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C/2025 R2 (SWAN)

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For other comets discovered by the SWAN instrument, see Comet SWAN.

C/2025 R2 (SWAN)
Comet C/2025 R2 (SWAN) imaged from the Siding Spring Observatory on 13 September 2025 with a 0.5 m (20 in) telescope.
Discovery[1][2]
Discovered bySOHO–SWAN
Vladimir Bezugly
Discovery date11 September 2025
Designations
SWAN25B, CK25R020[2]
Orbital characteristics[4]
Epoch10 October 2025 (JD 2460958.5)
Observation arc76 days
47 days (without STEREO-A)
Earliest precovery date13 August 2025
Number of
observations		666
Aphelion170 AU (inbound)[3]
166 AU (outbound)[a]
Perihelion0.5041 AU
Semi-major axis85 AU (inbound)[3]
83 AU (outbound)
Eccentricity0.9941 (inbound)[3]
0.9940 (outbound)
Orbital period≈787 years (inbound)[3]
≈757 years (outbound)
798±3 years[4][5] (one-size fits all)
Inclination4.4732°±0.00002°
335.3°
Argument of
periapsis		308.4°
Last perihelion12 September 2025
TJupiter0.937
Earth MOID0.048 AU
Jupiter MOID0.042 AU
Physical characteristics
Mean radius
0.3–1.0 km (0.19–0.62 mi)[6]
Comet total
magnitude (M1)		15±1
6.0[b]
(2025-09-26)

C/2025 R2 (SWAN), formerly known as SWAN25B, is a long-period comet discovered on 11 September 2025 by Vladimir Bezugly through SWAN imagery. As of 3 November 2025, the comet has a diffuse apparent magnitude of +8[7][b] and is observable in 50 mm (2.0 in) binoculars.[8] In the Northern hemisphere, it can now be seen about 20 degrees to the upper right of Saturn after sunset.

Observational history

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The comet was first spotted in images from the SWAN instrument onboard the Solar and Heliospheric Observatory (SOHO) by amateur astronomer Vladimir Bezugly on 11 September 2025. The presence of the comet was confirmed by other amateur astronomers, having an estimated magnitude of 7.4 and featuring a tail about 2 degrees long.[9] The comet upon discovery was located in the constellation of Virgo and it was better seen from the southern hemisphere, where it was higher in the sky after sunset.[8]

This is a time-lapse of C/2025 R2 (SWAN) captured in October 2025. It shows 40 minutes of movement. The core is going in the direction of the tail because the comet has already passed the sun and is leaving the inner solar system.

C/2025 R2 was not discovered earlier in part because of the Holetschek effect as the comet was less than 30 degrees from the Sun between August 7 – September 13. Additionally, since the comet wasn't detected by ground-based surveys while it was at opposition in February 2025,[c] the comet was probably inactive at the time, had a very low albedo, or its nucleus is less than a kilometer in radius.[6] Between August and September the comet had rapidly brightened from magnitude 11 to magnitude 8.[10] The comet peaked around an apparent magnitude of 6.2 on 16 September 2025, however it is currently unknown how quickly it will dim. The comet underwent an outburst later in September and its magnitude was reported to have reach mag 5.9. At that time it was in the same region of the sky as 3I/ATLAS and C/2025 K1 (ATLAS), the latter of which came very close to each other from the Earth's perspective on 29 September.[11] On 2 October, it passed near Zubenelgenubi.[8] It was around magnitude 6[12] during its closest approach to Earth on 20 October 2025. Coincidentally another comet, C/2025 A6 (Lemmon), was expected to be magnitude 4 around the same time. On 2 November 2025, a possible fragmentation event was observed from the Two-meter Twin Telescope (TTT3) of the Teide Observatory, with reports indicating that either the nucleus is splitting into two or it may be in the process of releasing a substantial fragment.[13]

C/2025 R2 is officially the 20th comet discovered through SOHO's SWAN instrument according to its discoverer, Vladimir Bezugly.[14][15][d]

Orbit

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C/2025 R2 (SWAN) came to perihelion one day after discovery on 12 September 2025 at a distance of 0.5 AU (75 million km) from the Sun.

Earth passed near the comet's meteoroid stream around 6 October 2025 and it had a chance of producing a weak meteor shower with a radiant only a few degrees from the Sun.[9] Meteor showers are more likely when a periodic comet is near perihelion as the debris from the previous passage will have only slowly spread out along the comet's orbit. But the distance to Earth's orbit (E-MOID) is a fairly large 0.048 AU (7.2 million km; 4.5 million mi), and the comet has such a long orbital period that the dust is likely too diluted to generate a shower.[e]

The comet made its closest approach to Earth at a distance of 0.26 AU (39 million km; 24 million mi) on 20 October 2025.[16][17] It will cross the celestial equator on 3 November 2025.

