The interruption in American-controlled access to space rankles some in Washington, including Senator Bill Nelson of Florida, a leading proponent of the space program. In an interview, Mr. Nelson said it was “inexcusable” for the country’s space program to be put in a position of dependence on such a politically volatile partner. “We’ve got a Russian prime minister who believes he’s czar,” he said of Vladimir Putin, referring to Russia’s military action in Georgia.

The United States has had periods in which its astronauts could not reach space: from the end of the Apollo program in 1975 to the beginning of shuttle flights in 1981, and for more than two years after the loss of the shuttles Challenger in 1986 and Columbia in 2003. But the coming interval could be the longest if the rollout of NASA’s new rockets is significantly delayed.

Even though the outlines of the gap have been known since soon after Dr. Griffin began running the agency in 2005, Cmdr. Scott J. Kelly of the Navy, an astronaut who has made two trips to orbit, warned in April that the prospect of a United States that could not send humans into space on its own rockets would come as a shock. “A large part of the American public is going to be surprised,” he said, adding that people would cry, “Who let that happen?”

Image Vitaly Davidov of the Russian space agency. Credit... James Hill for The New York Times

The Politics

The Bush administration chose to give up the nation’s own access to space for five years and move to the next phase of space travel. The administration decided to retire the shuttles and in January 2004 announced a sweeping “vision for space exploration.”

Under the plan, NASA would stop using the aging and risky shuttle fleet and move to a new launching program, Constellation, built around Ares rockets and Orion capsules that are designed to return astronauts to the Moon and even to explore near-Earth asteroids and Mars.

To get from one program to the other without inflating NASA’s $17 billion annual budget, the administration decided to wind down the shuttle program and ramp up Constellation. The decision has always been portrayed as difficult, but in recent months, criticism has flared. The Republican and Democratic presidential candidates, for example, have pledged to keep America flying.