When discussing the effects of living in space, or doing life-science experiments there, or investigating potential orbital manufacturing techniques, it is common parlance, even among space professionals, to describe the acceleration environment as “microgravity.” For example, here is an abstract of a paper that uses the word in the title, even though the abstract itself discusses the fact that there are gravity gradients within the International Space Station (ISS). This is unfortunate, because it is both misleading and a misuse of a word that is a specific scientific unit.
There is another phrase that people often use that is misleading, and wrong: zero gravity. This implies that people are floating around up there because the gravitational field has gone away, but there is no place in the universe in which there is no gravitational field. But in fact, in low Earth orbit, where the ISS resides, the acceleration of gravity is still about 90% of that on the Earth’s surface.
So let’s talk about what an orbit is, and what the gravitational environment for something in orbit is. One of the ways of describing it uses the analogy of a cannon ball. When you fire the projectile, it will go a certain distance before falling to the ground. If you fire it faster, it will go further. The faster it leaves the muzzle, the further it will go. If the Earth (as some claim to believe) was flat, it wouldn’t matter how fast you shot it; it would eventually hit the ground. But because the Earth is in fact an oblate spheroid, there is a speed at which the ground curves away from the cannon ball as fast as it is falling toward the center. It would be going so fast that it would continue all the way around the planet and never hit it. It would be in orbit, and endlessly falling. This is why “free fall” is a good description of what is happening.
When you are on the surface of the planet, it wants to pull you toward its center. Because the planet’s surface is keeping you from falling, you feel your own weight, a force that by Newton’s Law F=ma (where m is your body mass in kilograms and a is the acceleration of gravity, about 9.8 meters/second^2), is calculated as the product of those two values. But when you are falling, there is no force against gravity, so you don’t feel your weight; you could be said to be “weightless.”
Now let’s look at an orbit from a different point of view.
Let’s assume for the moment (though no such thing exists in the physical world) a point mass in orbit (we’ll assume a circular orbit for simplicity, though the principle would be the same if it was a non-circular ellipse). That is, all of the mass of the orbiting object is concentrated in a single point in space. There are two accelerations acting on it. One is the acceleration of gravity pulling it toward the center of the Earth, and remember, that is still about ninety percent of the amount at the surface. The other is the centripetal acceleration, or the outward acceleration created by the fact that it is moving in a circle. It’s the same acceleration that keeps water in a bucket that is being swung around on the end of a rope. In orbit, the two forces on the object are exactly balanced, one toward the planet, and the other away from it, and the net acceleration is zero, so it continues to follow the circular orbital path until it is disturbed by another force (by one of Newton’s other laws of motion), like atmospheric drag, or the thrust of a rocket.
So that’s for a point mass. What about for something with actual dimensions, like the ISS. Now while the entire object is in orbit, the orbital track is still defined by the center of mass, the two accelerations will change as you move toward or away from the planet from it aboard the station. As you move toward the planet, the gravitational acceleration gets a little stronger, and the centripetal acceleration gets a little weaker, and the net is an increase from zero. The same thing happens in the other direction, with greater centripetal, and less gravitational.
“Microgravity,” like microgram, or micro-anything, means literally one millionth of one Earth gravity; that is, one millionth of the gravitational acceleration at the surface of the Earth, which is approximately 9.8 meters per second per second. It is a convenient unit to use to describe the acceleration field on orbit which, within the ISS, ranges from zero at the center of mass, to several microgravities as one moves toward or away from the earth from that point. So where on the ISS is the local net acceleration exactly one microgravity? It turns out that it’s about eight and a half meters away from the center of mass. It is more further away, and less closer in. But in general, the value on ISS is not one microgravity, despite the misleading use of the term. This is why “weightlessness,” or “free fall” are better words to use to describe the orbital gravitational environment.
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