Engineers
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!