'Misty' is the code-name widely attributed to a stealthy imaging satellite launched by the space shuttle Atlantis in 1990. A US patent from Teledyne Industries details a method of suppressing the visibility of satellites by means of an inflatable, reflective ballon, which would be inflated in orbit, and pointed towards the earth to deflect radar, lasers and also to defeat optical searches.
Due to the covert and highly secret nature of this programme, it's not known what technology is used to hide the satellites, but this is a credible scheme.
This drawing (left), included in a patent application, shows how an inflatable space shield could help a satellite evade detection. Observers believe the satellite known as Misty may have used such a shield.