Comet 3I/ATLAS is showing signs of “non-gravitational acceleration”, a small extra push acting on it beyond the pull of the Sun’s gravity, according to NASA JPL navigation engineer Davide Farnocchia. 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.
Loeb suggests the object could lose “about a tenth of its mass” in a matter of a single month. “As it expels this material at a greater rate, it’s being kicked in the opposite direction,” Loeb wrote in a blog post. He estimated the comet could lose “about a tenth of its mass” in just a month.
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Loeb, in an interview with NBC News, also said that the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile observed a “deviation by four arcseconds in right ascension from the expected path.” “That’s very significant statistically,” he said, linking it to “the evaporation of about a sixth of the mass of the object,” which he described as “a significant fraction.”
Loeb predicted that “such a massive mass loss” should be detectable in the form of a large plume of gas surrounding 3I/ATLAS in November and December.
Has interstellar comet changed colours?
3I/ATLAS has also reportedly changed colour, turning bluer as it got closer to the Sun. Some reports claimed it changed colour several times, but Qicheng Zhang from the Lowell Observatory said that’s not the case.
“We don’t have any evidence for the gas coma changing colours,” Zhang told Space.com. “Our result just showed that the gas coma is likely still around and contributing substantially to the overall brightness.” Zhang explained that the comet appeared to “change colour” once, when its gas coma first became visible as it ejected gases in response to solar heating, according to NDTV.
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“As far as we know, the comet just ‘changed colour’ once when its gas coma first became visible/bright, and it’s still like that now (only brighter),” he said. “However, this was already beginning to happen by early September… numerous photos from amateur astronomers from around then already showing that the comet has a blue/green gas coma,” he added.
Loeb, who has previously suggested that some interstellar objects could be of artificial origin, did not rule out a technological explanation. “Alternatively, the non-gravitational acceleration might be the technological signature of an internal engine,” he said. “This might also explain the report on 3I/ATLAS getting bluer than the Sun.”
He noted that a comet’s dust would usually make it appear redder, not bluer, and speculated that the blue colour “could potentially be explained by a hot engine or a source of artificial light.” Even so, Loeb said a natural explanation is still likely. The colour could “be a signature of ionised carbon monoxide for a natural comet,” he said.
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3I/ATLAS loses its tail
Astronomers were surprised when new images showed the interstellar comet 3I/ATLAS had lost its tail after passing close to the Sun.
Usually, solar heat causes comets to release gas and dust, creating a glowing tail. But this time, the tail disappeared instead. Astrophysicist Avi Loeb noted that about 13% of the comet’s mass should have formed a tail, yet “no such tail is visible.”
NASA and the European Space Agency (ESA) will continue to track 3I/ATLAS as it moves toward Jupiter in March 2026. Discovered by a telescope in Chile in July, it is only the third known visitor from beyond our solar system after Oumuamua and Borisov. NASA said it poses “no threat” to Earth and will make its closest approach to the planet in December, passing within 269 million kilometres. On October 29, it appeared comet-like but brightened sharply and turned “distinctly bluer” as it neared the Sun. NASA reported that its speed has risen to 2,44,600 kmph, with a small, unexplained “kick” that can’t be explained by the Sun’s gravity alone.
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