There are different meanings of the word secret. As used in the title, the word "secret" is an adjective, or a "describing word", it gives more information about the noun that it describes.
As used in the title, the noun it is describing is "payload". The "payload" is the secret, not the launch.
You can tell this because of the order the author put the words in. If he had written "US payload top secretly launched into space", then that would have been a dichotomy worthy of note. You can tell the difference here because "secret" has become "secretly", an "adverb" - it is now describing the verb in the sentence, "launched".
In case it is not obvious, satellites are not very secret. It is impossible to secretly launch a satellite. Once launched, it is very hard to hide a satellite - you can simply look up and see it. Therefore, it makes no sense to hide that you are launching a satellite - as soon as you do launch it, people will know that you have launched it, and can track it.
On the other hand, those observers don't know what that satellite payload does, until it does it - perhaps not even then. Is it just taking pictures, or does it have a nuke on board to drop on Kazonistan? No-one knows, IT'S A SECRET.
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