What does it take for a comet to look like it’s from another world? For interstellar visitor 3I/ATLAS, the answer came in the form of this striking astrophotograph showing the comet’s shimmering green coma and multiple tails framed against the barred spiral galaxy NGC 4691, 70 million light-years away. Captured on Nov. 16 by New Mexico-based astrophotographer Satoru Murata using a 0.2-meter telescope, the image reveals both the comet’s long ion tail and its shorter anti-tail, the latter likely produced by excess dust streaming from its surface. Weeks earlier, both tails seemed to vanish a visual illusion caused by the comet’s orientation relative to Earth.
Closer inspection reveals smaller jets of gas erupting from the coma, a direct result of outgassing. As solar radiation heats up the comet’s icy interior, volatile compounds vaporize and escape through fissures in its surface, creating thrust that makes measurable non-gravitational acceleration. At perihelion, said Harvard astronomer Avi Loeb, 3I/ATLAS was exhibiting a radial acceleration of 135 kilometers per day squared and a transverse acceleration of 60 kilometers per day squared. It is just this sort of behavior that has driven speculation about artificial origins, but thermophysical modeling by Florian Neukart at Leiden University shows how the acceleration can be explained by conventional volatile-driven activity mainly CO and CO₂ with only sub-percent active surface coverage required. “Our results show that a conventional volatile-driven mechanism reproduces both the magnitude and the direction of the acceleration,” Neukart wrote, dismissing the need for exotic materials or engineered structures.
Murata’s image places the comet in dramatic juxtaposition with NGC 4691, underscoring its journey from the “frontier” region of the Milky Way. Estimated to be up to 7 billion years old,kj 3I/ATLAS is significantly older than our solar system. NASA’s Hubble Space Telescope has constrained its nucleus size to no more than 5.6 kilometers across, possibly as small as 320 meters, while detecting dust plumes consistent with other sun-bound comets. The velocity of this interstellar traveler-about 130,000 mph-represents gravitational boosts accumulated over billions of years in deep space.
Spectroscopic observations paint a deeper picture. NASA’s James Webb Space Telescope and China’s Tianwen-1 mission have detected a volatile-rich coma dominated by carbon dioxide, along with water, carbon monoxide, and unusual nickel signatures. Michio Kaku, a theoretical physicist, explains these peculiarities in terms of the extreme age of the comet, allowing it to have passed through different environments and accumulate different chemical profiles over billions of years. Radio detections from the MeerKAT telescope confirmed hydroxyl emissions, a breakdown product of water ice, further confirming its cometary nature.
Tracking such a fast-moving interstellar object requires precision. During its Oct. 3 flyby of Mars, ESA’s ExoMars Trace Gas Orbiter captured high-resolution images that improved trajectory predictions ten-fold. This rare use of a planetary orbiter for deep-space tracking allowed scientists to triangulate its position more accurately than Earth-based observations alone, enabling targeted follow-up with instruments such as ESA’s JUICE spacecraft and even encounters with its tail.
But despite the public’s fascination with alien spacecraft theories, experts have remained certain in their assessment. “No, it’s not an alien mothership,” Murata said. And NASA’s Slava Turyshev adds: Its hyperbolic orbit, volatile-driven acceleration, and dust tail are “textbook comet behavior.” The object will make its closest approach toward Earth on Dec. 19 at 270 million kilometers, affording a final window for observations before the object gets back to its one-way escape into interstellar space.
