[1] Following his graduation he was part of the specialization and improvement group for the ISS Russian Orbital Segment and Soyuz TMA-M spacecraft, he held this position until June 2015 when he was assigned to a backup crew.

The trio launched on Soyuz MS-09 from the Baikonur Cosmodrome on 6 June 2018 and spent approximately two days free flying in Low Earth orbit before the rendezvoused and docked to the ISS on 8 June, officially joined the Expedition 56 alongside American astronauts Andrew Feustel and Richard Arnold as well as Russian cosmonaut Oleg Artemyev.

[5] On 29 August an air leak was observed inside the station, this was later discovered to be caused by a hole aboard Soyuz MS-09, Prokopyev's spacecraft.

However, damage to the spacecraft extended the mission, and Prokopyev returned to Earth on 27 September 2023 with Soyuz MS-23 after spending a year in space.

[12] Prokopyev is married to Ekaterina Prokopyeva (née Negreyeva), who gave birth to their daughter, Anna, on 27 August 1997 and son, Timofei, on 23 February 2010.