This page of the Ratchet & Clank Rift Apart game guide explains how to catch up and stop the flying saucers appearing on the planet Savali . You'll learn about the circumstances under which a UFO can be encountered, how to lead up to its pursuit, and what is the reward for stopping a flying saucer.
You can encounter UFOs while traveling through the wilderness on the planet Savali. They do not attack the controlled character, but always run away once they spot you. You may choose to have an optional chase after them.
During the UFO chase, use Hoverboot by holding down R1. Additionally, you can accelerate by pressing L2 or R2.
During the chase, look for ramps that you can use to slide over and blast into the air at high speed. All UFOs should fly a route that involves passing various ramps, so don't worry if you miss one.
Once you are in the air, aim for the flying saucer and be ready to attach yourself to the UFO by pressing L1.
If you have followed all the steps correctly, you will land on a UFO. Touch the green pressure plate to access the object that the flying saucer was transporting. This could be a collectible or one of the shrines associated with the side quest available in Savali.
If a UFO has escaped, then return to the area where you first spotted it. The flying saucer will eventually return to the same location and you can start the chase all over again.