The Anti-Tail of 3I/ATLAS Extends Beyond The Distance To The Moon Spanning Half a Million Kilometers

As of December 15, 2025, 3I/ATLAS was roughly 270 million kilometers from Earth. By December 19, it will reach its closest approach at about 268.9 million kilometers—only marginally closer, but critically timed. The size of this anti-tail means it must have formed rapidly. To reach half a million kilometers in the roughly 45 days since perihelion, the material making up the structure would need to be moving sunward at a minimum relative speed of about 130 meters per second. That speed, sustained over that duration, is difficult to reconcile with conventional models of dust grains being gently pushed by solar radiation pressure or gas sublimating from ice pockets on a rotating nucleus.

In plain terms, comets typically grow tails that stream away from the Sun, shaped by radiation pressure and the solar wind. Anti-tails do exist, but they are usually optical effects caused by geometry—dust left behind in the comet’s orbit that appears sunward from certain viewing angles. What makes 3I/ATLAS different is scale, coherence, and apparent dynamism. An anti-tail of this magnitude, extending hundreds of thousands of kilometers and appearing tightly collimated, has no clear precedent in cometary observations.