Escape velocity: Why do rockets need to fly fast to escape the earth's gravitational pull?

Question

I can see why a rocket would need to fly fast in order to stay in orbit. But that's not the question; in such a case the rocket would balance the gravitational pull and not escape it.

If a rocket would fly, somehow, at a constant 1 m/s, wouldn't it (eventually) leave our solar system ?

Answer 1

Think of a spacecraft flying in orbit like a swinging pendulum. You need energy to get it started, but once it is oscillating, it will stay in that state for a long time. If you wanted to increase or decrease the pendulum oscillation that would require energy too. But once it is in a new oscillation cycle, it will tend to stay there.

Similarly, a spacecraft needs to burn a lot of fuel to get *into* or *out of* an orbit. Staying in a particular orbit requires very Little energy. If there were no momentum losses, then staying in orbit would require no energy.

An orbit is an ellipse with a massive body (like the Earth) at one focus. Ellipses are like flattened circles. The higher the eccentricity of the ellipse, the flatter it is. The Earth's orbit around the Sun has a very low eccentricity and is therefore quite circular.

So long as a spacecraft is flying in an ellipse shaped orbit then it needs almost no energy to stay there. If it needs to maneuver (like the shuttle docking with the ISS) then it needs to fire its thrusters to slowly change its orbit to match the object it is trying to rendezvous with.

Suppose you wanted to escape Earth's gravity. You fire thrusters for 10 seconds. Now your orbit is a flatter ellipse; your eccentricity just increased. However, you are still in orbit around the Earth. Part of your orbit may be far away from the Earth, but you are still under influence of Earth's gravity. You will realize that there is a critical speed -- called the escape velocity -- which will allow you to go so fast that the Earth cannot pull you back. This is like swinging a pendulum so fast that instead of rocking back and forth about a point, it just spins in circles.

The important concept here is that gravity does not stop pulling on you when you leave Earth's atmosphere. All spacecraft in Earth orbit are feeling its gravitational pull. However, they are all in a state of perpetual free fall. They are falling towards the Earth, but at the same time they are moving around it fast enough that they keep 'missing'. This is the same as if you took a ball on a string and spun it around in a circle. The string has tension; the ball is accelerating toward you. But its velocity is in the direction of travel (along the circumference) so it stays in a circular path.

Answer 2

You answered your own question. The rocket must fly very fast in order to stay into orbit, orbit is essentially falling. In order for the rocket to not fall back to earth, it needs to be going fast enough that it will not be pulled back to earth before it travels so far. You can't seperate the concept of an orbit from the concept of the rocket's high speed travel. The rocket doesn't escape the Earth's gravitational pull, it is just high enough to have less gravity, and to be able to orbit if it's going fast enough. If it didn't go up fast enough, it would not orbit. It would be a waste of fuel to just cruise up to space then plop back down. It would probably take a much larger rocket to do that. It's similar to a slingshot, or a bullet. The initial explosion sends the bullet flying so that the bullet can travel without using any more fuel.

Answer 3

You can absolutely travel at a slower speed, but you would have to continually provide force or consume fuel to keep going. Imagine you had a rope that went straight up, essentially forever. You could climb the rope at any speed you want and you'd get as far as you wanted to be. But the point is that you must keep climbing. You can't just let go. The escape velocity is the minimum speed that a vehicle with no further energy output needs to achieve to keep going. Think of shooting a bullet into the air. It can't go too slow, because it would just go slower and slower and fall back down. It has to go so fast that even though the earth is pulling it back, the force of gravity is getting weaker as it goes further away and gravity is never strong enough to "win" and stop it entirely.

Answer 4

Yes, if it could fly at 1 m/s, then it could eventually escape. But that is not what escape velocity means. Escape velocity is the speed you have to throw something at to escape Earth with no more thrust. It assumes coasting all the way. And coasting up means slowing down all the time. A rock thrown up goes only a few meters, slows to a stop and falls again. In theory, a rock thrown up from Earth at escape velocity would go up and up, slower and slower until it stopped at infinite distance after infinite time. Any slower and it stops sooner and falls back again. Faster and it is still going up with some speed after infinite time at infinite distance. If you had unlimited fuel and could keep pushing the rocket up enough to overcome gravity forever, you could slowly go up to any altitude, even to the Moon or Pluto or a star a billion light years away, but it would take a long time and require more fuel than exists in the universe.

Answer 5

Yes, you're right. In principle you can lift off as slowly as you like and eventually get away from the Earth, but it would be hopelessly wasteful in fuel. To take the extreme case, you could hover as long as you like without going anywhere, but you'd still be using up fuel. With the chemical rockets we've got today, we can only just make it into space, and it costs thousands of dollars per kg. of payload. One day no doubt, we'll have much more powerful and cheaper sources of rocket power; nuclear fusion perhaps, or vacuum energy, or tame black holes or antimatter. Then we'll be able to take off much more gently and we'll have plenty of spare power, and space travel will be much more comfortable, and space travellers won't need to be physically fit. But to use the minimum amount of fuel for lift-off, you need to accelerate to high speed as quickly as possible.

Answer 6

It has to do with the conservation of energy. The object trying to escape a planet's gravitational pull must have as much kinetic energy as the potential energy required to reach infinite height.

Kinetic energy is energy as the result of motion. Thus, to generate it you need SPEED. Lots of it. About 11.2 kilometers per second, or over 40,000 KPH.

Hope this helps.

Answer 7

It's a matter of effiency. In order to go into orbit slower you need more fuel and hence more weight. It becomes impractical to carry that mount of fuel unless you bring the weight down and the ability to generate lift remains the same as it currently is.

Answer 8

Ofcourse you can go at any velosity, but if you go at 1m/s, who will provide you that huge amount of fuel???

Answer 9

WE NEED THE ESCAPEVELOCITY TO GOING OUT OF THE GRAVITY OF TE EARTH