Intelligence officials are set to release a declassified U.F.O. report.

Politics|Intelligence officials are set to release a declassified U.F.O. report.

Top intelligence officials are expected to release a much-anticipated report Friday on aerial phenomena witnessed by Navy pilots in recent years — a document that is likely to do little to settle a growing debate over whether it is possible that alien spacecraft have visited earth.

The report will make clear there is no affirmative evidence that the unexplained phenomenon are space aliens. But for some of the reported sightings examined over the past two decades, the military and intelligence community have no explanation.

As long as the government cannot offer a strong public explanation, alternate theories of what the pilots saw and what video recordings of their flights show are likely to persist.

The report will rule out that the Navy pilots’ sightings were not a glimpse of a classified U.S. program. While not everyone is likely to believe the assertions, secrecy experts say the more categorical the report is, the more believable it will be.

That, experts have said, leaves three other categories of explanation: a secret technology developed by an adversarial power like Russia and China, an extraterrestrial technology or more ordinary explanation of a natural phenomenon or optical illusion.

The report is being made public because of a statute provision written by Senator Marco Rubio of Florida, the top Republican on the Senate Intelligence Committee, and passed by Congress as part of a massive spending bill.

The Pentagon and intelligence agencies now eschew the term U.F.O. and now refer to U.A.P., or unidentified aerial phenomena. It has been a bit of rebranding, both to cut down on public enthusiasm and remove the stigma of U.F.O. report to encourage pilots to report things that they see and scientists to study it.

But public excitement has not been dampened and scientific skepticism is likely to continue.

“Unfortunately, I don’t think that landscape is really going to change; it’s still an area where scientists mostly fear to tread,” said Chris Impey, an astronomy professor at the University of Arizona.

Since the late 1960s, most scientists and scholars have steered away from U.F.O. studies. Their reluctance has hampered the government’s ability to put conspiracy theories about aliens to rest.

Dr. Impey noted there are mundane explanations for the sightings. There are plausible, but dry, explanations for each of the Navy videos that are more likely than some sort of extraordinary technology, added Mick West, a science writer who has focused on debunking conspiracy theories.

In one video a sharp movement of the object is attributable to a shift in the camera’s movement. In another, an object that appears to be moving fast is shown to be actually moving much more slowly when a trigonometric calculation is applied. An image of a rapidly spinning object skimming over the clouds is caused by infrared glare, Mr. West said.

“If you just say, ‘Oh, it’s aliens,’ that’s actually quite a simple explanation,” Mr. West said. “The actual explanation is kind of complicated, which is why a lot of people miss it. But yet the mundane explanations exist for all of these videos.”

The release of the report, and public acknowledgment that the Pentagon and intelligence agencies cannot explain some of the videos, will likely set the stage for additional work and additional U.F.O. reports in coming in months.