What do scientists really mean when
they talk about "microgravity?" After all, in space,
you don't weigh anything, right?If you don't have the Futuresplash Plugin, or don't know if you do, go here for help.
What do scientists really mean when
they talk about "microgravity?" After all, in space,
you don't weigh anything, right?
Maybe, and then again, maybe not. Let's look at a couple animations. First, here's an animation of the effects of gravity on a person riding in a very untrustworthy elevator. Next, look at an animation on free fall using a ball, courtesy of the Mission Operations Lab at Marshall. Then come back to this page for more detailed explanation.
Gravity is a result of a basic property of all objects in the universe. The more massive the object, the greater its gravity. An object's gravitational field attracts another object with a force that is related to the distance between the centers of the two objects and their masses. The smaller the distance between the centers of the objects, and/or the greater their masses - the greater the attraction. Mathematically, this relationship is written:
F is the gravitational force between the two objects,
m1 and m2 are the masses of the two objects,
and r2 is the
square of the distance between the centers of the two objects.
The symbol, 
(the Greek letter
"a") means "is proportional to." As two objects
get further apart, their mutual attraction decreases pretty fast.
The relationship above is also known as "Newton's Law of
Universal Gravitation."
You can see that, mathematically, the gravitational force between two objects can get very small, but it can never be quite zero. So how small a gravitational force do you need to carry out "microgravity" experiments?
To scientists, a "Microgravity Environment" is one in which the apparent weight of a system is small compared to its actual weight due to gravity on Earth. "Small" is a relative term, however, a common example is an apparent weight of one millionth that on Earth's surface.
That's 0.000001 times less than on Earth - nearly, but not quite, weightless. Using the gravity relationship above, you can calculate how far away you need to be from Earth to achieve near-weightlessness. Remember that force is proportional to 1/r2, and that the distance (r) is measured as the distance between the centers of the objects. Therefore, for a laboratory on the surface of Earth, r becomes Earth's radius (6,370 kilometers) The math:
re:
Radius of Earth
Fe: Force at surface
of Earth
Fµ: Force
in microgravity environment (10-6*Fe)
rµ: distance
to microgravity environment
In other words, to achieve one millionth the force of gravity on the surface of Earth, you have to move your laboratory a distance equal to the square root of one million (one thousand) units further away from the center of Earth . That's 1000 Earth radii (or 6,370,000 kilometers)! The moon is only about 60 Earth radii away (384,401 kilometers)!!
Spacecraft in orbit around Earth (like the shuttle) are nowhere near far enough away to avoid Earth's gravitational field - they are only about 500 kilometers above the surface of the Earth (less than 1/10 of one Earth radius)!!!
So what's a scientist to do? Well, the scientist has to act like the girl in the elevator, and take a fall. A spacecraft does the very same thing the broken elevator does - it falls toward Earth due to gravitational attraction, but is moving fast enough, and in the right direction (i.e. away), to miss hitting it, thanks to the rockets used to launch it. Look again at this animation on free fall, with the ball.
Scientists call the environment within an orbiting spacecraft "microgravity," because the effects are the same as if the spacecraft were a thousand Earth radii away. In practice, however, there are still a multitude of accelerations affecting the spacecraft and everything in it - motions due to orbital thrusters, vibrations, people or cargo, etc. Fortunately, these accelerations tend to be much smaller, and often are negligible, compared to sitting on Earth.
Check out Experiments 1, 2, and 3 , which you can try in your class or at home, which demonstrates how orbiting spacecraft simulate microgravity. Then, try the QUIZ!
Adapted from NASA's "A Teacher's Guide With Activities", produced by the Microgravity Science and Applications Division, Office of Space Science and Applications, and NASA's Education Division, Office of Human Resources and Education.
author/curator: Linda
Porter
NASA Official: Gregory
S. Wilson
November 14, 1997