My estimate of the dynamical age is based on the (inbound) semi-major axis, which is ~4000 au for this comet. Comets with such a semi-major axis of a few thousand au can often become gravitationally unbound from the solar system by planetary perturbations in just a single apparition, where each such passage can randomly deliver a momentum kick up or down depending on exactly when the comet passes through the planetary region. Comets straight from the Oort cloud are essentially right on the edge of being gravitationally bound/unbound originally, so comets like C/2024 G3 are ~1 typical momentum kick in, while an older comet like C/2023 P1 with a semi-major axis of only ~60 au must have passed through the planetary region many, many more times to have received enough of these kicks to shrink its orbit down so much more.

For a proper analysis, you'd need to run a bunch of orbital simulations with clones of this comet on the same orbit, but shifted in perihelion time to measure the probability distribution for the momentum kick expected per apparition. It's quite hard to change the near-perihelion portion of a long period comet's orbit since gravitational perturbations tend to only significantly impact the aphelion distance/orbital period, whereas there aren't any significant perturbation sources stronger than the galactic tide/passing stars, which only really become significant out into the Oort cloud. Therefore, it's reasonably accurate to treat the dynamical evolution process of most long period comets as just random walks starting from the Oort cloud, and you can work out the probability distribution/expected value for the number of apparitions/momentum kicks any comet has had based on the typical kick size and how far the comet is from being unbound. That said, the typical kick size for most cometary orbits through the inner solar system that don't pass particularly close to Jupiter (or any other big planet) tends to be fairly similar. From having just looked at a bunch of these comets, I've found those with semi-major axes of several thousand au, like C/2024 G3, tend to be ~1 kick inward from the Oort cloud, so are statistically likely to have only made one to a few prior passages through the planetary region.

Note also that both old and young comets can be either dust or gas rich, although observational surveys seem to indicate older comets (especially at low q) overall tend to be less dusty than younger comets. Dynamically old comets also do tend to brighten more steadily than new ones, but that's more of a consequence of comets behaving differently on their first time through the inner solar system than afterward, and also because the dynamically new population contains a lot of tiny/unstable comets far smaller than what can survive near the Sun, whereas most of these tiny/unstable comets are destroyed within one apparition, leaving only the large/stable comets and a relatively smaller number of borderline surviving/less stable comets that may or may not survive again in the returning comet population.

Qicheng

On Sunday, December 29, 2024 at 08:41:46 p.m. UTC, Nico Lefaudeux via <nicolas.lefaudeux=[email protected]> wrote:

Hi Qicheng, hi all,

i am curious about which information can tell that the comat has "a dynamical age on the order of only ~1 orbit" ? I thought the orbit analysis could only tell the previous aphelion distance, and little about the previous perihelia.

Seeing that the comet seems to be very "gas depleted" and almost purely dusty, with a smooth light curve (without steps associated with volatiles "switching on"), i was thinking this was pointing?toward the comet having already seen at least a handful of very close perihelia (or many perihelia at larger distance).

i would be glad to learn more about how one can tell if a long-period comet has been near the sun only one or a few times compared to many times.

Nicolas

Le?sam. 28 déc. 2024 à?20:06, Qicheng Zhang via <qzalaska=[email protected]> a écrit?:

The comet's absolute magnitude is ~7, so it's technically brighter than the Bortle limit for all q. Of course, it's also dynamically old, but with a dynamical age on the order of only ~1 orbit, and disintegration isn't too abnormal for such comets, even when brighter than the Bortle limit. C/2021 A1 was a recent example that comes to mind. But many also do survive, like C/2002 V1.

A narrow dust spine (at least before perihelion, as in the present case) means there's now considerable dust being released at very low velocity, which requires that dust be from a source far from the nucleus (as outflowing gas would push any dust near the nucleus itself at high velocity into the main coma). That suggests the comet started to disrupt, with fragments breaking off and drifting away from the nucleus before crumbling further into the low velocity, spine-forming dust. This sort of structure is likely also responsible for the triangular shape the gas coma of a disrupting comet often takes, but this comet may be too dusty for its gas coma to be seen.

At the moment, the comet still has a large, primary coma, which indicates there's at least one large, active nucleus present. If the large coma fades out in favor of the spine (which is a distinct possibility, but far from guaranteed at this point), that would signal the end of this nucleus.

Qicheng

On Saturday, December 28, 2024 at 12:29:58 p.m. UTC, Nick James <comets@...> wrote:

Hi all,

I have been observing C/2024 G3 for the last few weeks using T70 at X07
in Chile. This instrument is able to get down to the mountain ridge
horizon which is around 4 degrees at that azimuth and the sky
transparecy at X07 is very good. T70 is a very short FL instrument
(actually a Samyang 135mm FL lens) but it is good for magnitude
estimates of bright comets although the astrometry I get from the images
is a bit ropey.

This morning (Dec 28.35) I had a total magnitude of 6.0 unfiltered using
Comphot and Gaia DR3 G mags. Michael's estiamate at around the same time
using T75 was 5.8 (taken from COBS). These are fitting the lightcurve
well so there is no sign of any abnormal activity at the moment.

Using astrometry from the MPC up to December 23 and my more recent
astrometry with the 135mm lens there appears to be no obvious
improvement to the residuals by including non-gravitational forces. This
was using Findorb.

My image from this morning is here:



Other images and data are here:



Here's hoping that this comet survives to provide a nice display
post-perihelion. Sadly, us northern observers won't get much of a chance
to see it so we will be relying on reports from the southern hemisphere.

Nick James. BAA Comet Section.

On 28/12/2024 07:42, Michael Mattiazzo via wrote:
> Howdy Folks,
>
>?
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> C/2024 G3 ATLAS has been kept under extensive observation by several observers, despite its poor location.
>
> Attached a light curve showing a steady brightening toward perihelion that could see it reach magnitude -3, if it survives.
>
> I’ve also attached? images from Dec 25 and 26 showing a curious spine feature (often seen in outbursting comets) but this comet is not in outburst.
>
> Hopefully there is sufficient nuclear material to keep going. It is a very dusty comet as seen with the parabolic hood.
>
> This situation reminds me of C/2002 V1 NEAT in March 2023
>
>?
>
> Cheers,
>
> Michael
>
>
>
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