Escaping Earth
But suppose the goal is to let the rocket escape from Earth’s gravity forever so it can fly off into the depths of space. That’s when scientists do a neat trick called staging. 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.
And just like there’s a minimum speed required to coast the bike, there’s a minimum speed a rocket needs to coast away into space: 25,020 mph (about 40,000 kilometers per hour).
Scientists call that speed the escape velocity. 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.
For example, Jupiter is the most massive planet in our solar system. It’s so big, it could swallow 1,000 Earths. 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.
But the extreme example is a black hole, 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.
