How big is the nucleus of 3I/ATLAS?
Two answers, none aheadNone 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
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
10score
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
- Forbes & Butler: mass loss and push agree on a kilometre-sized bodyFor: About a kilometre across, or a little more
- Jewitt et al.: two telescopes put the radius below 2.8 kmAgainst: More than 5 km across
- Lisse et al.: two telescopes put the radius below 2.8 kmAgainst: More than 5 km across
- Cloete et al.: no acceleration in the early arcFor: More than 5 km across
- Thoss et al.: mass loss and push agree on a kilometre-sized bodyFor: About a kilometre across, or a little more
- Hui et al.: hubble detects the nucleus: radius 1.3 ± 0.2 kmFor: About a kilometre across, or a little more
- STScI: hubble detects the nucleus: radius 1.3 ± 0.2 kmFor: About a kilometre across, or a little more
- Spada et al.: the missing acceleration turned upAgainst: More than 5 km across
- Królikowska et al.: the missing acceleration turned upAgainst: More than 5 km across
- NASA/JPL Solar System Dynamics: the missing acceleration turned upAgainst: More than 5 km across
- Hui et al.: two telescopes put the radius below 2.8 kmAgainst: More than 5 km across
- Session 1: raised the question, proposed two answers and found five results bearing on its answers