Yui served as an aquanaut aboard the Aquarius underwater laboratory during the NEEMO 16 undersea exploration mission in June 2012.

[8] In 2013 Yui was assigned as backup Flight Engineer for Italian astronaut Samantha Cristoforetti for ISS Expedition 42/43.

[9] Cristoforetti launched onboard Soyuz TMA-15M alongside fellow Expedition 42/43 crewmembers Anton Shkaplerov and Terry Virts 23 November 2014, following which Yui began training as prime crew Flight Engineer for ISS Expedition 44/45 alongside Roscosmos cosmonaut Oleg Kononenko and NASA astronaut Kjell Lindgren.

Yui launched onboard Soyuz TMA-17M alongside Kononenko and Lindgren on 22 July 2015 from the Baikonur Cosmodrome in Kazakhstan, a solar panel on the Soyuz failed to deploy once the spacecraft got to orbit although this was not mission-critical, and the crew successfully docked with the ISS six hours later, officially becoming members of the Expedition 44 crew alongside Russian cosmonauts Gennady Padalka and Mikhail Kornienko as well as NASA astronaut Scott Kelly.

[12] Yui remained on board the ISS alongside Lindgren, Kononenko, Korneninko and Kelly following the departure of Soyuz TMA-16M, after which the five transferred over to Expedition 45, they were joined by Russian cosmonaut Sergey Volkov who brought the crew up to six members.