On Tuesday, May 17, members of the US Congress gathered with military personnel to publicly discuss a rather unusual topic: UFOs. Or, as the US military now calls them to avoid the stigma of that name, unidentified aerial phenomena, or UAPs.

Although it is the first Congressional hearing on the topic in over half a century, it isn't entirely new airspace for Congress (so to speak). And for all the curiosity of elected officials grilling military intelligence about something academics generally don't take very seriously, the topic highlights a mundane question: Is it possible to look up at the sky and identify, with certainty, every vehicle—every perfectly mundane human-made drone or aircraft—that flies up there?

The answer is almost certainly no.

“There’s no illusion that we’re going to be able to, 100 percent, have perfect surveillance,” says Andrew Weinert, a member of the Homeland Protection and Air Traffic Control Division at MIT Lincoln Laboratory. “It would be great if we did.”

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