What are the long term affects of zero gravity on humans?

Ultimately, the complete long-term effects of microgravity on humans are unknown. This is simply due to the fact that the actual experiment (i.e. A truly long-term space-flight upwards of a year) has not been carried out yet.

The physical effects of microgravity on the bodies of astronauts and cosmonauts have been well-studied. Potentially, the most series problems occur with deterioration of weight-bearing structures (i.e. Muscles and bones).

From the Wikipedia article on "Human adaptation to space": "Microgravity results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip and femur. Elevated blood calcium levels from the lost bone result in dangerous calcification of soft tissues and potential kidney stone formation. It is still unknown whether bone recovers completely."

As has been noted elsewhere, however, thus far astronauts have recovered from these effects after sufficient time in Earth gravity. If the time of exposure to microgravity was increased though, this recovery is less certain. An issue of potentially greater concern for long-term space-flight is radiation.

While exposure to radiation is not a direct result of microgravity, it does occurs at much higher intensities outside the confines of Earth's protective atmosphere–i.e. In space, which is a microgravity environment. Therefore, it's not too big a stretch to say that high-levels of radiation are present as a result of being in a microgravity environment.

From the same Wikipedia article: "Without the protection of the Earth's atmosphere and magnetosphere astronauts are exposed to high levels of radiation through a steady flux of cosmic rays. A year in even low-earth orbit results in a dose of radiation 10 times that of the annual dose on earth resulting in a high risk of astronauts developing cancer. Gh levels of radiation can create 'chromosomal aberrations' in blood lymphocytes.

These cells are heavily involved in the immune system and so any damage may contribute to the lowered immunity experienced by astronauts. Over time immunodeficiency results in the rapid spread of infection between crew members, especially in such confined areas. Radiation has also recently been linked to a higher incidence of cataracts in astronauts.

Protective shielding and protective drugs may lower the risks to astronauts to an acceptable level, but data are scarce, and longer-term exposure will result in greater risks. " In addition to these risks, there are many more that may arise with long-term exposure to microgravity and space-flight, including psychological issues. Again, from the same Wikipedia article: "The enormous stress on the crew, coupled with the body adapting to other environmental changes, can result in anxiety, insomnia and depression."

All in all, the jury is still out. Which explains why this is such an active area of research as the United States and other countries looking towards missions requiring longer exposure to microgravity. There will likely be methods developed that will lessen most, if not all, of these effects.

But there is a lot of work yet to be done before this becomes a reality.

I do not believe there are any "For the most part, the effects of zero-gravity are reversible. Astronauts gradually recover from voyages into space and their bodies return to their normal function. The length of the recovery period varies in accordance with the duration of the mission.

Even when astronauts undergo strenuous exercise routines daily to try and maintain bone and muscle mass during a long space mission, some still have to carried on stretchers when they return to earth.

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