it finally managed to put a small, dummy payload in orbit last September. Orbital lost a Taurus rocket, carrying NASA's Orbiting Carbon Observatory , in February.

"Despite their best efforts, some truly private enterprises have not been able to deliver on plans of launching vehicles," Sen Richard Shelby , R- Ala. , told a Senate hearing last month. "However grandiose the claims of proponents (for commercial space flights) are, they cannot substitute for the painful truth of failed performance at present," Shelby said.