# Activity of Halley-type Comets 12P/Pons-Brooks and 13P/Olbers from SOHO/SWAN Observations

M R Combi, T Mäkinen, Jean-Loup Bertaux, Eric Quémerais, S Ferron

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M R Combi, T Mäkinen, Jean-Loup Bertaux, Eric Quémerais, S Ferron. Activity of Halley-type Comets 12P/Pons-Brooks and 13P/Olbers from SOHO/SWAN Observations. The Planetary Science Journal, 2025, 6, pp.306. 10.3847/psj/ae2475. hal-05438498

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# Activity of Halley-type Comets 12P/Pons-Brooks and 13P/Olbers from SOHO/SWAN Observations

M. R. Combi<fnref n="1" />, T. Mäkinen<fnref n="2" />, J.-L. Bertaux<fnref n="3" />, E. Quémerais<fnref n="3" />, and S. Ferron<fnref n="4" />

Received 2025 August 26; revised 2025 November 20; accepted 2025 November 24; published 2025 December 30

## Abstract

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyα camera on the Solar and Heliosphere Observatory (SOHO) observed the hydrogen coma of Halley-type comet (HTC) 12P/Pons-Brooks from SOHO's halo orbit around L1 throughout its 2023–2024 apparition from the beginning of 2023 November to just before the mid-November outburst. There was a gap in coverage during December and January when the comet was in the galactic plane, after which coverage then continued until 29 July. SWAN also observed another HTC comet, 13P/Olbers, from 2024 April 26 until 9 August. Water production rates were calculated from each image of both comets using the standard methodology with fluorescence rates calculated using the daily solar Lyα fluxes from the LASP database corrected for solar rotation. The water production rate of 12P reached a maximum of $1.7 \times 10^{30} \text{ s}^{-1}$ 7 days before the comet perihelion on 2024 April 21. Eight outbursts were detected with released water masses from $4.66 \times 10^8$ to $8.06 \times 10^9 \text{ kg}$. The comet's water production rate varied with heliocentric distance ($r$) with an exponent of -2.5 before perihelion and -2.9 after, covering heliocentric distances from 2.747 au before perihelion to 1.861 au after, when the activity is driven by water sublimation. The maximum production rate of 13P/Olbers was $1.1 \times 10^{29} \text{ s}^{-1}$, reached 27 days before perihelion on 2024 June 30. Its variation with heliocentric distance was more irregular and very scattered compared to that of 12P but generally increased with decreasing heliocentric distance. The variation of activity for 12P was generally very similar to that of the more famous HTC, 1P/Halley, in that both vary with heliocentric distance more similarly to most long-period Oort cloud comets than to most Kuiper belt Jupiter-family comets, many of which vary with very large negative slopes.

Unified Astronomy Thesaurus concepts: Comets (280); Comae (271); Comet origins (2203); Comet volatiles (2162); Ultraviolet sources (1741)

## 1. Introduction

Owing to an interesting coincidence, two ~70 yr period Halley-type comets (HTCs), 12P/Pons-Brooks and 13P/ Olbers, referred to as comets 12P and 13P hereafter, had their most recent perihelion passages in 2024. Comet 12P was “codiscovered” by Jean-Louis Pons in 1812 and then rediscovered by William Brooks during its next apparition in 1883. However, later historical studies show that it had likely been observed hundreds or even thousands of years earlier, long before the occurrences of periodic comets were realized. 12P was recovered during this apparition by Q. Ye et al. (2020) using the 4.3 m Lowell Discovery Telescope on 2020 June 10 and 17 when it was 11.89 au from the Sun. From their observations, Ye et al. also estimated the radius of the nucleus to be 17 ± 6 km, which would make it much larger than 1P/Halley. However, Ye et al. gave a caveat that this is really an upper limit owing to a large uncertainty in the subtraction of the dust coma model. Using a dust model, they estimate that 12P might have been active at a heliocentric distance as large as ~30 au. Finally, historical records had placed maximum visual magnitudes of about 3, and during this apparition, it reached magnitude 4, so accordingly, it has not faded very much or at all (S. Yoshida 2024a).

Comet 13P was discovered by Heinrich Olbers in 1815. It was recovered during its 1887 (M. Brooks 1887) and 1956 apparitions (E. V. Petersen & S. Lausten 1957) as comet 1956a. 13P was much less active than 12P and so is generally unremarkable. Its perihelion distance is much larger than the more famous ~70 yr period HTCs 1P and 12P, adding to its faintness. The recent apparition of comet 13P was recovered by A. Hale (2024) on 2024 August 13 with the Las Cumbres Observatory 1.0 m Schmidt-Cassegrain telescope at the South African Astronomical Observatory with orbits calculated with later observations using the Siding Spring Observatory in New South Wales, Australia. It was recovered at visual magnitude 21.6 when it was at a heliocentric distance of 4.1 au. There has been no published estimate of the size of the nucleus of 13P, but a very simple estimate from the visual magnitude at discovery, assuming that the observation was of the bare nucleus, would give it an upper limit radius of 2.5 km.

The main function of the Solar Wind ANisotropies (SWAN) instrument on the Solar and Heliosphere Observatory (SOHO) is making full-sky images of the H Lyα emission from the interplanetary medium to determine a 3D view of the solar wind as it ionizes the interstellar hydrogen atoms that pass through the solar system (J. L. Bertaux et al. 1995). Water is the main volatile gas sublimating from cometary nuclei that are active enough and pass close enough to the Sun. Solar UV light first photodissociates H₂O and then produces OH, yielding two H atoms each to form a large high-speed hydrogen coma that is millions of kilometers across and often detectable with SWAN

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Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹Dept. of Climate and Space Sciences and Engineering, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA; mcombi@umich.edu</footnote>

<footnote n="2">²Finnish Meteorological Institute, Box 503, SF-00101 Helsinki, Finland</footnote>

<footnote n="3">³LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France</footnote>

<footnote n="4">⁴ACRI-st, Sophia-Antipolis, France</footnote>
<table><caption>**Table 1**
Summary of SOHO/SWAN Observations</caption><thead><tr><th>Comet</th><th>12P</th><th>13P</th></tr></thead><tbody><tr><td>Perihelion date</td><td>2024 Apr 21.12</td><td>2024 Jun 30.95</td></tr><tr><td>q (au)</td><td>0.7808</td><td>1.175</td></tr><tr><td>No. of images</td><td>145</td><td>100</td></tr><tr><td>Start-end date (UT)</td><td>2023 Oct 31–2024 Jul 29</td><td>2024 Apr 26–1 Sep</td></tr><tr><td>r<sub>H</sub> range (au)</td><td>2.747–0.781–1.861</td><td>1.516–1.176–1.520</td></tr></tbody></table>

Note. $r_H$ = heliocentric distance (au) range. $q$ = perihelion distance (au).

when the comets are bright enough (M. R. Combi et al. 2004). Both the interstellar and cometary H atoms are illuminated through fluorescence by solar H Ly$\alpha$ photons.

SOHO, located in a halo orbit around the L1 Lagrange point between the Earth and Sun, provides an excellent vantage point to observe comets throughout the inner solar system portions of their orbits, typically for comets at $r < 3$ au from the Sun and those that typically have $m_V < 10$, at locations both north and south of the ecliptic. Since SOHO’s launch in 1995 December, water production rates have been calculated for nearly 100 comet apparitions (M. R. Combi et al. 2019; M. Combi et al. 2022), including multiple apparitions of several short-period comets.

Water production rates are calculated from SWAN images of the hydrogen coma using the time-resolved model (TRM) described by J. T. T. Mäkinen & M. R. Combi (2005) and tested using observations of comet C/1996 B2 (Hyakutake) by M. R. Combi et al. (2005). It has been applied to many comets since then (M. R. Combi et al. 2019; M. Combi 2022). The method accounts for the production and subsequent partial thermalization of H atoms produced in the photodissociation of H$_2$O and OH using results of Monte Carlo models for a range of comet production rates and heliocentric distances (M. R. Combi & W. H. Smyth 1988; M. R. Combi et al. 2000, 2004). The atoms are tracked to locations in the coma using the orbital calculation approach of H. U. Keller & R. R. Meier (1976). Because of the brightness of comet 12P, we were further able to apply a subset method described by M. R. Combi et al. (2005) and J. T. T. Mäkinen & M. R. Combi (2005) that analyzes the brightness variation in the coma in several consecutive images and models the dissociation time lag and travel time of H atoms in the coma to deconvolve the water production rate at the nucleus, enabling the times of outbursts and other short-term variations to be determined to the nearest day.

