You didn’t see a UFO. It was probably one of these things.

UFOs were, for decades, the stuff of science fiction and conspiracy theory circles. But the highest levels of the US government have started seriously considering these phenomena—redubbing them Unidentified Anomalous Phenomena, or UAPs. There have been hearings on Capitol Hill, Pentagon reports, and a NASA working group, all looking into more than 100 currently unexplained UAP sightings, often made by military pilots who caught something unfathomable on their sensors. And plenty of civilians see things they don’t know how to explain, either. 

Even if UAPs have gone mainstream, the vast majority of human sightings turn out to be perfectly explicable, though occasionally rare, phenomena. And Jonathan McDowell, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian, hears about them all. 

“I get a lot of social media questions and emails, and occasionally cold calls from random people who have seen something weird in the sky,” McDowell says. Around 90 percent of the conversations, he says, go something like this: ‘Is this space debris, Jonathan?’ No, it’s just a meteor, because it blew up in only two seconds. Or: ‘Is this a UFO?’ No, it’s a Falcon 9 [rocket] launch. ‘Is this aliens?’ Well, that depends on where you think Elon comes from.”

But it’s rarely ignorance or credulity that leads people to mistake a rocket launch or an aircraft for a UAP. Instead, it’s just how human perception works.

“Our ability to estimate how far away something is sucks when it’s not in a context where we have the usual clues,” McDowell says. A close-by insect moving in a peculiar way might be confused for something much farther. Or a shining light might appear close—when it’s actually Venus, 35 million miles or so away. 

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Even professionals can be fooled. Every once in a while, a satellite or spacecraft gets temporarily mistaken as a new asteroid. “If you have a spacecraft in a very high orbit around the Earth, the rate at which it’s moving across the sky is actually similar to an asteroid moving in orbit around the sun,” McDowell says. “There have been multiple cases where an object has been picked up by the asteroid surveys, given a temporary asteroid designation, and then it’s maneuvered. And we go, ‘Oh, that’s probably not an asteroid.’”

When it comes to the general public spotting what they think are UAPS, McDowell finds they usually turn out to be phenomena in three main categories: Rocket launches, spacecraft, and celestial objects. 

Rocket launches

If you’ve ever watched a rocket launch, in person or through a video, you can see a contrail as the craft shoots from the launchpad to the heavens. But once a rocket reaches space, its exhaust can lose that familiar linear shape, creating seemingly otherworldly sights under the right conditions.

The first weird thing rockets can create occurs about two minutes after launch, when they finally get above most of Earth’s atmosphere. No longer contained by thick air, the exhaust plume might spread out over hundreds of miles, according to McDowell, producing some bizarre forms that almost look oceanic. “Those are often described as jellyfish,” he says. “People are much less able to sort of recognize those as being rocket plumes and those often get reported as UFOs.”

Another type of rocket launch weirdness happens when rockets shut down and restart in space. This might be to change an orbit, or when a rocket vents its leftover propellant after delivering its payload. 

“You’ve got this big cloud of gas that then gets ejected from the rocket and forms ice crystals that reflect the sunlight,” McDowell says, ”so you get these big kind of comet-like clouds” moving through the sky. Even professional astronomers have been tricked by rocket fuel dumps, who reported them to the International Astronomical Union as new comet sightings. 

But the most striking rocket trail phenomenon are the striking, spiraling geometric patterns in the sky, such as those that appeared over Norway in 2009. At first glance, it is utterly unnatural. You might think it’s “a Stargate wormhole opening up in space and the aliens are invading,” McDowell says. But as weird as they look, the spirals are no portals. Instead, it’s the result of a spinning or tumbling rocket, which releases contrails “like a garden sprinkler.” In the case of the Norwegian spiral, it was actually a Russian military rocket maneuvering above Earth’s atmosphere. 

If you want to hunt down a bizarre rocket-exhaust plume for yourself, it’s important to realize the strange sights hinge on the relative position of the exhaust, the sun, and the observer: You’re most likely to catch one around dawn or dusk, when it’s dark for you on the ground, but the high-altitude rocket exhaust can catch the sun rays.

Spacecraft and satellites

There’s another category of artificial space objects that we commonly mistake for UAPs: Starlink satellite trains, which, in McDowell’s experience, “really freak people out.” 

SpaceX began launching its Starlink satellites in 2019, lofting between 22 and 60 of them at a time to provide broadband internet. As of September 2023, there are more than 4,700 Starlink satellites in orbit, according to McDowell’s personal satellite tracking website. It takes a couple of weeks after launch for Starlink satellites to fully separate from each other and move into their operational orbits at around 340 miles altitude. In their early days of flight, they can catch the sun and produce a bizarre geometric pattern in the sky—a long, bright straight line. 

“They march across the sky in this line, like little kids in the crocodile coming home from school,” McDowell says (using the British expression for pairs of kids in a line). “They’re close enough together that you can’t see them as separate dots. Even if you see them separately, to have them marching in lockstep across the sky as 20 different objects, that definitely looks like an alien invasion to your gut.”

To catch Starlink or other satellites as they fly overhead, McDowell recommends using the website heavens-above.com. “It tells you what time the satellite is going to go over, and if you click on the time, it gives you a nice star chart showing the path of that satellite across the sky, as seen from your location,” he says. The website doesn’t show Starlink satellites by default, because they’re so  numerous but you can click a link to see the Starlink constellation.

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The reentry of satellites, spacecraft, or space debris can also look pretty weird, too. It might not be easy to tell what’s happening, though. “That’s where it gets tricky because if you see bright stuff overhead in the sky breaking up, that can be one of two things,” McDowell says. “It can be a natural meteor, or it can be a reentering piece of space debris.”

They key distinction, he says, is that meteors shoot across the sky fairly quickly, then vanish. A deorbiting satellite or other debris will have multiple pieces that cross the sky as it breaks up over time. 

There is a particular type of reentry that is sometimes mistaken for UAPs or natural meteors—the return of a spacecraft like the SpaceX Crew Dragon. “That looks more like a fireball, like a natural meteor, except that it lasts much longer,” McDowell says. “And if it’s breaking up, that’s really bad news.”

Celestial objects 

The last type of thing McDowell commonly hears about being mistaken for UAPs are natural objects that are very, very far away—Venus, for instance. He estimates, before dash cams and cell phones started picking up meteors, space debris and the like, the planet caused about half of all UFO reports. “Venus is the classic UFO.”

When very faint, high clouds move at night, this foreground motion can trick human perception. The result is the sense that a bright light—in this case, the planet—is traveling across the sky, when in fact it’s only the clouds. 

Comets also sometimes trigger UAP questions, McDowell says. The Comet Nishimura, which makes its closest approach to Earth on September 12, could turn some heads, if it becomes bright enough to be visible to the naked eye. 

“People aren’t used to seeing comets, so if they haven’t heard in the news there’s a bright comet around, they might think that’s a UFO,” he says. 

As quick as people are to label many different sights in the sky as UAPs, McDowell notes there’s one common object in the night sky he rarely hears about: The International Space Station.

“I think it’s just that the number of times the ISS is passing over you is comparatively rare, maybe,” he says. “People don’t seem to worry about that as much for some reason.”

If you would like to catch the ISS or China’s Tiangong space station as they zoom overhead, heavens-above.com can also help you plot their course over your location so you can look up at the right time, according to McDowell. For the planets or other celestial objects, he recommends the interactive sky charts at skyandtelescope.org. 

“You can put in your position and the time of the night that it is and you get a map of the sky tuned for your experience,” he says. “You can see where the bright planets are relative to constellations.”