Astronomy Jargon 101: Escape Velocity

In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll finally get away with today’s topic: escape velocity!

If you really want to get away for a vacation, you’re gonna have to work for it. Getting into space isn’t all that hard. Staying in space in a whole different ballgame. Never coming back to Earth takes some real firepower.

The problem is that gravity, while weak, is persistent. You can get as far away from the Earth as you want, but the Earth’s gravity extends out to an infinitely large distance. Yes, at those great distances the gravity becomes exceedingly weak, but it’s not zero. And if you’re motionless, then eventually that weak gravity will drag you back down to the surface.

To really, truly leave the confines of the Earth you have to make sure that you have enough speed that the planet’s gravity can never, ever drag you back down. This is the idea behind escape velocity, the velocity needed to…well, escape.

The escape velocity is calculated by balancing the kinetic energy of an object trying to leave with its current potential energy. When those are equal, the speed is sufficient to never be pulled back. It’s a simple formula, too: the escape velocity is equal to the square root of 2 times the universal gravitational constant times the mass of the planet divided by the distance from the center of that planet.

At the surface of the Earth, that’s about 11.2 kilometers per second. That means if you can launch a rocket with at least that velocity, you’re guaranteed to stay in space forever (we are ignoring things like atmospheric drag, of course).

You can also do it the slow way. If you’re able to maintain constant acceleration, then you will escape. The further you get from Earth, the lower the escape velocity. If you stop, you come right back, but as long as you keep pushing forward eventually your speed will overcome the escape velocity and you’ll be on your way, free and clear.