Why does a rocket have to go 25,000 mph to escape Earth?

.

.

Gravity is the force that pulls you toward the ground. And that’s a good thing. Gravity keeps you on Earth; otherwise, you would float away into space.

.

Like a balloon

, one of the things I do is teach students how rockets overcome gravity. Here’s how it works:

. Then it shoots those hot gases out of a nozzle. It’s sort of like blowing up a balloon, letting go of it and watching it fly away as the air rushes out.

. The fuel is typically something flammable, usually hydrogen, methane or kerosene. The oxidizer is usually liquid oxygen, which reacts with the fuel and allows it to burn.

When going into space and escaping from Earth, rockets need lots of force, so they consume propellant very quickly. That’s a problem, because the rocket can’t carry enough propellant to keep thrusting forever; the amount of propellant needed would make the rocket too heavy to get off the ground.

So what happens when the propellant runs out? The thrust stops, and gravity slows the rocket down until it gradually begins to fall back to Earth.

, and begins to circle the planet.

.

Escaping Earth

. They launch with a big rocket, and then, once in space, discard it to use a smaller rocket. That way, the journey can continue without the weight of the bigger rocket, and less propellant is needed.

But even staging is not enough; eventually the rocket will run out of propellant. But if the rocket goes fast enough, it can run out of propellant and still continue to coast away from Earth forever, without gravity pulling it back. It’s like riding a bike: build up enough speed and eventually you can coast up a hill without pedaling.

(about 40,000 kilometers per hour).

. A rocket needs to go that fast so that the momentum propelling it away from Earth is stronger than the force of gravity pulling it back. Any slower, and you’ll go into an orbit of Earth.

Escaping Jupiter

Bigger, or more massive, objects have stronger gravitational pull. A rocket launching from a planet bigger than Earth would need to achieve a higher escape speed.

. So it requires a very high escape speed: 133,100 mph (about 214,000 kilometers per hour), more than five times the escape speed of Earth.

, an object so massive that its escape speed is extraordinarily high. So high, in fact, that even light – which has a speed of about 670 million mph (over a billion kilometers per hour) – is not fast enough to escape. That’s why it’s called a black hole.

. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

This article has been updated to correct the speed of light.