A lot of people disagree with my characterization of NASA's use of the word "microgravity" in lieu of "free fall" as a hoax. The chief objection seems to be that I am being pedantic. Well, here's what I have to say to people calling me "pedantic" --
Here are a few more (not at all pedantic) reasons why "microgravity" sucks and and "free fall" rules.
By the Book
Let's begin with some dictionary definitions of free fall.
From the Oxford English Dictionary:
The motion of an object under the influence of gravity alone, there being neither thrust nor appreciable drag acting on it
The state of being in a motion affected by no acceleration (force) other than that provided by gravity
The term free fall, then, doesn't say anything about the magnitude of the gravitation an object is experiencing. Really, it's just a condition in which gravity is essentially the only force the object is experiencing. When an object like the International Space Station (ISS) is in orbit, gravity is far and away the largest force on the object and not, as the prefix micro implies, the smallest.
Notice that this definition works even if gravity is quite large, as might happen if you were in orbit around dense object, like a neutron star. If the International Space Station (ISS) were orbiting a neutron star, astronauts would float about in exactly the same way they do in orbit around the earth (provided they weren't so close that tidal forces would be a problem, but now I'm being pedantic).
NASA would call both of those situations microgravity, even though the gravity in the vicinity of an Earth-orbiting space station is 90% of gravity on the Earth's surface (as NASA kindly points out on their microgravity information page), and the gravity in the vicinity of a space station orbiting a neutron star more be a billion times more than that of gravity on the Earth.
What's the Harm in "Microgravity"?
If you told me that you invented a new term for free fall, something like 'super fun force' perhaps, I would say that's stupid, but no harm done. Just tell people super fun force = free fall, and everything is fine.
But microgravity is a word made of parts that people recognize. They shouldn't have to look it up, and most people probably don't, in order to infer what it means. This leads to confusion, even at NASA.
Consider what NASA's own What is Microgravity page says under the section titled "Is There Gravity in Space?' [I put the most important part in bold.]
"The International Space Station orbits Earth at an altitude between 200 and 250 miles. At that altitude, Earth's gravity is about 90 percent of what it is on the planet's surface."
Now check out what NASA says on another web page Microgravity: Always a Bad Hair Day.
"[On the ISS, there] is gravity, however. Gravity is what keeps the ISS and Space Shuttle from speeding away into space. Earth's gravity and a small amount of aerodynamic drag from the atmosphere still exert some forces on the spacecraft and their contents, but they are very small. That's why it's called microgravity, rather than zero gravity."
Is gravity at the ISS 90% of Earth's surface gravity, or is it "very small"? It can't be both. Even NASA writers can't keep it straight. Thanks, microgravity misnomer.
The damage goes much further. Let's get back to the dictionaries to see how.
From the Oxford English Dictionary, microgravity is:
A condition of very weak gravity, spec. that existing in a spacecraft in orbit.
But as I pointed out above, gravity isn't inherently weak when you're in orbit. You could be in orbit around a black hole and still float inside your space ship.
In fact, for most orbits, the magnitude of gravity is substantial. It's very hard to go into orbit around something when gravity is actually micro. Case in point - the Rosetta Mission was an amazing technical achievement in large part because the European Space Agency managed to put a spacecraft in orbit around an asteroid where the relevant gravitational forces were actually very small (micro or less, I suspect).
Not only is the Oxford English Dictionary definition of microgravity wrong, it doesn't even agree with itself. Is microgravity a condition where the gravity is very weak, or is it the condition that corresponds to a space craft being in orbit where gravity is usually substantial (unless you're visiting an asteroid)? I believe the error comes about because the first half of the OED definition restates the literal meaning of the prefix 'micro' combined with the word 'gravity', while the second half echoes NASA's misuse of the compound word 'microgravity.' Put them together and it's gibberish.
One of good Sir Isaac's greatest contributions to physics is noting that an object in motion will stay in motion, in a straight line, unless acted on by an outside force. How the heck are we supposed to reconcile that with microgravity? If gravity were indeed small in circumstances where NASA likes to use the term, then motion should be very nearly straight. Newton, I think, would take exception to calling the force of gravity "micro" in this enchanting little NASA graphic showing objects orbiting the Earth.
