3I/ATLAS has reappeared after passing behind the Sun, with new images showing a single intact body and a pronounced anti-tail. Harvard astronomer Avi Loeb suggests the jets creating the anti-tail could be natural dust emissions or, more speculatively, thrusters from a technological spacecraft accelerating the object away from the Sun. Most astronomers regard the object as a CO2-rich comet and point to historical precedents for sunward tails, but continued observations as it nears Earth in late December should clarify its nature.
Harvard Astronomer Suggests Interstellar Object 3I/ATLAS Could Be Firing Thrusters — Or Just a Comet
3I/ATLAS has reappeared after passing behind the Sun, with new images showing a single intact body and a pronounced anti-tail. Harvard astronomer Avi Loeb suggests the jets creating the anti-tail could be natural dust emissions or, more speculatively, thrusters from a technological spacecraft accelerating the object away from the Sun. Most astronomers regard the object as a CO2-rich comet and point to historical precedents for sunward tails, but continued observations as it nears Earth in late December should clarify its nature.
07:25 PM, 11/15/2025Science
Interstellar visitor back in view
Mysterious interstellar object 3I/ATLAS has reemerged from behind the Sun, giving astronomers another chance to study the rare visitor. Most researchers currently interpret the object as a natural comet, likely rich in carbon dioxide ice, on a highly eccentric trajectory that will bring it near Earth just days before Christmas as it departs the solar system.
New images and the anti-tail
Recent observations from the Nordic Optical Telescope in the Canary Islands show a single intact body and no clear evidence of breakup after its perihelion (closest approach to the Sun) two weeks earlier. The images also reveal a pronounced "anti-tail," a feature that appears to point sunward and is thought to result from the concentration of larger dust grains that are less affected by solar radiation pressure.
Two competing interpretations
Harvard astronomer Avi Loeb has highlighted these features but offered two possible explanations. In addition to a natural-comet interpretation, Loeb has suggested the jets that form the anti-tail could conceivably be thrusters on a technological spacecraft. He told NBC News and discussed in blog posts that thrusters oriented with their exhaust toward the Sun would accelerate the object away from the Sun, producing a post-perihelion boost instead of a gravitational-slowing assist.
"Technological thrusters which point their exhaust towards the Sun would accelerate away from the Sun," Loeb wrote. "This post-perihelion maneuver might be employed by a spacecraft that aims to gain speed rather than slow down through the gravitational assist from the Sun."
Scientific caution and alternative explanations
The broader astronomical community remains skeptical of the alien-spacecraft interpretation. Many researchers point out that sunward-facing enhancements and anti-tails have known natural causes. Pennsylvania State University astronomer Jason Wright noted in a September 29 blog post that similar features have been observed before and can arise from large, slowly moving dust grains that are not swept away by the solar wind. Wright also cited a 1974 study of the "anomalous tail of Comet Kohoutek" as a historical precedent.
Size, mass loss, and what comes next
Loeb has estimated that, based on the apparent mass loss, 3I/ATLAS could have a surface-area equivalent to a sphere roughly 14.3 miles (≈23 km) in diameter, about four times larger than his earlier estimate. He argues that if hypothetical technological thrusters used much higher exhaust velocities, the required propellant could be only a small fraction of the craft's mass — a point he uses to keep the possibility open, albeit speculative.
For now, most scientists favor the natural-comet explanation, but researchers emphasize continued monitoring as the object moves away from the Sun and approaches its closest Earth pass in late December. Additional observations over the coming weeks should help distinguish between natural outgassing and other causes of the observed features.
On scientific openness
Loeb has framed his approach as one of scientific openness and caution: "The foundation of science is the curiosity, the humility to learn," he told NBC News. Regardless of the final interpretation, 3I/ATLAS offers a rare opportunity to study material from another star system and to test our models of cometary behavior.