"One of the major obstacles to long-term space missions in the threat of severe bone loss in astronauts. In the microgravity environment of space, astronauts lose on average 1% to 2% of their bone mineral density every month. For a short-duration flight, bone loss is a fairly minor consequence.
On a long-duration space flight, such as those planned for missions to Mars and beyond, bone loss can be a serious impediment. This loss may not hinder astronauts while they are in orbit, but upon return to Earth, their weakened bones will be fragile and at an increased risk of fractures. At this time, it is unknown whether this bone loss will eventually reach a plateau, or whether it will continue indefinitely." [1]
2) ISS astronauts spend a couple of hours every day exercising to keep the worst of the space-related medical issues at bay... but it's simply no substitute for real gravity.
Space has all sorts of weird effects on the human body: [2]
"One of the major obstacles to long-term space missions in the threat of severe bone loss in astronauts. In the microgravity environment of space, astronauts lose on average 1% to 2% of their bone mineral density every month. For a short-duration flight, bone loss is a fairly minor consequence.
On a long-duration space flight, such as those planned for missions to Mars and beyond, bone loss can be a serious impediment. This loss may not hinder astronauts while they are in orbit, but upon return to Earth, their weakened bones will be fragile and at an increased risk of fractures. At this time, it is unknown whether this bone loss will eventually reach a plateau, or whether it will continue indefinitely." [1]
2) ISS astronauts spend a couple of hours every day exercising to keep the worst of the space-related medical issues at bay... but it's simply no substitute for real gravity.
Space has all sorts of weird effects on the human body: [2]
[1] https://www.asc-csa.gc.ca/eng/astronauts/space-medicine/bone...
[2] https://www.smithsonianmag.com/science-nature/nasas-twins-st...