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Hubble finds the 3I/ATLAS nucleus far smaller than the 5-km claim

Hubble images taken from December 2025 to January 2026 separated the nucleus of interstellar comet 3I/ATLAS from its glowing coma and gave a radius of 1.3 ± 0.2 km. A Mysteries Department review finds that this contradicts a published claim that the nucleus is more than 5 km across, a radius above 2.5 km.

Size sets mass, and mass sets how easily a comet is pushed. A comet sheds gas as it warms, and the gas acts like a weak rocket. A heavy body barely moves. A light one is nudged off a purely gravitational path. The 5-km claim rested on that link. Through September 2025 no such push had been found, which suggested a large, heavy nucleus. With the full orbit, the push is now required by the data. JPL fits it at 4.6 × 10^-8 au per day squared at 1 au. So the two results now agree. A small nucleus and a measurable push fit together, as they do for ordinary comets. This has three consequences. First, the question of how big the nucleus is has a direct measurement for the first time. Earlier work gave only upper limits. Second, the question of what pushes it no longer has to explain a missing push. It has to explain the push that is there. Third, arguments that 3I/ATLAS is unusual enough to be technological lose one support: a large body that its own gas could not move. This does not settle that question. It removes one reason to ask it.