ELI5: If gravity gets weaker the further we travel from earth, then what’s the meaning of the term “Earth surface escape velocity”?

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It’s the speed an object incapable of self-propulsion needs to be moving at the surface of the Earth directly up in order to be able to keep going without being pulled back down. Easier to think of it in terms of firing a projectile that carries no fuel or means of propulsion after it’s launched (rather than a rocket or missile which continues to burn fuel and accelerate as it climbs). Imagine a gun pointed straight up. If a bullet was fired from that gun moving less than 11,186 m/s, it will ultimately turn around and return to Earth. If it’s moving at or greater than 11,186 m/s, it will keep going “forever” (until it encounters another gravitational influence or object).


Let’s do this as a real ELI5. Gravity pulls on you no matter how far away you get. You’ll always be pulled towards to earth no matter how far away you are. But not much. This fact isn’t just true about the earth, it’s also true about every other planet in the solar system- you are being pulled by Neptune right now! Do you feel it? Probably not because Neptune is really far away (and it’s pulling everything else on earth too). Every time you get twice as far away, the gravity is four times less. When you’re 10 times further away, gravity is 100 times less. The idea of escape velocity is that once you’re moving fast enough away, gravity will get weaker and weaker and while it will slow you down, it won’t be able to slow you down enough to pull you back. To escape from Earth, you need to be moving at 11km per second (7 miles per second) to escape.


If you go really fast, you can get really far away from Earth before gravity pulls you back down. Go even faster, and you can get farther away, where the gravity is weaker. And it takes a longer time to pull you back. If you go REALLY REALLY fast, you can reach a special speed where gravity can’t pull you back. You’ll be able to go so far away, it will be too weak to ever stop you. It will still slow you down at first, but not enough to ever bring you back to Earth. We call this the Escape Velocity, because if you can go this fast (and you’re not pointed directly at the ground), you can “escape” Earth’s gravity and travel into outer space.


Escape velocity is the speed needed to go from the current altitude to infinity while unpowered (so no rockets, etc). Surface escape velocity means that the altitude is 0. On the Earth that is about 11kps. That means that from the surface to go up and never return you’d need a speed of 11kps with engines off.


Gravity does indeed get weaker the further you are from the surface of an object. That means the escape velocity *at that distance* is lower, too; I suspect this is where the question arises. If you imagine building an enormous catapult at a height of *x1* that could throw a boulder straight up with an initial velocity *v1*, at some height *x2* which is higher than *x1*, gravity will have acted on the boulder to bring its velocity down to a new velocity *v2*, in accordance with the laws of physics. If that boulder were thrown with an initial velocity *v1* greater than the escape velocity, then its velocity later, though decreased by gravity, will still be higher than the (also lower) escape velocity at the new height.