MOSCOW, March 13. /TASS/. U.S. astronaut Joseph M. Acaba will fly to the International Space Stations (ISS) as a third crew member of the Soyuz MS-06 spaceship. His flight will be financed by Russia’s Rocket and Space Corporation Energia as debt repayment to US’ Boeing under the joint project Sea Launch, a source in the Russian space industry told TASS on Monday.

"Joseph Acaba has been appointed as a member of the main crew of the Soyuz MS-06 spaceship due to be launched to the International Space Station on September 13. Shannon Walker has been appointed as a member of backup crew. Most likely, she will be subsequently chosen as a main crew member of the Soyuz MS-08 spacecraft due to fly to the ISS in March 2018," the source said.

A spokesman for the cosmonaut training center told TASS on Monday the two U.S. astronauts are to begin pre-flight training but did not specify which crews they have been appointed to.

According to earlier reports, under an amicable agreement reached by Energia and Boeing as part of debt repayment under the Sea Launch project, the Russian corporation will give the American side five seats aboard Soyuz spacecraft, in particular one seat in 2017, one seat in 2018, and an option on three seats in 2019. Energia’s debt to Boeing was 330 million US dollars, as was ruled by a California court in 2015. In the summer of 2015, the sides reached an amicable agreement where Energia undertook to repay its debt by means of works and new projects.

Acaba and Walker were named NASA astronaut candidates in May 2004. Both have already made a spaceflight aboard a Russian Soyuz spaceship.