NASA announced this week its intention to purchase six more seats aboard Russian Soyuz spacecraft for U.S. astronauts bound for the International Space Station.

File photo of a Soyuz launch to the International Space Station. Photo credit: NASA/Bill Ingalls
 
The notice released Monday indicates the space agency will book transportation on Soyuz launches to the space station through the end of 2017, with return trips to Earth reserved through early 2018.

NASA and international partners typically send crews on six-month expeditions on the space station, launching and landing on Soyuz capsules provided by Roscosmos, or the Russian federal space agency.

The proposed sole-source deal also includes rescue and training services provided by Roscosmos. The seats may also be used by European, Japanese and Canadian astronauts, whose transportation to the space station is contracted with Russia through NASA.

Soyuz capsules remain docked at the space station to serve as an emergency lifeboat for their three-person crews during each half-year rotation. The vehicles launch from the Baikonur Cosmodrome in Kazakhstan and return to parachute-assisted landings on the remote Kazakh steppe.

"NASA needs to secure crew transportation with a known reliability for the near-term to ensure a continued U.S. presence aboard the ISS until the sustained availability of a U.S. commercial vehicle," NASA officials wrote in a "presolicitation" notice posted on a federal government procurement website. "The intent of this proposed action is to provide the government the ability to procure these services until an alternate U.S. provider demonstrates full operational capability."

The synopsis posted online Monday says NASA expects the first crewed commercial demonstration flight to the International Space Station to occur in the fall of 2017. Operational crew rotation missions by U.S. commercial crew vehicles will only begin after a successful demonstration flight.

File photo of a Soyuz descent module resting on its side after landing in Kazakhstan. Photo credit: NASA/Bill Ingalls
 
NASA is working with Boeing Co., SpaceX and Sierra Nevada Corp. in public-private partnerships to develop commercial space taxis to ferry astronauts to and from the space station, ending the U.S. space agency's reliance on Russian Soyuz transportation.

The companies are in the home stretch of existing agreements to mature the design of their commercial space transport systems. NASA plans to award contracts to one or two firms no later than September to proceed with the final stages of development, including demonstration flights in low Earth orbit.

Boeing's CST-100 and Sierra Nevada's Dream Chaser space plane will initially launch on United Launch Alliance Atlas 5 rockets. SpaceX's Dragon spacecraft will launch atop the company's own Falcon 9 rockets.

"Until a U.S. commercial vehicle is sustained, continued access to Russian crew launch, return, and rescue services is essential for planned ISS operations and utilization by all ISS partners," NASA wrote in the presolicitation document.

NASA spokesperson Trent Perotto said the space agency has not requested or received a price quote from Roscosmos for the extension of crew transport services through spring 2018.

NASA signed a $424 million contract with Russia in April 2013 covering seats for six astronaut flights launching in 2016 with landing services through June 2017. The deal was worth about $70.7 million per astronaut seat.

Follow Stephen Clark on Twitter: @StephenClark1.