Yet the big unknown, according to Kanas, does not involve who astronauts will not be able to talk to or what gifts they will not be able to get, but instead what they will not be able to clearly see: planet Earth.

CNN: Mars on the brain? Red Planet pioneers to face cosmic mind trip

They may be driving while not able to see road signs clearly anymore or may be wearing glasses for driving which are no longer suitable.

BBC: NEWS | Health | Many drivers 'have poor eyesight'