But at the end of the experiment, "less than half of the mice made it -- but that was to be expected," Sychov told Russian news agencies.
"Unfortunately, because of equipment failure, we lost all the gerbils."
The TsSKB-Progress space research center's department head, Valery Abrashkin, said on the day the mission took off in April that the study was aimed at determining how bodies adapt to weightlessness "so that our organisms survive extended flights".
The space adventure has been widely praised by Russian state media as a unique experiment that no other country has yet pulled off.
Russia last sent mice into space in 2007 for a much shorter duration of 12 days.
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France's Center National d'Etudes Spatiales (CNES) space center said 15 of the 45 mice came from a French research lab that is cooperating with the study.
CNES life science department head Guillemette Gauquelin-Koch said the project took "a further decisive step in human adaptation to weightlessness".