Discovery & Orbit
Discovered on July 1, 2025 by the ATLAS sky survey (Asteroid Terrestrial-impact Last Alert System) at its Río Hurtado station in Chile, initially catalogued as C/2025 N1. It is only the third confirmed interstellar object ever observed passing through the Solar System, after 1I/'Oumuamua (2017) and 2I/Borisov (2019). Its orbit is the most extreme of the three — and of any object yet recorded — with an eccentricity of roughly 6.1 and a hyperbolic excess velocity near 58–60 km/s, confirming it is not gravitationally bound to the Sun and is simply passing through. It reached perihelion (closest approach to the Sun) on October 29, 2025; per NASA, it came no closer to Earth than about 1.8 AU (170 million miles), posing no danger to the planet.
Composition & Activity
Unlike 'Oumuamua, which showed no detectable activity at all, 3I/ATLAS behaves like a genuine comet — developing a coma and tail as it neared the Sun, with outgassing dominated by carbon dioxide at greater distances and shifting to water-driven sublimation closer in. Spectroscopy has found hydroxyl (OH) emissions consistent with ordinary cometary water loss. Its estimated nuclear radius is under about 10 km, and its measured kinematic age suggests it may be one of the oldest comets ever observed — plausibly billions of years older than the Solar System itself, having drifted through interstellar space since long before the Sun and planets formed.
The Technosignature Claims
Harvard astrophysicist Avi Loeb (profiled elsewhere on this site) proposed shortly after discovery that 3I/ATLAS could be an artificial object rather than a natural comet, and has continued to argue the case since — pointing to its extreme trajectory and a roughly 5-degree tilt from the ecliptic plane, an unusually high nickel-to-iron ratio in some spectral readings, a claimed measurable non-gravitational acceleration, a symmetric arrangement of small jets he suggests could be thrusters rather than natural outgassing, and a periodic ~16-hour brightness variation he has publicly called a "heartbeat." Loeb maintains a running personal classification system (informally called the "Loeb Scale") and has placed 3I/ATLAS at a mid-level tier reserved for objects meeting some technosignature criteria, while stating plainly that "the simplest hypothesis is that 3I/ATLAS is a comet."
The Mainstream Response
NASA's lead scientist for small bodies, Tom Statler, has responded directly: "It looks like a comet. It does comet things." Dedicated radio technosignature searches — including the Allen Telescope Array shortly after discovery and the 100-meter Green Bank Telescope near closest approach, run by the SETI Institute's Breakthrough Listen initiative specifically because a real interstellar probe would be a legitimate, worth-checking possibility — found no artificial radio emissions of any kind. The broader astronomical community's consensus, based on the spectroscopy, imaging, and orbital dynamics gathered so far, is that 3I/ATLAS is behaving exactly as a natural, ancient interstellar comet would be expected to. The dispute remains empirical and, in principle, resolvable with further data — which is precisely why observation of it has continued.