View Full Version : Science/Space geeks: ISS microgravity
We've all seen the demos of "weightlessness" inside the ISS with astronauts playing with water bubbles, etc. However, something stuck me as odd watching the astronauts inside SpaceX's Dragon capsule after docking. The "Gravity indicator" (a blue plush toy) that was in the capsule gets knocked around a couple of times and fairly quickly settles down instead of bouncing/floating around the interior of the capsule as I would expect. I know the ISS' orbit is based upon the CG of the ISS, so there should be some tiny ballistic trajectory issues above and below the CG of the station, but nothing that would amount to what we're seeing in the video - I would think.
Anyone have an explanation?
The following YouTube video starts at 5:05 - watch until 6:00 to see the toy tapped once, then settle down only to be launched by a second bump, yet it settles down quickly again. This isn't like anything like we've seen in the "weightless" demos that have come from the ISS in the past and franky I have no explanation.
O2
https://www.youtube.com/watch?v=Ct80WPZl1h0&t=305s
...and here I'll get "The world is flat" and "NASA is a hoax" reply out of the way... :)
O2
BushMasterBoy
03-04-2019, 17:08
Soft foam toy in a plastic lined crew capsule. Static electricity attraction!
Gravity isn't an on/off thing. There are varying levels. Looks like there isn't much gravity going on in there but it's likely not zero.
Static electricity attraction!
Even though the breathing gas isn't pure O2, static electricty is something that would be designed out.
O2
Gravity isn't an on/off thing. There are varying levels. Looks like there isn't much gravity going on in there but it's likely not zero.
Yes I know it's not an on/off thing. What I tried to make clear is that other videos have clearly shown that for all intents and purposes gravity is zero on the ISS (water bubbles hang in mid-air, for exmaple). But this video appears to show some significant microgravity (sounds like an oxymoron :)).
O2
BushMasterBoy
03-04-2019, 17:30
The space station has a very dry atmosphere. There is still a weak force attraction. Nice argument though.
I'll go with the microgravity thing.
I googled it and it's real. Defined as "very weak gravity, as in an orbiting spacecraft."
I'll phrase it a different way...
Yes, there's microgravity on the ISS. I'm wondering a why we see evidence of DIFFERENT microgravities on it. In one case (water bubble demos) it's virtually zero. In this case with the plush toy, it has a fairly significant effect.
O2
DenverGP
03-04-2019, 17:55
It's movement looks weird because the zero-g indicator is tethered to one of the armrests. During launch it got kind of stuck down by the footrests.
https://www.youtube.com/watch?v=ohMhV1WRxcc
Fine. Then I think it fluctuates every time Jer mashes the gas on his Tesla.
I'm trying not to say anything since I have no idea, but could those zero gravity experiments have taken place in a different area of the station that has less gravity? I assume (but don't know) that there is some sort of rotation to simulate gravity and anything toward the outside of the rotation would appear to have more gravity than the center. I'll have to watch the video later to see what the astronauts were doing while the globe was settling
OldFogey
03-04-2019, 18:04
Since anything that has mass also has gravity, I'd say we are all lending gravity to the situation.
I bet if you tweeted your question to Neil D Tyson he'd answer you.
DenverGP
03-04-2019, 18:12
I'll have to watch the video later to see what the astronauts were doing while the globe was settling
Check the video I added to my reply above... you can see the tether if you look closely.
ChickNorris
03-04-2019, 18:21
Yep. There's a tether.
The turbo-encabulator is out of calibration.....
you can see the tether if you look closely.
Crap. Yes, you can see it laying across the handrest in front of it just before he gives it the second boot. Plus now that you mention it you can see the vector change just before it would hit the camera.
But still, it does seem to keep drifting "down" to the footrest area each time.
O2
I assume (but don't know) that there is some sort of rotation to simulate gravity and anything toward the outside of the rotation would appear to have more gravity than the center.
Nope, ISS has no rotation and no simulated gravity. Well, none to speak of since it does rotatate slowly in order to keep it's orientation to the Earth constant.
O2
DenverGP
03-04-2019, 18:49
Any other drift/movement is most likely due to air currents, etc.
OtterbatHellcat
03-04-2019, 19:06
The space station has a very dry atmosphere. There is still a weak force attraction. Nice argument though.
Making this completely clear out of the gate. I had an astronomy class in high school and I don't know shit about space exploration. I'm going for it, Irving.
Okay, so I figure the no static thing is a big deal for sure. Wouldn't stupid dry air invite static? Another thing I wondered, was it would seem everything electrical would be sealed panels and connections, yes one might have to work on something at some time, but it would get resealed. Wouldn't the human component require some percentage of humidity in the air being breathed?
ChickNorris
03-04-2019, 19:54
Dynamic vs. Static equilibrium...
where all of the forces acting on it zero out & that's why it settles where it does.
Great-Kazoo
03-04-2019, 20:00
The turbo-encabulator is out of calibration.....
Or a bad Central Scrutiziner
Some intelligent dudes think there is like 0 to 5% of gravity at space station, but if you do
"Gimmrr" equation gmm/r^2 it is probably about ~~95% of g at spacestation. (Too lazy to run it)
It feels like zero gravity because of free fall. It stays on the orbit, because of cen tri peta l force.
Its always been manufactured?
