* An object in a stable orbit: A satellite or asteroid that orbits a planet without hitting it. These objects are constantly falling towards the planet, but their sideways motion keeps them from actually making contact.
* An object passing through a Lagrange point: Lagrange points are points in space where the gravitational forces of two large bodies (like a planet and a star) balance out. An object at a Lagrange point can remain relatively stationary, even though it's close to a planet.
* An object moving too fast: If an object is moving at a very high speed, it might simply pass through a planet's atmosphere without actually colliding. This is similar to how a bullet can pass through a piece of paper without tearing it.
* An object that is simply too small: Tiny particles of dust or gas might not even register as collisions when they hit a planet.
Let me know if you had something specific in mind, and I can give you a more tailored answer!
