In early July, astronomers spotted a mysterious object, later dubbed 3I/ATLAS after it was confirmed to be the third-ever interstellar visitor cruising through our solar system.
Last week, the object, which is now generally believed to be a comet, reached its closest point to the Sun, or its perihelion, brightening up at an unexpected rate and turning “distinctly bluer.”
And it may be getting a major boost that’s unaccounted for by the Sun’s gravitational pull as well. As NASA’s Jet Propulsion Laboratory navigation engineer Davide Farnocchia detailed in a recently filed report, 3I/ATLAS is showing signs of “non-gravitational acceleration.”
While that may sound like an alien spacecraft is lighting up its afterburners, there’s a far more mundane and natural explanation.
As Harvard astronomer Avi Loeb laid out in a blog post last week, the acceleration is likely the result of 3I/ATLAS losing significant amounts of mass, forming an elongating plume of dust and gas. Put simply, as it expels this material at a greater rate, it’s being kicked in the opposite direction.
Loeb suggests the object could lose “about a tenth of its mass” in a matter of a single month.
During an NBC News interview on Monday, Loeb explained that the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile observed a “deviation by four arcseconds in right ascension from the expected path.”
“And that’s very significant statistically,” he said, calculating the boost corresponding to the “evaporation of about a sixth of the mass of the object,” which is a “significant fraction.”
“Such a massive mass loss should be detectable in the form of a large plume of gas surrounding 3I/ATLAS during the upcoming months of November and December 2025,” he wrote in his blog, referring to the time the object will reemerge from behind the Sun.
At that time, we should be able to observe a “cloud of gas that carries five billion tons [of material] or more,” he told NBC.
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