Whether or not you subscribe to Elon Musk's view that the survival of humanity is dependent on interplanetary travel (and the eventual colonisation of Mars), it's hard to ignore the fact that humans might soon be travelling very long distances through space.

NASA has investigated the feasibility of sending astronauts to near-Earth asteroids and recently announced a manned mission to Mars by mid-2030, while the Mars One program has an ambitious plan to establish a Martian colony by 2025. (Here's the roadmap).

Of course, conquering the distance between Earth and Mars will require a plethora of sophisticated technology and means spending A LOT of time in space - anywhere from 150 to 300 days depending on the speed of the launch, planetary alignment, and how frivolous the spacecraft operators are with fuel. And once there, (presuming the mission and landing were successful) settlers' would still have to live in a planet with one-third the gravity of earth. 

To ensure these future missions can be carried out safely, researchers are trying to learn more about the potential health effects of long-term space flight.   

In addition to the loneliness and boredom that might accompany a journey to Mars, it is fairly well-understood that spaceflight affects almost all of our physiological systems. For instance, prolonged exposure to microgravity can cause bone loss, and result in a greater chance of post-mission fractures. It can also cause muscle fibres to shrink, leaving people feeling weaker, and less coordinated. (For a detailed breakdown of how the human body responds to spaceflight check out this inforgraphic from NASA's Human Research Program).

Spaceflight has also been linked to decreased functioning of the immune system. Fifteen of the 29 Apollo astronauts contracted bacterial and viral infections either during their missions, or upon returning to earth. This heightened risk of illness could seriously jeopardise a mission, but little is known about what causes it.

Now, a team of researchers from France has suggested that spaceflight may cause the immune systems of mice to age prematurely. The team simulated the effects of microgravity on mice by suspending their hind limbs (an often-used practice to simulate spaceflight called hind limb unloading)

The team found that "limb disuse" led to changes in the microstructure of the suspended mices' bones, and a decrease in the number of B-cell progenitors in the bone marrow. B-cells are white blood cells and they play a critical role in producing antibodies to protect us from infection.

The researchers say the modifications observed in the B cells are "similar to those observed in aged mice". Their results were published in the Journal of the Federation of American Societies for Experimental Biology.

In August 2014, NASA released results from two studies looking at changes in the immune systems of 28 crew members aboard the International Space Station.

The results of one study showed that the functions of certain cells decrease, while the functions of other cells are heightened, causing the immune system to get "confused".

According to the NASA press release:

"When cell activity is depressed, the immune system is not generating appropriate responses to threats. This may also lead to the asymptomatic viral shedding observed in some crew members, which means latent, or dormant, viruses in the body reawaken, but without symptoms of illness. When activity heightens, the immune system reacts excessively, resulting in things like increased allergy symptoms and persistent rashes, which have been reported by some crew members."

The French study and the NASA results highlght the fact that more research is needed to understand these immune responses and other physiological changes, in order to develop countermeasures to protect space voyagers.

The NASA researchers say possible countermeasures to protect immunity could include re-designed spacecrafts with greater radiation shielding, nutritional supplements and new drugs.    

Source: PBS