In our solar system, everything belongs to the sun. While they might take years and years to complete one loop, every planet and moon, asteroid and comet, every bit of crushed ice and rock, is gravitationally bound to our star, fixtures of a perpetual cosmic carousel. But every once in a while, something else comes along that, based on careful measurements of orbital mechanics, is unmistakably untethered: an interstellar visitor. The object arrives from the realm between stars, and after a quick sojourn in our solar system, it goes back out to the unknowable depths.

Such enigmatic travelers are called interstellar objects, and they bring with them a glimpse of a part of the cosmos that we’ve never seen before. Each time one shows up it kicks off an observing spree, with astronomers mustering other telescopes, on the ground and in space, to scrutinize the visitor. The object appears as little more than a fuzzy speck of light, but astronomers try to uncover its true nature before it slips away, from fundamental properties (dimensions, chemical composition) to the wild possibilities (signs of alien technology).

Only three interstellar objects have ever been discovered, with the latest appearing just this month, from the direction of somewhere in the center of the Milky Way. 3I/ATLAS, as the object is known, is currently traveling just inside the orbit of Mars, captivating telescopes around the world. These are rare events—for now.

A new telescope, scheduled to begin full operations later this year, is expected to find many more such transient objects. The Vera C. Rubin Observatory, perched on a mountaintop in the Chilean desert, is designed to scan the sky night after night and capture faint glimmers in the darkness—including sunlight reflecting off a fast-moving celestial body. Its observations produce time-lapse views, allowing astronomers to track those objects and study their orbits for any oddities. Rubin didn’t discover 3I/ATLAS but happened to snap dozens of pictures of the object, before and after its interstellar nature became apparent. Those images are already helping astronomers form a clearer picture of the visitor.