The world’s largest single-dish radio telescope—the Five Hundred-Meter Aperture Spherical Telescope (FAST) in southern China—is about to start operation after more than three years of testing and commissioning. While the trials have focused on detecting neutron stars, one of the goals during the telescope’s operational lifetime will be to search for signals generated by intelligent extraterrestrials. Since Chinese officials claim that FAST is already three times as sensitive as the Arecibo observatory in Puerto Rico, the second-largest single-dish telescope in the world, we surely expect new discoveries.

We don’t know, of course, whether the giant telescope will detect signs of extraterrestrial technology. But we’d love to have an explanation for what’s been called the Great Silence, also known as the Fermi Paradox: If there is intelligent life out there, why don’t we see any evidence of it?

There are many possible answers, including the idea that we live in a kind of designated nature preserve, or zoo. Or, if you like Star Trek, maybe the aliens are applying their version of the prime directive and trying not to interfere with life on Earth.

When I was in New Mexico recently for a workshop on extant life on Mars, I also visited the International UFO Museum and Research Center in Roswell. While the museum did have some interesting exhibits, including artwork and depictions of aliens in science fiction movies, its focus on the famous 1947 Roswell UFO incident seemed to suggest a government cover-up of an alien visitation. Never mind that the Roswell event likely has a much more mundane explanation: the crash of a high altitude balloon like the ones used by the U.S. Air Force for Project Mogul.

The problem with many so-called alien encounters is that the claims are based on eyewitness reports that are not always reliable. While I think scientists sometimes too easily dismiss such accounts, the underlying truth is that any science investigations have to be based, by their very nature, on experiments and reproducibility. And when it comes to alien visitations, those standards can be difficult or impossible to apply.

The credibility of UFO reports is not helped by the fact that 99 percent of them can be easily explained as natural phenomena. Others are outright hoaxes. As for the rest of the one percent, some events are stubbornly difficult to explain, which is why the government has long investigated UFOs. Not that the investigators turned up evidence of aliens. Atmospheric phenomena like sprites—which produce dancing flashes of bright light when lightning is exciting the electrical field above a storm—are still routinely mistaken for UFOs.

Where does this leave us? I still think UFO claims deserve serious scientific investigation. Even if we find no alien spacecraft, it will benefit science to discover previously unknown natural phenomena. And considering that we still have no answer for the Great Silence, we have to leave open the possibility that aliens have been visiting Earth, or are close by. Scientists have to keep an open mind.

Even though standard scientific methods have trouble evaluating claimed encounters, we should not stigmatize the research. From a practical viewpoint, science may be better suited to analyzing alien artifacts or possible alien objects in space, like the recent debate about the interstellar asteroid ʻOumuamua. As our observatories improve, and better telescopes like FAST come online, we may find ourselves with many more such mysteries to solve.