This is a very important subject.
Recommended reading: Nick Kanas, Dietrich Manzey. Space Psychology and Psychiatry. Springer, 2008.
To quote (pp.193-194):
From the very beginning of long-duration space flight, the provision of
psychological in-flight support to crewmembers has been an important counter-
measure in Russia [Grigoriev et al., 1987; Kanas, 1991]. For this purpose, a psychological support group was established that coordinated different activities in order to counter feelings of monotony, isolation, and behavioral health issues like asthenia. Such activities have included surprise presents and favorite foods delivered via re-supply vehicles, increased on-board music and lighting, increased contact with people on Earth, and ground-crew counseling or psychotherapy [Kanas, 1991, 1998]. In addition, the arrival of visiting astronauts and cosmonauts has helped break the monotony and provided stimulation and assistance in performing mission activities.
Based on this experience, a similar system has been established by NASA for its space station support activities [Flynn, 2005; Sipes and Vander Ark, 2005].
Minimum psychological support measures (after Kanas, Manzey/ISS MORD 2000):
- Personal packages by re-supply flights (not applicable to Mars mission)
- Uplink of audio news once a week
- Uplink of written news summaries once every 2 days
- Uplink of sports/concerts/news videos
- Videos, books, music, computer games
- Onboard ham radio (not applicable to Mars mission)
- Daily uplink of e-mails from family and friends
- Private two-way videocalls contacts with family and friends (15 minutes or more for each crewmember per week)
- Private two-way videocalls with members of the psychological support group (10 minutes or more for each crewmember once a fortnight)
- Psychological intervention if necessary
- Family support during the mission as necessary
When all else fails, there are risperidone and other anti-psychotics etc. in medkits.