# Forbes 2025: The size of 3I/ATLAS from non-gravitational acceleration

> John C. Forbes, Harvey Butler, 2025. 1 records on Mysteries Department cite it. The mass-balance size estimate for 3I; smaller than the 1.3 km Hubble…

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# The size of 3I/ATLAS from non-gravitational acceleration

John C. Forbes, Harvey Butler, 2025. _arXiv_.

Converts the acceleration into a mass-loss rate and matches it to JWST, SPHEREx, ALMA and TRAPPIST production rates, finding a diameter of about 0.8-1.05 km for an assumed density of 0.5 g per cubic centimetre, with a large systematic uncertainty.

**1I/'Oumuamua** Preprintdoi: **[10.3847/2515-5172/ae3747](https://doi.org/10.3847/2515-5172/ae3747)** Checked against the original0

## What it's used for here

The mass-balance size estimate for 3I; smaller than the 1.3 km Hubble radius, within its stated systematics.

## Cited by on this site 1 record

[1I/'Oumuamua](https://mysteriesdept.com/mysteries/oumuamua)

The mystery article

MYS

## Cite it

```
Forbes, J. C., & Butler, H. (2025). The size of 3I/ATLAS from non-gravitational acceleration. arXiv. https://doi.org/10.3847/2515-5172/ae3747
```

[Read it free at arxiv.org](https://arxiv.org/abs/2512.18341)

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Preprint

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arXiv-nonexclusive

Kept

27 Oct 2026, for editors

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Against the published original

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