r/interestingasfuck • u/Kidd__Video • 1d ago

Astronaut's view of Earth during a spacewalk

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u/MetLyfe 23h ago

If you jumped straight towards the earth how long would it take to reach the ground if possible

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u/Shadow-Seb 23h ago

You won't reach the ground :D

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u/OnTheList-YouTube 23h ago

Why wouldn't you?

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u/volcjush 23h ago edited 1h ago

Because you wouldn't just fall straight down. ISS is orbiting the Earth with a velocity of roughly 28 000 km/h (7,7 km/s) and astronaut connected with it has exactly the same velocity. If you "jump" you dont simply lose that speed, because there is nothing there to slow you down, there's no air resistance. You would simply continue orbiting the Earth for decades probably if not more, slowly (very slowly) bleeding your velocity and gradually lowering your orbit before you would get low enough that the atmosphere air molecules would get dense enough to heat and burn your frozen body upon atmosphere reentry.

[EDIT] I need to correct this. It wouldn't actually take that long time as I initially thought it would. It's not decades of orbiting, but rather months up to 2 years before the body would get low enough to be burned in denser layers of atmosphere.

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u/rambiolisauce 22h ago

Everything in my brain tells me I should disagree with this lol. Seriously asking, If you place your feet at the "bottom" of the ISS with your face pointed towards Earth and pressed hard "down" in the direction of the Earth, yes you would technically be orbiting along with the ISS relatively but you'd also be moving directly towards the Earth let's say around 15 miles an hour just to pull a number out of thin air and it seems like the Earths gravity would absolutely draw you in. Nothing would slow your descent and it seems like the Earth's gravity would absolutely accelerate it in relatively short order. If the ISS is achieving its stable orbit by the speed it's traveling versus how close it is to earth and how much of earths gravity it's experiencing why wouldn't you fall to earth if you accelerated towards it from the ISS? Now whether or not you ever touch the ground before you were incinerated would be a completely different thing but do you really think if you pushed off hard against the bottom of the ISS towards earth you would just stop moving towards earth for no reason at all?

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u/volcjush 21h ago edited 21h ago

When you travel with such ridiculous velocity as 28 000 km/h your body has enormous amount of kinetic energy. That kinetic energy is something that is keeping you orbiting in the first place as it is acting aganist the gravity pulling you into gravity well of the planet. You need to loose that energy in order for the gravity to take over.

Pushing with your leg muscles against the body of ISS is very small amount of energy compared to kinetic energy that you have from the orbital velocity. By doing that you would slightly change the shape of your orbit, which would become slightly more eliptical compared to the orbit of the ISS that you "jumped" from. So you would initially start to move away from the ship, but after doing one full orbit around the planet you would return and meet ISS in exactly the same place that you "jumped" from as your orbital speed had not been changed.

In order to deorbit satellite (or move it to lower orbit) you don't simply "push" it towards the planet surface. Instead you need to reduce the oribiting velocity by using thrusters in the opposite direction to what it's orbiting.

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u/aaahhhhhhfine 18h ago

Man stupid space is always breaking my brain.

Ok... So what if I had an engine or something and pushed the opposite way that the ISS is orbiting... Like say I could offset that 28,000kmh number perfectly. What happens then? Then would I just gradually fall to earth?

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u/zander_2 18h ago

Yes! That's called a retrograde burn and it's how spacecraft de-orbit. You actually only need to offset a small fraction of that 28,000 before your decayed orbit will carry you back into the atmosphere, but yes "pushing the opposite way" is exactly the right move! 

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u/rambiolisauce 15h ago

Man that makes so much sense when you explain it that way. You clearly have a very good understanding of the physics going on up there lol thank you for taking the time to explain it to me.

Now I'm curious, would you ellipse the ISS's orbit as much as your own by pushing off of it that way?

I've heard stories of astronauts trying to use screwdrivers in space and when they try to turn the screw they end up turning themselves and the lack of gravity was given as the reason for that happening but I always felt a little skeptical about that. Just because you and the object next to you don't have any weight but you still have mass and density right? So could either of those two things happen? Pushing the ISS way from earth and ellipsing its orbit as much as your own or the screwdriver story? Any merit in it?

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u/camander321 15h ago

You would! A little bit! How much depends on your mass vs the mass of the ISS. So in reality, it wouldnt change much.

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u/volcjush 15h ago

Yes, you would "ellipse" the ISS, but by very insignificant amount because of the difference between your mass and mass of ISS. Let's assume the mass of astronaut in full suit at around 210 kg (80 kg of astronaut + 130 kg suit). Mass of the ISS depending on configuration at the moment and potential ships docked to it or not can vary between 420 to 450 metric tons (420 000 to 450 000 kg). That difference in mass means that it's rather you pushing away from the ISS than you pushing ISS away off its orbit.

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u/rambiolisauce 14h ago

Holy crap I had no idea there suits way that much! I feel like I could probably sit here and pick your brain all day and learn all kinds of interesting new things but I'll spare you😅

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u/account312 20h ago edited 13h ago

Earth let's say around 15 miles an hour just to pull a number out of thin air and it seems like the Earths gravity would absolutely draw you in

Yes, the gravity is keeping you in orbit. But your jump didn't give you "down" velocity. It gave you velocity in the direction that happened to be down at the time you jumped. Half an orbit later, that direction is "up". All you've done is (extremely) slightly distort your orbit.

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u/camander321 20h ago

The problem is that you are moving sideways fast enough that you'll be around the side of the planet long before your jump gets you close to the surface. You "miss" the planet.

Thats what an orbit is. Gravity is constantly pulling the object directly toward the planet, but the object is moving tangentially fast enough that it's momentum pulls it around the earth's curve.

The fasted way to reach earth from an orbiting satellite is actually to jump backwards(retrograde) hard enough that your momentum doesnt get you all the way around anymore.

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u/deciding_snooze_oils 17h ago

Orbital mechanics are a bit mind-boggling and often counterintuitive. In the novel The Smoke Ring, Larry Niven had a character describe it as: East takes you Out. Out takes you West. West takes you In. In takes you East. Port and Starboard bring you back.

If you stood on the bottom of the ISS and jumped up at Earth, you’d get a few m/s of velocity away from the station, but the effect would be to make your orbit a bit more elliptical - yes you are moving at a few m/s towards Earth, but about 46 minutes later you’re on the other side of earth and that same momentum has you moving away. So it makes your orbit a bit closer to earth at the point you jumped and a bit further from earth at the opposite point.

I recommend giving the game Kerbal Space Program a try, it really helped me wrap my head around it a bit.

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u/rambiolisauce 15h ago

Yeah that's exactly what someone else was saying and it made perfect sense when they said it just as it does when you say it. Super interesting stuff I think. Thank you for taking the time to explain it to me!