The brighter comets of 2021

2025 February 7

Jonathan Shanklin

This report describes and analyses observations of the brighter or more interesting comets at perihelion during 2021, concentrating on those for which visual observations were obtained. Magnitude parameters are given for all comets with observations. Any evolution in the magnitude parameters of those periodic comets with multiple returns is discussed. Additional information on the comets discussed here, and on other comets seen or at perihelion during the year, may be found on the Section visual observations web pages.

Introduction

Ninety-five comets or potential comets were assigned year designations for 2021, while 50 previously numbered periodic comets returned to perihelion. 245 comets found by the SOHO satellite, including several archival finds, and 11 from STEREO were credited during 2021. 218 of these were members of the Kreutz group, 15 were members of the Meyer group, one was of the Marsden group, none were of the Kracht group, and 22 were not associated with any known group. One of these objects was given a designation (2021 D1). One Marsden-group comet returned to perihelion and therefore could be numbered. 342P/SOHO returned to perihelion.

There were five amateur discoveries (2021 J1, L3, O1, U3, X1) for which Alain Maury, Georges Attard, Gennady Borisov and Hideo Nishimura gained the Edgar Wilson Award.¹ The awards were belatedly announced in 2024 in MPEC 2024-S182 with a concerned commentary in CBET 5451.²

Twenty-seven periodic comets were numbered during the year. One comet was reported as visible to the naked eye (2021 A1) and five others reached binocular brightness, though large binoculars were required for the majority of these.

The remainder of this report covers only the comets that were at perihelion. When periodic comets have visual or electronic observations at five or more returns and have not previously been analysed in detail over the past decade, the secular behaviour of the comet is considered, even though it may not qualify as a ‘brighter’ comet during the present return. Any evolution in behaviour is of interest, as is observation of a steady state.

Orbital elements for all the comets discovered and returning during the year can be found on the JPL Small-Body Database Browser,³ which will also generate ephemerides. Discovery details and some information for the other comets found or returning during the year are available on the BAA Comet Section visual observations web pages,⁴ which also contain links to additional background information. The raw visual observations for the year are on the same web pages in ICQ format and in the Comet Observations (COBS) database.⁵ The full data set from COBS is used for the multi-return analyses presented here, but otherwise only those observations submitted to the Section – through the visual observations coordinator or through COBS – are included, along with all observations submitted to The Astronomer (TA) magazine. Additional images of the comets are presented in the Section image archive.⁶

The comets given a discovery designation

2020 R4 (ATLAS)

The ATLAS (Asteroid Terrestrial-impact Last Alert System) team discovered a 19th-magnitude comet in images taken with the 0.5-m Schmidt at Mauna Loa on Sept 12.50. It was posted on the Possible Comet Confirmation Page (PCCP) as A10qhkr and subsequently confirmed as cometary by several astrometrists. There were pre-discovery images of the comet in Catalina Sky Survey data from August and Pan-STARRS data from August and September. [CBET 4849, MPEC 2020-S33, 2020 Sept 16.] It was subsequently re-reported by ATLAS as A10wNNY and by Catalina as C23NY71 in 2021 Apr and as A10wVgj in May. These multiple postings may have been a result of the Minor Planet Center (MPC) providing out-of-date orbital elements.

The comet was at perihelion at 1.0 astronomical units (au) in 2021 Mar and has a period of around 1,000 years. Unfortunately, it was not an optimum return, though post perihelion the comet approached us to 0.5 au in late April. It was initially given a rather faint absolute magnitude, but these comets often appear brighter to visual observers. Imaging by Taras Prystavski on Dec 12 suggested that it was at least three magnitudes brighter than originally expected. J. J. Gonzalez observed it at 9th magnitude in January, though his observation is considerably brighter than the mean curve. It was observed from the UK during April, when it was around 9th magnitude. During the moderately close approach, it was a fast-moving object. It was reported to be in outburst on Apr 30, with the outburst having started about a week previously.

There is a lot of scatter in the observations, which obscures any convincing evidence of the outburst. This may therefore be an artefact of the close approach, as there is no sign of an increase in degree of condensation (DC), which usually accompanies an outburst. In addition, the Moon was full on Apr 27 and in the same quarter of the sky, which may also have compromised observations. After perihelion, the comet faded rapidly, and it was last seen in early June.

2020 T2 (Palomar)

Dmitry A. Duev found a comet of 19th magnitude in images taken with the Zwicky Transient Facility camera on the 1.2-m Oschin Schmidt on 2020 Oct 7.51. It was posted on the PCCP as ZTFDD01. Pre-discovery images back to 2019 Dec were found. [CBET 4870, MPEC 2020-U170, 2020 Oct 22.] It was subsequently independently discovered by the Catalina Sky Survey on Oct 22.5 and posted on the PCCP as C1G21V1. It was at perihelion at 2.1 au in 2021 Jul.

