NASA said on Friday it was weighing an option to buy two additional astronaut seats aboard a Russian rocket as a contingency plan against further delays in the launch systems being developed by Elon Musk’s SpaceX and Boeing Co.

A possible purchase “provides flexibility and back-up capability” as the companies build rocket-and-capsule launch systems to return astronauts to the International Space Station (ISS) from US soil for the first time since NASA’s Space Shuttle program went dark in 2011.

The US space agency has since had to rely on Russia’s Roscosmos program to ferry astronauts to the orbital space station at a cost of roughly $80 million per seat, NASA has said.

After 2019 there are no seats available on the spacecraft for U.S. crew, and a NASA advisory panel recommended on Friday that the US space program develop a contingency plan to guarantee access to the station in case technical problems delay Boeing and SpaceX any further.

A NASA spokesperson on Friday characterized a solicitation request NASA filed on Wednesday as a contingency plan.

NASA said it could buy a seat for one astronaut in the fall and another seat in the spring of 2020.