OtterbatHellcat
03-04-2019, 20:21
I've blown an hour searching ISS oxygen humidity levels, found a lot of shit that didn't really answer my question. Information yes.
I figure the next time I saddle up for my ride into orbit, I'll remember to take a thermometer that reads humidity as well.
ChickNorris
03-04-2019, 20:21
Some intelligent dudes think there is like 0 to 5% of gravity at space station, but if you do
"Gimmrr" equation gmm/r^2 it is probably about ~~95% of g at spacestation. (Too lazy to run it)
It feels like zero gravity because of free fall. It stays on the orbit, because of cen tri peta l force.
; ) Derived
; ) Derived
Thanks.
dv/dt "chain" on the bottom reminds me of high school AP physics.
Today is highschool rememberance day, because even 90210 star passed.
ChickNorris
03-04-2019, 20:32
Here's to pouring out derivatives for Dylan.
: ) like beer only different.
ChickNorris
03-04-2019, 20:33
Dang... there's peanut butter in the chocolate again.
Apologies OP.
Here's to pouring out derivatives for Dylan.
: ) like beer only different.
Hardy Har Har!
https://youtu.be/98unLjZRc_8
Duplicate. Can't load video
DenverGP
03-04-2019, 20:56
Weird, was working earlier... here's a link:
https://www.youtube.com/watch?v=ohMhV1WRxcc
The toy sticking down is probably due to the airflow requirements in the capsule. Since there is "no" gravity convection is not really a thing in space and any CO2 generated by the crew would hang around them and suffocate them. To fight this the air flow on station and other vehicles is kept high so the CO2 is removed and scrubbed. In tightly confined spaces with low air flow the crew will also have personal work fans.
Gravity is something on the order of something times 10 to the negative 3. Humidity is kept low but high enough so that static is not an issue.
BushMasterBoy
03-04-2019, 21:25
I believe gravity is ruled by the inverse square law. Neutral gravity points in the solar system are called La Grange points.
https://en.wikipedia.org/wiki/Inverse-square_law
http://audiokarma.org/forums/index.php?media/ancient-aliens-meme-hair-guy-0131-600x630.16082/full&d=1533750866
Circuits
03-04-2019, 22:46
Don't forget tidal pull.
No, not the moon (though that's a tiny factor, as well), but the only part of the ISS that's in true "zero-g" is its center of mass. The further in or out you go from the center of mass of the combined system, the further you are from a true free fall orbit, and that exerts a small tidal pull. It's even used to stabilize some satellites that want to have a particular aspect always facing down towards the gravity well.
Not saying that's what's happening in the video, but it's another possible cause for things to be settling in a microgravity environment.
DenverGP
03-04-2019, 23:35
Yes, earths gravity is still pulling on the ISS quite strongly, but the ISS station's orbit at 17,500 mph gives it centrifugal force that cancels out the pull of earth's gravity. So the ISS experiences almost perfect zero G.
I believe gravity is ruled by the inverse square law. Neutral gravity points in the solar system are called La Grange points.
https://en.wikipedia.org/wiki/Inverse-square_law
When I first learned about La Grange points it blew my mind.
As far as the water globules v. plush toy, inertia/density may provide an explanation. Yes, they are both accelerating towards the same point, but the forces acting on them(airflow?) have different effects based on their ratio of mass:volume. Given enough time, those water globules would likely settle in the same direction as the plush toy, but for obvious reasons, they don't allow that to happen.
The toy sticking down is probably due to the airflow requirements in the capsule.
Now here's an answer that could explain it. I know it's conjecture, but it makes sense. In the Dragon capsule I could see the airflow being designed to go from the astronaut's heads to their feet, which would explain the plush toy settling in down towards the footrests over and over. I can also envision that when the astronauts do weightless demos, they'd go out of their way to assure there were as little air movement as possible in the area where they're doing the demos.
O2
Ps. Someone had previously stated "air currents". As a standalone explanation I rejected that because, at least to me, that implies a natural phenomenon such as convection which does not happen in zero G. If you ment to imply more, my apologies.
DenverGP
03-07-2019, 19:15
https://twitter.com/elonmusk/status/1101789828574777346?lang=en
Link to a video on twitter from Elon showing the toy floating in the capsule.
DenverGP
03-07-2019, 19:18
https://www.youtube.com/watch?v=sFEYktWdy2I
Another vid of it a bit after the hatch was opened
Aloha_Shooter
03-09-2019, 20:40
The water globules form due to the surface tension of the water but they float due to the lack of gravity. In microgravity situations, there actually is a very tiny gravitational force that acts as a stabilizing force. This has been used by spacecraft to maintain a "vertical" configuration (i.e., one axis pointed radially to the center of the Earth) by using a long pole with a weight at the end (could also be a spacecraft payload). The differences in gravitational forces at either end of the pole are sufficient to keep the spacecraft from tumbling.
There ARE air currents and eddies in the space station and probably the Dragon capsule due to the movement of astronauts and the life support systems actively trying to move air. If they didn't move the air, the CO2 exhaled would linger around the astronauts and they'd suffocate quickly.
The motion of "Ripley" looks to me like classic zero-g/microgravity with air currents from the opened hatch and the life support systems causing it to move around. Note that once it is free to move, it will be affected by it's own angular and linear momentum in 3D free space which may not correspond exactly to the capsule's motion causing apparent motion within the capsule.
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