A summary of the SWAN observations of comets 12P and 13P and their orbital parameters are given in Table 1. In the remainder of this paper, the production rate uncertainties, $dQ$, are $1\sigma$ values from the fit of the variation of the model spatial distributions as scaled to the observation as well as the fitted interplanetary hydrogen background and noise in the data. Systematic uncertainties that result from the solar Ly$\alpha$ flux as obtained from LASP,$^5$ the calibration of SWAN itself, and the model parameters, combined with faint field stars not able to be explicitly detected, are on the order of up to ~30%.

## 2. 12P

Comet 12P was detectable with SWAN over a time period lasting 250 days. Figure 1 shows the water production rate ($Q$)

determined from each single image of 12P plotted as a function of time from perihelion in days. Table 2 gives the single-image water production rates as well as other observational parameters. These derived production rates essentially correspond to an average production rate over the previous 2–3 days due to the timescale of the H$_2$O and OH photodissociation process and the filling time of the H coma out to several single-degree field-of-view pixels. Transient events are similarly delayed. Figure 2 shows $Q$ plotted versus heliocentric distance. The straight lines show the two power-law exponents of $-2.5$ for preperihelion and $-2.9$ postperihelion, though the postperihelion value for most of the earlier data is closer to that same $-2.5$, the numerical value of the latter is influenced by the relatively large values at 1.9 au, and, through most of the postperihelion time, the exponent is similar to the preperiehelion one.

Figures 1 and 2 indicate the presence of several outbursts. The initial outburst from November 1 through November 16 seen by SWAN appears to correspond to the two outbursts beginning on November 1 followed by another on November 14. These are more clearly seen in the visual magnitude observations compiled by S. Yoshida (2024a). After this, the comet moved into the galactic equator, and water production rates could not be calculated from November 17 until early 2024 January. The comet could be seen in many of the images, but not enough of the coma could be distinguished from nearby stars, including bright stars such as $\alpha$ Lyr, to get an adequate coma separation for the modeling procedure to extract production rates.

One of the features of the TRM when the coma is sufficiently bright over an extended region and period of time is the ability to analyze multiple images together to deconvolve the H$_2$O production at the nucleus with 1 day resolution. The method accounts for the dissociation times of H$_2$O and OH as well as the propagation times of H atoms out to various distances in the coma. The veracity of the method has been successfully compared with other higher spatial resolution observations (M. R. Combi et al. 2005). The results of this are shown in Figure 3, where the dates of several outbursts have also been noted.

The beginning daily H$_2$O production rates from the deconvolved SWAN results show an almost continuous rise with only a small discontinuity, beginning on 2023 October 31 and continuing through November 16 with a peak on November 14. This smearing of two outbursts is caused by a combination of the dissociation times of the parent H$_2$O and OH producing H and the expansion of H atoms through the low spatial resolution images of SWAN, which are not able to be fully deconvolved by the method. The time/space smear of this early outburst is exacerbated by the even longer lifetimes of H$_2$O, OH, and H at heliocentric distances of greater than 2 au.

The dates of the start, peak, and end of each of eight outbursts found in the SWAN results are given in Table 3. From the water production rates during each outburst, the total mass of water released above the quiescent level was calculated and is given in the third column. The values range from $4.66 \times 10^8$ to $8.06 \times 10^9$ kg. Sixteen outbursts were cataloged by M. Gritsevich et al. (2025) combining their continuum magnitude observations with those of E. Jehin et al. (2023, 2024a, 2024b). Two of these generally correspond with two of those seen in the deconvolved SWAN results for the

<footnote>$^5$ https://lasp.colorado.edu/data/timed_see/composite_lya/version3/composite_lya.dat</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Water production rate of comet 12P as a function of time from perihelion. The open symbols give the water production rate per second from single images, where diamonds are preperihelion and squares are postperihelion. The error bars give the $1\sigma$ formal random fitting errors for each value. There is a potential ~30% uncertainty from the model parameters and calibration not included in the plotted error bars.</figcaption></figure>

peaks on January 30 and April 3. In their paper, Gritsevich et al. also show the results of their dust modeling of each outburst using three scenarios each of dust initial conditions with different dust porosities for 100% and 50% active areas giving six values total of the dust produced from each of their 16 outbursts driven by H$_2$O or CO$_2$. In all cases, the CO$_2$-driven outburst masses were larger than the H$_2$O-driven modeled outbursts by factors of a few.

Generally, the total dust masses from Gritsevich et al., even from their H$_2$O-driven outbursts, are much larger than the water results. Their various models for their February 2 outburst range from $1.813 \times 10^{10}$ to $2.636 \times 10^{11}$ kg, and those for their April 2.95 outburst range from $1.194 \times 10^{11}$ to $1.758 \times 10^{12}$ kg, where the water masses released were only $4.66 \times 10^8$ for the January 30 outburst and $1.35 \times 10^9$ kg for the April 3 outburst.

D. Jewitt & J. Luu (2025) reported the results of a study of seven outbursts in 12P from 2023 July 21 to 2024 January 18. Two of their noted outbursts, C and D, occurred during the period from November 1 to 15, where the SWAN results could not distinguish two separate events but the visual magnitudes from S. Yoshida (2024b) did. Because we could not recover the time of the return of water activity to the quiescent level, it was not possible to determine the water mass. All of their other

outbursts occurred before the SWAN observations or during times when SWAN could not get observations because of interference with bright background stars in our 1° pixels. However, it is worth noting that the range of dust masses released during all of their outbursts, from $1.0 \times 10^9$ to $6.8 \times 10^9$ km, are all more in the range of the water masses determined from the SWAN results, rather than any of those from Gritsevich et al. D. Jewitt & J. Luu (2025) address the issues in their dust modeling and those of Gritsevich et al.

The range of SWAN water production rates generally agrees rather well with several other observational datasets, so it is difficult to understand a dust mass outburst release of 3 or more orders of magnitude larger than the water released. This includes their H$_2$O-driven models, which are factors of 5 or more smaller than their CO$_2$-driven models.

The vast majority of the total water mass loss over the apparition can be estimated from the run of daily water production rates throughout the continuous coverage −90 to +102 days from perihelion. This includes most of the SWAN-identified outbursts. The value obtained is $4.4 \times 10^{10}$ kg, which puts the modeled outburst dust mass-loss rates from Gritsevich et al. even more into question.

