Long Term Weightlessness

Effects on the Human Body

The human body has evolved under 1 g (one earth gravity).  It is not well adapted to working in a zero gravity environment.  In the short term (the first three to four days of spaceflight), almost one half of all astronauts experience motion sickness.   For some it may be quite bad, making it difficult to carry out their mission.   The probable cause is that the inner ear which gives us our sense of balance, and its feedback to our muscles which normally would respond to the force of gravity to keep us upright, require gravity to tell us which way is "up".  Without this frame of reference, visual clues confuse our sense of balance, and cause us to become nauseated.   This is very similar to the motion sickness caused by virtual reality systems, where the body is confused by apparent motion, without actually experiencing the forces its expects to feel.   The drug promethazine is commonly given by intramuscular injection to minimize the effects of weightlessness-induced motion sickness.

However, after a few days the human body becomes adjusted to this effect of weightlessness, and it no longer interferes with astronauts' work.  But long term effects of the lack of gravity become evident after a few weeks in space.  In the absence of gravity, our muscles no longer need to work hard to move our mass.  An unused muscle loses muscle tone.   This includes one of the most important muscles in our body – the heart.   Astronauts who spent long terms in space aboard the Russian space station Mir returned to earth unable to support their own body mass, and had to be carried from the re-entry vehicle.

One of the most serious problems caused by long term weightlessness is loss of calcium from bone.  Without the force of gravity, the body decreases its bone mass.  The reaction is similar to what happens to people on Earth who are bedridden.  Bone is alive.  Under stress, bone grows stronger, which is why weight bearing exercise is critical for the proper development of the human body.  When bones are not stressed, they lose bone density at the rate of up to 1 or 2 percent per month.   Despite trying to exercise vigorously while in space, astronauts have returned from long term (six months or more) stays on the Mir space station with up to 15% loss of bone mass.   This is a serious health problem.

One possible solution is to create artificial gravity by rotating the spacecraft.   If you tie a weight to a string, and swing it, the weight travels in a circular path.  The weight would fly off in a straight line, were it not for the string.  There is a force along the string caused by centripetal acceleration.  Gravity too is a force that causes acceleration – remember Newton's second law: F = ma.  Centripetal acceleration simulates gravity.

While we could just spin our spacecraft along its axis, this will probably cause serious motion sickness.  Studies on earth show that humans can only tolerate rotational motion of about 2 revolutions per minute (rpm) or less without becoming motion sick (high rotational rates cause currents in the fluids of the inner ear resulting in dizzyness).  If we assumed a diameter for our spacecraft of 10 metres, then as you can see from the following graph, you'd have to rotate at a speed of almost 10 rpm to simulate 1 g.  This is too high a rotational velocity to be tolerated by humans.

The graph inset shows the rotational rate required to achieve 1 g at small radii.   The larger graph shows that to create artificial gravity with a tolerable rotation rate (2 rpm, shown with the heavy black line) requires a radius slightly over 200 m.

Click to see how to use a spreadsheet to calculate this graph.

We need a radius nearly 250 m to keep the rotation rate down to where it can be tolerated by humans.  We can't build a spacecraft of that radius – doing so would be impractically large.  However, our whole spacecraft doesn't have to be that big.  The solution is to use a much larger radius than the size of the spacecraft. One method of getting a very large radius of rotation is to connect two masses, the crew living quarters being one, and perhaps the spent rocket booster the other, by a very long tether.  Then the connected masses could be spun about their centre of mass to provide the necessary artificial gravity.

While it may be simple in principle to do this, the technological difficulty in linking two masses by a cable may be hard to accomplish.  Certainly the cable has to be strong enough.  If the cable broke, then the rocket booster and the crew compartment would move away from each other in straight lines in opposite directions with a speed near 190 km per hour (for a 250 m radius cable).