Why do astronauts stay on the International Space Station for 6 months?

Answer 1

I am not an expert on the matter but I will give you what I know.

Once the astronauts and necessary supplies are on the Station it is relatively inexpensive to stay there since it is a self-contained living environment. It is the travel to and from the station that is expensive, and even more so for the people than for supplies. Consider also that launch is still the most dangerous part of space travel.

(On a side note, this was the fundamental flaw in the design of the Space Shuttle. And I'm saying this as a big fan of the space program. I came of age in the 80s and there was no greater pride than the Shuttle. But the expense and danger of launching what was essentially a semi trailer truck into space with people on board, whether loaded or not, was considerably higher than what would have been if the people were launched in a much smaller vehicle with equipment and supplies launched separately. This separate launch system is what is being designed for future United States manned space flight.)

I know there is a higher level of exposure to radiation on the Station than on the ground, which limits how long they can stay. It is my understanding that this exposure is what currently limits our desire to travel to Mars. Otherwise the technology exists to make the trip.

There is also the atrophy of muscles while in micro gravity. They exercise rigorously every day but over time there is still a measurable amount of loss of strength and flexibility that increases with time in space.

They are not just sipping coffee while up there. They have a full schedule of scientific experiments they work with every day. The physical exertion necessary might be less than sitting in an office cubicle (except when working with the physical maintenance and repair of the Station itself) but they are working nonetheless.