Weightlessness or microgravity is a new experience for the human body. Because of it in space, all the human systems that have evolved to cope with the constant tug of Earth's gravity no longer function in the same way. But as long as a spacecraft contains a carefully controlled atmosphere to enable normal breathing, adequate temperature, and sufficient shielding to guard against the dangerous radiation levels in space, humans can survive under microgravity conditions.

Imagine that you are on your way to Mars. After launch, thrusters provide power to carry the spacecraft out of Earth's atmosphere. You are now weightless! Your feet rise from the floor and you are ready to turn somersaults in the cabin, walk along the walls and ceiling, and balance bulky objects, even other crewmembers, on the tip of your finger. 
 

STS-99 Crew

In weightlessness, there is no up or down as we know it. You don't even know the orientation of your body at first because it has no weight for you to feel and sense where it is. In space, your body becomes confused by the sudden change in what it has learned to expect.

When entering weightlessness, nearly all astronauts are troubled to some extent by a condition called space motion sickness, which is similar to car sickness or seasickness. Because the human brain on Earth has learned how to process signals about the position of different parts of the body in relation to the world around it, this sudden input of confusing signals causes many astronauts to feel sick. Within the inner ear, there is a balance organ called the vestibular organ and information from it, together with information from your senses of sight and touch and information from your muscles and joints, all integrated together, helps your brain focus on the position of your body relative to the downward pull of gravity. In space, where there is virtually no gravity, the signals from the vestibular organ combined with what is seen and felt by the other body sensors are all giving conflicting information to your brain. Space sickness usually goes away after a few hours or days of acclimating to the weightless environment. The medical results from Skylab suggested that space motion sickness cannot be predicted, but can be alleviated somewhat by medications.
 

The most serious consequence of weightlessness, however, is the deconditioning (weakening) of physiological systems such as the cardiovascular system. On Earth, the heart must operate against gravity to sustain blood flow and proper functioning of the cardiovascular system. Under zero-gravity conditions, the heart lessens its pace to achieve an equilibrium appropriate to decreased demands. Reduced output of the heart, decreased heart rate, decreased heart size, and diminished blood volume regulation result.

Astronaut Bonnie Dunbar exercising on the bicycle

Diminished gravity is also a significant problem in the way it affects the musculoskeletal system. Reduced weight-bearing in space leads to bone "disuse" symptoms including loss of calcium, nitrogen, and phosphorus and decreased bone size and volume. As discovered on the Mir space station, 1.2 % of bone mass in the lower hip and spine is lost per month in microgravity! Decreased muscle tone and strength, weakened reflexes, and decreased tolerance for physical work are further negative consequences of a zero-g environment. The last thing we want for future Mars explorers is for them to arrive on the planet unable to walk or work because of a weakened heart or muscular system.

Bed-rest tests on Earth, which simulate some of the effects of zero g on the body, are used in conjunction with studies on the astronauts living in space to help determine how we can help keep crews fit during their flight to Mars. Exercise and nutritional supplements are currently used to help counteract some of the negative effects of zero g.

Artificial gravity?

One possibility that has been suggested as a countermeasure to the effects of weightlessness is the use of artificially produced gravity aboard spacecraft. Artificial gravity could be accomplished either by rotating the entire vehicle or by including an onboard centrifuge.

Rotating a vehicle to the degree that it can be developed to approximate a normal gravitational environment would produce the more comfortable living arrangement for long-duration space crews. Unfortunately, a large vehicle is necessary to produce rotation simulating Earth gravity. Click here to read about one design for an artificial gravity rotating spaceship to Mars. There are also rotational sickness problems with the rotation of the ship. If you go closer to or away from the center of rotation people will feel it and probably not like it.

How can crews adapt from zero g to Mars g?
 

On its surface, Mars has one-third the gravitational force of Earth. Long-term exposure to Mars gravity can be as harmful as long-term exposure to microgravity when astronauts are planning to return to a 1-g environment on Earth.

The Martian gravity is low enough that bone integrity will probably still be affected, and it is too high to be ignored. There will be symptoms that occur due to the transition between zero g and Mars g. How severe the transition will be is unknown. Will it last a few days or longer? Mars explorers who are on a short surface stay mission will need to accomplish their surface activities quickly since any adaptation problems could impact the fulfillment of their mission. The design of any mission will have to include a strategy to deal with this possibility.  To read the Mars Society of CalTech Human Exploration of Mars Endeavor mission design plan click here.

It is thought that Martian gravity may perhaps be sufficient to counter some of the deconditioning that endangers astronaut health during the in-flight zero-g environment. If this is the case, expeditions that include a longer surface stay may be better because they would deflect some of the physical conditions caused during in-flight zero-g exposure. Crews might be able to rebuild their tolerance to gravity in this gentler gravity environment, thereby restoring their bodies to near-preflight conditions before embarking on the mission back to Earth. However, the effect of prolonged exposure to Mars g over time is still an unknown factor. Astronauts who live on the surface of Mars for a long period of time may have unanticipated reactions to prolonged exposure to 1/3 g. Click here to look at the effects of gravity on the Earth versus Mars.

Questions to think about:

 
• How do you feel after staying in bed for several days with a cold? How do your muscles feel?

• How do you think the 1/3 g-force on Mars will feel to astronauts after a 6-month flight in zero g?

• How do you think you would adapt to living in Mars g for a long period of time? What changes in your body might occur? 

• How hard would it be to readapt to Earth gravity after an extended period of time on Mars?

Keeping Fit