28 July 2014
Zero gravity is one of those words that appears in science fiction before science and engineering had an actual need for it. Zero gravity, also called zero g or microgravity, is the state of weightlessness experienced in outer space (and, as we shall see, at the center of the earth).
The term is much older than you might expect. It first appears back in 1938 in the story “If Science Reached Earth’s Core,” in the October issue of Thrilling Wonder Stories:
Starting at the zero-gravity of earth’s core, accumulative acceleration is easily built up in a four-thousand-mile tube.
Since gravity is the attraction between two masses, if we could go to the center of the earth, we would feel no pull from the earth’s mass. The planet’s mass would surround us, and the pulls in all different directions would cancel each other out—we would be at zero-gravity.
For spacecraft in orbit, the mechanism is different, but the effect is the same. In earth orbit, the planet’s gravity is still tugging at a spacecraft, but the craft is traveling fast enough that it “falls around” the earth. The craft’s forward motion cancels out the effect of the earth’s gravity and things and people float.
The shorter zero g dates to 1952 and Arthur C. Clarke’s novel Islands in the Sky:
She was escorted by an elderly woman who seemed quite at home under zero “g” and gave Linda a helpful push when she showed signs of being stuck.
The abbreviation G or g is standard physics notation for the force of gravity and has been in use since at least 1785.
Today, space scientists tend to use the term microgravity to describe most real-world zero-gravity situations. In orbit, the effects of earth’s gravity are not completely cancelled out, and other astronomical bodies, notably the moon and the sun, will exert some, albeit very weak, gravitational influence. These minute gravitational forces are not technically “zero,” so the term microgravity is substituted. Use of microgravity dates to the Skylab missions. From the February 14, 1975 issue of Science:
The experiments chosen to fly on the various Skylab missions are best characterized as a mixed bag of studies designed to observe the effect of microgravity on a variety of phenomena.
Sources:
“G, n.,” Oxford English Dictionary, second edition, 1989.
“microgravity, n.,” Oxford English Dictionary, third edition, December 2001.
“zero-g, zero-gee n.,” “zero gravity n.,” Prucher Jeff, Brave New Words: The Oxford Dictionary of Science Fiction, 2007.