The highest altitude nuclear explosion prior to market creation appears to be the third Argus test at 794 kilometers, but information is scarce. If that number is wrong, this market resolves based on the correct number.

The explosion must be human-caused. A natural or alien one does not count.

This resolves based on altitude above Earth, so an explosion on the moon or Mars would count. (Unless they get a lot closer, in which case I may not be able to resolve this market promptly.)

The explosion must be at least 10 tons of TNT equivalent.