Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected].
Why does a rocket have to go 25,000 mph (about 40,000 kilometers per hour) to escape Earth? – Bo H., age 10, Durham, New Hampshire
There’s a reason why a rocket has to go so fast to escape Earth. It’s about gravity – something all of us experience every moment of every day.
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.
But gravity also makes it difficult to leave Earth if you’re a rocket heading for space. Escaping our planet’s gravitational pull is hard – not only is gravity strong, but it also 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, the rocket has to make thrust – that is, create force – by burning propellant to make hot gases. 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.
On July 16, 1969, a Saturn V rocket sent Apollo 11 and three American astronauts on their way to the Moon.
Heritage Images/Hulton Archive via Getty Images
More specifically, the rocket propellant consists of both fuel and oxidizer. 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.
A rocket provides the spacecraft with a sideways push (right arrow), gravity pulls it toward Earth (down arrow), and the resulting motion (red arrow) puts the spacecraft into orbit (yellow path).
ESA/ L. Boldt-Christmas
Fortunately, scientists can launch the rocket with some sideways momentum so that it misses the Earth when it returns. They can even do this so it continuously falls around the Earth forever. In other words, it goes into orbit, and begins to circle the planet.
Many launches intentionally don’t completely leave Earth behind. Thousands of satellites are orbiting our planet right now, and they help phones and TVs work, display weather patterns for meteorologists, and even let you use a credit card to pay for things at the store or gas at the pump. You can sometimes…
