At 0713 GMT on 20 April 2017, a Soyuz FG rocket was launched from the Baikonur Cosmodrome, near Tyuratam in Kazakhstan. The launch was carrying the Soyuz MS-04 crewed spacecraft travellign to the International Space Station. However, due to a decision of Roscosmos only two crew would be carried (the Russian ISS crew has been limited to just two cosmonauts until MLM Nauka module is launched). This resulted in the crew carried by this Soyuz flight being just Russian cosmonaut Fyodor Yurchikhin and US astronaut Jack Fischer with the third seat being occupied by a cargo container. The docking would take place about six hours after the launch at 1318 GMT using the new four orbit fast approach technique.
Apart from a crew reduction, there were a few other unusual elements for this flight. The spacecraft originally intended to be used for this mission, Soyuz No 734, was replaced with Soyuz No 735. Media reports said the spacecraft’s replacement was due to a leak in the Descent Module’s thermal protection system that was discovered as the Soyuz MS-04 underwent testing at Baikonur. However, other media reports believe the replacement is related with the execution of NASA contracts for delivering crews to the ISS.
Another change was that the Soyuz-FG launch vehicle’s third stage (second core stage) had its engines swapped out as a precaution due to an earlier failure in December 2016.