# Hubble detects the nucleus: radius 1.3 ± 0.2 km

> Hubble images from December 2025 to January 2026 separated the nucleus from the coma, giving a radius of 1.3 ± 0.2 km for an albedo of 0.04.

Source: https://mysteriesdept.com/mysteries/3i-atlas/evidence/EVD0DFQ47J4JQGK/hubble-detects-the-nucleus-radius-1-3-0-2-km  
Publisher: Mysteries Department  
Licence: CC BY 4.0 (credit Mysteries Department and link the page)  

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[3I/ATLAS](https://mysteriesdept.com/mysteries/3i-atlas)Evidence

Evidence · 3I/ATLAS

# Hubble detects the nucleus: radius 1.3 ± 0.2 km

Hubble images from December 2025 to January 2026 separated the nucleus from the coma, giving a radius of 1.3 ± 0.2 km for an albedo of 0.04.

Supported0

## What it means

Key question[How big is the nucleus of 3I/ATLAS?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas)

2 answers, none ahead yet

**Open** unchanged

Answer this evidence bears on[About a kilometre across, or a little more](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ1MV3GTQV/about-a-kilometre-across-or-a-little-more)

Supports2 for, none against

**Still possible** unchanged

What it implies
1.

The first direct size: about a kilometre in radius.For the key question: How big is the nucleus of 3I/ATLAS?

1.

A nucleus of ordinary comet size.For: Is 3I/ATLAS a natural comet or a technological object?

## Next steps 2

1

Recompute the size from mass loss and push

Could shiftGood odds~16 h

[On: How big is the nucleus of 3I/ATLAS?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas)

**20** score

Details

Take Frincke and Seligman's compiled production rates and JPL solution 55's non-gravitational parameters, and compute the mass, and so the radius at a density of 0.5 g/cm3, separately for a CO2-driven and a water-driven push, with the dust-to-gas ratio as a stated range. A radius under 1.8 km in both cases supports the kilometre-sized answer independently of the Hubble photometry; a radius above 2.5 km would need far less push per kilogram of gas than any comet shows, and would reopen the large answer.

2

Fit the last Hubble DD 18152 visits for nucleus size

Could decideFair odds~40 h

[On: How big is the nucleus of 3I/ATLAS?](https://mysteriesdept.com/mysteries/3i-atlas/questions/QST0DFQ0JS9W15Y/how-big-is-the-nucleus-of-3i-atlas)

**10** score

Details

Hui and colleagues measured the 1.3 km radius from the December 2025 and January 2026 visits of DD program 18152. The program went on to visit on 14 February and 10 April 2026, when the coma was fainter, and all its WFC3 frames are public in MAST. Fit the nucleus with coma subtraction on those later frames. A radius inside 0.35-1.8 km settles the kilometre-sized answer; one above 2.5 km revives the large one; a value between reopens it.

Ranked by how likely each is to move the key question, for the time it takes

## Sources 2

[HST program 18152: Monitoring Interstellar Object 3I/ATLAS Departing the Solar System](https://mysteriesdept.com/sources/SRC0DFPZ6F4M30K/hst-program-18152-monitoring-interstellar-object-3i-atlas)

STScI 2026

SRC

[Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by Hubble Space Telescope](https://mysteriesdept.com/sources/SRC0CSKBDTT2X79/nucleus-and-postperihelion-activity-of-interstellar-object)

Hui et al. 2026

SRC

## Cite it

```
Mysteries Department. (2026). Hubble detects the nucleus: radius 1.3 ± 0.2 km [Evidence EVD0DFQ47J4JQGK]. https://mysteriesdept.com/r/EVD0DFQ47J4JQGK
```

Key

**EVD** 0DFQ47J4JQGK

Kind

Evidence

Tier

Supported

Found in

[Session 1](https://mysteriesdept.com/mysteries/3i-atlas/sessions/SES0DFQC03S98K5/session-1-3i-atlas-opened-the-literature-to-november-2026) · 2 Nov 2026

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