In its early years, NASA's efforts to propel Americans into space were driven by an overarching imperative of competing with the Soviet Union during the Cold War. Half a century later, with the final U.S. space shuttle mission set to blast off on Friday, Russian vehicles will be the only option for American astronauts.

NASA plans to continue sending four to six astronauts annually to the International Space Station, and it will be paying Russia for the privilege. The Wall Street Journal reported that Russia has already begun tripling the cost of seats on Soyuz crew capsules, which will run the price tag to between $43 million and $56 million per seat.

We are not in a very comfortable situation, and when I say uncomfortable, that is a euphemism, Jean-Jacques Dordain, director general of the European Space Agency, told the Wall Street Journal. We made a collective mistake.

The shift parallels an exodus of NASA astronauts as astronauts recognize that, with seats on the remaining shuttles scarce, the wait for spaceflight is a long one. NASA currently employs 61 astronauts, down from 150 in 2001.

Some of the departing astronauts are leaving to work on what promises to be the next frontier in American space exploration: privately designed and financed shuttles. Earlier this year, NASA awarded a total of $269.3 million to five U.S. aerospace companies who are developing technology to ferry astronauts to the space station by 2016.

We are working aggressively to get our own crew capability, said William Gerstenmaier, NASA's associate administrator for space operations.