Finally, from the single-image data (Figure 1), it appears that there was another outburst at about 90 days after
<table><caption>**Table 2**
SOHO/SWAN Observations of 12P and Water Production Rates</caption><thead><tr><th>ΔT<br>(days)</th><th>R<br>(au)</th><th>Δ<br>(au)</th><th>g<br>(s<sup>-1</sup>)</th><th>Q<br>(10<sup>28</sup> s<sup>-1</sup>)</th><th>δQ<br>(10<sup>28</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>-171.804</td><td>2.747</td><td>2.883</td><td>0.002399</td><td>2.59</td><td>0.26</td></tr><tr><td>-170.804</td><td>2.735</td><td>2.874</td><td>0.002387</td><td>2.58</td><td>0.27</td></tr><tr><td>-169.804</td><td>2.723</td><td>2.866</td><td>0.002431</td><td>2.85</td><td>0.15</td></tr><tr><td>-167.804</td><td>2.699</td><td>2.849</td><td>0.002506</td><td>4.75</td><td>0.13</td></tr><tr><td>-166.804</td><td>2.687</td><td>2.840</td><td>0.002511</td><td>4.40</td><td>0.15</td></tr><tr><td>-165.804</td><td>2.676</td><td>2.832</td><td>0.002459</td><td>3.10</td><td>0.21</td></tr><tr><td>-162.804</td><td>2.640</td><td>2.805</td><td>0.002315</td><td>4.02</td><td>0.18</td></tr><tr><td>-159.811</td><td>2.604</td><td>2.778</td><td>0.002304</td><td>12.21</td><td>0.07</td></tr><tr><td>-158.811</td><td>2.592</td><td>2.769</td><td>0.002244</td><td>10.52</td><td>0.07</td></tr><tr><td>-157.810</td><td>2.580</td><td>2.760</td><td>0.002183</td><td>5.56</td><td>0.16</td></tr><tr><td>-156.811</td><td>2.568</td><td>2.750</td><td>0.002122</td><td>8.42</td><td>0.10</td></tr><tr><td>-155.804</td><td>2.556</td><td>2.741</td><td>0.002118</td><td>30.66</td><td>0.11</td></tr><tr><td>-154.811</td><td>2.544</td><td>2.731</td><td>0.002110</td><td>12.01</td><td>0.11</td></tr><tr><td>-153.810</td><td>2.532</td><td>2.722</td><td>0.002093</td><td>22.70</td><td>0.04</td></tr><tr><td>-152.810</td><td>2.520</td><td>2.712</td><td>0.002142</td><td>18.49</td><td>0.05</td></tr><tr><td>-151.810</td><td>2.508</td><td>2.702</td><td>0.002223</td><td>19.25</td><td>0.04</td></tr><tr><td>-150.810</td><td>2.496</td><td>2.693</td><td>0.002475</td><td>19.22</td><td>0.04</td></tr><tr><td>-149.810</td><td>2.483</td><td>2.683</td><td>0.002555</td><td>17.52</td><td>0.04</td></tr><tr><td>-148.810</td><td>2.471</td><td>2.673</td><td>0.002565</td><td>13.77</td><td>0.05</td></tr><tr><td>-147.810</td><td>2.459</td><td>2.662</td><td>0.002597</td><td>26.81</td><td>0.03</td></tr><tr><td>-146.810</td><td>2.447</td><td>2.653</td><td>0.002625</td><td>14.81</td><td>0.04</td></tr><tr><td>-145.810</td><td>2.435</td><td>2.643</td><td>0.002612</td><td>16.28</td><td>0.05</td></tr><tr><td>-111.579</td><td>2.005</td><td>2.264</td><td>0.002257</td><td>8.93</td><td>0.13</td></tr><tr><td>-110.579</td><td>1.992</td><td>2.252</td><td>0.002280</td><td>9.88</td><td>0.14</td></tr><tr><td>-109.579</td><td>1.979</td><td>2.241</td><td>0.002329</td><td>11.25</td><td>0.12</td></tr><tr><td>-108.579</td><td>1.966</td><td>2.229</td><td>0.002365</td><td>16.11</td><td>0.11</td></tr><tr><td>-92.480</td><td>1.756</td><td>2.047</td><td>0.002604</td><td>5.23</td><td>0.19</td></tr><tr><td>-91.480</td><td>1.743</td><td>2.036</td><td>0.002565</td><td>5.59</td><td>0.17</td></tr><tr><td>-90.480</td><td>1.730</td><td>2.025</td><td>0.002492</td><td>4.47</td><td>0.21</td></tr><tr><td>-89.475</td><td>1.717</td><td>2.014</td><td>0.002403</td><td>5.80</td><td>0.19</td></tr><tr><td>-88.475</td><td>1.703</td><td>2.003</td><td>0.002310</td><td>6.53</td><td>0.22</td></tr><tr><td>-85.475</td><td>1.664</td><td>1.971</td><td>0.002297</td><td>6.51</td><td>0.19</td></tr><tr><td>-83.475</td><td>1.637</td><td>1.950</td><td>0.002331</td><td>13.18</td><td>0.09</td></tr><tr><td>-82.475</td><td>1.624</td><td>1.940</td><td>0.002363</td><td>12.05</td><td>0.09</td></tr><tr><td>-81.451</td><td>1.610</td><td>1.930</td><td>0.002364</td><td>12.46</td><td>0.09</td></tr><tr><td>-80.451</td><td>1.597</td><td>1.920</td><td>0.002455</td><td>10.95</td><td>0.10</td></tr><tr><td>-79.451</td><td>1.584</td><td>1.910</td><td>0.002458</td><td>21.22</td><td>0.09</td></tr><tr><td>-78.451</td><td>1.571</td><td>1.900</td><td>0.002454</td><td>10.72</td><td>0.14</td></tr><tr><td>-77.451</td><td>1.557</td><td>1.890</td><td>0.002498</td><td>9.19</td><td>0.12</td></tr><tr><td>-76.446</td><td>1.544</td><td>1.881</td><td>0.002480</td><td>8.96</td><td>0.09</td></tr><tr><td>-75.446</td><td>1.531</td><td>1.871</td><td>0.002507</td><td>8.08</td><td>0.10</td></tr><tr><td>-71.446</td><td>1.477</td><td>1.835</td><td>0.002568</td><td>7.45</td><td>0.10</td></tr><tr><td>-70.423</td><td>1.464</td><td>1.826</td><td>0.002581</td><td>6.84</td><td>0.10</td></tr><tr><td>-69.423</td><td>1.451</td><td>1.817</td><td>0.002553</td><td>7.55</td><td>0.10</td></tr><tr><td>-68.423</td><td>1.437</td><td>1.808</td><td>0.002409</td><td>9.58</td><td>0.09</td></tr><tr><td>-67.423</td><td>1.424</td><td>1.800</td><td>0.002387</td><td>10.92</td><td>0.07</td></tr><tr><td>-66.423</td><td>1.411</td><td>1.792</td><td>0.002354</td><td>14.00</td><td>0.06</td></tr><tr><td>-58.394</td><td>1.305</td><td>1.733</td><td>0.002303</td><td>14.78</td><td>0.07</td></tr><tr><td>-57.394</td><td>1.292</td><td>1.726</td><td>0.002306</td><td>12.94</td><td>0.07</td></tr><tr><td>-56.388</td><td>1.279</td><td>1.719</td><td>0.002309</td><td>12.82</td><td>0.07</td></tr><tr><td>-55.388</td><td>1.266</td><td>1.713</td><td>0.002322</td><td>12.47</td><td>0.06</td></tr><tr><td>-54.388</td><td>1.253</td><td>1.707</td><td>0.002346</td><td>14.61</td><td>0.06</td></tr><tr><td>-53.365</td><td>1.240</td><td>1.701</td><td>0.002366</td><td>16.81</td><td>0.05</td></tr><tr><td>-52.365</td><td>1.227</td><td>1.696</td><td>0.002282</td><td>18.09</td><td>0.06</td></tr><tr><td>-51.365</td><td>1.214</td><td>1.690</td><td>0.002308</td><td>16.25</td><td>0.06</td></tr><tr><td>-50.365</td><td>1.201</td><td>1.685</td><td>0.002342</td><td>17.53</td><td>0.06</td></tr><tr><td>-49.359</td><td>1.188</td><td>1.680</td><td>0.002396</td><td>18.49</td><td>0.04</td></tr><tr><td>-48.359</td><td>1.176</td><td>1.675</td><td>0.002375</td><td>20.02</td><td>0.05</td></tr><tr><td>-47.359</td><td>1.163</td><td>1.670</td><td>0.002365</td><td>13.33</td><td>0.09</td></tr><tr><td>-46.336</td><td>1.150</td><td>1.666</td><td>0.002373</td><td>19.43</td><td>0.04</td></tr><tr><td>-45.336</td><td>1.138</td><td>1.661</td><td>0.002329</td><td>17.90</td><td>0.05</td></tr><tr><td>-44.336</td><td>1.126</td><td>1.657</td><td>0.002260</td><td>19.37</td><td>0.07</td></tr><tr><td>-43.336</td><td>1.113</td><td>1.654</td><td>0.002258</td><td>21.14</td><td>0.07</td></tr><tr><td>-38.308</td><td>1.053</td><td>1.637</td><td>0.002448</td><td>27.44</td><td>0.02</td></tr><tr><td>-35.302</td><td>1.018</td><td>1.629</td><td>0.002364</td><td>29.84</td><td>0.04</td></tr><tr><td>-33.210</td><td>0.995</td><td>1.625</td><td>0.002266</td><td>60.29</td><td>0.02</td></tr><tr><td>-31.215</td><td>0.973</td><td>1.621</td><td>0.002182</td><td>63.69</td><td>0.02</td></tr><tr><td>-30.215</td><td>0.963</td><td>1.620</td><td>0.002145</td><td>76.76</td><td>0.02</td></tr><tr><td>-29.215</td><td>0.952</td><td>1.618</td><td>0.002097</td><td>67.84</td><td>0.02</td></tr><tr><td>-28.238</td><td>0.942</td><td>1.617</td><td>0.002104</td><td>73.50</td><td>0.02</td></tr><tr><td>-27.238</td><td>0.932</td><td>1.616</td><td>0.002121</td><td>71.30</td><td>0.02</td></tr><tr><td>-26.243</td><td>0.923</td><td>1.615</td><td>0.002126</td><td>84.10</td><td>0.03</td></tr><tr><td>-25.243</td><td>0.913</td><td>1.614</td><td>0.002163</td><td>76.63</td><td>0.02</td></tr><tr><td>-24.243</td><td>0.904</td><td>1.613</td><td>0.002191</td><td>95.40</td><td>0.02</td></tr><tr><td>-23.243</td><td>0.895</td><td>1.612</td><td>0.002153</td><td>91.20</td><td>0.02</td></tr><tr><td>-22.268</td><td>0.886</td><td>1.612</td><td>0.002127</td><td>91.16</td><td>0.02</td></tr></tbody></table>

