NASA classifies it as space debris. It was first spotted by Astronaut Gordon Cooper during his twenty-two-orbit Mercury-Atlas 9 flight in 1963. A spacecraft or satellite or some sort, he believed, much larger than any other man-made satellite of the time.
It was seen, on radar, by multiple witnesses at a NASA tracking station near Perth, Australia. The only official explanation given was that Cooper had breathed in too much CO2, giving him hallucinations. Which was all the confirmation UFO-cover-up-conspiracists needed. It became known as TBK—The Black Knight.
Then in 1987 the Endeavour Mission, STS-088, turned in these photos:
So what TBKF is it?