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Temporary weightlessness

Test crew floats weightless in KC-135Engineers and scientists experience about 20 to 30 seconds of weightlessness during each parabola aboard NASA's KC-135 aircraft, an effective and inexpensive means of testing experiments before they go to space. Because everything floats, test equipment must be bolted or taped to the deck, as with the apparatus here for testing liquid cages.

What astronauts experience in space isn't really zero-gravity. NASA scientists call it microgravity or low-g, but it's really free fall or weightlessness.

Gravity goes to the edges of the universe -- it's why planets circle the sun, stars clump together to form galaxies, and Space Shuttles stay in orbit.

So what is happening on a spacecraft or when Kornfeld and Antar run experiments on the KC-135 (as seen at top)?

As a spacecraft orbits a planet, it really falls endlessly in a circle (or ellipse) that is a delicate balance between the satellite's forward motion and the planet's gravitational pull. Because everything is falling together, nothing has weight.

Well, almost no weight. Unless an object is at the precise center of a satellite's mass, it will try to pull ahead or fall back into a slight different orbit. And that means that the object will experience a small amount of acceleration against a wall. And even at the Shuttle's altitude, a trace of atmosphere is left and gently drags on the Shuttle which will cause an object to drift inside the Shuttle.

NASA scientists call this microgravity since usually it is equivalent to about 1/1,000th or less of one Earth gravity (the range depends on the location in the spacecraft and other factors). The term is apt since Albert Einstein said that acceleration caused by gravity is equivalent to any other push.

Free fall can be duplicated, briefly, on Earth, by dropping an object. Like falling off a cliff, it's not the first step that gets you, or the long trip down, but the stop at the end.

NASA has drop tubes in which molten droplets of material fall for about 2 to 3 seconds before hitting a bucket of oil to capture them safely and cool them off.

For larger experiments, or to train astronauts, NASA uses a KC-135, a military tanker version of the Boeing 707 jetliner. The pilots guide these jets on carefully designed parabolic trajectories that resemble a roller coaster ride.

At the top, the pilot throttles back and noses over, letting the plane dive to give everyone about 20 to 30 seconds of free fall (actually, it varies between 0.01 g to 0.001 g; it's not nearly as good or as long as being in orbit). They do this 40 times on each mission, so they get about 13 minutes of microgravity time -- at a personal price.

People riding the NASA KC-135 often get extremely sick doing this. That's why the plane is also called The Vomit Comet.

The things you do for science!


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