<table><caption>**Table 2**
(Continued)</caption><thead><tr><th>ΔT<br>(days)</th><th>R<br>(au)</th><th>Δ<br>(au)</th><th>g<br>(s<sup>-1</sup>)</th><th>Q<br>(10<sup>28</sup> s<sup>-1</sup>)</th><th>δQ<br>(10<sup>28</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>-21.268</td><td>0.877</td><td>1.611</td><td>0.002113</td><td>91.62</td><td>0.02</td></tr><tr><td>-20.268</td><td>0.869</td><td>1.611</td><td>0.002189</td><td>108.60</td><td>0.02</td></tr><tr><td>-19.268</td><td>0.861</td><td>1.610</td><td>0.002246</td><td>92.96</td><td>0.02</td></tr><tr><td>-18.269</td><td>0.854</td><td>1.610</td><td>0.002271</td><td>108.90</td><td>0.02</td></tr><tr><td>-16.325</td><td>0.840</td><td>1.609</td><td>0.002358</td><td>120.30</td><td>0.02</td></tr><tr><td>-15.325</td><td>0.833</td><td>1.609</td><td>0.002392</td><td>116.50</td><td>0.01</td></tr><tr><td>-14.325</td><td>0.827</td><td>1.609</td><td>0.002353</td><td>132.10</td><td>0.01</td></tr><tr><td>-13.325</td><td>0.821</td><td>1.609</td><td>0.002339</td><td>154.40</td><td>0.02</td></tr><tr><td>-11.325</td><td>0.810</td><td>1.608</td><td>0.002268</td><td>120.80</td><td>0.01</td></tr><tr><td>-10.325</td><td>0.805</td><td>1.608</td><td>0.002112</td><td>150.10</td><td>0.01</td></tr><tr><td>-9.325</td><td>0.801</td><td>1.608</td><td>0.002047</td><td>151.70</td><td>0.01</td></tr><tr><td>-8.325</td><td>0.797</td><td>1.608</td><td>0.001990</td><td>159.40</td><td>0.01</td></tr><tr><td>-7.307</td><td>0.793</td><td>1.607</td><td>0.001942</td><td>169.50</td><td>0.02</td></tr><tr><td>-4.306</td><td>0.785</td><td>1.606</td><td>0.001952</td><td>111.40</td><td>0.01</td></tr><tr><td>-3.306</td><td>0.783</td><td>1.606</td><td>0.001983</td><td>81.54</td><td>0.03</td></tr><tr><td>-2.306</td><td>0.782</td><td>1.605</td><td>0.002009</td><td>75.69</td><td>0.03</td></tr><tr><td>-1.286</td><td>0.781</td><td>1.604</td><td>0.002005</td><td>73.69</td><td>0.03</td></tr><tr><td>-0.286</td><td>0.781</td><td>1.604</td><td>0.002007</td><td>64.64</td><td>0.04</td></tr><tr><td>0.715</td><td>0.781</td><td>1.603</td><td>0.002027</td><td>58.85</td><td>0.04</td></tr><tr><td>1.715</td><td>0.782</td><td>1.602</td><td>0.002055</td><td>58.35</td><td>0.04</td></tr><tr><td>2.715</td><td>0.783</td><td>1.601</td><td>0.002137</td><td>51.92</td><td>0.03</td></tr><tr><td>3.740</td><td>0.784</td><td>1.600</td><td>0.002259</td><td>49.67</td><td>0.04</td></tr><tr><td>4.740</td><td>0.786</td><td>1.599</td><td>0.002232</td><td>50.08</td><td>0.04</td></tr><tr><td>5.740</td><td>0.788</td><td>1.598</td><td>0.002352</td><td>48.61</td><td>0.03</td></tr><tr><td>6.743</td><td>0.791</td><td>1.597</td><td>0.002360</td><td>46.74</td><td>0.03</td></tr><tr><td>7.743</td><td>0.795</td><td>1.595</td><td>0.002352</td><td>49.28</td><td>0.04</td></tr><tr><td>8.743</td><td>0.798</td><td>1.594</td><td>0.002411</td><td>36.79</td><td>0.04</td></tr><tr><td>10.770</td><td>0.807</td><td>1.591</td><td>0.002379</td><td>35.12</td><td>0.04</td></tr><tr><td>11.772</td><td>0.812</td><td>1.589</td><td>0.002245</td><td>35.23</td><td>0.04</td></tr><tr><td>12.772</td><td>0.817</td><td>1.587</td><td>0.002142</td><td>35.20</td><td>0.07</td></tr><tr><td>13.772</td><td>0.823</td><td>1.585</td><td>0.002003</td><td>47.74</td><td>0.05</td></tr><tr><td>14.798</td><td>0.830</td><td>1.584</td><td>0.001958</td><td>52.45</td><td>0.04</td></tr><tr><td>15.798</td><td>0.836</td><td>1.581</td><td>0.001948</td><td>57.35</td><td>0.04</td></tr><tr><td>16.798</td><td>0.843</td><td>1.579</td><td>0.001956</td><td>45.94</td><td>0.04</td></tr><tr><td>17.801</td><td>0.850</td><td>1.577</td><td>0.002010</td><td>47.73</td><td>0.04</td></tr><tr><td>18.746</td><td>0.857</td><td>1.576</td><td>0.002047</td><td>59.23</td><td>0.03</td></tr><tr><td>19.746</td><td>0.865</td><td>1.574</td><td>0.002099</td><td>60.66</td><td>0.03</td></tr><tr><td>20.772</td><td>0.873</td><td>1.572</td><td>0.002146</td><td>38.77</td><td>0.04</td></tr><tr><td>21.772</td><td>0.882</td><td>1.570</td><td>0.002160</td><td>36.04</td><td>0.05</td></tr><tr><td>22.772</td><td>0.890</td><td>1.568</td><td>0.002206</td><td>25.50</td><td>0.04</td></tr><tr><td>23.775</td><td>0.899</td><td>1.566</td><td>0.002222</td><td>25.96</td><td>0.04</td></tr><tr><td>24.774</td><td>0.909</td><td>1.564</td><td>0.002302</td><td>18.97</td><td>0.05</td></tr><tr><td>25.775</td><td>0.918</td><td>1.562</td><td>0.002367</td><td>20.47</td><td>0.05</td></tr><tr><td>26.802</td><td>0.928</td><td>1.560</td><td>0.001957</td><td>22.40</td><td>0.06</td></tr><tr><td>27.802</td><td>0.938</td><td>1.558</td><td>0.002499</td><td>25.37</td><td>0.05</td></tr><tr><td>28.802</td><td>0.948</td><td>1.557</td><td>0.002549</td><td>17.94</td><td>0.05</td></tr><tr><td>29.802</td><td>0.958</td><td>1.555</td><td>0.002547</td><td>17.78</td><td>0.04</td></tr><tr><td>35.830</td><td>1.024</td><td>1.547</td><td>0.002330</td><td>12.62</td><td>0.06</td></tr><tr><td>36.833</td><td>1.036</td><td>1.546</td><td>0.002219</td><td>10.26</td><td>0.08</td></tr><tr><td>37.832</td><td>1.047</td><td>1.546</td><td>0.002132</td><td>13.29</td><td>0.06</td></tr><tr><td>38.833</td><td>1.059</td><td>1.545</td><td>0.002132</td><td>12.62</td><td>0.06</td></tr><tr><td>39.833</td><td>1.071</td><td>1.544</td><td>0.002131</td><td>13.54</td><td>0.06</td></tr><tr><td>40.859</td><td>1.083</td><td>1.544</td><td>0.002193</td><td>10.35</td><td>0.06</td></tr><tr><td>41.858</td><td>1.095</td><td>1.544</td><td>0.002213</td><td>8.84</td><td>0.07</td></tr><tr><td>42.859</td><td>1.107</td><td>1.544</td><td>0.002296</td><td>8.11</td><td>0.07</td></tr><tr><td>43.861</td><td>1.120</td><td>1.545</td><td>0.002354</td><td>9.22</td><td>0.06</td></tr><tr><td>44.861</td><td>1.132</td><td>1.546</td><td>0.002365</td><td>8.10</td><td>0.09</td></tr><tr><td>45.861</td><td>1.144</td><td>1.547</td><td>0.002410</td><td>8.76</td><td>0.08</td></tr><tr><td>46.861</td><td>1.157</td><td>1.549</td><td>0.002391</td><td>6.28</td><td>0.10</td></tr><tr><td>48.888</td><td>1.182</td><td>1.551</td><td>0.002474</td><td>7.01</td><td>0.10</td></tr><tr><td>49.888</td><td>1.195</td><td>1.553</td><td>0.002426</td><td>8.65</td><td>0.09</td></tr><tr><td>50.888</td><td>1.208</td><td>1.560</td><td>0.002419</td><td>5.96</td><td>0.09</td></tr><tr><td>53.891</td><td>1.246</td><td>1.860</td><td>0.002458</td><td>6.98</td><td>0.08</td></tr><tr><td>90.431</td><td>1.729</td><td>1.874</td><td>0.002537</td><td>1.80</td><td>0.34</td></tr><tr><td>91.431</td><td>1.742</td><td>1.888</td><td>0.002506</td><td>2.65</td><td>0.24</td></tr><tr><td>92.431</td><td>1.756</td><td>1.977</td><td>0.002476</td><td>4.01</td><td>0.26</td></tr><tr><td>98.431</td><td>1.834</td><td>1.993</td><td>0.002524</td><td>6.62</td><td>0.11</td></tr><tr><td>99.451</td><td>1.848</td><td>1.546</td><td>0.002528</td><td>3.01</td><td>0.24</td></tr><tr><td>100.451</td><td>1.861</td><td>2.009</td><td>0.002502</td><td>5.03</td><td>0.18</td></tr></tbody></table>

