To some veterans of the American space program, the liftoff of the Space Shuttle Endeavor Monday morning was bittersweet.
After decades of American dominance in space exploration, the next-to-last shuttle flight brings country to the threshold of a period that experts are calling "The Gap," -- the first significant stretch of time in decades during which the U.S. will be unable, on its own, to put astronauts into space.
"I don't like it at all," said Rep. C.A. Dutch Ruppersberger, a Maryland Democrat who has led oversight of the space program. "The previous administrations have not made space a priority. It's expensive. Now we're in this situation."
If the fears of some in Congress come true, a period of unprecedented drift for the space program could follow the final Shuttle launch, now scheduled for July. With no American vehicle capable of carrying astronauts into space, the U.S. will be forced to pay the Russians a steadily escalating price -- eventually hitting $62.7 million per seat -- to carry Americans and international partners to the International Space Station through 2016.
Meanwhile, NASA Administrator Charles Bolden told ABC News that the U.S. will be relying on a relatively young collection of private companies to build the rockets that will restart American-led missions to the space station, which he estimates will begin launching by 2015.
"Everybody knew it was coming," Bolden said of The Gap. "The primary hurdle it creates is that people will become comfortable with it. We tend to be short-sighted and our memory is short."
NASA officials are quick to note that under the Bush administration's space initiative, known as Constellation, The Gap would have lasted eight years. A six-year gap, if all goes as planned, would pass more quickly than the eight-year gap between the end of the Apollo program and the launch of the first space shuttle in 1981.
The public posture of NASA officials has been to focus on a modernized program that relies far more on private companies to handle the increasingly routine work of hoisting satellites and servicing the space station, while dedicating U.S. government resources to planning the more complex task of taking astronauts deeper into space. Bolden says NASA will be developing a separate, heavy-lift rocket to explore deep space and eventually, maybe, take astronauts to an asteroid, the moon, and Mars.
But privately, political leaders are bemoaning what could be a deeply unsettling period during which the U.S. will have no way to put humans into space -- and efforts to reach more distant destinations appear hazy and uncertain. Bolden's critics told ABC News they see significant hurdles for NASA after the shuttle retires. They are worried that without a clear destination or proven spacecraft to get there, it could be a long, long time before a manned U.S. rocket heads for the heavens.
"What used to be a gap is now a cliff," said Michael D. Griffin, who served as NASA administrator from 2005 to 2009 under President Bush. "What really is happening here is the destruction of an American institution that has been preeminent in the world for the past 40 years. I believe it's tragic."