The interstellar Comet 3I/ATLAS is on its way out of the inner solar system after passing its closest point to the Sun (perihelion) on October 30. Observations reveal that it is currently behind the Sun on its hyperbolic trajectory and will offer better views from Earth again during its closest approach to our planet on December 19. However, the most puzzling characteristic of this interstellar visitor is a lack of tail despite reaching perihelion.
Comets are composed of ice, dust and gas and as sunlight heats them up, the ice sublimates and gets released along with gas and dust to create a tail. That’s something that has been observed on the native Comet Lemmon, designated C/2025 A6, which was discovered on January 3 and will reach perihelion on November 8. Several pictures of the Comet have surfaced in the past few weeks as it moved in its 1,350 year orbit with a tail millions of kilometres long.
Dr. Avi Loeb, a physicist at Harvard University, shared new pictures captured by Spain’s R. Naves Observatory highlighting the lack of a cometary tail on 3I/ATLAS. Loeb has divided the scientific community for suggesting, based on almost a dozen anomalies he has flagged, that this comet could be alien technology.
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Why doesn’t Comet 3I/ATLAS have a tail?
Most scientists are trying to find a natural explaination for 3I/ATLAS’s lack of cometary tail while steering clear of the alien theory. According to the SETI Institute, which is dedicated to finding life beyond Earth, spectroscopic measurements have revealed that Comet 3I/ATLAS’s coma (cloud of gas and dust surrounding the nucleus) is dominated by carbon dioxide or CO2.
Its gas plume also has minimal traces of water, just 4% by mass, says Loeb. Since carbon dioxide behaves differently, its ice turns into gas directly (that’s sublimation) and causes evaporative cooling on the surface by absorbing heat. Since this lowers the local temperature, the water-ice doesn’t sublimate, leaving no tail behind. Normally, comets also achieve acceleration as they reach perihelion because the ejecting gases act as thrusters. Surprisingly, 3I/ATLAS is accelerating without a tail.
Another possible explanation from SETI is – “Over millions of years in interstellar space, cosmic radiation can alter ice chemistry, producing physical behaviors distinct from those seen in native comets.”
On the other hand, Loeb argues citing his calculations that the comet must have lost more than 13% of its mass near perihelion. “For a typical comet, this should have resulted in a massive coma with dust and gas that would have been pushed by the solar radiation pressure and the solar wind to the shape of a typical cometary tail pointing away from the Sun. No such tail is visible in the new images from November 5, 2025,” he wrote in his blog.
The apparent lack of a tail, Loeb says, could provide an alternative explanation for 3I/ATLAS’s acceleration, possibly a non-human propulsion mechanism. He also stated that future observations from various telescopes on December 19, when the comet gets close to Earth, will provide a clear picture on what’s causing 3I/ATLAS’s acceleration.
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