The loss of bone and muscle mass they experience in space is so profound that they cannot stay any longer.
But what about the health impact of forthcoming suborbital flights for space tourists who are not fit, highly-trained individuals?
Yet there will be few GPs experienced enough in space medicine to provide advice.
Past research tells us that spaceflight causes changes in the physiology of the human body, but how it might affect underlying medical conditions in an unfit, 50-year-old space tourist is not yet clearly known.
Dr David Green, senior lecturer in human and aerospace physiology at Kings College London, predicts that in the next two years or so significant numbers of people will be taking up places on suborbital flights in a specially-designed spacecraft.
This means they will dip out of Earth’s atmosphere, experience weightlessness for around four minutes and then descend back to Earth’s surface.
The speed of the acceleration and deceleration involved in that flight could be an issue for some, Dr Green says.
“It’s highly likely you will feel sick or be sick and that’s a real concern.
“Also, there will be an issue making sure everyone gets back in their seats after floating about.
“Going back to Earth, everything will feel heavier. You could knock yourself unconscious.”
The most common problems during a spaceflight have been shown to be motion sickness, fatigue, dehydration, loss of appetite and back pain.
During the massive vertical acceleration and deceleration of spaceflight, it is hard for the heart to pump blood to the brain.
“If you have underlying cardiovascular disease that could be exposed,” says Dr Green.
Dr Jon Scott, aseniorscientistatQinetiQand