Black Knight satellite

The Black Knight satellite is a piece of space debris confused for an extraterrestrial UFO with claims attached to it being a 13,000 years old satellite which was known to Nikola Tesla and is currently being covered up by NASA, and a whole bunch more. Outside of ufology, this conspiracy theory has gained some attention within QAnon.

Space debris
It turns out that this supposed satellite was most likely space debris from the Space Shuttle mission in 1989, specifically a thermal blanket that was lost during the mission when two of the astronauts went outside of the vehicle. As the blanket tumbled towards the Earth, it eventually got the shape that the picture in its specific angle shows, aided by the fact that it was irregularly shaped and already crumpled.

Conspiracy
The conspiracy theory surrounding this blanket is an absolute indiscernible mess of a bunch of different unrelated stories.

Like any good conspiracy theory, it involved Nikola Tesla. In particular, the sounds he heard during his 1899 experiments with radio signals. He heard strange pulsating waves. While unknown to him, he was probably tuning in to pulsars.

It also includes the anecdote of an astronaut named Gordon Cooper who claimed he saw a UFO during his 1963 15th orbit in Mercury 9. There is no evidence that this happened. NASA's mission transcripts and Cooper's personal copies don't show any such report being made during the orbit. But since an astronaut said it, it must be true!

The age-estimate conspiracy theorists often cite via the popular method of PIDOOMA (Lunan actually said it was 12,600 years old), also using the method to conclude that the satellite had come from  system. He would later admit that it was made in "outright error":