NASA decided to adopt to new situation, which was caused by decision made by Roscosmos to reduce number of their permanent residents from three to two. Agency announced that in the second half of 2017 number of American astronauts will be increased to four.
Astronauts Shannon Walker and Joe Acaba were probably already designated by NASA to occupy seats inside Soyuz spacecrafts during flights planned for second half of 2017. These places were not planned for NASA during MS-06 and MS-08 missions. After announcing its decision about reducing number of Russian cosmonauts, Roscosmos freed two seats. These places were given to Boeing as part of agreement after Sea Launch trial. Earlier this year NASA purchased these places from boeing. Agency decided for rather risky strategy and will use these seats now to keep present research schedule up to date. Previously places were considered as backup option to be used if Boeing and SpaceX will fail to keep the schedule of their CST-100 and manned Dragon spacecraft and NASA will not be able to deliver to ISS their astronauts in 2019 inside American made vehicles. To clarify situation, NASA did not purchased places for their astronauts for flights planned to 2018 and in spite of delay confirmed by SpaceX and Boeing, declares that it will not be necessary.
Astronauts Shannon Walker and Joe Acaba already started their training at Gagarin Cosmonaut Training Center on March 13, 2017. It was not officially confirmed by NASA, but according to Gagarin Cosmonaut Training Center Shannon Walker will fly to ISS during MS-06 mission.
Previous experience of both astronauts with ISS missions is rather confirmation of the fact, that they will visit Station in following months. Both will be capable to take active participation in scientific experiments or service procedures necessary to keep ISS operational. It is worth to remind, that research program for 2017 seems to be quite busy and NASA would like to make the best use of the last years of ISS.
Joe Acaba (born 1967) is experienced astronaut with 137 days, 19 hours, and 22 minutes in space and two missions in his career (STS-119 and Expedition 31/32). He is hydrogeologist and he was chosen to NASA Astronaut Group 19 in may 2004. Shannon Walker (born 1965) is physicist and was chosen to Astronaut Corps in this same group as Joe Acaba. She has spent 163 days, 7 hours and 11 minutes in space and has one mission on her account: Expedition 24/25.