Escape Velocity
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Basic Information

Escape Velocity is the speed needed to get beyond the gravitational pull of an object. It is most commonly heard in relation to spacecraft leaving a planet, but the term also applies to leaving a moon, a star system or even a galaxy.

The further you get from the source of gravity, the lower the escape velocity requirement is. For example, if you made a giant cannon to launch a projectile into space, it would need to fire the projectile at 11.2 km per second for it to leave the Earth and not fall back due to gravity. If, instead, you built a rocket, and put the cannon on the rocket, you could launch the rocket into low-earth orbit at about 8 km per second. From such an orbit, the cannon would only have to fire about 10.9 km per second to send it's projectile beyond earth's gravity. If you built the cannon on the moon, it would only need to fire a projectile 2.4 km per second to escape the gravities of both the earth and the moon. In other words, the hurdle to escaping gravity can be solved in two ways: by extreme speed applied once, or by applying much lower thrust over long durations. This concept explains how multi-stage rockets work, propelling progressively smaller masses at higher speeds from higher elevations.

See Also: Delta V

In our Solar System

Because the Earth has slightly more gravity than Venus, and a lot more gravity than Mars or Mercury, any spacecraft capable of leaving the earth should also be able to leave any of those planets, and use less fuel in the process. Likewise, such a ship would be able to reach the escape velocity to leave our solar system, provided it had enough fuel / reaction mass conserved to apply thrust while out at the distance around the orbits of Neptune or Uranus.

Speaking of Uranus, a starship leaving it or Neptune would need an escape velocity around twice what is needed to get away from earth. Saturn would take three times as much thrust as earth. Jupiter would take over five times as much (nearly 60 km/second). Again, the further away you get from a planet, the lower the speed you need to escape it, but initial take off from large planets will really burn up fuel.


To launch yourself right out of the plane of our galaxy would take a speed of around 1,000 km / second. If you headed out further down the arm of the galaxy, you'd be able to escape with much less acceleration, but at the cost of spending thousands of years traveling within the galaxy first, and having to ration out your fuel / power.

The escape velocity of a black hole at or within its event horizon exceeds the speed of light. Unless your ship has an FTL stardrive, you're just hosed.



Game and Story Use

  • Most Space Opera and Sci Fi games just abstract or hand wave space travel, so the specific data above is probably only of real use to a Hard Science game, and (given that most of the data concerns points in our own solar system) especially one set Twenty Minutes Into The Future.
    • The concepts, however, can be applied to lighter games, without bogging your campaign down in technical detail:
      • A really large planet is harder to escape the gravity of, so a big starship would take a much more distant orbit around it, and then use shuttlecraft to ferry personnel and supplies too and from the planet.
      • A ship could get caught in the gravity well of an uncharted massive compact halo object, and have to burn most of its fuel to escape. It's left with just enough power for life support and emergency course-corrections, so once the ship gets back to populated space, it'll have to rely on a space tugboat to refuel it before it can establish a stable orbit anywhere.
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