“The comet’s behavior at perihelion is an acid test,” Loeb told Newsweek, referring to the point at which it comes closest to the Sun.
In his Medium Q&A on 3I/ATLAS at Perihelion, Loeb elaborated on the comet’s surprising blue color, which defies expectations.
“The appearance of 3I/ATLAS as bluer than the Sun is very surprising. Dust is expected to redden scattered sunlight, and its surface should be colder than 5,800 Kelvin, making it redder—not bluer—than the Sun,” he said.
This unusual blue hue, Loeb added, could indicate gas emissions contributing to the comet’s brightness—an anomaly that he now lists as the ninth unexplained property of this strange object.
Cornell Study: Rapid Brightening of 3I/ATLAS Ahead of Perihelion
Eight Other Oddities: A “Loeb Scale” Ranking of 4
Before the latest discovery, Loeb had cataloged eight peculiar features of 3I/ATLAS that placed it at 4 out of 10 on what he calls the “Loeb scale”—a ranking system gauging the likelihood of non-natural origin.
These features include:
- A trajectory aligned within 5 degrees of the ecliptic plane of the planets.
- A sunward jet (anti-tail) observed in July and August 2025, not explainable by geometry.
- A large nucleus unlike any known comet.
- An arrival path that passed near Mars, Venus, and Jupiter within a fine-tuned distance.
- Timing that rendered it unobservable from Earth during its closest pass to the Sun.
Now, with its blue perihelion color, Loeb has added a ninth anomaly to the list.
NASA: 31/ATLAS No Threat to Earth, But Plenty of Mysteries
While speculation continues about its origins, NASA has confirmed that “comet 3I/ATLAS poses no threat to Earth and will remain far away.”
