In the summer of 1947, a top-secret U.S. military balloon developed to spy on the Soviet nuclear program crashed in the desert near Roswell, N.M. The military gave only incomplete accounts of what happened, sowing decades of conspiracy theories (and a tourism industry) that built up around Roswell as the site of an alien crash landing.

Since then, Americans’ passion for alien visitation has proved tough to shake, even when the evidence is clear that no spaceships have touched down or crash landed. After the Cold War, a pair of Air Force reports that aimed to come clean about the experiments near Roswell did little to debunk any belief in the potential for aliens.

The government’s latest report on U.F.O.s, which the Pentagon now wants to call unidentified aerial phenomena, is unlikely to settle anything. Due out on Friday, the report’s expected assertion that no classified American programs exist to explain the observations will most likely be dismissed by those primed to disbelieve government pronouncements. Its failure to find affirmative evidence of alien spaceships will largely be ignored by those most passionate about theories of extraterrestrial visitation.

It will also serve as the latest in a history of efforts by the government to confront public eagerness to know more about U.F.O.s. Officials have sometimes sought to be transparent about what they know, according to documents and interviews, but in other instances allowed confusion and conspiracy theories to take root as a useful cover-up for top-secret military programs.

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