Notes. ΔT: days from perihelion, 21.12 April 2024.

r: heliocentric distance (au).

Δ: comet-SOHO distance (au).

g: solar Lyα g-factor (photons s⁻¹) at 1 au.

Q: water production rates for each image (s⁻¹).

δQ: internal 1σ uncertainties.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Water production rate in comet 12P as a function of heliocentric distance, with preperihelion to the left and postperihelion to the right. Crosses give the water production rate per second from single images. The error bars give the $1\sigma$ formal random fitting errors for each value. There is an additional ~30% uncertainty from the model parameters and calibration. The solid lines give the best-fit power laws with slopes of -2.5 and -2.9 pre- and postperihelion, respectively.</figcaption></figure>

perihelion, or about 2024 July 20, which has not been reported in other observations to date or seen in the visual magnitudes. The quality of the extended coma in the original images did not yield acceptable uncertainties for the deconvolved daily values to be obtained during this period.

Given the similarities of comets 12P and 1P/Halley, having orbital periods in the range of 70–75 yr and generally similar levels of water production normalized to similar heliocentric distances, it is interesting to compare these rates and consider similarities and differences. 1P/Halley admittedly had larger peak production rates owing to its much closer perihelion distance (0.59 au) compared with 12P (0.78 au). As mentioned already, Q. Ye et al. (2020) estimated the mean radius of the nucleus of 12P to be 17 ± 6 km, which is 3 times the mean radius of 1P at 5.5 km (H. U. Keller et al. 1987). However, as already mentioned, they caution that the contribution of the dust coma to the total IR magnitude is highly uncertain, so this could very well be overestimated. In fact, a size comparable to that of Halley cannot be ruled out (Q. Ye 2025, private communication).

The question of the size of 12P's nucleus has been discussed by D. Jewitt & J. Luu (2025) and J. Li et al. (2025). Li et al. calculated that based on OH radio observations on active areas, the minimum size for the nucleus would correspond to a nucleus of radius 2.22 km, which agrees rather well with the minimum active area from the SWAN observations discussed below. Jewitt & Luu argued for a nucleus radius of about 5.6 km based on the total power required to produce the mass and velocity of one of their dust outbursts (D). This is nearly identical to the radius of 1P/Halley.

The H₂O production rates at 1 au in 12P were $4.7 \times 10^{29}$ and $1.52 \times 10^{29} \text{ s}^{-1}$ averaging closest preperihelion and postperihelion pairs of values, respectively. Those for comet 1P/Halley using comparable coma models (M. R. Combi et al. (1993) were $3.6 \times 10^{29}$ and $3.4 \times 10^{29} \text{ s}^{-1}$, respectively, in the

range of 0.45–1.30 times that of 12P, or roughly comparable. Therefore, it is most likely that the active fraction of about 10% is also roughly comparable to that of 1P/Halley. If the actual radius were 17 km, it would make the surface area of 12P 9 times that of 1P/Halley and the active fraction very small indeed.

A number of observations of the H₂O production rate have been reported in the literature and provided in private communications. The H₂O production rate just after the peak of the November 14 outburst of $1.63 \times 10^{29} \text{ s}^{-1}$ is in excellent agreement with the OH production rate (1.41 ± 0.33) × $10^{29} \text{ s}^{-1}$ reported by E. Jehin et al. (2024a) from Trappist-North observations on November 15 assuming a 0.82 yield of OH from H₂O (M. R. Combi et al. 2004). Two and three days later, L. Ferellec et al. (2024) reported much lower OH production rates from ground-based long-slit optical observations of OH of (0.571 ± 0.060) × $10^{29}$ and (0.535 ± 0.055) × $10^{29} \text{ s}^{-1}$ with different instrumentation and aperture size. As shown in Figure 3, it is possible that the difference between the ground-based filter photometry OH observations of E. Jehin et al. (2024a) and those of L. Ferellec et al. (2024) is actually that the latter follows the post–November 14 outburst peak water production, returning back down to a nominal level below $6.5 \times 10^{29} \text{ s}^{-1}$. E. Jehin et al. (2024b) reported a production rate on 2024 February 25 of (1.54 ± 0.43) × $10^{29} \text{ s}^{-1}$ for OH, which is also consistent with our water production rate on that day to well within the expected uncertainties.

There have been other observations that provide water production rates in 12P. M. A. Cordiner et al. (2025) derived a water production rate from Atacama Large Millimeter/ submillimeter Array (ALMA) observations on 2024 May 4 of (4.17 ± 0.09) × $10^{29} \text{ s}^{-1}$, which is excellent agreement with the SWAN results of $4.78 \times 10^{29} \text{ s}^{-1}$ from the single-image value and $4.93 \times 10^{29} \text{ s}^{-1}$ from the daily deconvolved value.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Water production rate of comet 12P as a function of time from perihelion in days deconvolved to the production time of water at the nucleus. The triangles give the water production rate per second. The error bars give the $1\sigma$ fitting errors for each value, which are much larger because of the deconvolution procedure. There is also an overall ~30% uncertainty from the model parameters and calibration. The peaks of eight individual outbursts, determined to the nearest day, are noted. The durations of the outbursts are given in Table 3. The filled squares are values from IR observations by E. Gibb et al. (2025, private communication), and the filled diamond is from ALMA observations by M. Cordiner et al. (2025, private communication). The crosses are from J. Li et al. (2025). The filled circles are from E. Jehin et al. (2024a, 2024b). The filled triangles are from L. Ferellec et al. (2024).</figcaption></figure>

