Tristan da Cunha:

In 1958 as part of an experiment, Operation Argus, the United States Navy detonated an atomic bomb 160 kilometres (100 mi) high in the upper atmosphere about 175 kilometres (109 mi) southeast of the main island.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". When did the upper atmosphere detonate an atomic bomb?
unanswerable