“Houston, we have a problem with the natural comet hypothesis,” said Harvard astrophysicist Avi Loeb as he analyzed the latest post-perihelion data on interstellar comet 3I/ATLAS. Its recent behavior-from colossal jets and extreme mass loss to puzzling compositional signals-has re-energized debate on whether this is just a volatile-rich visitor from another star system or something far more exotic.
3I/ATLAS, the third confirmed interstellar object after 1I/ʻOumuamua and 2I/Borisov, swung past the Sun on October 29 at the scorching rate of 68 kilometers per second. New images from British astronomers Michael Buechner and Frank Niebling revealed a massive anti-tail stretching 620,000 miles sunward and a “smoking” trail extending 1,860,000 miles away. Loeb calculated that, for a natural comet, the outflow velocity of these jets about 0.4 kilometers per second would require sustained activity over one to three months. The implied mass loss rate near perihelion soared to roughly 2 million kilograms per second, a four-order-of-magnitude jump from August’s modest 150 kilograms per second.
This surge is a thermodynamic enigma: it would take an absorbing surface area of roughly 1,600 square kilometers, to sublime the estimated 5 billion tons of CO₂ that would be released during perihelion equivalent to a sphere 23 kilometers in diameter, far larger than the 5.6-kilometer upper limit inferred from Hubble’s July imaging. Such a discrepancy implies the comet may have fragmented into at least 16 pieces, increasing its surface area enough to power the observed jets. Yet photographs from Spain on November 5 revealed no debris tail, furthering the mystery.
Adding to the complexity, the MeerKAT radio telescope in South Africa detected “radio absorption lines by hydroxyl radicals” on Oct. 24, the first radio signal ever recorded from 3I/ATLAS. Hydroxyl radicals form when water molecules are broken apart by sunlight, supporting the cometary composition hypothesis. Earlier Webb telescope spectroscopy indicated the coma was dominated by CO₂, with water comprising only 4% of its mass an unusually low fraction compared to solar system comets.
Not least among these is the comet’s bizarre non-gravitational acceleration. Measured at 1.1×10⁻⁶ au/day² radially and 3.7×10⁻⁷ au/day² transversely, it’s consistent with the “rocket effect” from asymmetric outgassing. For a natural comet ejecting gas at 300 meters per second, the observed acceleration implies a loss of more than 13% of the nucleus’s mass. However, Loeb notes that chemical rockets, with exhaust speeds of 3–5 kilometers per second, or ion thrusters reaching 10–50 kilometers per second, could produce similar jets with far less mass loss. “Technological thrusters require a much smaller mass loss in order to produce the observed jets around 3I/ATLAS,” he wrote, adding that alien technology could employ even higher exhaust velocities, preserving the object’s integrity.
From an engineering perspective, such propulsion would radically change the energy budget. Sublimation-driven jets are bound by the latent heat of volatiles-600 J/g for CO₂ and 2,835 J/g for H₂O-whereas advanced thrusters could avoid such limits altogether. If the jets are artificial, their symmetry, velocity profiles, and spectral signatures could be rather unlike natural outgassing, a distinction upcoming spectroscopic campaigns try to resolve.
Detection and tracking of 3I/ATLAS depend on a co-ordinated network of telescopes on the ground and in space. High-resolution imaging with Hubble constrains nucleus size, whereas infrared spectroscopy with the James Webb Space Telescope probes volatile composition; radio observations from MeerKAT add another layer, revealing molecular breakdown products. These facilities together comprise a multi-wavelength diagnostic suite that tests Loeb’s competing hypotheses. The trajectory of the comet adds to the intrigue: its retrograde orbit aligns within 5 degrees of the ecliptic-a statistical rarity of only 0.2% probability-and its arrival was timed such that it would pass near Mars, Venus, and Jupiter while hidden from Earth at perihelion.
Together with its enormous size, estimated at around a million times the mass of ʻOumuamua, and extreme blue brightening close to the Sun, these anomalies have fired up speculation about non-natural origins. On December 19, 2025, 3I/ATLAS will have its closest approach to Earth at 269 million kilometers. In addition, high-resolution imagery of its jets and coma will be captured both from the ground by observatories and from space using Hubble and Webb. If the object remains intact without the expected massive gas cloud, then the case for artificial propulsion will firm up. Instead, if it reveals itself to be a fragmented, volatile-rich comet displaying telltale signs, that would be the “fireworks” scenario. Whichever the case, it is going to deepen our insight into interstellar visitors and the engineering-natural or technological-that drives them.
