Soyuz spacecraft. Credit: NASA

NEW YORK — NASA has signed a new $335 million contract with Russia to buy six extra seats on Soyuz spacecraft to launch American and partner astronauts into space after the space shuttle fleet is retired, the U.S. space agency announced April 6.

The new deal allows NASA to pay the Russian Federal Space Agency for six round-trip rides to and from the international space station in 2013 and 2014. That averages to about $55.8 million per trip — a slight increase from the $51 million NASA paid for seats on the Russian-built Soyuz spacecraft through 2012.

After the scheduled retirement of NASA’s three-orbiter space shuttle fleet this fall, American astronauts will have to rely on Russia for transportation to the international space station until U.S. commercial firms can build spaceships capable of carrying humans.

U.S. President Barack Obama is hoping the commercial ventures will come through.

In his 2011 budget request, Obama proposed that NASA cancel its current plans for a space shuttle replacement vehicle and instead invest in the private sector to encourage companies such as Hawthorne, Calif.-based Space Exploration Technologies (SpaceX) to develop spacecraft to carry astronauts to low Earth orbit. That would free the agency to focus on more-ambitious missions to the Moon, asteroids or Mars, NASA officials have said.

The Space Shuttle Discovery launched April 5 on one of NASA’s final four shuttle missions scheduled before the fleet retires.

“We’re having redundant services,” NASA spokesman John Yembrick said April 6. “We always plan on purchasing a Soyuz vehicle to make sure we have access to the space station while commercial is progressing toward cargo and eventually crew capability. We’re making sure that we’re going to have access.”

Since no commercial firm has yet launched humans into space, NASA cannot be sure when to expect alternative options to be ready. And Russia requires a good deal of advance notice to make sure it will have Soyuz seats available, Yembrick said.

The new contract provides for the six astronauts to launch on four Soyuz vehicles in 2013 and return on two vehicles in 2013 and two in 2014. That will allow for roughly the same number of NASA crew members on the space station in 2013 as there are now.

“Right now we’re keeping the crew of six with our partners and Russia to the same level it is now,” Yembrick said, explaining that future space station crews will be composed of about the same proportions of Americans, Russians, Europeans, Canadians and Japanese as they are now.

The contract also covers necessary astronaut training and preparation for the flights, and crew rescue if it is needed. The fee allows each astronaut to pack about 50 kilograms of cargo on the trip to space, and about 17 kilograms for the return trip, plus trash.

NASA already has contracts in place with two American companies to provide unmanned cargo shipments to the space station in coming years.

The space agency has agreed to pay SpaceX a total of $1.6 billion for 12 cargo delivery flights to the station using its Falcon 9 rockets and Dragon space capsules. The first Falcon 9 rocket is targeted to fly a test mission no sooner than May 8.

SpaceX has said that its Dragon capsule is designed to function as a crewed vehicle as well.

NASA also has signed a $1.9 billion deal with Dulles, Va.-based Orbital Sciences Corp. for eight cargo shipments using the company’s unmanned Cygnus spacecraft and its Taurus 2 rocket. The first flight of a Cygnus vehicle is targeted for late March 2011.