It was Carl Sagan who used to say, “Extraordinary claims require extraordinary evidence.” That adage now looms over the world of astronomy as Harvard astrophysicist Avi Loeb puts forward a bold hypothesis: The recently discovered interstellar object 3I/ATLAS is not a comet, but rather a nuclear-powered spacecraft. It is a claim based on evidence, not fantasy evidence that, if verified, would rewrite the place of humanity in the universe.
1. A Distant Stranger from Outside the Solar System
Spotted on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile, 3I/ATLAS is the third interstellar object confirmed to enter our solar system after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. NASA classifies it as a comet, but its birth moving in the direction of the Galactic Center and its hyperbolic motion moving about 60 km/s designate it as an unbound traveler from the depths of space. Present estimates give its size as between 10 and 20 kilometers, although Loeb’s calculations indicate that it may be much smaller if its light is intrinsic.
2. The Frontal Glow That Contradicts Comet Physics
It all started with a Hubble Space Telescope photo of the object on July 21, 2025. Loeb observed a clear glow in front of the direction of motion, not following like would be the case for sunlight-powered outgassing. The most straightforward explanation is that the nucleus of 3I/ATLAS is emitting most of the light, he wrote. Photometry shows the light profile cannot be accounted for by sunlight reflected from the comet or normal cometary activity. If the luminosity’s source is less than 100 meters in diameter, as Loeb estimates, then the overall energy production on the order of gigawatts must have an internal source of power.
3. Eliminating Natural Sources of Power
Loeb methodically rules out competing natural explanations. A black hole in the early universe would produce only ~20 nanowatts, orders of magnitude too weak. A piece of radioactive supernova wreckage is conceivable but statistically improbable in view of the rarity of such material in interstellar space. Interstellar gas friction heating misses momentum and density requirements. That leaves, in his opinion, a “central, compact, high-power source” with nuclear propulsion as its most natural technological analog.
4. Nuclear Propulsion and Travel Interstellar
Nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP) are the only two methods in human engineering that can maintain multi-gigawatt outputs for deep-space missions over prolonged periods of time. NTP systems use a propellant like hydrogen to achieve very high temperatures, and the hot gas is expelled out of a nozzle to create thrust. NEP systems generate electricity by using nuclear heat to power ion or Hall-effect thrusters. Both might, in principle, produce the long-term luminosity Loeb infers, particularly if waste heat were emitted in the forward direction.
5. A Trajectory Too Precise to Ignore
Trajectory modeling contributes to the mystery. Loeb estimates that just one of 500 random interstellar visitors would be so close to planetary orbits, and that passing close to Mars, Venus, and Jupiter in a single transit has a probability of roughly one in 20,000. Discussion in a recent study of astrodynamics reveals the object’s orbital plane is tilted only ~5° from the ecliptic, retrograde a geometry that renders capture with chemical rockets almost impossible, but facilitates efficient planetary flybys.
6. Possibility of Advanced Maneuvers
The same report observes that 3I/ATLAS will be totally behind the Sun as seen from Earth at perihelion on October 29, 2025, at 1.35 AU. The configuration might hide a high-thrust “reverse Solar Oberth” maneuver burning engines at the point of closest approach to decelerate into a bound solar orbit. Such a trajectory would be undetectable to Earth-based telescopes and could establish a rendezvous with Jupiter or even the inner planets. The ΔV needed for intercepting Mars or Jupiter off its current trajectory is <5 km/s, within the capabilities of a nuclear-powered vessel.
7. Artificial Illumination Detection
If 3I/ATLAS is self-illuminating, then its light curve will have a flux–distance slope of α = –2, not α = –4 for reflected sunlight. Methods used to determine artificially lit objects in the Kuiper Belt might be used here, quantifying brightness variations as the object’s distance changes. Spectroscopy may also be able to identify artificial illumination e.g., LED-like emission spectra versus natural thermal emission.
8. The Problem of Timely Observation
Chances to study are short-lived. Loeb has called on NASA to assign the Mars Reconnaissance Orbiter’s HiRISE camera to photograph the object on its October flyby, when it will be 17 million miles away from Mars, and to utilize the Juno spacecraft for observations on its 2026 Jupiter flyby. But mission constraints and instrument limitations do not permit rapid retasking. The Vera C. Rubin Observatory, which is coming online this year, can potentially see more such interstellar visitors, but its survey cadence is not designed for high-speed intercept planning.
9. Machine Learning and Future ISO Detection
Automated detection breakthroughs are essential. Machine learning classifiers that have been trained on LSST-like simulated data attained F1 scores of 0.9987 for identifying interstellar objects versus asteroids, allowing candidate flagging in nearly real-time. Tools like these may, in the future, enable earlier identification of outliers such as 3I/ATLAS, raising the chances of launching a dedicated intercept mission before the object disappears.
Whether 3I/ATLAS is an atypical comet or an artificial probe, the next few months will determine. As Loeb points out, “If it’s technological, it would have a huge impact on humanity’s future. We need to determine how to react to that.”