<table><caption>**Table 3**
Dates and Durations of Outbursts in Comet 12P</caption><thead><tr><th></th><th>Outburst<br>Start Date</th><th>Outburst<br>Peak Date</th><th>Outburst<br>End Date</th><th>Water Mass<br>(kg)</th></tr></thead><tbody><tr><td>1</td><td>Oct 31</td><td>Nov 14</td><td>&gt;Nov 16<sup>a</sup></td><td>...</td></tr><tr><td>2</td><td>Dec 31</td><td>&gt;Jan 5<sup>a</sup></td><td>&gt;Jan 5<sup>a</sup></td><td>...</td></tr><tr><td>3</td><td>Jan 24</td><td>Jan 30</td><td>Feb 1</td><td>4.66 × 10<sup>8</sup></td></tr><tr><td>4</td><td>Mar 27</td><td>Mar 29</td><td>Mar 31</td><td>1.21 × 10<sup>9</sup></td></tr><tr><td>5</td><td>Mar 31</td><td>Apr 3</td><td>Apr 5</td><td>1.35 × 10<sup>9</sup></td></tr><tr><td>6</td><td>Apr 8</td><td>Apr 10</td><td>Apr 12</td><td>4.25 × 10<sup>9</sup></td></tr><tr><td>7</td><td>Apr 30</td><td>May 2</td><td>May 9</td><td>3.20 × 10<sup>9</sup></td></tr><tr><td>8</td><td>May 23</td><td>May 30</td><td>May 31</td><td>8.06 × 10<sup>9</sup></td></tr></tbody></table>

**Note.**

J. Li et al. (2025) observed radio OH emissions in 12P in a large beam of 11.6. Finally, E. Gibb et al. (2025, private communication) have determined water production rates from mid-IR observations with the NASA IRTF telescope on Maunakea using long-slit spectrophometry. While these follow the general trend of the other results, a few of them are somewhat larger. All of these results are plotted with the deconvolved daily average water production rates in Figure 3.

In order to examine the nature of the outburst activity, Figure 4 shows a plot of the active area as a function of time from perihelion calculated using the typical water-dominated fast rotator model of J. J. Cowan & M. F. A'Hearn (1979) using the single-image production rates. Also shown on the plot are the times of the peaks of the eight outbursts identified in Figure 3 as well as three horizontal lines corresponding to three nucleus surface area sizes that are 100% active. Quite clearly, with an active area varying by nearly 2 orders of magnitude, the outbursts are accompanied by the release of many icy grains/chunks that sublimate away over time after

<footnote><sup>a</sup> The outburst starting on October 31 has a peak on November 14, but the data end on November 16, so the end is unknown, and the mass cannot be calculated. For the outburst starting on December 31, neither the peak nor the end are known for a mass be calculated.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Active area of comet 12P as a function of time in days from perihelion. The water production rate gives the active area in square kilometers assuming the fast rotator model of J. J. Cowan & M. F. A'Hearn (1979). The short vertical lines show the peak times of the numbered outbursts shown in Figure 3 and given in Table 3. The lower solid horizontal line is the active area for a nucleus with a radius of 1.26 km, which would correspond to the smallest possible 100% active nucleus radius. The upper horizontal line corresponds to a 100% active nucleus of 5.6 km radius (D. Jewitt & J. Luu 2025). The middle solid horizontal line gives the calculated active area for a 100% active nucleus of radius 2.2 km from the early preperihelion activity given as the minimum possible size by J. Li et al. (2025).</figcaption></figure>

each event. Such behavior has been seen in past outbursting comets. The minimum active area of ~20 km² is reached at around 50 days after perihelion and then again before and after the July 20 outburst called out in the single-image production rates. This would correspond to a 100% active 2.22 km radius nucleus (J. Li et al. 2025) or, more likely, a ~15% active nucleus of radius 5.6 km (D. Jewitt & J. Luu 2025), which, as discussed earlier, would make the nucleus of 12P similar to that of 1P/Halley.

Note that for most of the apparition, the activity of 12P is well above this minimum active area. This would imply that in addition to the release of icy particles during outburst events, particles are likely being released at some level throughout the times between outbursts. Such activity seems to be strongest in the month leading up to and including the peak production rate 8 days before perihelion, where outbursts 4, 5, and 6 are accompanied by a large and continuous release of icy particles.

## 3. Comet 13P

The results of the derived single-image water production rates are shown in Figure 5 and given in Table 4. Oftentimes the comet was in regions with many faint background stars that

could not be singled out and identified so they could be effectively subtracted. This essentially puts extra sources both within the observed coma and near it and contributes to the wide scatter in the results well beyond the formal $1\sigma$ errors determined from the fitting of the coma model plus IPM background to the SWAN images. The preperihelion results give a heliocentric distance dependence that is basically flat, whereas postperihelion, the value decreases with increasing heliocentric distance with a weak, very rough exponent of only -0.7 as is shown in Figure 6.

E. Jehin et al. (2024b) reported a production rate for OH of (1.62 ± 0.79) × 10²⁸ s⁻¹ on 2024 July 9, when the H₂O production rate from SWAN was found to be (3.39 ± 0.35) × 10²⁸ s⁻¹. This is somewhat larger than the 2 × 10²⁸ s⁻¹ that would be expected based on the OH measurement. The only other publicly reported observation by Jehin's group preceded the SWAN observations. D. Schleicher (2025, private communication), on the other hand, has reported a water production rate of 5.75 × 10²⁸ s⁻¹ on 2024 July 30 when SWAN finds a value of (6.82 ± 0.27) × 10²⁸ s⁻¹, which is quite reasonable agreement, especially considering the scatter in the SWAN results beyond the 1σ model fitting analysis uncertainties. Schleicher also
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Water production rate of comet 13P as a function of time from perihelion. The diamonds give the water production rate per second from single images. The error bars give the $1\sigma$ formal random fitting errors for each value. There is a potential ~30% uncertainty from the model parameters and calibration.</figcaption></figure>

reports a value from ground-based optical range filter photo-metric observations of $4.47 \times 10^{28} \text{ s}^{-1}$ on 2024 August 27 when SWAN finds a value of $(5.02 \pm 0.38) \times 10^{28} \text{ s}^{-1}$, which is also in quite reasonable agreement.

Because of the large scatter in the SWAN-derived water production rates, it is difficult to make a direct comparison of the variation of water production with the optical lightcurve. Over the time period covered by the SWAN observations, the visual magnitude varied from magnitude 8 to magnitude 6 at perihelion and back to magnitude 8 after perihelion. During this time, the heliocentric distance changed from 1.5 to 1.17 au at perihelion and back to 1.5 au after. The variation of visual magnitude from 8 to 6 corresponds to a factor of about 6, whereas illumination varying as the inverse square of the heliocentric distance would give a factor of only 1.65. However, the scatter of the water production rate is extremely large, with a scatter of nearly a factor of 8, and unfortunately probably masks the actual variation over the time covered.

## 4. Summary Discussion

The SOHO/SWAN camera images the entire sky in the far-UV emission of H Lyα nearly every day with a 1° scale resolution from the position of SOHO in a halo orbit around the L1 Lagrange point. As a result, images of the Lyα comae

of comets 12P and 13P were obtained and analyzed for many months from the last 2 months of 2023 and well into 2024, yielding water production rates on 145 and 100 days, respectively. There were the expected gaps in the coverage owing to bright stars when the comets were in the galactic equator. Because it was much fainter and less productive, there was considerable scatter in the water production rates for 13P, but in any case, it varied as expected with heliocentric distance, and the SWAN results were generally consistent with water production rates inferred from other observations.

