Weird Science

C∕2025 R2 (SWAN) Comet

Polish ver­sion is here

The autumn of 2025 bro­u­ght an intri­gu­ing pair of comets to astro­no­mers’ atten­tion: C/2025 A6 (Lem­mon) and C/2025 R2 (SWAN). Both objects, visi­ble almost simul­ta­ne­o­u­sly in the night sky, allo­wed obse­rvers to wit­ness rare phe­no­mena such as sud­den out­bur­sts, rapid chan­ges in acti­vity, and pos­si­ble nuc­leus frag­men­ta­tion. Among the two, comet C/2025 R2 (SWAN) pro­ved par­ti­cu­larly fasci­na­ting. It was disco­ve­red acci­den­tally by an ama­teur astro­no­mer and was soon stu­died in detail by some of the world’s lar­gest obse­rva­to­ries.

Comet C/2025 R2 was disco­ve­red on Sep­tem­ber 11, 2025, by Vla­di­mir Bez­u­gly in ima­ges taken by the SWAN (Solar Wind Aniso­tro­pies) instru­ment abo­ard the SOHO spa­ce­craft. Altho­ugh SWAN was ori­gi­nally desi­gned to moni­tor solar wind varia­tions, it has repe­a­te­dly con­tri­bu­ted to the disco­very of new comets. The pro­vi­sio­nal desi­gna­tion SWA­N25B refers to the twen­tieth object detec­ted using this instru­ment.

At the time of disco­very, the comet was loca­ted in the con­stel­la­tion Virgo. It was best obse­rved from the Sou­thern Hemi­sphere and had an appa­rent magni­tude of about 7.4m. Its bri­ght­ness incre­a­sed ste­a­dily over the fol­lo­wing days, quic­kly attrac­ting the atten­tion of both ama­teur and pro­fes­sio­nal obse­rvers.

Altho­ugh the comet had been tra­ve­ling thro­ugh the inner Solar Sys­tem for some time, it rema­i­ned unde­tec­ted for seve­ral mon­ths. The rea­son was the so-cal­led Holet­schek effect, a con­fi­gu­ra­tion in which a comet appe­ars very close to the Sun in the sky, making it nearly impos­si­ble to obse­rve from Earth. Between August 7 and Sep­tem­ber 13, 2025, C/2025 R2 (SWAN) stayed within 30 degrees of the Sun, lost in the glare of day­li­ght.

During its oppo­si­tion in Febru­ary 2025, the comet was not recor­ded by any gro­und-based survey. It was likely inac­tive at the time, cove­red by a dark, low-albedo dust layer, or sim­ply too small to reflect eno­ugh sun­li­ght to be detec­ted. Only as it appro­a­ched the Sun in August did its nuc­leus sud­denly become active. Within a few weeks, its bri­ght­ness incre­a­sed from 11m to 8m, and by mid-Sep­tem­ber it had rea­ched 6.2m, beco­ming an easy tar­get for bino­cu­lar obse­rva­tion.

At the end of Sep­tem­ber 2025, the comet sur­pri­sed obse­rvers with a sud­den out­burst. Its bri­ght­ness rose to about 5.9m, accom­pa­nied by noti­ce­a­ble struc­tu­ral chan­ges in the tail. During the same period, it appe­a­red close in the sky to two other comets, 3I/ATLAS and C/2025 K1 (ATLAS), cre­a­ting an espe­cially stri­king and com­plex cele­stial field. On Novem­ber 2, 2025, the Two-meter Twin Tele­scope (TTT3) at the Teide Obse­rva­tory detec­ted a signal sug­ge­sting a pos­si­ble sepa­ra­tion of the comet’s nuc­leus. Ana­ly­sis of sub­se­qu­ent data indi­ca­ted that C/2025 R2 (SWAN) may have under­gone par­tial frag­men­ta­tion, split­ting into two or more com­po­nents. Such an event often marks the end of a comet’s active phase. The frag­men­ta­tion likely occur­red aro­und mid-Octo­ber.

Obse­rva­tions

Octo­ber 19, 2025 – Jaworzno (Poland)
urban area, high light pol­lu­tion

That eve­ning, the comet was cle­arly visi­ble within the boun­da­ries of the con­stel­la­tion Scu­tum (the Shield). Photo 1 shows the comet’s vivid gre­e­nish gas-and-dust coma sur­ro­un­ding its nuc­leus.

Ilustracja: Photo 1 – without labels

Photo 1 – without labels

Ilustracja: Photo 1 – with labels

Photo 1 – with labels

On Sep­tem­ber 12, 2025, the comet rea­ched peri­he­lion, its clo­sest point to the Sun. At that moment, it was 0.5 astro­no­mi­cal units away, rou­ghly 75 mil­lion kilo­me­ters. A month later, on Octo­ber 20, it pas­sed clo­sest to Earth at a distance of 0.26 astro­no­mi­cal units, or about 39 mil­lion kilo­me­ters. Despite cros­sing Earth’s orbi­tal path, the enco­un­ter with the dust rele­a­sed by the comet did not pro­duce a visi­ble meteor sho­wer, as the stream of par­tic­les was too dif­fuse to gene­rate noti­ce­a­ble light phe­no­mena in the atmo­sphere.

C/2025 R2 (SWAN) is clas­si­fied as a dyna­mi­cally old comet, ori­gi­na­ting from a distance of aro­und 170 astro­no­mi­cal units, much clo­ser than the typi­cal Oort Cloud popu­la­tion. This sug­ge­sts it has visi­ted the inner Solar Sys­tem before. Its orbit is ellip­ti­cal, with a period esti­ma­ted between 750 and 780 years.

Spec­tro­sco­pic obse­rva­tions con­duc­ted on Octo­ber 18, 2025, at the Uni­ver­sity Obse­rva­tory Jena using the FLE­CHAS échelle spec­tro­graph reve­a­led a rich emis­sion spec­trum span­ning 5000–7500 Å. Distinct spec­tral lines of C2, NH2, and ato­mic oxy­gen were iden­ti­fied, indi­ca­ting strong out­gas­sing acti­vity.

Unlike comet C/2025 A6 (Lem­mon), the spec­trum of C/2025 R2 (SWAN) sho­wed no detec­ta­ble sodium emis­sion, sug­ge­sting dif­fe­ren­ces in sur­face com­po­si­tion or ther­mal evo­lu­tion between the two objects.

C/2025 R2 (SWAN) rema­ins an excel­lent exam­ple of how much can still be lear­ned about the evo­lu­tion of long-period comets thro­ugh the com­bi­ned efforts of ama­teur and pro­fes­sio­nal astro­no­mers. Out­bur­sts, gas emis­sions, and frag­men­ta­tion events pro­vide valu­a­ble insi­ghts into sub­li­ma­tion pro­ces­ses, ther­mal stres­ses, and the inter­nal struc­ture of come­tary nuc­lei, hel­ping us bet­ter under­stand these fra­gile wan­de­rers from the outer­most regions of the Solar Sys­tem.

Photo 1 Para­me­ters:

  • Canon EOS 60D
  • Total expo­sure time: 8 minu­tes (stack of 32 RAW fra­mes at 15s each)
  • ISO: 1000
  • Lens: zoom
  • Aper­ture: f/4

Fur­ther rea­dings:

Marek Ples

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