Orbital period

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The comet is dynamically old having come from 170 AU (about 6 times farther out than Neptune) and therefore did not come directly from the Oort cloud region. As the comet was discovered near perihelion, the closest approach to the Sun is well known. The orbital period and aphelion (farthest distance from the Sun) is somewhat constrained by the low spatial resolution of about 1° per pixel STEREO-A observations in August, and without the STEREO-A observations, there is a modest 47-day observation arc for the orbit determination. The inbound orbital period was about 787 years and the outbound orbital period is about 757 years.[3]

Physical characteristics

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Spectra of the central part of the coma of C/2025 R2 (SWAN) were taken with the Échelle spectrograph FLECHAS at the University Observatory Jena on 18 October 2025. In the wavelength range between 5000 and 7500 Å many emission features were detected in the spectrum of C/2025 R2 (SWAN), the most prominent ones are those of C2, NH2, and [OI]. In contrast to C/2025 A6 (Lemmon), no Na emission was detected in the spectrum of the coma of C/2025 R2 (SWAN).[18]

See also

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Notes

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  1. ^ Using JPL #6, the comet is expected to come to aphelion (farthest distance from the Sun) around the year 2405 at about 165.7 AU.
  2. ^ a b Since the surface brightness of a diffuse coma+nucleus is harder to detect than a point-like star of the same magnitude, a comet realistically needs reach an apparent magnitude of ≈4.5 to be visible to the naked eye under a rural dark sky. Magnitude 6 is a good target for 40mm binoculars and magnitude 8 is a good target for 70mm binoculars.
  3. ^ In February 2025, C/2025 R2 was about 2.4 AU (360 million km) from Earth and 3.4 AU (510 million km) from the Sun.[6]
  4. ^ This figure includes 273P/Pons–Gambart, which was recovered and identified through SWAN imagery in 2012. Therefore, C/2025 R2 is actually only the 19th comet discovered through SWAN alone.
  5. ^ Comet C/1861 G1 (Thatcher) with an orbital period of 422 years is known for the April Lyrids meteor shower. C/1911 N1 (Kiess) with an orbital period of ~2067 years is known for the Aurigids meteor shower.

References

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  1. ^ V. Bezugly; M. Masek; R. D. Matson; et al. (15 September 2025). D. W. Green (ed.). "Comet C/2025 R2 (SWAN)". Central Bureau for Astronomical Telegrams. 5606. Bibcode:2025CBET.5606....1B.
  2. ^ a b V. Bezugly; M. Masek; H. Sato; et al. (15 September 2025). "Comet C/2025 R2 (SWAN)". Minor Planet Electronic Circulars. 2025-R102. ISSN 1523-6714.
  3. ^ a b c d e Horizons output. "Barycentric Osculating Orbital Elements for SWAN (C/2025 R2)". Retrieved 29 October 2025. (Solution using the Solar System's barycenter (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0)
    Inbound period (PR) = 2.874E+05 / 365.25 days = 787 years
    Outbound period (PR) = 2.765E+05 / 365.25 days = 757 years
  4. ^ a b "C/2025 R2 (SWAN) – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Jet Propulsion Laboratory. Archived from the original on 15 September 2025. Retrieved 29 October 2025.
  5. ^ "C/2025 R2 (SWAN) Orbit". Minor Planet Center. Archived from the original on 16 September 2025. Retrieved 29 October 2025.
  6. ^ a b c W. G. Levine; Y. Kim; T. B. Spahr; T. Linder; Y. R. Fernández; et al. (2025). "Assessing the Potential Visibility of Comets like C/2025 R2 (SWAN) by NEO Surveyor". Research Notes of the American Astronomical Society. 9 (10): 262–264. Bibcode:2025RNAAS...9..262L. doi:10.3847/2515-5172/ae0f12.
  7. ^ "COBS Observation list: C/2025 R2". Crni Vrh Observatory. Archived from the original on 10 October 2025. Retrieved 9 October 2025. Comet C/2025 R2 (SWAN) Lightcurve
  8. ^ a b c E. Irizarry (14 September 2025). "New comet C/2025 R2 (SWAN) is becoming more visible". EarthSky.org. Retrieved 15 September 2025.
  9. ^ a b B. King (15 September 2025). "New Comet SWAN25B Pops Out from Behind the Sun". Sky & Telescope. Retrieved 15 September 2025.
  10. ^ Q. Zheng (15 September 2025). "SWAN25B was only recently reported as a comet in SOHO/SWAN data, but had actually been visible in STEREO-A HI1 imagery since August". Cometary.org. Retrieved 15 September 2025.
  11. ^ Nemiroff, R.; Bonnell, J., eds. (29 September 2025). "Two Camera Comets in One Sky". Astronomy Picture of the Day. NASA. Retrieved 29 October 2025.
  12. ^ S. Yoshida. "C/2025 R2 (SWAN)". www.aerith.net. Retrieved 24 September 2025.
  13. ^ M. Serra-Ricart; J. Licandro; M. R. Alarcon (2 November 2025). "C/2025 R2 (SWAN): Possible Fragmentation Observed". The Astronomer's Telegram. 17469: 1.
  14. ^ J. Rao (22 September 2025). "Could new comet C/2025 R2 (SWAN) become visible to the naked eye in October? Here's what we know". Space.com. Retrieved 23 September 2025.
  15. ^ D. Dickinson (15 September 2025). "New Bright Comet SWAN Could Perform a Surprise October Show". Universe Today. Retrieved 18 September 2025.
  16. ^ "Earth approach on 20 October 2025". JPL Horizons. Archived from the original on 15 September 2025. Retrieved 15 September 2025.
  17. ^ "Project Pluto: SWAN25B". Project Pluto (find_orb). Archived from the original on 15 September 2025. Retrieved 14 September 2025.
    (P 455 AND Q 117 +/- 60)
  18. ^ M. Mugrauer; K. U. Michel; E. Loesch (29 October 2025). "Follow-Up Imaging and Spectroscopy of Comet C/2025 R2". The Astronomer's Telegram. 17462: 1. Bibcode:2025ATel17462....1M.
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Astronomy Pictures of the Day

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