Correspondence

More on Central Venous Pressure in Space

N Engl J Med 1993; 329:1822December 9, 1993

Article

To the Editor:

Buckey et al. (June 24 issue)1 promulgate a common misconception about the effects of gravity on astronauts in orbit. The term “zero gravity” is used to describe the environment inside the space shuttle. Gravity, however, is not zero either inside or outside the shuttle.

While in orbit, the astronauts and the spacecraft are moving under the direct influence of gravity, which is far from zero. Without the force of gravity, the shuttle would hurtle into space. True zero gravity or weightlessness can exist only in a region of space far from any planet or star. The term “microgravity,” which is sometimes used to describe the environment in Earth orbit, is also misleading.

The phenomenon of apparent weightlessness occurs because the shuttle is in free fall -- that is, it is falling toward Earth with an acceleration equal to the acceleration of gravity. The beginning of apparent weightlessness is experienced by the average earthbound person when riding in an elevator. As the elevator accelerates downward, one feels momentarily lighter. The reason for this sensation is that the floor of the elevator is not pushing up on one's feet with a force equal to one's weight, which is what normally happens. The opposite sensation occurs when the elevator accelerates upward. Should the cable break and the elevator fall freely, one would float (briefly) toward the ceiling in an apparently weightless condition. Replacing the term “zero gravity” with the term “free fall” would not affect the arguments made by Buckey and his coworkers but would be more accurate.

Lawrence B. Coleman, Ph.D.
University of California, Davis, Davis, CA 95616

1 References

1
Buckey JC, Gaffney FA, Lane LD, Levine BD, Watenpaugh DE, Blomqvist CG. Central venous pressure in space. N Engl J Med 1993;328:1853-1854
Full Text | Web of Science | Medline

Author/Editor Response

Dr. Buckey replies:

To the Editor: Dr. Coleman is correct. The shuttle is in a continuous free fall while in orbit, and gravity is not zero in the shuttle.

For the purpose of work in human physiology, however, true zero gravity and free fall are equivalent; the terms “zero gravity,” “weightlessness,” and “microgravity” have been used interchangeably. Sometimes physicists use them this way, too. When a representative of the American Physical Society testified to Congress in July about the space station, he also used the terms “microgravity” and “zero gravity” when referring to orbital research.1

Jay C. Buckey, M.D.
University of Texas Southwestern Medical Center, Dallas, TX 75235

1 References