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Harvard astrophysicist Avi Loeb has shared new insights into the mysterious interstellar object 31/ATLAS, which has recently passed its perihelion—the point in its orbit closest to the Sun. According to Loeb, the Manhattan-sized object is now hidden from Earth’s view as it travels behind the Sun.

A Cosmic Visitor at Perihelion

The European Space Agency (ESA) reported that 31/ATLAS came within 130 million miles of the Sun on October 30, just beyond Earth’s own orbital distance of 93 million miles. “As frozen gases vaporized from its surface, the comet shed dust and ice into space, forming a luminous coma,” said an ESA scientist in a press release.

For now, the comet remains on the far side of the Sun and will stay invisible from Earth until December, when it will reemerge on the other side of its orbit.

Read more from the ESA

See the scientific paper: “Rapid Brightening of 3I/ATLAS Ahead of Perihelion”

Is 31/ATLAS an Alien Spacecraft?

The bizarre journey of 31/ATLAS has stirred both scientific curiosity and online speculation. Loeb, known for his bold hypotheses, has entertained the possibility that 3I/ATLAS might not be entirely natural.

“While 3I/ATLAS is most likely a comet of natural origin, we should remain open to alternative explanations,” Loeb wrote in a Medium essay. His comments echo his earlier work on the interstellar object ‘Oumuamua, which he suggested might have been artificial.

Public fascination has fueled theories that 31/ATLAS could be alien technology or a probe exploring our solar system. However, most astronomers maintain it is a cometary body—though an unusually massive and resilient one.

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Timeline of Key Events

• Sep 20: Astronomers spot a mysterious green glow around 3I/ATLAS, likely from rare gases.
• Sep 23: New theory suggests it could be a “planet-forming seed.”
• Sep 25: Loeb notes the comet shows no non-gravitational acceleration, suggesting great mass—or propulsion.
• Sep 28: The comet passes through the region of the 1977 Wow! Signal.
• Sep 29: Survives a solar storm intact.
• Sep 30: Slips behind the Sun, disappearing from view.
• Oct 1–7: NASA’s Mars orbiters and ESA’s ExoMars capture its glow and detect water vapor.
• Oct 19: Keck Observatory spots a rare anti-tail pointing toward the Sun.
• Oct 21: Loeb speculates the object could use the Oberth effect to alter its trajectory unseen.
• Oct 30: Reaches perihelion at 130 million miles from the Sun.

What Comes Next after Perihelion for 31/ATLAS?

A fleet of telescopes—including NASA’s Hubble Space Telescope and ESA’s Mars Express—has been tracking the object closely. Estimates suggest its nucleus measures between 1,000 feet and 3.5 miles wide.

Next up, the ESA’s Jupiter Icy Moons Explorer (JUICE) mission will monitor 31/ATLAS between Nov. 2 and Nov. 25, when it’s expected to be in a “very active state” after perihelion. Due to its current position beyond the Sun, ESA scientists don’t anticipate receiving JUICE’s data until February 2026.

“The data we’ll get will help us understand how interstellar objects behave when interacting with our Sun,” said an ESA mission engineer. “It’s a rare opportunity to study an outsider up close.”

Learn more about ESA’s JUICE mission