The Discovery of Comet C/2025 F2 SWAN
By Michael Mattiazzo
The Solar Wind Anisotropy instrument (SWAN) is an ultraviolet camera on board the Solar and Heliospheric Observer (SOHO) spacecraft. SOHO was launched by NASA in 1995. It is in a stable orbit at the L1 Lagrange point, some 1.5 million kms from the Earth towards the Sun. Its primary purpose is to study the Sun using various instruments but as a side benefit, the SWAN instrument is a very good comet detector. Comets shine brightly in ultraviolet light due to the sublimation of water ice when near the Sun, via the Lyman Alpha emission line of ionised Hydrogen. SWAN takes a daily image of the entire sky at low resolution. The data is provided in a form suitable for comet tracking and can be downloaded at http://swan.projet.latmos.ipsl.fr/
I co-discovered comet SWAN by searching this publicly
available data on the SWAN Project website. Vladimir Bezugly (Ukraine) and Rob
Matson (USA) were other hunters credited with the discovery.
On 2025 May 30, I noticed a steadily brightening, moving object that did not
correspond with any known comets. The new comet was faintly detectable as early
as March 22 with a notable brightening over the days that followed.
Cropped animation showing the daily movement of the comet
in SWAN data between March 24 to 28
Above: SWAN all-sky map image on 2025 March 27 indicating positions of C/2025 F2 SWAN, Earth and Sun. Note that each rectangle corresponds to a 30x30 degree patch of sky.
It takes plenty of patience and experience when looking at the data because there are many false positives due to its very low resolution and many background sources, especially in the region of the Milky Way. Comets are most likely to be found approaching the Sun, so this region should be monitored carefully.
Once you think you have found a possible candidate, you
need to eliminate known comets.
The second challenge is to then find the comet in the night sky. SWAN data is
usually made publicly available after about a 3-day delay, therefore you need to
forecast where the comet might currently be situated in the night sky. It may be
more favourable for southern or northern observers. In this instance, it was not
visible from my location south of the equator, so I had to attempt a ground
confirmation using a remote telescope. The SWAN positions indicated that the
comet was situated only 30 degrees from the Sun. This provides a challenge as
most remote observatories don’t allow its telescopes to slew to such a low
altitude.
The SWAN data is also presented in ecliptic coordinates, which don’t make a lot of sense to astronomers, but it is easy to convert to equatorial coordinates using planetarium software, such as Guide 9, which can display ecliptic and equatorial coordinates simultaneously.
Guide 9 representation of the SWAN comet tracker map on
2025 March 27
You can see that on March 27, the comet (circled at far left) was situated in Pegasus and about to enter the square of Pegasus. Once you have a search area, it is best to use a camera and telephoto lens to photograph a wide area of sky. The more area you cover, the better your chances of confirming it.
Not having much success with weather using a remote telescope, I sent an email to the comets mailing list on April 1, requesting assistance from the comet community who quickly responded. Quicheng Zhang found it on the morning of 2025 April 3 from Mormon Lake, Arizona using a 40 mm refractor. He saw a compact source of approximate magnitude 12 some 2.5 degrees west of predictions. Once detected and measured, the positions of the comet are then posted onto the Possible Comet Confirmation Page (PCCP) for immediate follow up by astronomers around the globe. Remote telescopes have become especially useful tools for this purpose. After a few days of astrometric data, a reasonable orbit was able to be established which enabled identification of pre-discovery images taken by PANSTARRS survey as early as October 2024.
This enabled a precise orbital calculation which indicated that the comet would be closest to the Sun on 2025 May 1 at 0.33AU, near the orbit of Mercury. On the same day, it would be closest to the Earth at 0.96AU. These conditions were favourable for a bright binocular comet, but maximum brightness was uncertain at this time as the comet experienced an outburst on April 6.
This is my 10th discovery credit for SWAN comets since 2004 and I do check the data on most days. SWAN is credited for 20 comets as well as recoveries of several others. Comet SWAN is not named after me as I did not use my own equipment, as per IAU naming guidelines.
My interest in comets were sparked by the arrival of comet Halley in 1986. Then in 1987, a comet by the name of Bradfield became visible in small scopes. I had the privilege of meeting Bill Bradfield after attending an Astronomical Society of South Australia (ASSA) meeting in 1995, after his comet discovery of that year. Overall, he managed to visually discover a total of 18 comets as an amateur, between 1972 to 2004, that will never likely to be repeated given today's technology. He was very keen to pass on his knowledge and experience to me. Bill was a life member of ASSA and was inducted into the ASSA Hall of Fame in 2013, and best described as a gentleman, scholar and a mentor.
I had taken up comet hunting in 1997 when I moved to Wallaroo, South Australia where my greatest success was making an independent visual discovery of comet C/2000 W1 Utsunomiya-Jones but was 24hrs too late and missed the cut.
In 2002, SWAN became a threat to the visual comet hunter when a couple of Japanese amateurs picked up C/2002 O6 and SWAN data was then becoming publicly available on the internet as a comet tracker map. After that time, I concentrated on SWAN hunting and have since discovery credit for C/2004 H6, C/2004 V13, C/2005 P3, P/2005 T4, C/2006 M4, C/2015 C2, C/2015 P3, C/2020 F8, C/2021 D2 and finally C/2025 F2.
Follow me on southern comets facebook page where I post regular updates of my recent comet observations or southern comets homepage, updated less frequently, but has a terrific record of my comet observations since 1996 as well as finder charts available for the brighter comets.