astronaut

After their arrival, astronauts mounted the samples on the exterior of the station, exposing them to space for 283 days.

It has since sought to bring other countries into its efforts and signed a deal with Pakistan in February to recruit the first foreign "taikonauts" -- a term used for astronauts in China's space programme.

It will be an uncrewed mission, carrying cargo to the ISS, potentially paving the way for the spacecraft to be used for future astronaut missions.

Sen Kelly, who served in the Navy and is a former astronaut, says: "Our laws are clear. You can refuse illegal orders."

Both Blue Origin and SpaceX have submitted bids to build the lunar lander that will return astronauts to the moon, a project that was initially awarded to SpaceX.