Because it was much brighter and more productive, there was much less scatter in the results for 12P. A maximum water production rate a few days before perihelion was $2 \times 10^{30} \text{ s}^{-1}$. The baseline production rate between 0.78 and 2.74 au was consistent with water-sublimation-driven activity having exponential slopes of -2.5 and -2.9 over that range before and after perihelion, respectively. The baseline water production variation was, however, interrupted by a number of outbursts, of which eight were found in the SWAN results, yielding water mass ejections ranging from $4.66 \times 10^8$ to $8.06 \times 10^9 \text{ kg}$. These water mass ejections are generally comparable to, though not coincident with, the dust ones found by D. Jewitt & J. Luu (2025).
<table><caption>**Table 4**
SOHO/SWAN Observations of 13P and Water Production Rates</caption><thead><tr><th>ΔT (days)</th><th>r (au)</th><th>Δ (au)</th><th>g (s<sup>-1</sup>)</th><th>Q (10<sup>28</sup> s<sup>-1</sup>)</th><th>δQ (10<sup>28</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>-64.233</td><td>1.516</td><td>2.262</td><td>0.002384</td><td>6.39</td><td>0.10</td></tr><tr><td>-63.233</td><td>1.507</td><td>2.258</td><td>0.002347</td><td>4.55</td><td>0.17</td></tr><tr><td>-62.233</td><td>1.498</td><td>2.254</td><td>0.002548</td><td>2.22</td><td>0.25</td></tr><tr><td>-61.233</td><td>1.489</td><td>2.250</td><td>0.002560</td><td>2.18</td><td>0.28</td></tr><tr><td>-60.233</td><td>1.481</td><td>2.246</td><td>0.002549</td><td>2.81</td><td>0.19</td></tr><tr><td>-59.233</td><td>1.472</td><td>2.242</td><td>0.002582</td><td>2.65</td><td>0.22</td></tr><tr><td>-58.233</td><td>1.463</td><td>2.238</td><td>0.002599</td><td>4.72</td><td>0.11</td></tr><tr><td>-57.233</td><td>1.455</td><td>2.234</td><td>0.002603</td><td>4.63</td><td>0.12</td></tr><tr><td>-56.233</td><td>1.446</td><td>2.229</td><td>0.002553</td><td>3.32</td><td>0.18</td></tr><tr><td>-55.233</td><td>1.438</td><td>2.225</td><td>0.002416</td><td>3.45</td><td>0.20</td></tr><tr><td>-53.233</td><td>1.421</td><td>2.216</td><td>0.002210</td><td>4.62</td><td>0.18</td></tr><tr><td>-52.233</td><td>1.413</td><td>2.211</td><td>0.002138</td><td>4.71</td><td>0.17</td></tr><tr><td>-51.289</td><td>1.406</td><td>2.207</td><td>0.002085</td><td>7.22</td><td>0.13</td></tr><tr><td>-49.231</td><td>1.390</td><td>2.197</td><td>0.002058</td><td>12.21</td><td>0.08</td></tr><tr><td>-48.231</td><td>1.382</td><td>2.192</td><td>0.002092</td><td>7.44</td><td>0.16</td></tr><tr><td>-38.230</td><td>1.310</td><td>2.139</td><td>0.002352</td><td>9.04</td><td>0.10</td></tr><tr><td>-37.230</td><td>1.304</td><td>2.133</td><td>0.002053</td><td>6.70</td><td>0.17</td></tr><tr><td>-36.206</td><td>1.297</td><td>2.127</td><td>0.002447</td><td>5.14</td><td>0.20</td></tr><tr><td>-35.206</td><td>1.291</td><td>2.122</td><td>0.002475</td><td>4.73</td><td>0.19</td></tr><tr><td>-29.206</td><td>1.257</td><td>2.087</td><td>0.002316</td><td>6.91</td><td>0.15</td></tr><tr><td>-28.202</td><td>1.251</td><td>2.082</td><td>0.002225</td><td>8.29</td><td>0.09</td></tr><tr><td>-27.202</td><td>1.246</td><td>2.076</td><td>0.002134</td><td>11.07</td><td>0.10</td></tr><tr><td>-26.202</td><td>1.241</td><td>2.070</td><td>0.002097</td><td>8.60</td><td>0.11</td></tr><tr><td>-25.202</td><td>1.237</td><td>2.064</td><td>0.002055</td><td>4.91</td><td>0.25</td></tr><tr><td>-24.202</td><td>1.232</td><td>2.058</td><td>0.002051</td><td>6.69</td><td>0.27</td></tr><tr><td>-23.202</td><td>1.228</td><td>2.053</td><td>0.002046</td><td>4.92</td><td>0.36</td></tr><tr><td>-22.176</td><td>1.223</td><td>2.047</td><td>0.002096</td><td>3.90</td><td>0.43</td></tr><tr><td>-20.176</td><td>1.215</td><td>2.035</td><td>0.002190</td><td>3.05</td><td>0.36</td></tr><tr><td>-17.176</td><td>1.204</td><td>2.018</td><td>0.002283</td><td>6.50</td><td>0.16</td></tr><tr><td>-11.301</td><td>1.188</td><td>1.987</td><td>0.002227</td><td>6.19</td><td>0.33</td></tr><tr><td>-10.301</td><td>1.186</td><td>1.982</td><td>0.002215</td><td>6.12</td><td>0.22</td></tr><tr><td>-9.300</td><td>1.184</td><td>1.977</td><td>0.002190</td><td>5.57</td><td>0.23</td></tr><tr><td>-8.301</td><td>1.182</td><td>1.972</td><td>0.002195</td><td>5.63</td><td>0.23</td></tr><tr><td>-6.300</td><td>1.179</td><td>1.962</td><td>0.002248</td><td>7.07</td><td>0.25</td></tr><tr><td>-5.326</td><td>1.178</td><td>1.957</td><td>0.002239</td><td>5.31</td><td>0.23</td></tr><tr><td>-4.326</td><td>1.177</td><td>1.953</td><td>0.002217</td><td>5.72</td><td>0.20</td></tr><tr><td>-3.326</td><td>1.177</td><td>1.948</td><td>0.002181</td><td>5.13</td><td>0.23</td></tr><tr><td>-2.326</td><td>1.176</td><td>1.944</td><td>0.002139</td><td>5.70</td><td>0.27</td></tr><tr><td>-1.326</td><td>1.176</td><td>1.940</td><td>0.002121</td><td>5.24</td><td>0.23</td></tr><tr><td>-0.326</td><td>1.175</td><td>1.936</td><td>0.002115</td><td>5.31</td><td>0.24</td></tr><tr><td>0.674</td><td>1.176</td><td>1.932</td><td>0.002105</td><td>6.82</td><td>0.30</td></tr><tr><td>1.674</td><td>1.176</td><td>1.929</td><td>0.002104</td><td>4.93</td><td>0.25</td></tr><tr><td>2.674</td><td>1.176</td><td>1.925</td><td>0.002127</td><td>5.12</td><td>0.29</td></tr><tr><td>3.674</td><td>1.177</td><td>1.921</td><td>0.002149</td><td>4.83</td><td>0.25</td></tr><tr><td>4.670</td><td>1.178</td><td>1.918</td><td>0.002218</td><td>3.43</td><td>0.39</td></tr><tr><td>5.670</td><td>1.179</td><td>1.915</td><td>0.002267</td><td>4.43</td><td>0.24</td></tr><tr><td>6.670</td><td>1.180</td><td>1.912</td><td>0.002303</td><td>5.02</td><td>0.25</td></tr><tr><td>7.670</td><td>1.181</td><td>1.909</td><td>0.002303</td><td>5.97</td><td>0.18</td></tr><tr><td>8.670</td><td>1.183</td><td>1.907</td><td>0.002324</td><td>6.48</td><td>0.17</td></tr><tr><td>9.670</td><td>1.185</td><td>1.904</td><td>0.002303</td><td>4.25</td><td>0.24</td></tr><tr><td>10.670</td><td>1.187</td><td>1.902</td><td>0.002265</td><td>3.39</td><td>0.35</td></tr><tr><td>11.670</td><td>1.189</td><td>1.900</td><td>0.002191</td><td>5.46</td><td>0.21</td></tr><tr><td>12.670</td><td>1.191</td><td>1.898</td><td>0.002178</td><td>6.45</td><td>0.23</td></tr><tr><td>13.670</td><td>1.194</td><td>1.897</td><td>0.002143</td><td>5.00</td><td>0.32</td></tr><tr><td>14.670</td><td>1.197</td><td>1.895</td><td>0.002128</td><td>3.76</td><td>0.39</td></tr><tr><td>15.670</td><td>1.200</td><td>1.894</td><td>0.002099</td><td>4.44</td><td>0.28</td></tr><tr><td>16.646</td><td>1.203</td><td>1.893</td><td>0.002123</td><td>6.04</td><td>0.21</td></tr><tr><td>17.646</td><td>1.206</td><td>1.893</td><td>0.002168</td><td>5.79</td><td>0.26</td></tr><tr><td>18.646</td><td>1.209</td><td>1.892</td><td>0.002273</td><td>4.26</td><td>0.27</td></tr><tr><td>19.646</td><td>1.213</td><td>1.892</td><td>0.002305</td><td>3.73</td><td>0.29</td></tr><tr><td>20.646</td><td>1.217</td><td>1.892</td><td>0.002403</td><td>4.00</td><td>0.22</td></tr><tr><td>21.646</td><td>1.221</td><td>1.892</td><td>0.002412</td><td>5.43</td><td>0.21</td></tr><tr><td>22.670</td><td>1.225</td><td>1.893</td><td>0.002402</td><td>5.99</td><td>0.24</td></tr><tr><td>23.646</td><td>1.229</td><td>1.893</td><td>0.002432</td><td>5.93</td><td>0.12</td></tr></tbody></table>

