wrote:
Low orbits decay - I assume higher ones do as well, just more slowly.
Could we place a satellite such that it would still be up there, say,
a million years from now? How about 100 million years?

Orbits decay because of air resistance. And tidal forces become
significant when air resistance is negligible.

The Moon has been orbiting the Earth for more than 100 million years.

Placing a satellite in orbit around the Earth at twice the distance of
the Moon is not a particularly hard thing to do. Of course, a small
satellite might get hit by an asteroid, or somehow driven out of its
orbit by gravitational perturbations, but the odds do favor such a
satellite staying up for a very long time.

An early Russian space probe became an "artificial planet", orbiting
the Sun. That would be a way of putting something out in space for a
very long time.

John Savard