"On 2 August, a few days after the completion of the eight-day flight of the Nauka module and its docking with the ISS, Alexander Kuznetsov was taken to a hospital with suspected stroke," the source said, adding that the 61-year-old was discharged and undergoing rehabilitation.
Another source told Sputnik that the scientist had been readmitted to the hospital.
"Alexander Alexandrovich was hospitalized again," the source said.
Kuznetsov spent the entire duration of the module's flight at the Mission Control Center, virtually without rest.
Nauka docked with the ISS on 29 July after 14 years of waiting on the Earth and eight days of orbital flight. This is the first Russian module sent to the station in 11 years. The module was originally scheduled to go into orbit in 2007, but the launch was postponed multiple times for various reasons.