<table><caption>**Table 4**
(Continued)</caption><thead><tr><th>ΔT (days)</th><th>r (au)</th><th>Δ (au)</th><th>g (s<sup>-1</sup>)</th><th>Q (10<sup>28</sup> s<sup>-1</sup>)</th><th>δQ (10<sup>28</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>25.647</td><td>1.239</td><td>1.896</td><td>0.002409</td><td>4.88</td><td>0.25</td></tr><tr><td>26.646</td><td>1.243</td><td>1.897</td><td>0.002385</td><td>7.33</td><td>0.14</td></tr><tr><td>27.646</td><td>1.248</td><td>1.899</td><td>0.002333</td><td>4.30</td><td>0.27</td></tr><tr><td>28.646</td><td>1.254</td><td>1.901</td><td>0.002300</td><td>3.79</td><td>0.26</td></tr><tr><td>29.646</td><td>1.259</td><td>1.904</td><td>0.002274</td><td>6.83</td><td>0.27</td></tr><tr><td>30.646</td><td>1.264</td><td>1.907</td><td>0.002256</td><td>3.70</td><td>0.36</td></tr><tr><td>31.647</td><td>1.270</td><td>1.909</td><td>0.002265</td><td>4.39</td><td>0.29</td></tr><tr><td>32.647</td><td>1.276</td><td>1.913</td><td>0.002265</td><td>3.44</td><td>0.33</td></tr><tr><td>33.647</td><td>1.282</td><td>1.916</td><td>0.002288</td><td>4.47</td><td>0.26</td></tr><tr><td>35.647</td><td>1.294</td><td>1.924</td><td>0.002335</td><td>3.19</td><td>0.31</td></tr><tr><td>36.647</td><td>1.300</td><td>1.929</td><td>0.002317</td><td>3.89</td><td>0.29</td></tr><tr><td>37.647</td><td>1.307</td><td>1.933</td><td>0.002306</td><td>3.02</td><td>0.42</td></tr><tr><td>38.647</td><td>1.313</td><td>1.938</td><td>0.002296</td><td>3.32</td><td>0.30</td></tr><tr><td>39.647</td><td>1.320</td><td>1.943</td><td>0.002298</td><td>2.39</td><td>0.42</td></tr><tr><td>40.647</td><td>1.327</td><td>1.949</td><td>0.002292</td><td>2.47</td><td>0.41</td></tr><tr><td>41.647</td><td>1.333</td><td>1.955</td><td>0.002306</td><td>2.86</td><td>0.35</td></tr><tr><td>42.647</td><td>1.341</td><td>1.961</td><td>0.002318</td><td>5.45</td><td>0.26</td></tr><tr><td>44.647</td><td>1.355</td><td>1.973</td><td>0.001956</td><td>5.59</td><td>0.26</td></tr><tr><td>45.647</td><td>1.362</td><td>1.980</td><td>0.001958</td><td>6.98</td><td>0.34</td></tr><tr><td>46.647</td><td>1.370</td><td>1.987</td><td>0.001960</td><td>5.07</td><td>0.45</td></tr><tr><td>47.647</td><td>1.377</td><td>1.995</td><td>0.001962</td><td>3.68</td><td>0.41</td></tr><tr><td>48.647</td><td>1.385</td><td>2.002</td><td>0.001964</td><td>4.91</td><td>0.38</td></tr><tr><td>49.647</td><td>1.393</td><td>2.010</td><td>0.001966</td><td>4.96</td><td>0.31</td></tr><tr><td>50.647</td><td>1.401</td><td>2.018</td><td>0.001968</td><td>3.56</td><td>0.43</td></tr><tr><td>51.647</td><td>1.409</td><td>2.027</td><td>0.001970</td><td>4.01</td><td>0.39</td></tr><tr><td>52.647</td><td>1.417</td><td>2.036</td><td>0.001972</td><td>3.91</td><td>0.36</td></tr><tr><td>53.647</td><td>1.425</td><td>2.044</td><td>0.001973</td><td>2.17</td><td>0.49</td></tr><tr><td>54.647</td><td>1.433</td><td>2.054</td><td>0.001975</td><td>2.30</td><td>0.63</td></tr><tr><td>55.670</td><td>1.442</td><td>2.063</td><td>0.001977</td><td>3.40</td><td>0.44</td></tr><tr><td>56.647</td><td>1.450</td><td>2.073</td><td>0.001978</td><td>5.02</td><td>0.38</td></tr><tr><td>57.670</td><td>1.458</td><td>2.083</td><td>0.001980</td><td>8.66</td><td>0.45</td></tr><tr><td>58.670</td><td>1.467</td><td>2.093</td><td>0.001982</td><td>3.85</td><td>0.47</td></tr><tr><td>61.692</td><td>1.493</td><td>2.125</td><td>0.001987</td><td>2.96</td><td>0.55</td></tr><tr><td>62.693</td><td>1.502</td><td>2.136</td><td>0.001988</td><td>3.52</td><td>0.48</td></tr><tr><td>63.693</td><td>1.511</td><td>2.147</td><td>0.001990</td><td>3.68</td><td>0.49</td></tr><tr><td>64.693</td><td>1.520</td><td>2.158</td><td>0.001991</td><td>2.55</td><td>0.56</td></tr></tbody></table>

**Notes.** ΔT: days from perihelion, 30.95 June 2024.

r: heliocentric distance (au).

Δ: comet-SOHO distance (au).

g: solar Lyα g-factor (photons s<sup>-1</sup>) at 1 au.

Q: water production rates for each image (s<sup>-1</sup>).

δQ: internal 1σ uncertainties.

The tracking of the heliocentric distance dependence and the outbursts were enabled by the nearly daily coverage of SWAN. This positioning enables SWAN to observe comets both north and south of the ecliptic as well as reasonably close to the Sun in the sky. Comparisons with other observations of water production rates were generally consistent, with some differences that are present in most other comparisons of comet observations.

M. M. Knight et al. (2024) found two active areas in CN images of 12P from which they calculated the nucleus rotation rate of ~57 hr. Because of the time/space averaging of the large-aperture SWAN observations, it is not possible to distinguish relative emissions from the rotations of the active areas to the nucleus as a whole or to see if the very small active area of 12P means that the two active areas constitute all or most of the water-driven activity.
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Water production rate of comet 13P as a function of heliocentric distance (au). The diamonds give the water production rates before perihelion, and the squares give the values after perihelion. Although the power-law fits are not very meaningful, the dashed line gives the fit of a slope of +0.1 before perihelion and the dashed-dotted line a slope of -0.7 after to show the trends. See the caption in Figure 5 for the uncertainties and the text for discussion.</figcaption></figure>

## Acknowledgments

We thank Quanzhi Ye for useful discussions regarding the nucleus radius determination of comet 12P/Pons-Brooks and David Schleicher and Emmanuel Jehin for generously sharing their data on comet 13P with the community. We also thank Martin Cordiner, Erika Gibb, and Boncho Bonev for sharing the results of their observations of comet 12P before publication. We also thank both reviewers whose comments led to revisions that greatly improved the paper. SOHO is an international mission between ESA and NASA. M.R.C. acknowledges support from NASA grant 80NSSC23K0030 from the Solar System Observations Program. T.T.M. was supported by the Finnish Meteorological Institute (FMI). J.-L.B. and E.Q. acknowledge support from CNRS and CNES. We obtained orbital elements from the JPL Horizons website (http://ssd.jpl.nasa.gov/horizons.cgi). For classification of the dynamical ages of the comets, we used the Minor Planet Center website (https://www.minorplanetcenter.net/db_search) tool. The composite solar Lyα data were taken from the LASP website at the University of Colorado (https://lasp.colorado.edu/lisird/data). We acknowledge the personnel who

have been keeping SOHO and SWAN operational for nearly 30 yr.

## ORCID iDs

M. R. Combi https://orcid.org/0000-0002-9805-0078

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