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Can someone elaborate on why escaping a planet's gravity is about velocity and not just attaining a certain height from the surface? More concretely, why isn't

Can someone elaborate on why escaping a planet's gravity is about velocity and not just attaining a certain height from the surface? More concretely, why isn't getting to space as easy as taking a balloon to a certain height?

pradn 8y ago
This is the simplest way to explain it, I think. https://www.youtube.com/watch?v=xfZHUwyh59k https://www.youtube.com/watch?v=xfZHUwyh59k
baq 8y ago
The absolutely best thing you can do to really internalize what achieving orbit make is to play a bit of Kerbal space program. No sarcasm, it's an eye opening experience the first time you make it.
dredmorbius 8y ago
Gravity + tyranny of the rocket equation. http://www.physlink.com/Education/AskExperts/ae158.cfm http://www.physlink.com/Education/AskExperts/ae158.cfm Escape velocity is the inverse of terminal velocity of an object falling from infinite height (discounting air resistance). https://youtube.com/watch?v=uWjdnvYok4I https://youtube.com/watch?v=uWjdnvYok4I
pavel_lishin 8y ago
It's because you need to not just get to space, you need to be in orbit. If you just take a magic balloon up to a certain height, once you're up there and let go fo the balloon - you just start falling back down. You need to have a lot of sideways velocity in order to actually get into orbit, to be revolving around whatever body you're trying to get off of. (It didn't used to make sense to me, either.)
- ythn 8y ago
  Couldn't you theoretically escape a planet's gravity without going into orbit? I mean, gravity weakens by the square of distance, so it wouldn't be long before Earth didn't have much influence on you
  - aphextron 8y ago
    No, you would just be in a different kind of orbit. You would be in Earth orbit until reaching escape velocity, at which point you would be in Solar orbit.
    - alistproducer2 8y ago
      so are you saying that even though I could escape Earth's gravity going at sub-orbital velocities that I would still need to be going fast enough to avoid being pulled into the sun?
      - aphextron 8y ago
        Disclaimer: the entirety of my orbital mechanics knowledge derives from Kerbal Space Program. I cant recommend it enough if you're really interested in building an intuitive understanding for this stuff. But no, it's not that you would be pulled into the sun. You would simply be in a solar orbit just like the earth, unless you continued accelerating and reached solar escape velocity (~600km/s). You can think of gravity like successively larger "wells" which you must escape from if you want to go anywhere. You can't just point a spaceship at a planet and go there.
        kuschku 8y ago
        > You can't just point a spaceship at a planet and go there. Disclaimer: You can, you just need so much power it’s like flying with a helicopter from your bedroom to the kitchen, absolutely wasteful.
        a_t48 8y ago
        The main question at that point becomes: how can you carry that much fuel?
        Florin_Andrei 8y ago
        Depends on how efficient the engine is. If you're riding a nuclear rocket, you don't need that much fuel.
- hodgesrm 8y ago
  See here for early 20th Century experiment validating the outcome when you let go of the balloon: https://lizaknapp.files.wordpress.com/2015/10/pooh-with-bees.jpg https://lizaknapp.files.wordpress.com/2015/10/pooh-with-bees...
- ecto 8y ago
