How does the orbit of the Voyager and New Horizons probes manage to get into interstellar space?

Question

I understand that every probe launched has been launched into an orbit. The further you want to go into the solar system, the faster it needs to be moving. And everything orbits the sun until it finds a different mass to "capture" it.

But I noticed that Pluto is REALLY far away and the orbit of New Horizons is not a circle nor an elliptical shape and it doesn't look like the probe will ever come back - it will just keep going. It doesn't even look like it is orbiting the sun! How is this kind of orbit possible?

Same with the voyager probes. It looks like they will keep going into interstellar space and never come back. Is their trajectory even remotely some kind of a circular orbit around the sun at this point? If not, how is this kind of trajectory possible?

Answer 1

This relates to the concept of escape velocity, which is the speed needed to escape an object's gravity well. Even though gravity has an infinite reach, the fact that its force reduces quadratically with distance means that it only takes a finite amount of energy to climb out of a planet's gravity well. For some fixed energy cost, you can get arbitrarily far away from the planet. If your speed is higher than escape velocity, you can get as far away from the planet as you like, and still have some speed carrying you away - in other words, you are never coming back.

You are correct that cyclical orbits are round and repeating. But not everything in space has to be in a cyclical orbit. Objects that are moving at least as fast as escape velocity are not in a cyclical orbit, they are on an escape trajectory. New Horizons has sufficient speed that it is on an escape trajectory from the sun - it will leave the solar system and never come back, although it may fall into orbit somewhere else around another celestial body.

Answer 2

I think your problem is just a matter of terminology. Almost all of the orbits you hear about are elliptical. Some are almost exactly circular, but that counts as a special case of elliptical. Orbits like that keep going round and round, and stay within a certain distance of the primary. You might have the impression that's part of the meaning of "orbit".

But technically, it is still called an orbit if it is not a closed curve. Orbits where the object doesn't come back are hyperbolas, rather than ellipses. These space probes are on hyperbolic orbits. If you remember your geometry, a hyperbola has two asymptotes that the curve approaches as it extends off to infinity. By now, the space probes are so far from the sun that they are very close to the hyperbolic asymptote, and basically moving in a straight line out of the solar system.