Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Biological Effects of Weightlessness

s / On October 4, 2004, the privately funded SpaceShipOne won the ten-million-dollar Ansari X Prize by reaching an altitude of 100 km twice in the space of 14 days. [Courtesy of Scaled Composites LLC.]

The usefulness of outer space was brought home to North Americans in 1998 by the unexpected failure of the communications satellite that had been handling almost all of the continent's cellular phone traffic. Compared to the massive economic and scientific payoffs of satellites and space probes, human space travel has little to boast about after four decades. Sending people into orbit has just been too expensive to be an effective scientific or commercial activity. The downsized and over-budget International Space Station has produced virtually no scientific results, and the space shuttle program now has a record of two catastrophic failures out of 113 missions.

Within our lifetimes, we are probably only likely to see one economically viable reason for sending humans into space: tourism! No fewer than three private companies are now willing to take your money for a reservation on a two-to-four minute trip into space, although none of them has a firm date on which to begin service. Within a decade, a space cruise may be the new status symbol among those sufficiently rich and brave.

Space sickness

Well, rich, brave, and possessed of an iron stomach. Travel agents will probably not emphasize the certainty of constant spacesickness. For us animals evolved to function in g = 9.8 m/s2, living in g = 0 is extremely unpleasant. The early space program focused obsessively on keeping the astronaut-trainees in perfect physical shape, but it soon became clear that a body like a Greek demigod's was no defense against that horrible feeling that your stomach was falling out from under you and you were never going to catch up. Our inner ear, which normally tells us which way is down, tortures us when down is nowhere to be found. There is contradictory information about whether anyone ever gets over it; the "right stuff" culture creates a strong incentive for astronauts to deny that they are sick.

Effects of long space missions

Worse than nausea are the health-threatening effects of prolonged weightlessness. The Russians are the specialists in long-term missions, in which cosmonauts suffer harm to their blood, muscles, and, most importantly, their bones.

The effects on the muscles and skeleton appear to be similar to those experienced by old people and people confined to bed for a long time. Everyone knows that our muscles get stronger or weaker depending on the amount of exercise we get, but the bones are likewise adaptable. Normally old bone mass is continually being broken down and replaced with new material, but the balance between its loss and replacement is upset when people do not get enough weightbearing exercise. The main effect is on the bones of the lower body. More research is required to find out whether astronauts' loss of bone mass is due to faster breaking down of bone, slower replacement, or both. It is also not known whether the effect can be suppressed via diet or drugs.

t / U.S. and Russian astronauts aboard the International Space Station, October 2000.

The other set of harmful physiological effects appears to derive from the redistribution of fluids. Normally, the veins and arteries of the legs are tightly constricted to keep gravity from making blood collect there. It is uncomfortable for adults to stand on their heads for very long, because the head's blood vessels are not able to constrict as effectively. Weightless astronauts' blood tends to be expelled by the constricted blood vessels of the lower body, and pools around their hearts, in their thoraxes, and in their heads. The only immediate result is an uncomfortable feeling of bloatedness in the upper body, but in the long term, a harmful chain of events is set in motion. The body's attempts to maintain the correct blood volume are most sensitive to the level of fluid in the head. Since astronauts have extra fluid in their heads, the body thinks that the over-all blood volume has become too great. It responds by decreasing blood volume below normal levels. This increases the concentration of red blood cells, so the body then decides that the blood has become too thick, and reduces the number of blood cells. In missions lasting up to a year or so, this is not as harmful as the musculo-skeletal effects, but it is not known whether longer period in space would bring the red blood cell count down to harmful levels.

Reproduction in space

For those enthralled by the romance of actual human colonization of space, human reproduction in weightlessness becomes an issue. An already-pregnant Russian cosmonaut did spend some time in orbit in the 1960's, and later gave birth to a normal child on the ground. Recently, one of NASA's public relations concerns about the space shuttle program has been to discourage speculation about space sex, for fear of a potential taxpayers' backlash against the space program as an expensive form of exotic pleasure. Scientific work has been concentrated on studying plant and animal reproduction in space. Green plants, fungi, insects, fish, and amphibians have all gone through at least one generation in zerogravity experiments without any serious problems. In many cases, animal embryos conceived in orbit begin by developing abnormally, but later in development they seem to correct themselves. However, chicken embryos fertilized on earth less than 24 hours before going into orbit have failed to survive. Since chickens are the organisms most similar to humans among the species investigated so far, it is not at all certain that humans could reproduce successfully in a zero-gravity space colony.

u / The International Space Station, September 2000. The space station does not rotate to provide simulated gravity. The completed station will be much bigger.

Simulated gravity

If humans are ever to live and work in space for more than a year or so, the only solution is probably to build spinning space stations to provide the illusion of weight, as discussed in section 9.2. Normal gravity could be simulated, but tourists would probably enjoy g = 2 m/s2 or 5 m/s2. Space enthusiasts have proposed entire orbiting cities built on the rotating cylinder plan. Although science fiction has focused on human colonization of relatively earthlike bodies such as our moon, Mars, and Jupiter's icy moon Europa, there would probably be no practical way to build large spinning structures on their surfaces. If the biological effects of their 2 - 3 m/s2 gravitational accelerations are as harmful as the effect of g = 0, then we may be left with the surprising result that interplanetary space is more hospitable to our species than the moons and planets.

Optional Topic: More on Apparent Weightlessness

Astronauts in orbit are not really weightless; they are only a few hundred miles up, so they are still affected strongly by the Earth's gravity. Section 10.3 of this book discusses why they experience apparent weightlessness. More on Simulated Gravity For more information on simulating gravity by spinning a spacecraft, see section 9.2 of this book.

Last Update: 2009-06-21