  Imagine one of those games where you put a nickel in and watch it circle around a slope until it falls into a hole (for charity!). What's happening is the nickel's forward momentum being lost, resulting in succumbing to the gravity well. Launching a rocket to orbit is like starting from the bottom of the well, and spinning around the slope really fast until the nickel is returned to your hand.
  - cecilpl2 8y ago
    That's the best ELI5 analogy for orbit I've ever heard!
  - nojvek 8y ago
    Xkcd explains orbital speed in a much detailed explanation. https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/ The gist is that gravity pulls you. To go to space you have to escape gravity. Even if you go thousands of miles above earth, gravity will still pull you. To counteract gravity, you need to go really fast sideways. When you spin a ball attached to a string, the faster you spin, the more the ball pulls the string away from your hand. If you spin it really really fast, the string breaks eventually and the ball flies away. The string is gravity.
mrfusion 8y ago
Here read this. https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
- alistproducer2 8y ago
  That was a good read. Thanks
aphextron 8y ago
You are in space right now, you just happen to have 100km of atmosphere above you. When we say "go to space" we really mean "go to orbit". To go to orbit, you need tangential velocity to get away from the body you're currently attracted to. We use rockets to gain that velocity because no other technology we know of can provide it with the required thrust/weight ratio.
nilkn 8y ago
Escape velocity is actually only necessary for launching ballistic objects such that they will never return to the surface of the planet over an infinite amount of time (assuming no other interactions occur). It's not necessary when you have a method of providing continual propulsion and acceleration. Speed is mostly irrelevant if you can provide propulsion indefinitely. Escape velocity is an important concept because, realistically, you can't actually do that or it's extremely cost inefficient to attempt to do so.
- larkeith 8y ago
  Escape velocity is the total amount of acceleration necessary to escape a gravity well, aka the minimum amount of energy.
  - nilkn 8y ago
    No, it's explicitly defined as a speed or velocity, not as acceleration and not as energy.
    - larkeith 8y ago
      A velocity is the total of acceleration over time (as was referred to). A mass (the spacecraft) moving at a velocity has a given amount of energy, and in this context may be used interchangeably.
      - mannykannot 8y ago
        This is just pointlessly pedantic. You are going to get into all sorts of problems if you arbitrarily substitute any of velocity, acceleration or energy for any of the others. Having unstated dependencies is only going to lead to avoidable confusion. The key insight revealed in the term 'escape velocity' is that it is independent of the mass of the body. It is also independent of how quickly it acquires that velocity.
- aphextron 8y ago
  >Speed is mostly irrelevant if you can provide propulsion indefinitely Except you can't, so speed is very definitely not irrelevant.
abecedarius 8y ago
Besides what others have said, the surface of the Earth is already quite high (distant from the center). Getting above the atmosphere only adds ~1% to your height (if you're 40 miles up). OTOH the Earth spins at only a small fraction of the speed you'd need: in low Earth orbit you're going around in a little over an hour, versus 24 for the spin.
Doxin 8y ago
It basically amounts to falling down, except you're moving sideways at a fast enough clip to miss the earth entirely. Just being up the right distance isn't enough, You need the sideways velocity to make sure you don't hit the planet.

