Yes, astronauts landed on the moon. But that doesn't mean NASA has been completely honest.
|Free fall aboard NASA's vomit comet, where gravity is anything but micro.|
OK, maybe they didn't invent it. I'm not sure if it's possible to figure out exactly who coined the term. Still, some time in the late seventies or early eighties, NASA (and a lot of other scientists) enthusiastically embraced the the idea of microgravity. The problem is, NASA is using it wrong - and I believe that's a very bad thing.
When Microgravity is Macro-wrong
Microgravity is a perfectly fine term, if a bit vague, in some circumstances. If you accept the somewhat egocentric view that the gravitational force we experience standing on the earth is the benchmark, and that the term micro is the prefix for one millionth, then microgravity a million times weaker than our gravity does exist when you are very far from massive bodies like the Earth and Sun.
By that definition, these folks are not in microgravity.
The picture was taken when these astronauts (who are oddly unidentified in the NASA stock photo I found) were aboard the International Space Station. As NASA points out on their microgravity information web page, the acceleration due to Earth's gravity at the ISS is about 90% of what we experience on the ground. In other words, people on the ISS are existing in something much more like gravity than microgravity.
Maybe, what NASA means by microgravity is that the astronauts are weightless - except that is as wrong as saying they are in microgravity. Weight is the force acting on an object that is the product of the object's mass and the acceleration due to gravity.
That is, no matter where you are F = m x a
And if you replace the acceleration term a with the acceleration to to gravity (g), you get
F = m x g = weight
So a hundred pound person on Earth weighs about ninety pounds on the ISS.
No, you can't put them on a scale to measure that weight. That doesn't mean they're weightless, it just means scales are the wrong instruments to use to weigh people in orbit.
If not Microgravity, Then What?
Astronauts and everything else that isn't tied down on the ISS appear to float about not because they are in "microgravity" or even small gravity (as NASA prefers to define micro). Nor are they floating about because they are "weightless." They aren't actually floating at all. They are falling.
You can experience the very same thing, briefly, by jumping in the air. Are you momentarily experiencing microgravity before you come crashing back down? No. Has your weight changed? No.
You are, however, in free fall. As are the ISS astronauts. They just get to fall for a much longer amount of time.
Once upon a time, free fall was in fact the accepted term for what many people now (completely erroneously) call "microgravity." Free fall also happens to be completely correct, from a physics point of view. So why did NASA abandon a perfectly good, scientifically reasonable term and replace it with a nonsensical word? I wish I knew, but I'm guessing it was an attempt to make things sound more sciency.
What Difference Does it Make, It's Just a Word?
Why does it matter? Because, dagnabit it, words are important. They can help teach and provide insight, or they can mislead and do real intellectual harm, as I feel is the case with "microgravity."
As NASA uses it, microgravity implies several things that are fundamentally at odds with basic, high school level physics. It flies in the face of the inverse square law of gravity. It devalues a very useful and scientific prefix. It hides the connection between everyday experiences (bouncing on a trampoline, riding a roller coaster, throwing a baseball) and the awesomeness of orbiting the planet in perpetual free fall.
When a child asks "why do astronauts float in space?" if you answer "because of microgravity," instead of "because they are in free fall," you have missed an opportunity to teach them something about both space and the world around them. Instead of planting a beautiful seed of knowledge, you have given them a meaningless bit of utterly useless nonsense, at best. At worst, you've taught them something their future physics instructor is going to have to unteach in high school.
I'm sure you don't tell kids rain is the tears of angels, or the moon disappears each month because a giant monster is eating it. Why tell them that astronauts float because they are in microgravity? Why not just tell them the truth and say because astronauts are in free fall?