This artist's concept shows a SpaceX Crew Dragon docking with the International Space Station, as it will during a mission for NASA's Commercial Crew Program. SpaceX

SpaceX has been selected by NASA for another crewed flight to the International Space Station, marking the second such mission that Elon Musk's company has been granted an order for. The move, part of NASA's Commercial Crew Transportation Capability (CCtCap) Program, is aimed at ending America's reliance on Russia for ferrying astronauts to the space station.

This is the fourth and final guaranteed order the space agency will make under the CCtCap contracts. SpaceX received its first order last November, while Boeing — the other company participating in the program — received two orders in May and December 2015.

"The order of a second crew rotation mission from SpaceX, paired with the two ordered from Boeing will help ensure reliable access to the station on American spacecraft and rockets," Kathy Lueders, manager of NASA’s Commercial Crew Program, said in a statement released Saturday. "These systems will ensure reliable U.S. crew rotation services to the station, and will serve as a lifeboat for the space station for up to seven months."

Since the U.S. ended its space shuttle program in 2011, Russia’s Soyuz rockets have the only means of ferrying astronauts to and from the ISS. Under its current arrangement, NASA has to shell out nearly $80 million for every seat on the Soyuz rocket.

However, Russia's Roscosmos recently announced that it does not have any plans to send U.S. astronauts to the ISS after 2018. This means that unless SpaceX or Boeing are ready with their flagship capsules by then, U.S. presence on the space station would be directly impacted — at least temporarily.

As things stand now, SpaceX plans to begin testing its Crew Dragon capsule sometime next year, while Boeing CST-100 Starliner's first uncrewed and crewed test flights are scheduled to take place in December 2017 and February 2018 respectively.

"We’re making great progress with Crew Dragon, with qualification of our docking adapter and initial acceptance testing of the pressure vessel qualification unit completed" Gwynne Shotwell, SpaceX president and chief operating officer, said in the statement. “We appreciate the trust NASA has placed in SpaceX with the order of another crew mission and look forward to flying astronauts from American soil next year."

It is still not clear which company would be the first to send astronauts to the ISS. In its statement, NASA said that the decision would be taken "at a later time."