David,

Back in early September, before 3I/ATLAS went into conjunction, Jaeger
and Rhemann were reporting a coma diameter of around 5 arcmin from their
deep images. See, for example, this one:

https://britastro.org/cometobs/3i/3i_20250924_mjaeger.html

Unfortunately the image itself does not quote the field of view so it is
hard to judge but there is definitely a significant coma. At the time of
that image (2025 Sept 24) the comet was around 2.5 au from the Earth. If
you do the geometry an apparent diameter of 5 arcmin at 2.5au is a coma
diameter of around 500,000 km so that is consistent with the NASA statement.

3I/ATLAS has now passed through conjunction and it will move rapidly
into our morning sky over the next week or two. By the time of the next
new moon (Nov 20) it will be 25 deg above the horizon when the Sun is 15
deg down as seen from southen England.

Nick.

On 02/11/2025 21:02, Douglas Heggie wrote:
> Dear David,
>
> I caught your enquiry to the baa comet list the other day, but the name
> rang a bell.  Did we once know each other in connection with John
> Murray's Iceland trips in the 1960s?  Please forgive me if I am totally
> mistaken here.
>
> On  your question, I would not have thought a diameter (or radius) of
> 350,000km  was so incredible.  At a distance of 1AU (say) that means an
> angular size of about 0.0023 radians, or about 8 arc minutes, which is
> quite reasonable for the coma of a bright comet.
>
> Best wishes,
> Douglas
>
> Postal Address: 3 St Ninian's Terrace, Edinburgh EH10 5NL, UK
>
>
>
> ------------------------------------------------------------------------
> *From:* baa-comet@simplelists.com <baa-comet@simplelists.com> on behalf
> of David Reynolds - djr.mars2 at btinternet.com (via baa-comet list)
> <baa-comet@simplelists.com>
> *Sent:* 31 October 2025 23:52
> *To:* baa-comet@simplelists.com <baa-comet@simplelists.com>
> *Subject:* [BAA Comets] 3IATLAS
>
> Can anyone shed some light on a statement in Astronomy Now about our
> interstellar visitor, 3IATLAS, please.
>
> In the October edition there is a one page piece (p10) on 3I/ATLAS.  The
> credit from NASA/Caltech to the SPHEREx image of the comet states that
> the gas cloud around it extends out to around 350,000km.  This seems to
> me to be an enormous distance; is it a misprint? This is which is really
> a statement of the size of the comet? In comparison, the diameter of the
> Earth is about 12,800km.  Can you throw any light on this statement; it
> can't be true surely?
>
> Nick, I enjoyed your talk at Kelling Heath last month, many thanks.
>
> Regards
> David Reynolds