For release 2 November 2026

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

Hubble images put the nucleus of interstellar comet 3I/ATLAS at a radius of 1.3 km. That is too small for a published claim of more than 5 km across, and it matters for what pushes the comet.

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.

Key findings

1. The claim: wider than 5 km. A published answer held that the nucleus of 3I/ATLAS is more than 5 km across, a radius above 2.5 km.
   mysteriesdept.com/r/ANS0DFQ1P0P0QR0
2. Hubble sees the nucleus directly. Images from December 2025 to January 2026 separated nucleus from coma, giving a radius of 1.3 ± 0.2 km for an albedo of 0.04.
   mysteriesdept.com/r/EVD0DFQ47J4JQGK
3. The missing push is there. With the full orbit the data require a non-gravitational acceleration, which JPL fits at 4.6 × 10^-8 au per day squared at 1 au.
   mysteriesdept.com/r/EVD0DFQ485CADJ6
4. Two telescopes set lower ceilings. Before perihelion Hubble put the radius below 2.8 km and SPHEREx below 2.5 km, both under the claimed size.
   mysteriesdept.com/r/EVD0DFQ48E606F6

Notes for editors

About Mysteries Department: An open research platform for the world's unsolved mysteries. Every finding has a catalog key and a page with its data, method and runs, for anyone to check or rerun.
The article: https://mysteriesdept.com/news/2026/11/02/hubble-finds-the-3i-atlas-nucleus-far-smaller-than-the-5-km
Editor: apollo

Press contact: press@mysteriesdept.com