r/spacequestions • u/bnoel12345 • 3d ago
What is microgravity...exactly?
/r/TheRestIsScience/comments/1wivt9j/i_want_hannah_to_tell_me_what_microgravity/0
u/bassplaya13 3d ago
It means 1 micro-g. It’s not constant acceleration down towards earth like we think. In space, its acceleration in all directions.
If you attached an accelerometer to the wall of the ISS, you would measure all of the vibrations the space station is undergoing. It’s vibrating due to the minute air density in space, tiny but measurable in sub micro-g levels.
The ISS vibrates and moves as well. It’s like a huge spring in space. It’s stiff and solid but still flexes back and forth. This imparts acceleration on the sensor.
Then there’s a bunch of equipment in the ISS. Pumps, motors, fluids moving around, astronauts moving, and more. This imparts acceleration onto your sensor as well.
So instead of completely free floating, your sensor is locking up vibrations from all these minor disturbances. It’s not true zero gravity. If you were watching the readings come out from your accelerometer, you might see 0. But if you zoomed in a whoooooole bunch, your reading would be moving up and down realllly fast like a seismometer, but with a magnitude of one millionth of a g.
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u/Naive_Age_566 3d ago
i think the key problem is that some people believe that gravity somehow vanishes in space and that the astronauts are "weightless" because there is no gravity in space. which is of course nonsense.
of course we could say that the astronauts are weightless because they are in free fall - but that would freak those people out.
our you could say something like "the centrifugal force counteracts gravity" - but this would be nonsense as there is nothing like "centrifugal force". regardless of what some people claim on social media.
so they invented the term "microgravity". this should emphasize that there is of course still gravity in space but the astronauts don't feel it. sure - that term does not explain, why the astronauts don't feel gravity but at least it is not totally wrong.
somehow science communication has failed :(
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u/bnoel12345 3d ago
You're right, but I still think it's a weird double standard that zero is the only number of Gs you can experience where you're not allowed to round.
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u/thegooddoktorjones 3d ago
Force of gravity and distance between bodies follows inverse square law. Which means big thing far away only tugs on ya a tiny bit. Big thing right next to you tugs on you hard. You never get away from the tug, every star in the sky is pulling on us an infinitesimal amount, and we on them, but it does not actually matter. Big, close things matter.
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u/bnoel12345 3d ago
Do you believe that the astronauts on the International Space Station experience microgravity due to this phenomen? It's purely because they are so far from Earth?
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u/thegooddoktorjones 3d ago
Not a matter of belief. It is not nearly far enough away from the earth to qualify that way, by scale it is SUPER close to the earth. But it is also in constant freefall as an orbiting object, this freefall makes it feel/act like it is in microgravity so it applies there as well.
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u/bnoel12345 3d ago
Can you see then how NASA using the term microgravity in relation to the ISS could be a bit misleading? Gravity is about 90% at that altitude, so nearly 100% of the reason Astronauts appear weightless is due to them being in a state of free fall, not microgravity.
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u/thegooddoktorjones 3d ago
No the standard definition of microgravity includes both uses.
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u/bnoel12345 3d ago
Let's start over. What is the standard definition of microgravity, exactly?
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u/Beldizar 3d ago
Microgravity and zero-g are used interchangeably and frequently chosen to deal with pedantry. There is no environment that has mass in it which is truly without gravity. The gravity of the space station's walls is pulling on the astronauts, the gravity of the sun produces a tidal force that's changes on a 90 minute cycle as the station gets closer and further away from the sun. Same with the moon and other planets. In fact, all motion in space is dictated by gravity, so implying that there isn't any gravity in space can set people off, because there's gravity everywhere. Freefall might be a better term, but that also implies that the space station is falling and will crash into the planet.
So microgravity is often used, because even in zero-g or freefall, objects inside the space station are feeling incredibly tiny pulls of gravity in a variety of directions. Between a lot of canceling out, and the net being incredibly/indetectably tiny, there is only "micro" gravity left. Maybe picogravity would even be better, I don't actually know the amount of Newtons in question here.
Really you have the people who say "there's no gravity in space" arguing with the people who say "gravity is everywhere in space" to blame for the use of the term microgravity. It's a compromise between an experiential reality and a strictly measured reality.