Why do rockets have to travel at 25,000 mph to escape Earth?

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Why do rockets have to travel at 25,000 mph (about 40,000 km/h) to escape Earth? – Bo H., 10, Durham, New Hampshire

There's a reason rockets have to fly so fast to escape Earth. It’s about gravity – something we all experience every moment of every day.

Gravity is the force that pulls you toward the ground. This is a good thing. Gravity keeps you on Earth; otherwise, you would drift off into space.

But if you were a rocket heading into space, gravity would also make it difficult for you to leave Earth. It's difficult to escape Earth's gravity—not only is it strong, but it extends far away from Earth.

like a balloon

As a rocket scientist, one of the things I do is teach students how rockets overcome gravity. Here's how it works:

Essentially, a rocket must produce thrust, or force, by burning propellant to produce hot gases. It then blasts these hot gases out of the nozzle. It's a bit like blowing up a balloon, letting it go, and watching it fly away with a surge of air.

More specifically, rocket propellant consists of fuel and oxidizer. The fuel is usually a flammable substance, usually hydrogen, methane or kerosene. The oxidizer is usually liquid oxygen, which reacts with the fuel and causes it to burn.

Rockets require a lot of power when entering space and escaping from Earth, so they consume propellant quickly. This is a problem because a rocket cannot carry enough propellant to maintain thrust forever; the amount of propellant required would make the rocket too heavy to leave the ground.

So what happens when the propellant is exhausted? Thrust ceases and gravity slows the rocket until it gradually begins to fall back to Earth.

Fortunately, scientists can use some lateral momentum to launch a rocket so that it doesn't miss Earth when it returns. They could even do it so that it keeps falling around the Earth forever. In other words, it enters orbit and begins orbiting the Earth.

Many launches deliberately don't leave Earth entirely behind. There are now thousands of satellites orbiting our planet, helping phones and televisions work, showing weather patterns to meteorologists, and even letting you use your credit card to pay for things at the store or pay for gas at the gas station. Sometimes you can see these satellites in the night sky, including the International Space Station.

escape earth

But let’s say the goal is to get the rocket out of Earth’s gravity forever so it can fly deep into space. That's when scientists resort to a clever trick called staging. They launch with big rockets and then, once in space, dump them and use smaller rockets. This way, the journey can continue without the weight of a larger rocket and require less propellant.

But even a stage show isn't enough; eventually the rocket will run out of propellant. But if a rocket goes fast enough, it can run out of propellant and still continue forever away from Earth without being pulled back by gravity. It's like riding a bike: add enough speed and eventually you can glide up a hill without pedaling.

Just like the minimum speed required for a bicycle to glide, a rocket needs a minimum speed to glide into space: 25,020 mph (about 40,000 km/h).

Scientists call this speed the escape velocity. A rocket needs to go so fast that the momentum pushing it away from the Earth is greater than the force of gravity pulling it back to Earth. Any slower and you'd be in Earth orbit.

Escape from Jupiter

Larger or more massive objects have stronger gravitational pull. Rockets launched from planets larger than Earth would need to achieve higher escape velocities.

For example, Jupiter is the most massive planet in the solar system. It is so big that it can swallow 1,000 earths. So it would require a very high escape velocity: 133,100 mph (about 214,000 kilometers per hour), more than five times the Earth's escape velocity.

But the extreme example is a black hole, which is so massive that its escape velocity is very high. So high, in fact, that not even light traveling at 370 million miles per hour (about 600 million kilometers per hour) is fast enough to escape. That's why it's called a black hole.

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