Video: Soyuz MS-01 leaves the space station for landing

A Soyuz rocket is sitting on a historic launch pad in Kazakhstan after a railroad journey at sunrise Wednesday positioned the crew ferry craft for liftoff Friday with a three-man crew destined for the International Space Station, including two spaceflight veterans who will spend nearly one year in orbit.

Investigators believe a leak of propellant inside the Crew Dragon spacecraft’s propulsion system led to the capsule’s explosion April 20 during a ground test at Cape Canaveral, and a senior SpaceX official said Monday that delays are making it “increasingly difficult” to fly astronauts on the commercial spaceship before the end of the year.

An unexpected, quickly-planned spacewalk is on tap Monday to troubleshoot a problem preventing the International Space Station’s robot arm transporter from reaching a worksite where it can be locked down, a requirement before a Russian Progress cargo ship can dock later in the week.