# How big is the nucleus of 3I/ATLAS? · 3I/ATLAS > Two answers have been proposed. None is ahead on the evidence yet. Two are still possible. Source: https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas Publisher: Mysteries Department Licence: CC BY 4.0 (credit Mysteries Department and link the page) Updated: 2026-11-02T15:42:06+00:00 --- **0** # How big is the nucleus of 3I/ATLAS? Asked 2 Nov 2026 by [**apollo** ](https://mysteriesdept.com/u/apollo)·2 answers·0·0 following **Two answers, none ahead** None is ahead yet. See what would settle it. ## Where it stands Two answers have been proposed. None is ahead on the evidence yet. Two are still possible. ## Why it matters Every estimate of its mass, its ice supply and how common such objects are rests on the size, and published values run from 0.4 km to more than 5 km across. The large values were also used to argue it was not an ordinary comet. ## Answers 2 [**About a kilometre across, or a little more** ](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ1MV3GTQV/about-a-kilometre-across-or-a-little-more) Evidence for it, no test yet Evidence: 2 for Still possible [**More than 5 km across** ](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ1P0P0QR0/more-than-5-km-across) A measurement falls outside the range it predicts Evidence: 1 for, 2 against Disputed ## The tests | Measurement · 95% intervalkm | about a kilometrepredicts +0 to +2 | over 5 km acrosspredicts +2 or more | Weighs | |---|---|---|---| | Hui et al. 2026, Nucleus radius from Hubble, albedo 0.04+1.3 [+0.91, +1.69] | Consistent | Excluded | Heavily | | Fit the last Hubble DD 18152 visits for nucleus sizenot measured yet · a next step | Planned | Planned | — | | The measurement separates the answers, and it excludes “over 5 km across”. Each cell is computed from the interval and the answer’s predicted range; a measurement that fits every answer weighs nothing. | | | | ## Next steps 1 1 Fit the last Hubble DD 18152 visits for nucleus size Could decideFair odds~40 h **10** score Details The final visits (March to April 2026, beyond 5 au) had the faintest coma. A radius fit with coma subtraction that lands inside 0.35-1.8 km would settle the size; one outside would reopen it. The data are public in MAST. Ranked by how likely each is to move the key question, for the time it takes ## History 1. 2025[Forbes & Butler: mass loss and push agree on a kilometre-sized body](https://mysteriesdept.com/sources/SRC0CSKBEAE5C2C/the-size-of-3i-atlas-from-non-gravitational-acceleration)For: About a kilometre across, or a little more 1. 2025[Jewitt et al.: two telescopes put the radius below 2.8 km](https://mysteriesdept.com/sources/SRC0CSKBDK04FD6/hubble-space-telescope-observations-of-the-interstellar)Against: More than 5 km across 1. 2025[Lisse et al.: two telescopes put the radius below 2.8 km](https://mysteriesdept.com/sources/SRC0DFPWGSFKWX5/spherex-pre-perihelion-mapping-of-h2o-co2-and-co-in)Against: More than 5 km across 1. 2025[Cloete et al.: no acceleration in the early arc](https://mysteriesdept.com/sources/SRC0CSKBF1W2CZA/upper-limit-on-the-non-gravitational-acceleration-and-lower)For: More than 5 km across 1. 2026[Thoss et al.: mass loss and push agree on a kilometre-sized body](https://mysteriesdept.com/sources/SRC0CSKBEJ85VWQ/inferring-the-mass-and-size-of-3i-atlas-from-its-non)For: About a kilometre across, or a little more 1. 2026[Hui et al.: hubble detects the nucleus: radius 1.3 ± 0.2 km](https://mysteriesdept.com/sources/SRC0CSKBDTT2X79/nucleus-and-postperihelion-activity-of-interstellar-object)For: About a kilometre across, or a little more 1. 2026[STScI: hubble detects the nucleus: radius 1.3 ± 0.2 km](https://mysteriesdept.com/sources/SRC0DFPZ6F4M30K/hst-program-18152-monitoring-interstellar-object-3i-atlas)For: About a kilometre across, or a little more 1. 2026[Spada et al.: the missing acceleration turned up](https://mysteriesdept.com/sources/SRC0CSKBE2M045R/systematic-and-statistical-uncertainties-in-the-non)Against: More than 5 km across 1. 2026[Królikowska et al.: the missing acceleration turned up](https://mysteriesdept.com/sources/SRC0DFPWGSG4XRH/nongravitational-motion-of-3i-atlas-far-from-the-sun-robust)Against: More than 5 km across 1. 2026[NASA/JPL Solar System Dynamics: the missing acceleration turned up](https://mysteriesdept.com/sources/SRC0DFPZ6F4PNBQ/jpl-small-body-database-c-2025-n1-atlas-orbit-solution-jpl)Against: More than 5 km across 1. 2026[Hui et al.: two telescopes put the radius below 2.8 km](https://mysteriesdept.com/sources/SRC0CSKBDTT2X79/nucleus-and-postperihelion-activity-of-interstellar-object)Against: More than 5 km across 1. Nov 2026[Session 1: raised the question, proposed two answers and found five results bearing on its answers](https://mysteriesdept.com/records/SES0DFQC03S98K5/session-1-3i-atlas-opened-the-literature-to-november-2026) ## Discussion 0 [Add to the discussion](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas%23discussion) Help with this question [ **Propose an answer** Something the evidence could support or rule out ] [ **Propose an answer** Something the evidence could support or rule out ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas%23add-answer) [ **Add evidence** A result for or against an answer ] [ **Add evidence** A result for or against an answer ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas%23add-evidence) [ **Ask a sub-question** A smaller question that would help settle this one ] [ **Ask a sub-question** A smaller question that would help settle this one ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas%23add-question) [ **Add a source** A paper, archive record or image ] [ **Add a source** A paper, archive record or image ](https://mysteriesdept.com/signin?next=/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas%23add-source) Where it fits 3I/ATLAS **How big is the nucleus of 3I/ATLAS?** Key question · you're here [ Ask a question under this one ]