# Why is the escape velocity of Earth so fast?

1. Jun 28, 2004

### syano

Help me understand this please.

Why is the escape velocity of Earth so fast? Doesn’t the gravitational pull of the Earth weaken as you get farther away from the center of the Earth?

I can jump a couple feet into the air… I’m am going upwards nowhere near 27k miles an hour… if I could leverage my feet on air, couldn’t I apply the same amount of force to go another few feet in the air until I escape the Earth? High altitude balloons can almost escape Earth and are going nowhere near 27k miles an hour. Yet I know, (because I have heard it many times before) that the escape velocity of Earth is about 27,000 miles an hour.

Why can’t you go upwards at 1 mile an hour until you escaped Earth?

What’s the scoop?

Thanks,

2. Jun 28, 2004

### Fuego

You might want to read this, it explains it better than I can.

3. Jun 28, 2004

### syano

Thanks, I get it now… I think. So the Space Shuttle (when taking off) doesn’t go 25 thousand miles per hour for any extended period of time… if any at all. People refer to escape velocity as just a simple way to refer to something? Instead of saying you can escape Earth by going one mile an hour provided you continue to apply “x” amount of force for “t” amount of time.

Is that right?

4. Jun 28, 2004

### arildno

As long as you could continously apply a force on the spaceship to maintain a given upwards velocity, you could get as far as you like away from the earth.
The crucial issue with escape velocity, is that this is the velocity you'll need to have to escape earth once no other forces than gravity acts on the spaceship.
(It depends on the distance from the center of the earth at the place where this condition (no other forces than gravity) occurs)

5. Jun 28, 2004

### Staff: Mentor

Yes, you are right, but its more than just a convention. If at any time you stopped applying a force to an object moving up at 1mph (below whatever altitude the escape velocity is 1mph), it'll fall back to earth. An object at escape velocity will never fall back to eart.

6. Jun 28, 2004

### enigma

Staff Emeritus
Hi Syano.

The Shuttle NEVER goes 25K mph. Take a look at http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html. It explains how you transfer from orbit to orbit in space.

To be in orbit, you need to be going ~18K mph or ~7.75 km/sec. That's roughly the velocity of the Shuttle. In the graphic on the website, that's orbit 'A'. If you want to go higher, you do a maneuvering burn. When you do the burn, you're at the intersection of orbit 'A' and orbit 'B'. You are going faster, so you'll go into an orbit which takes you farther out of the gravity well (orbit 'B'). As you go farther out, you'll be slowing down (gravity pulls you back). The maximum distance away (intersection of 'B' and 'C') depends on how fast you were going at the intersecion of 'A' and 'B'. You're still in what is called a bounded orbit (meaning you'll always come back) so long as you don't hit escape velocity. If you hit escape velocity, that means that the intersection of orbits 'B' and 'C' is an infinite distance away... your velocity of ~11 km/sec in low Earth orbit (LEO) was just enough to get you to zero velocity an infinite distance away. If you have more velocity than that, you'll go into a parabolic orbit.

Hope that makes some sense.

Last edited by a moderator: Apr 21, 2017
7. Jun 29, 2004

### LURCH

IOW, "escape velocity" is the speed you must be going in order to coast away from Earth. That's why is referred to as a "velocity" rather than an "acceleration".