Astronomers tracking the mysterious interstellar visitor 3I/ATLAS have revealed that the object has performed a dramatic tail reversal, now pointing away from the sun.

The change comes just months after Hubble Space Telescope images captured an unusual 'anti-tail,' a jet of particles streaming toward the sun instead of away from it.

New high-resolution observations from the Nordic Optical Telescope in the Canary Islands confirm that the anti-tail seen in July and August 2025 vanished and a new one formed in the opposite direction by September.

The shift occurred because the comet's dust and ice particles react differently to sunlight. 

Early on, large, slow-moving dust grains scattered light sunward, creating the anti-tail. But as 3I/ATLAS moved closer to the Sun, rising temperatures ejected more ice fragments and longer-lived dust particles, producing the tail that now points away.

Ground-based observations will be impossible through October as the object passes behind the Sun, hidden from Earth's view.

Researchers at the University of California and the University of Oslo found that 3I/ATLAS is shedding material at a rate proportional to the solar radiation striking its surface.

Meanwhile, NASA's space telescopes previously detected the comet losing about 330 pounds of material per second, made up of 87 percent carbon dioxide and nine percent carbon monoxide.

Harvard professor Avi Loeb and his colleague Eric Keto have developed a model that offers a possible natural explanation for the comet's strange behavior.

Their findings suggest that as 3I/ATLAS moves closer to the Sun, different types of ice on its surface sublimate, turning directly from solid to gas.

Farther from the sun, about 279 million to 372 million miles away, CO2 ice drives the unusual sunward jets. But as the comet gets nearer, water ice begins to dominate, changing how material is released and causing the tail to flip into the classic shape now observed.

Loeb shared the study in a Wednesday blog post, writing: 'The total amount of mass lost from 3I/ATLAS during July through October 2025 amounts to about two million tons. 

'This is less than 0.00005 of the comet's total mass, which is estimated at over 33 billion tons based on its stable trajectory.'

The gas cloud surrounding 3I/ATLAS could be produced by stripping away only a 1.6-inch-thick surface layer from a 3.1-mile-wide object, a ratio similar to the length of your palm compared to the length of Manhattan Island.

'Needless to say, we cannot infer the true nature of 3I/ATLAS from the skin layer it has shed so far,' Loeb added. 

'My colleague, Adam Hibberd, noted that if the object were an alien spacecraft slowing down, and the anti-tail represented braking thrust, this shift from anti-tail to tail would be entirely expected near perihelion.'

However, NASA and most scientists maintain that 3I/ATLAS is a natural comet, though one that originated outside our solar system.

Unlike the first interstellar visitor ʻOumuamua, which showed no signs of gas or dust, and the second, 2I/Borisov, which behaved like a normal comet, 3I/ATLAS displays unique features, including its anti-tail, extreme color changes, and a massive surrounding coma.

'As the surface of 3I/ATLAS is exposed to at least 33 gigawatts of solar radiation at perihelion, observations during its closest approach to Earth on December 19, 2025, will offer the most important clues about its nature,' Loeb said. 'If it continues to show all the features of a natural comet, that will confirm its origin.'

The Harvard professor issued a chilling warning as the object was set to move behind the sun on Tuesday.

The object, dubbed 3I/ATLAS, will be exactly on the opposite side of the sun relative to Earth, constituting a so-called `solar conjunction,' tomorrow, which Avi Loeb said would be 'an opportune time for technological action.'

Loeb explained that in space travel, the best time to speed up or slow down a spacecraft is when it's closest to a large object, since firing the engine at that point, known as the Oberth effect, gives the biggest change in speed.

'If 3I/ATLAS is a massive mothership, it will likely continue along its original gravitational path and ultimately exit the Solar system,' the professor shared in a Sunday blog post.

'In that case, the Oberth maneuver might apply to the mini-probes it releases at perihelion towards Solar system planets.'

3I/ATLAS will reach its best window for such Oberth maneuvers just eight days after it slips behind the sun, which will put it the closest to the sun at about 126 million miles away, he added.

While Loeb has floated the idea that 3I/ATLAS could be of extraterrestrial origin, NASA has long maintained that the object is simply a comet from a distant galaxy.

However, that conclusion has not stopped Loeb from calling for more data before the case is closed.