There are obvious problems for humans trying to survive in space, and some that are not as obvious. Obvious issues include those associated with simply having the necessary substances for life--air, water, and food. These do not exist in space and must be transported from Earth. When astronauts live for...
There are obvious problems for humans trying to survive in space, and some that are not as obvious. Obvious issues include those associated with simply having the necessary substances for life--air, water, and food. These do not exist in space and must be transported from Earth. When astronauts live for extended periods on the International Space Station air and water are conserved and recycled; food must be continuously brought from Earth. Wastes are ejected from the ISS.
Health issues that are not as apparent are those associated with extremely low gravity (I'll refer to this as 0g, though it is not exactly that), and the exposure to radiation. Scientists know that extended time in 0g results in loss of bone density and changes to the cardiovascular system. Bones are not working against the usual 1g of Earth; the lack of typical stress leads to loss of mass. The heart and blood vessels have evolved to work in Earth's gravity; when that is not present there can be loss of muscle tissue, including heart muscle. Blood pressure takes time to re-adapt back on Earth. Heart problems have been noted in returning astronauts. One unexpected issue that has appeared is the effect of 0g on the eye and vision, with swelling of the optic nerve and increase in far-sightedness--see the Times article below for more detail. Efforts to limit the effect of 0g include having the astronauts run while strapped down to a treadmill, in an effort to simulate the effect of gravity. Possibilities in the future could include spinning part of a spacecraft to simulate gravity. But at this time, when they return to Earth after an extended time in space, astronauts need to rest while their bodies readjust to 1g.
Exposure to cosmic radiation in the form of high-energy, ionizing cosmic ray (HZE) nuclei is a huge problem for humans in space. The shielding of shuttles and the ISS is an attempt to limit the amount of radiation to which the astronauts are exposed, but it is not currently the same as the shielding effect of Earth's atmosphere. Cancer of various types is a risk, as are effects on the immune system; other long-term effects are currently not known, and studies on both the ISS astronauts and other life forms taken to the ISS have been on-going. It has been difficult to produce shielding that is both effective and light enough to launch into space. This is a problem currently being worked on as a manned flight to Mars is being planned. NASA guidelines are that the level of exposure to HZE should at most double an astronaut's life-time cancer risk, but we do not yet have shielding that could protect astronauts at that level for the several years it will take for a Mars mission.