pradn 8y ago

This is the simplest way to explain it, I think. https://www.youtube.com/watch?v=xfZHUwyh59k https://www.youtube.com/watch?v=xfZHUwyh59k

baq 8y ago

The absolutely best thing you can do to really internalize what achieving orbit make is to play a bit of Kerbal space program. No sarcasm, it's an eye opening experience the first time you make it.

dredmorbius 8y ago

Gravity + tyranny of the rocket equation. http://www.physlink.com/Education/AskExperts/ae158.cfm http://www.physlink.com/Education/AskExperts/ae158.cfm Escape velocity is the inverse of terminal velocity of an object falling from infinite height (discounting air resistance). https://youtube.com/watch?v=uWjdnvYok4I https://youtube.com/watch?v=uWjdnvYok4I

pavel_lishin 8y ago

It's because you need to not just get to space, you need to be in orbit. If you just take a magic balloon up to a certain height, once you're up there and let go fo the balloon - you just start falling back down. You need to have a lot of sideways velocity in order to actually get into orbit, to be revolving around whatever body you're trying to get off of. (It didn't used to make sense to me, either.)

ythn 8y ago
Couldn't you theoretically escape a planet's gravity without going into orbit? I mean, gravity weakens by the square of distance, so it wouldn't be long before Earth didn't have much influence on you
- aphextron 8y ago
  No, you would just be in a different kind of orbit. You would be in Earth orbit until reaching escape velocity, at which point you would be in Solar orbit.
  - alistproducer2 8y ago
    so are you saying that even though I could escape Earth's gravity going at sub-orbital velocities that I would still need to be going fast enough to avoid being pulled into the sun?
    - aphextron 8y ago
      Disclaimer: the entirety of my orbital mechanics knowledge derives from Kerbal Space Program. I cant recommend it enough if you're really interested in building an intuitive understanding for this stuff. But no, it's not that you would be pulled into the sun. You would simply be in a solar orbit just like the earth, unless you continued accelerating and reached solar escape velocity (~600km/s). You can think of gravity like successively larger "wells" which you must escape from if you want to go anywhere. You can't just point a spaceship at a planet and go there.
      - kuschku 8y ago
        > You can't just point a spaceship at a planet and go there. Disclaimer: You can, you just need so much power it’s like flying with a helicopter from your bedroom to the kitchen, absolutely wasteful.
        a_t48 8y ago
        The main question at that point becomes: how can you carry that much fuel?
        Florin_Andrei 8y ago
        Depends on how efficient the engine is. If you're riding a nuclear rocket, you don't need that much fuel.
hodgesrm 8y ago
See here for early 20th Century experiment validating the outcome when you let go of the balloon: https://lizaknapp.files.wordpress.com/2015/10/pooh-with-bees.jpg https://lizaknapp.files.wordpress.com/2015/10/pooh-with-bees...
ecto 8y ago
Imagine one of those games where you put a nickel in and watch it circle around a slope until it falls into a hole (for charity!). What's happening is the nickel's forward momentum being lost, resulting in succumbing to the gravity well. Launching a rocket to orbit is like starting from the bottom of the well, and spinning around the slope really fast until the nickel is returned to your hand.
- cecilpl2 8y ago
  That's the best ELI5 analogy for orbit I've ever heard!
- nojvek 8y ago
  Xkcd explains orbital speed in a much detailed explanation. https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/ The gist is that gravity pulls you. To go to space you have to escape gravity. Even if you go thousands of miles above earth, gravity will still pull you. To counteract gravity, you need to go really fast sideways. When you spin a ball attached to a string, the faster you spin, the more the ball pulls the string away from your hand. If you spin it really really fast, the string breaks eventually and the ball flies away. The string is gravity.

mrfusion 8y ago

Here read this. https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/

alistproducer2 8y ago
That was a good read. Thanks

aphextron 8y ago

You are in space right now, you just happen to have 100km of atmosphere above you. When we say "go to space" we really mean "go to orbit". To go to orbit, you need tangential velocity to get away from the body you're currently attracted to. We use rockets to gain that velocity because no other technology we know of can provide it with the required thrust/weight ratio.

nilkn 8y ago

Escape velocity is actually only necessary for launching ballistic objects such that they will never return to the surface of the planet over an infinite amount of time (assuming no other interactions occur). It's not necessary when you have a method of providing continual propulsion and acceleration. Speed is mostly irrelevant if you can provide propulsion indefinitely. Escape velocity is an important concept because, realistically, you can't actually do that or it's extremely cost inefficient to attempt to do so.

larkeith 8y ago
Escape velocity is the total amount of acceleration necessary to escape a gravity well, aka the minimum amount of energy.
- nilkn 8y ago
  No, it's explicitly defined as a speed or velocity, not as acceleration and not as energy.
  - larkeith 8y ago
    A velocity is the total of acceleration over time (as was referred to). A mass (the spacecraft) moving at a velocity has a given amount of energy, and in this context may be used interchangeably.
    - mannykannot 8y ago
      This is just pointlessly pedantic. You are going to get into all sorts of problems if you arbitrarily substitute any of velocity, acceleration or energy for any of the others. Having unstated dependencies is only going to lead to avoidable confusion. The key insight revealed in the term 'escape velocity' is that it is independent of the mass of the body. It is also independent of how quickly it acquires that velocity.
aphextron 8y ago
>Speed is mostly irrelevant if you can provide propulsion indefinitely Except you can't, so speed is very definitely not irrelevant.

abecedarius 8y ago

Besides what others have said, the surface of the Earth is already quite high (distant from the center). Getting above the atmosphere only adds ~1% to your height (if you're 40 miles up). OTOH the Earth spins at only a small fraction of the speed you'd need: in low Earth orbit you're going around in a little over an hour, versus 24 for the spin.

Doxin 8y ago

It basically amounts to falling down, except you're moving sideways at a fast enough clip to miss the earth entirely. Just being up the right distance isn't enough, You need the sideways velocity to make sure you don't hit the planet.