Why do astronauts no disappear into the distance when they release their hold on their space craft (like the ISS) which is travelling at incredible speeds through space?


There is no air resistance, I get that. And the space craft/ISS is in orbit, I *kind* of get that. But if astronauts still experience acceleration in space, why do they not experience deceleration when they are no longer getting “pulled” by their vehicle?

BONUS QUESTION: at what point does acceleration forces stop? A space ship accelerates in space, all crew are pushed back into their seats, but when does that feeling dissipate if they remain at that new speed?

In: Physics


Mostly, it’s because of the law of physics that says an object in motion stays in motion, unless acted upon by an outside force. If the spaceship is traveling at 1,000 mph, then so is the astronaut holding onto the spaceship. If he lets go of the spaceship, he is still traveling at 1,000 mph, because no other outside force caused him to slow down.

The ISS uses 8,000 pounds of propellant a year to *maintain* its speed, which is less than a car idling. That small boost is the only thing you lose when you let go of it, so it’ll *eventually* move away from you, but very slowly.

There is no acceleration forward or back, only towards the centre of the earth, that is what creates the orbit (like how yo-yo string must be tense to keep it going round and round). And that acceleration acts on both the ISS and the astronaut by precisely the same amount, because they’re in the same place. So there is no reason an astronaut should fly away from the iss as there is no difference in acceleration.

The acceleration pushing crew back in the seat come from engines being fired. This disappears the exact moment the engines are turned off.

The astronaut would “fly away” if the space ship was firing big engines when they let go

>But if astronauts still experience acceleration in space, why do they not experience deceleration when they are no longer getting “pulled” by their vehicle?

I think the crucial thing to understand here is that the ISS is not undergoing any acceleration. It doesn’t have rockets propelling it around the earth, it’s simply in orbit – so whether the astronaut is sitting/touching/attached to the ISS doesn’t actually matter as neither the astronaut nor the ISS is changing speed. They are both zipping around the earth every 90 or so minutes, and will continue to do so forever*.


Consider the fact that the earth spins such that – at the equator – a person is zipping around relative to the center of the earth at around 1,000 miles per hour. Not only that, but the whole earth is zipping around the sun at about 20 miles *every second*. Relative to the sun, every time you blink you’ve moved the distance to your local supermarket and back, yet you can happily jump up into the air, blinking away with frankly reckless abandon, and you don’t land 50 miles away. Why? Because…


>at what point does acceleration forces stop? A space ship accelerates in space, all crew are pushed back into their seats, but when does that feeling dissipate if they remain at that new speed?


Another important thing to understand is that acceleration is simply a change in speed. It’s not a force itself, it’s just the consequence of a force acting upon something. So it’s actually no different to a car – if you floor it as you join a fast-moving road full of traffic, you might get thrust back into your seat a little – because the car is moving faster than you are, and you’re essentially being ‘bumped’ forwards by it, like a tractor pushing a broken down car. But once you hit 70mph and stay at 70mph, this feeling goes away because you’re now travelling the same speed as the car – it doesn’t need to push you forwards. Slam on the breaks and the opposite happens; The car slows down but you don’t (which is why seat belts are very important – that make sure you slow down at the same rate as the car, rather than fly through the wind screen and then suddenly stop when you hit a wall or lamp-post or another car). The feeling of accelerating (or decelerating) is just the feeling of being pushed forwards or pulled back by something travelling a different speed to you.


It’s no different in space. In fact, it’s easier to do in space, because the lack of wind resistance means that when something is travelling at X speed, it stays at that speed unless another force acts upon it (though it’s worth remembering that ‘speed’ is relative to *something* – as mentioned above, everyone is travelling very, very fast relative to the sun, even when they’re asleep – yet two people asleep next to each other are considered to have a speed of 0mph relative to one another).


On earth, our thick, dense atmosphere creates so much air resistance (and the floor creates friction, too) so anything travelling at any speed that isn’t just 0mph (relative to the ground) needs constant force acting on it keeping it there – jet engines in a plane, wheels turning in a car, leg muscles whilst walking etc. This is also how docking space craft is, without intending to understate its difficulty, relatively easy. Once you have matched speeds between two crafts (or a human doing a space walk and a craft), you will stay the exact same distance apart. You can then slooowwwlllyyy inch towards your target and, despite the fact you’re flying around the earth at about 4.5 miles per second, your speed relative to each other only changes as much as your thrust.


(* The ISS is in low enough orbit that, actually, there *is* a small amount of air resistance which means the ISS is constantly losing a little altitude. Every now and then the ships that are docked to it will ‘boost’ it back up a little, wherein it’ll start very slowly falling again and they’ll boost it back up again, the cycle repeating. This is an unavoidable part of being at the orbital level they’re at, but it’s also handy for satellites of any kind to have this sort of actively maintained trajectory so that when they reach the end of their life or otherwise we lose control of it, it’ll slowly descend and burn up in earth’s atmosphere rather than stay up there for thousands of years. The GPS satellites, by comparison, are much higher up and their orbits won’t naturally decay for millenia – if that – but then, being so much further away, are less likely to actually cause a problem even if we do lose control of them.)

Because the astronaut is traveling at the same speed as the vehicle they are riding in.

The room you are in currently traveling in all sorts of different directions (Earth rotation around its own axis earth orbiting the sun and the sun itself traveling in respect to the stars and the galaxy etc) but if you jump up right now you won’t be smashed into a wall.

The thing that you have difficulty with is that you don’t feel speed.

On earth you are always slowed down by friction but outside the atmosphere, Newton rules.

what you do feel is acceleration. getting faster and slowing down or changing the direction.

If you have ever ridden a reasonably good train you will not have felt anything about how fast the outside world moves by unless you look out the window.

You aren’t pulled by the train unless it is accelerating, if it is just going at constant speed down straight tracks you won’t feel anything.

Because they’re travelling at the same speed as the vehicle.

> why do they not experience deceleration when they are no longer getting “pulled” by their vehicle?

Acceleration requires a force. There’s no force acting on them that would slow them down, so they don’t slow down. Objects in motion remain in motion unless an unbalanced force acts upon them.

> A space ship accelerates in space, all crew are pushed back into their seats, but when does that feeling dissipate if they remain at that new speed?

At the point that the ship stops accelerating

It’s impossible to detect your own velocity without a reference point to compare to, because velocity itself doesn’t produce a force, acceleration does. That’s why it was such a debate about whether the Earth was moving around the Sun or not, people really thought we’d be able to tell if the Earth was moving that fast. But you can’t. You are currently travelling at around 30 km/s, but you feel like you’re motionless, because the Earth isn’t accelerating.

Same reason the flight attendant doesn’t get splattered against the back of the airplane despite the fact that it’s moving 500 mph and she’s not strapped in. They are both moving the same speed.

You are standing on a bus, the bust departs.

You feel the acceleration and you have to grab something to not lose balance. As soon as the bus is going at a fix speed, you can stand and balance on one foot if you want.

Acceleration is a change of speed or a change of direction(turn), if you have fix speed and not turning, you experience a balance, and no force is felt onto you.

For the astronauts; once the rocket engine is shut town, they are “on the bus” that is tiding at fix speed. Even if you step outside the ship, you still travel the space with it.

When in space you are still traveling at all that speed so If you are sitting outside the space station and let go you likely aren’t drifting away that said you would have no method of returning without some sort of rocket or way to pull you back to the station.

Perhaps an interesting real-life (ISS) video regarding acceleration in space:


Jump to 3:00-ish for when the burn actually happens.