How can I rotate the UFO around the planet with acceleration?

How can I rotate the UFO around the planet with acceleration?

This is currently how I’m moving my player:

transform.RotateAround(targetPlanet.position, transform.right, moveInput.y * rotateSpeed);
transform.RotateAround(targetPlanet.position, transform.forward, -moveInput.x * rotateSpeed);

The problem is that there is no acceleration/decceleration which I would like.

Movie_002

How could I add this?