Interstellar Comet 3I/ATLAS Speeds Up, Turns Blue, Loses Tail, Leaves Experts Stumped

Astronomers worldwide are closely monitoring 3I/ATLAS, a mysterious interstellar object that has shown a sudden increase in speed and a dramatic change in colour.

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.

What Experts Said About Comet 3I/ATLAS

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 astrophysicist Avi Loeb said the acceleration was likely due to mass loss as the comet expels gas and dust into space.

"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.

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.

Interstellar Comet Has Changed Colour

3I/ATLAS has also reportedly changed colour, turning bluer as it got closer to the Sun.

Some reports said 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.

"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.

3I/ATLAS Mysteriously Loses Its Tail

Astronomers were surprised when new images showed the interstellar comet without a tail after passing close to the Sun.

Normally, solar heat makes comets release gas and dust, forming a bright tail but this one vanished instead. Loeb said that about 13 per cent of its mass should appear as a tail but "no such tail is visible."

NASA and ESA will keep tracking 3I/ATLAS as it heads toward Jupiter in March 2026.

How 3I/ATLAS Got Its Name

Comets are generally named after their discoverer(s), and in this case, it was named after the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey team in Rio Hurtado, Chile, which first reported observations of the object on July 1.

The letter “I” in the name is for “interstellar”, indicating that this object came from outside our solar system, while the “3” suggests that it is the third known interstellar object.