Possibly gravitational slingshots or maybe they're not much more advanced than us and are using ion thrusters , they could be in their slowing down phase as they enter the Solar System.
When the Comet / Craft completes its orbit of the Sun we will see if it changes course or speeds up like Oumuamua , either one of those will be interesting.
When the Comet / Craft completes its orbit of the Sun we will see if it changes course or speeds up like Oumuamua , either one of those will be interesting.