Avi Loeb of Harvard is urging caution and more data before declaring the interstellar object 3I/ATLAS a routine comet. He points to an unusual chemical signature, close planetary flybys, and a potentially large size — up to 3.1 miles across and roughly 33 billion tons — as reasons to keep alternative hypotheses open. HiRISE images taken during an October Mars flyby were delayed by a government shutdown but may be released soon. The object will pass within 170 million miles of Earth on December 19, and a March Jupiter encounter could yield additional data.
Avi Loeb Doubles Down on 3I/ATLAS Origins — Calls for More Data and Rebuts Critics
Avi Loeb of Harvard is urging caution and more data before declaring the interstellar object 3I/ATLAS a routine comet. He points to an unusual chemical signature, close planetary flybys, and a potentially large size — up to 3.1 miles across and roughly 33 billion tons — as reasons to keep alternative hypotheses open. HiRISE images taken during an October Mars flyby were delayed by a government shutdown but may be released soon. The object will pass within 170 million miles of Earth on December 19, and a March Jupiter encounter could yield additional data.
02:23 AM, 11/18/2025Science
Harvard Astronomer Urges Caution and More Data on Mysterious Interstellar Visitor
As scientists and the public await high-resolution images from NASA’s Mars Reconnaissance Orbiter, Harvard astronomer Avi Loeb remains open to the possibility that the interstellar object 3I/ATLAS could be an artificial probe rather than a routine comet.
Most researchers currently favor a straightforward explanation: 3I/ATLAS appears to be a comet rich in carbon dioxide. Loeb, however, highlights several anomalies — an unusual chemical signature, unusually close encounters with multiple planets, and a very large estimated size — and argues these factors justify continued investigation of an artificial origin.
“The verdict is still out, and I’m very much looking forward to getting more data in the coming weeks,” Loeb told NewsNation. “That’s the way science should be done. It’s like a detective story.”
Loeb has criticized colleagues who offer a definitive conclusion now, saying that ruling out possibilities before all time-sensitive data are available stifles curiosity. He has also called for faster public access to observations, noting that sharing data promptly helps the broader community plan follow-up measurements.
Key observational milestones include a close approach to Earth on December 19 — about 170 million miles at closest pass — which will provide a better viewing opportunity as 3I/ATLAS departs the solar system. The object is also expected to pass near Jupiter in March, a rendezvous that could allow spacecraft in that region to collect additional information.
Images taken by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter during 3I/ATLAS’s October flyby of Mars were delayed amid a recent U.S. government shutdown. Loeb has expressed hope those frames will be released soon; he believes they could help refine estimates of the object's size, which he has suggested might reach about 3.1 miles (5 km) across with an estimated mass on the order of 33 billion tons.
While Loeb has acknowledged that a cometary explanation remains the most likely scenario, he refuses to rule out more exotic hypotheses — including the possibility that the object could be maneuvering — and stresses that hypotheses should be tested against publicly shared data rather than authoritative pronouncements.
“The truth about the nature of 3I/ATLAS will be revealed by publicly shared data and not by any storyline of gatekeepers,” he wrote in a recent blog post.
Many in the astrophysics community remain skeptical of Loeb’s more speculative suggestions, noting that unusual observations often have natural explanations once additional data are analyzed. Still, the release of HiRISE imagery and continued monitoring around the December and March encounters are expected to significantly narrow the range of plausible interpretations.
What to watch next: the HiRISE image release, further spectroscopic and astrometric observations over the coming weeks, and the object’s December 19 Earth flyby and March Jupiter encounter.