First objective is to have at least one of the Officers (the Commander or the 1st Officer) on his post; at any time, there's always someone in charge.
Second objective is to have at least two other crew members in "Task Period", so that no complex or heavy task becomes impossible to get done.
On the worries side, working and living as a couple on such a confined environment and for a long duration raises several questions.
As pregnancy in space has never been addressed (at least that we know of...) it is not desirable to deal with such a big question mark during a mission meant to take its crew to another planet, both because of the foetus' development and the inherent risks to a crew member, the mother.
Furthermore, to avoid a complete segregation among the crew (Indians vs. Brazilians), the task teams are not coincidental with the couples, having men and women in different shifts thus also keeping eventual jealousies aside (never a husband stays a Task Period alone with the other's wife).
The psychology behind such a mission must deal with highly trained professionals carrying with them the most primitive feelings and reactions.
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