Overview
The "Black Knight" is an object that conspiracy theorists claim is an ancient, alien-made satellite. Proponents link it to "long delayed echoes" heard by radio researchers in the 1920s and NASA photos from 1998 showing a dark object in orbit.
Rebuttal
NASA explains the 1998 object as a thermal blanket that was lost during a spacewalk during the STS-88 mission to the International Space Station.