On February 26th, amateur astronomer Michael Mattiazzo (Australia) discovered a previously unknown comet using publicly available data from the SOHO/SWAN instrument! At the time of discovery, the object (C/2021 D1) was already near perihelion, and close to maximum brightness (mag +11). Due to the comet’s location (poor solar elongation), despite its brightness, it had escaped previous detection by ground-based minor planet surveys (e.g. ATLAS, CSS, Pan-STARRS). Fortunately, comet SWAN is moving east towards increasingly favorable skies. Sadly, however, it is slowly fading (currently ~ +12 mag), and is expected to have reached mag +15 by June of this year. The comet currently displays a somewhat condensed appearance, although a faint tail has been reported.
Michael Mattiazzo discovered C/2021 D1 (SWAN) on February 26th, using near-real-time Comet Tracker images from the SOHO/SWAN website. More specifically, he found the comet in data from February 19th – 23rd, where it was situated only tens of degrees NE of the Sun, in the Pegasus constellation. Located aboard the SOHO (Solar and Heliospheric Observatory) spacecraft, the SWAN (Solar Wind Anisotropies) cameras image the sky in the Lyman-α band, a wavelength that happens to be particularly sensitive to comets. In fact, although SWAN mainly focuses on the solar wind, it has also been used to study comets (e.g. Combi et al., 2000). In order to make this possible, the instrument must first block the overwhelming glare of the Sun and Earth. As a consequence of its build-up, SWAN is capable of imaging the sky at low solar elongation, a blind spot for most ground-based minor planet surveys. However, due to the very poor resolution of its images (of ~1°), only comets brighter than ~12 mag are possible to detect using this data. Despite this, SWAN has enabled the discovery of fourteen comets (not including two co-discoveries), as well as one recovery! Most SWAN comets nowadays are found at low solar elongation, by amateur astronomers regularly studying the data. This was the case of C/2021 D1 (SWAN). Due to the faint nature of this comet in SWAN data (Fig. 2), it could not be confirmed using these images alone. Ground-based observations were hence necessary to confirm the comet.
Following the comet’s discovery, attempts at constraining a rough orbit were made. This was particularly challenging, not only because of the short observation arc, but also due to the very low resolution of the SWAN Comet Tracker images. Consequently, preliminary estimations varied widely. Indeed, some proposed solutions suggested that it might have been a sunskirting comet, or that it was related to the Great Comet of 1686 (C/1686 R1). Other solutions suggested that it was headed for a close approach with Earth in early-March (which sadly did not happen). The uncertainty in the comet’s trajectory and location made it initially difficult to locate the comet from Earth. Astrophotographer Nicolas Lefaudeux (France) took a wide-field image of the region on February 27th, however early attempts at locating the comet in his image failed.
On February 28th, after multiple attempts since the comet’s discovery, astrophotographer Michael Jäger (Austria) became the first to recover C/2021 D1 (Fig. 3). Jäger measured the comet to be ~10.5 mag in his confirmation image (Fig. 3). He further described the comet as displaying a condensed appearance, with an apparent diameter of 3.5′, and even showing a very faint tail. The comet was also independently recovered by Krisztián Sárneczky (Hungary). Following Jäger’s announcement, other astrophotographers (e.g. Luca Buzzi and Nick James) were also able image the comet that same day (Fig. 4). Based on these new observations, astronomer Alan Hale (U.S.A.) was able to locate the comet in Lefaudeux’s image.
Since the comet’s recovery, with the increasing observations and growing observation arc, the comet has been found to be periodic (P= 76.9 years), with a perihelion on that occurred on T= February 27th at q= 0.89 AU. Fortunately, the comet is slowly moving towards more favorable skies. However, it is also slowly fading. By June the comet will likely have faded below mag +15, assuming no outbursts occur.
Acknowledgements: I wish to thank Michael Jäger and Nick James for granting me permission to use their images of C/2021 D1 (SWAN).
Copyright (c) 2021 Trygve Prestgard. All rights reserved.