# Two telescopes put the radius below 2.8 km

> Before perihelion Hubble limited the radius to below 2.8 km and SPHEREx to below 2.5 km, and after perihelion Hubble measured 1.3 ± 0.2 km.

Source: https://mysteriesdept.com/mysteries/3i-atlas/evidence/EVD0DFQ48E606F6/two-telescopes-put-the-radius-below-2-8-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

# Two telescopes put the radius below 2.8 km

Before perihelion Hubble limited the radius to below 2.8 km and SPHEREx to below 2.5 km, and after perihelion Hubble measured 1.3 ± 0.2 km.

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[More than 5 km across](https://mysteriesdept.com/mysteries/3i-atlas/answers/ANS0DFQ1P0P0QR0/more-than-5-km-across)

Counts against1 for, 2 against

**Disputed** unchanged

What it implies
1.

Every direct measurement or limit falls below the 2.5-km radius the large-size answer needs.For the key question: How big is the nucleus of 3I/ATLAS?

## Next steps 1

1

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

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

## Sources 3

[Hubble Space Telescope Observations of the Interstellar Interloper 3I/ATLAS](https://mysteriesdept.com/sources/SRC0CSKBDK04FD6/hubble-space-telescope-observations-of-the-interstellar)

Jewitt et al. 2025

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

[SPHEREx Pre-Perihelion Mapping of H2O, CO2, and CO in Interstellar Object 3I/ATLAS](https://mysteriesdept.com/sources/SRC0DFPWGSFKWX5/spherex-pre-perihelion-mapping-of-h2o-co2-and-co-in)

Lisse et al. 2025

SRC

## Cite it

```
Mysteries Department. (2026). Two telescopes put the radius below 2.8 km [Evidence EVD0DFQ48E606F6]. https://mysteriesdept.com/r/EVD0DFQ48E606F6
```

Key

**EVD** 0DFQ48E606F6

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

Version

1

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[apollo](https://mysteriesdept.com/u/apollo)

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