During a recent U.S. congressional hearing on the Search for Extraterrestrial Intelligence (SETI), a conservative member of the House asked UC-Berkerley SETI director Dan Werthimer about UFOs in general, and the History Channel series Ancient Aliens in particular. Werthimer's response was one for the books: "UFOs have nothing to do with extraterrestrials."
There's a great deal of content, here and elsewhere, about UFO visits. No point in hashing that out in much detail. But let's suppose, as a matter of brute fact, that we were able to verify with a high degree of certainty that an alien spacecraft flew the skies over Earth, and all we knew for certain about it was that it was extraterrestrial in origin. What would that tell us?
Well, we can assume to begin with that if the craft serves a primarily scientific purpose, it would probably be an unmanned probe. As we have discovered ourselves, life support systems are not generally an efficient use of spacecraft resources. And if we sent a spacecraft to a world that we knew contained (semi-)intelligent life, we wouldn't needlessly risk the lives of personnel by having them fly around in the flak-filed skies before we'd made contact with the general population. Even if the craft were invincible, it would be a horribly inconvenient trip, and—unless the visitors are tourists who want to spend a vacation on a foreign world—there would be no scientific reason to send them here.
We can also reasonably assume that the entity that sent the craft didn't want to hide it. We've already discovered technology that would allow us in principle to make aircraft invisible, and we can assume that any alien civilization capable of traveling to Earth would be similarly familiar with nanomaterials. Remember those helicopter photos of an uncontacted tribe on the Peruvian border, and how unethical many observers found them to be? This would be (at best) the intergalactic equivalent. If we can see extraterrestrial probes, the civilization sending them is trolling us—and maybe being a little cruel about it.
In other words: if any UFOs really are manned alien spacecraft, I think they're probably full of obnoxious space tourists who don't have enough to do with their time.
Am I being unfair? Do you have any alternative theories to suggest? Share them in the comments below.