Among other uses, the clandestine CORONA program used Thor-Agena from June 1959 until January 1968 to launch United States military reconnaissance satellites operated by the Central Intelligence Agency (CIA).

The launch vehicle detonation produced a large circular cloud that appeared over the southwestern United States.

Due to its mysterious nature, appearing at a very high altitude and being visible for hundreds of miles, the cloud attracted widespread attention and was published by the news media.

[9][10] Prof. James MacDonald at the University of Arizona Institute for Atmospheric Physics investigated the phenomena and linked it to the Thor launch vehicle launch after contacting military personnel at Vandenberg Air Force Base.

When the launch records were later declassified, the United States Air Force released a memo explaining that the cloud was the result of a military operation.