Question: How are the stars, the planets and the comets able to stay in 'predictable' orbits, as if they know their limits and boundaries and realize 'when' to make a 'U-turn' for the orbit?

Jarvist Moore Frost answered on 15 Mar 2012:

They don’t do ‘U-turns’ like they’re driving around the block, objects in orbits are constantly falling towards whatever they’re in orbit around, but at the same time are travelling forward at just the right speed that the object is constantly moving out of the way.

If they had started a little bit faster, they would have simply ended up in a slightly higher orbit travelling a little bit faster. Our Moon is still ‘settling’ into its orbit, every year it moves a few centimetres further away from us.

So it’s not that we did a calculation & worked out exactly where Jupiter (or any other planets) should be in their orbit, but we measured exactly what its orbital parameters are and can then predict exactly where it will be in thousands of years.

Comets and other objects (in particular, spy satellites!) are in highly elliptic orbits where they’re not going very fast sideways, and instead have an orbit where they fall very close to their orbiting body (and travel very very fast) and then fling themselves out a long way away again.