1962 Operation Fishbowl.

Some say they were trying to either map the Firmament, or to break through it.

Official version: Operation Fishbowl was a series of high-altitude nuclear tests in 1962 that were carried out by the United States as a part of the larger Operation Dominic nuclear test program. The Thor missiles were used to lift warheads into near-space.

https://en.wikipedia.org/wiki/Operation_Fishbowl