When you get the chance to go on vacation, take it by October 29, because you never know what may come. That was Harvard astronomer Avi Loeb speaking about the interstellar comet 3I/ATLAS. His statement is both shocking and far-fetched, but it recognizes the once-in-a-lifetime spectacle and controversy the third verified object beyond our own planet.
Discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 3I/ATLAS follows in the wake of 1I/‘Oumuamua and 2I/Borisov. Its hyperbolic trajectory, with an eccentricity between 6.1 and 6.2, confirms its interstellar origin. The comet will never return; after a brief passage through our celestial neighborhood, it will continue into deep space. What makes it stand out is not just its speed initially about 61 km/s but its unusual chemistry, polarization, and a rare anti-tail, a dust feature pointing toward the Sun rather than away from it.
Spectroscopic observations using the Keck Cosmic Web Imager showed an “extreme abundance ratio” of nickel relative to cyanide, well above what has been observed in the comets of the solar system. The nickel seems concentrated in the innermost 2,000 km of the coma, and bound to polycyclic aromatic hydrocarbons, where it would be photo-dissociated by the sun. These refractory metals are hard to reconcile at the observed temperatures, and the comet formation environment comes into question is this 10-billion-year-old remnant of an older planetary system?
Its current position behind the Sun, in solar conjunction, gives observers fits. Perihelion, or its best approach to the Sun, at 1.4 AU or so, occurs on October 29, and terrestrial telescopes are blinded by solar glare. Spacecraft orbiting Mars and potentially Jupiter are the only observers likely keeping vigil on it, though the faintness of the object has stretched even refined telescopes, including ESA’s ExoMars Trace Gas Orbiter, to the limit.
Harvard astronomer Avi Loeb has controversially suggested that 3I/ATLAS might be an alien mothership performing an Oberth maneuver near perihelion. The Oberth effect, proposed by Hermann Oberth in 1929, boosts a spacecraft’s kinetic energy via a propulsive burn near a massive body. In the speculative hypothesis by Loeb, the mini-probes of a mothership would be doing the reverse Oberth maneuver slowing down near perihelion to hit Earth. He estimates the necessary change of angular momentum per unit mass as ~(0.36 AU)*(68 km/s), which depends upon probe mass and fuel. Such a maneuver, if observed, would cause inexplicable shifts of the trajectory or mass alterations.
By engineering standards, a solar Oberth maneuver is challenging. Research for the NASA Interstellar Probe mission indicates that to shield a propulsion stage for a close solar pass requires sophisticated thermal protection, possibly including active cooling systems, such as hydrogen-fed heat shields that are also solar thermal engines. With solid rocket motors of ~300 second specific impulse, even the benefit of the maneuver over a Jupiter gravity assist is slight, and the stakes are thermal loads, mass limitations, and narrow timing are substantial. Existing technology prefers Jupiter assists to solar dives for fast interstellar missions.
Few scientists dismiss the alien technology theory. “It looks like a comet. It does comet things. It very, very strongly resembles, in just about every way, the comets that we know,” noted Tom Statler, the lead scientist of NASA’s solar system small body program. These peculiar characteristics the anti-tail shape, the metal coma are interesting but not new to cometary physics.
When 3I/ATLAS reappears above the Sun later this December, astronomers will once again make multi-wavelength observations. NASA’s Hubble and Webb Space telescopes, SPHEREx, and other assets should refine its composition, rate of activity, and nucleus size, presently between 440 meters and 5.6 kilometers. These measurements will refine the understanding of cometary diversity all along planetary systems and possibly uncover the source of the peculiarities as the products of exotic chemistry or the vagaries of interstellar travel.
For the time being, the comet bears down towards perihelion, unconcerned by the speculation of humans. If it makes a gravity assist or just presses on with its outward journey, 3I/ATLAS gives us a fleeting, fast-moving insight into the material and dynamics of worlds long beyond our own.
