Dark disc-shaped UFO caught by Curiosity on Mars

On December 4, 2015 (Sol 1183), the rover Curiosity has photographed with his camera Rear Hazcam a dark disc-shaped UFO. Images published on NASA website were impressive and unexplained.

Scott C. Waring explains:

This is a UFO from the most recent photos from Curiosity rover this week. It has a dark disk and is not an anomaly stuck to the lens. I checked other photos from the same camera and it is not there, therefore its not on the lens. It is flying in a horizontal position and observing the rover from a distance.

Source of the photo: