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I was listening to Skeptics Guide to the Universe the other day, and Steve Novella mentioned that the escape velocity for the galaxy is 544 km/s.

If an object fails to reach escape velocity on Earth, it will return to Earth. What will be the fate of an object, say the Voyager spacecraft which is currently traveling at 17 km/s, that fail to meet the 544 km/s.

I'm having a hard time wrapping my head around what galactic escape velocity means.

Does this mean that Voyager is incrementally slowing down by the pull of the galaxy? Given enough time, will it end up in the supermassive black hole in the center?

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  • $\begingroup$ Escape velocity is simply the speed at which an objected placed at infinity, and acting only under the influence of gravity from a second object, will collide with that second object. Objects launched at less than that velocity will inevitably return to that second obeject, unless gravity or some other force intervenes. $\endgroup$ – Wayfaring Stranger Dec 23 '14 at 16:11
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If you fail to reach escape velocity from the earth, you won't break free of its gravitational well, in other words, you will keep orbiting the center of mass of the earth. If the path of your orbit intersects the surface of the earth, then you will, you know, crash.

If you fail to reach escape velocity for the galaxy, you'll keep orbiting the center of mass of the galaxy, just like the other couple hundred billions of stars in the galaxy. The galaxy has no hard surface to crash into, so you'll keep orbiting until you come close enough to another orbiting body to perturb your orbit (which could, in the right circumstances, leave you with escape velocity and kick you out of the galaxy.

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Voyager is traveling at 17 km/s with respect to the sun. Since the sun is orbiting the galaxy, it is likely that Voyager will also orbit the galaxy for 5 billion years or so, until the collision with Andromeda happens.

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Escape velocity is only relevant for unpowered objects, ie. a lump of space rock. Any powered craft can move at any speed, and escape the gravity of any object (black holes excepted).

For example, the escape velocity of the Sun is around 617.5 km/s, yet the "slow" solar wind has a speed of only around 400km/s, but manages to leave the Sun with ease. Since the solar wind accelerates from the Sun, there must be a constant force acting on the solar wind particles.

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  • $\begingroup$ Please correct me if I'm wrong, but isn't the Voyager spacecraft an unpowered object ATM? $\endgroup$ – John Dvorak Dec 23 '14 at 11:39
  • $\begingroup$ Indeed. I was responding to the previous paragraph. $\endgroup$ – iantresman Dec 23 '14 at 12:19

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