Microgravity and a Missed Opportunity
If people were stop using the deceptive and generally wrong term "microgravity," it opens the way to one of the deepest insights in the whole idea of gravity. That comes from asking the question, "If the ISS and everything on it is in free fall under the influence of (substantial) gravity, why is it that very heavy things and very light things fall together?"
An elephant and a mouse in free fall travel together, not because they experience the same force, but because they experience very different forces. And those forces differ in a way that makes their descent match perfectly. It took the brilliance of Galileo to first recognize and understand the amazing significance of heavy and light objects falling together.
The reason, as I'm sure you all know, is that unlike all the other fundamental forces, gravity is proportional to mass. That's both cool, and for many of us, very counter-intuitive. That's why this famous video of Apollo 15 Commander David Scott dropping a hammer and feather on the moon is so fascinating.
Free fall is interesting precisely because all the effects that NASA erroneously ascribes to the lack of gravity are actually illustrations of the very characteristic that makes gravity unique.
Micro? Compared to What?
Two other aspects make gravity particularly interesting. One is the fact that gravity is always attractive. Electrical forces can be repulsive or attractive, as can the forces between magnets, but not gravity.
Another interesting aspect is that gravity is an infinite range force - that is, while the force of gravity goes down as two bodies move apart, it never goes to zero. This isn't unique to gravity, the same is true for the forces between electrically charged objects. However, electrical charge comes in two types, positive and negative, and they tend to cancel each other out on large scales.
When you combine gravity's infinite range with its universal attraction, it turns out that as you move farther and farther into space gravity becomes the dominant force affecting you. Physicists fairly recently discovered that there's another force in the universe that's pushing everything apart, associated with the still mysterious dark energy. But other than that, gravity is the main force that controls your motion in the depths of space. Yes, the force is very small compared to what you feel while standing on Earth, but there's nothing besides our our ego-centrism that makes our gravity a sensible reference standard.
Gravity way out there may be micro compared to Earth surface gravity, but it's mega compared to just about every other force almost everywhere else in the universe.
One of the most useful guidelines in physics and science in general has been Occam's Razor, which is often written "Entities must not be multiplied beyond necessity". It means that it's better to have fewer theories that explain many phenomena than a whole bunch of disparate theories for each phenomenon.
For example, while a pendulum looks different from a mass hanging on a spring, and you could develop the physics of pendulums and springs separately, it's much more satisfying and powerful to think of them both as examples of simple harmonic oscillators. Similarly, the discovery of atoms provides an elegant framework to allow us to understand chemistry in a way that was not possible when all we had was an table of distinct, seemingly unrelated, elements.
Free fall is the motion of an object when all the forces on an object are much smaller than the force of gravity. Microgravity is the condition where all the forces on an object are smaller than gravity and gravity is weak.
So microgravity is just a special case of free fall. It's such a special case that no human has ever actually experienced it.
You can calculate the nearest place to us where gravity is micro with the equation
Where G is just a number called the gravitational constant, me is the mass of earth, and the r^2 is the distance from the center of the Earth squared. When you're standing on the Earth, r is roughly 6400 kilometers and g is 9.8 meters per second squared. For the force of gravity to be a million times smaller, you have to go a thousand times farther from the center of the Earth, to about 6,400,000 kilometers out. That's sixteen times greater than the distance from the Earth to the moon and sixteen thousand times the orbital altitude of the ISS. No person, and few space craft, have traveled that far.
Occam's Razor suggests that one concept is better than two, especially when one of the concepts is rarely applicable and the other is always applicable. Why then do people insist on talking about microgravity when we could erase the word from existence and be none the worse for it? Following Occam's advice, we should simplify and clarify things by dispensing with the word "microgravity" entirely.
On Being "Pedantic"
Here's how Dictionary.com defines pedantic:
And here's an example of being pedantic about free fall:
The ISS isn't really in free fall. It's also experiencing drag as it passes through the extremely thin atmosphere far above the Earth. That's why, if left alone, its orbit would gradually decay and it would eventually come crashing down. If the ISS were truly in free fall, it would stay in orbit forever.
This is not pedantic:
Microgravity does not equal free fall. Microgravity, as NASA so often uses it, is so utterly meaningless that, as Wolfgang Pauli would say, it's not even wrong.