escape velocity question?

Question

cant you just eventually leave a planets gravitational pull at any speed, it'll just take more time?? if so, then how is esacape velocity different?

Answer 1

> cant you just eventually leave a planets gravitational pull at any speed,...

Yes, you can. GP99 is mistaken. What he says is a common misconception about escape velocity.

If you have some sort of long-lasting engine that can keep generating thrust, then you CAN get continually farther away from the earth (even to the moon or beyond) as long as you can keep the engines running. Even if you're only moving at 10 mph.

The problem comes when you turn your engines off.

If you are going at a speed lower than escape velocity at the moment you shut down the engines, then what will happen is: your momentum will continue to carry you forward for a while, but then the earth's gravity will slow you to a stop, then pull you back down.

On the other hand, if you are going at a speed FASTER than escape velocity at the moment you shut the engines off, you can continually "outrun" gravity just by coasting. Gravity DOES still continually pull on you even in this case, and in fact it continues to slow you down a bit. However, you're moving so fast that, before gravity can slow you to a stop, you've already moved out to a distance where the gravity is much weaker. So the gravity never quite stops you. You keep going (with engines off) forever.

You never technically "leave" a planet's gravitational pull, even if you're far away from it. So it's not as though escape velocity lets you crash through some kind of "gravity barrier." It's just that escape velocity lets you continually "outrun" gravity even without propulsion. If you have an endless supply of propellant, you don't need to go at escape velocity.

Answer 2

Escape velocity is the velocity you need to escape in an instantaneous launch, with air resistance ignored. You can escape at a low speed but this costs much more propellant than a short high-acceleration launch, since at low acceleration and low speed you're approaching the total-waste situation of hovering against 1 g by rocket thrust.
EDIT: A clarification of Rick's answer, since "escape velocity" is generally understood to be referred to the surface of the planet: As you gain altitude the escape velocity at that altitude decreases. You can escape at the end of a slow ascent if your speed is >= that reduced escape velocity, and the speed needn't be anywhere near the surface escape velocity.

Answer 3

No, if you leave with too low of a velocity, the planet's gravitational effects will accelerate you back towards it.

So, unless your velocity is above the 'escape velocity', you'll just get pulled back down.