It barely qualifies as a brighter comet, just reaching 10th magnitude in 2021 Jun. Even though still relatively bright, it was not followed by BAA observers after 2021 October, when it was a southern-hemisphere object. It then remained at a poor solar elongation until 2022 Mar.

2021 A1 (Leonard)

Nick James published a preliminary report on this comet in the Journal.⁷ This includes a fine selection of images, some showing exquisite detail in the tail. It includes observations made up to late January. Here, a summary is given, with further conclusions based on observations made subsequently.

Gregory Leonard discovered a comet of 19th magnitude in images taken with the 1.5-m reflector of the Mt Lemmon Survey on 2021 Jan 3.54. It was placed on the PCCP as C4AGJ62. There were pre-discovery observations from the Catalina Sky Survey (2020 Dec, 2021 Jan), Mt Lemmon Survey (2020 Apr, Nov), Pan-STARRS (2020 Apr, May, Jun, Aug) and the Szeged Asteroid Program, Hungary (2020 Nov). [CBET 4907, MPEC 2021-A99, 2021 Jan 10.]

The comet reached perihelion at 0.6 au in 2022 Jan. It passed 0.23 au from Earth on 2021 Dec 12 and 0.0286 au from Venus on 2021 Dec 18. In early October, Michael Mattiazzo suggested on the Comets Mailing List (groups.io/g/comets-ml) that it was a very dusty comet, although some later studies suggested that the dust-to-gas ratio was normal. He suggested that the dust could produce strong forward scattering between Dec 9 & 20 and also noted that Earth crossed the orbital plane of the comet on Dec 8, which could enhance the tail. The comet was at less than 25° solar elongation from Dec 11–17 and again from Jan 21 to Feb 18. The Moon was full on Nov 19, Dec 19, and Jan 17.

Michael Mattiazzo imaged the comet on Oct 3, estimating the G magnitude at 13. J. J. Gonzalez observed it visually on Oct 7, estimating it at 11.2. It was reported as being visible in SWAN images in early 2021 Nov. On Dec 2.3, it was 6.7 in 20 × 80 binoculars from central Cambridge, with a 45-arcminute tail. On Dec 8.2, it was 5.5 in 8 × 40 binoculars from central Cambridge, and was on track to reach about 4th magnitude, though some pundits suggested that it was disintegrating. The comet was rapidly moving south, and Stephen Getliffe made the last UK observation on Dec 11.22, when it was 5.4 in his 108-mm reflector.

Some imagers reported the comet being brighter than expected on Dec 14 and this might have been a result of the predicted forward scattering, although there was also a factor-of-roughly-three increase in water-vapour production at the same time. J. J. Gonzalez reported it at 3.3 in 10 × 50 binoculars from his mountain location on Dec 15.75. Observations after that became a little scattered, with much speculation about outbursts, and the comet certainly became much brighter than expected from the light curve. Activity continued to be intermittent, with the comet fading to 4.5 on Dec 22/23 (Goiato & Gonzalez), but it was 3.5 on Dec 23/24 (Amorim). Alexandre Amorim reported seeing jets in his 90-mm refractor.

Southern-hemisphere observers made naked-eye observations in early January, but the comet was rapidly fading. On Jan 28, Chris Wyatt estimated it as 8.5 in his 25-cm reflector. It was imaged in late February and early March after conjunction, and the images suggested that the comet was disintegrating, with no obvious central condensation. David Seargent then suggested that the disintegration might have already started by late January as his image on Jan 22 showed the tail more prominent than the head. Graham Wolf observed it on Feb 28, estimating it at 11th magnitude. An image taken by Nirmal Paul on 2022 Mar 31 only showed a ghostly outline of the tail.

Visual observers began to report a short tail in November and there was significant tail development during December. Several observers reported lengths of up to 9 degrees around Dec 28. The tail was still around 5 degrees long in early January, but it rapidly disappeared, leaving nothing significant to visual observers after mid-month. The degree of condensation followed a similar pattern, becoming nearly stellar in late December, before relaxing to a more diffuse coma.

Using all the observations, the standard equation produces a good fit to the observations throughout the apparition. A better fit is obtained if the observations used are restricted to those made more than 30 days prior to perihelion and this still gives a good fit to the observations made more than 20 days after perihelion. The comet was however systematically brighter than this curve from mid-December to early January; this encompasses the period when forward scattering might have played a part but also continues until shortly after perihelion. It seems clear that there was a series of events during this period, probably linked to enhanced outgassing. Whether this led to a complete disintegration of the nucleus or just to a shutdown of further activity is not clear. No discrete centres of condensation were reported, and images still show a gas coma as late as Jan 23, suggesting that complete disintegration had not happened up to this point. Thereafter, images show a steadily more diffuse object, disappearing rather like Lewis Carroll’s Cheshire Cat.

Members can view the full illustrated article in PDF format by returning to the previous page. Not a member? Why not join today?