A new comet approaching from sunward could make a fine dusk appearance in October.
Hello. A bright new comet is now emerging from the Sun’s glare at dusk. It all started with discussion on the boards this past Friday September 12th, about a comet seen in the Solar Wind Anisotropies (SWAN) images near the Sun. SWAN is based on the joint NASA/ESA SOHO space observatory, which observes the Sun from the sunward L1 Lagrange point.
The new comet was initially named SWAN25B. The find comes courtesy of Ukrainian amateur astronomer Vladimir Bezugly, who first noticed the comet moving across the field of view in SWAN images.
"This is a milestone, the 20th official SWAN comet so far," Bezugly told Universe Today. "It was an easy comet for detection due to sufficient brightness in the UV (ultraviolet) band and location in the SWAN images, exactly in its center. But it was difficult due to the very close location to the Sun and angular motion, which is very close to the Sun's motion in SWAN images." Plus, hot pixels can cause spurious artifacts, mimicking the moving dot of a hidden comet.
Currently at a respectable +7.3 magnitude sporting a 2 degree-long tail, the comet seems to have spent the past month trailing in the sunward direction, thus eluding detection. This phenomenon is known as the Holetschek effect, an observational bias favoring the discovery of comets with close passages near the Sun post-perihelion. It’s currently swamped by twilight for northern hemisphere observers low in the dusk near Mars and Spica, but that could soon change in October, as the comet makes a 0.25 Astronomical Unit (AU) pass near Earth in October.
What can we expect from the comet in the coming weeks? "I remembered the words of the famous Comet Hunter David Levy," says Bezugly. "Comets are like cats; that have tails, and do precisely what they want."
Roger Hellot caught this fine image of Comet R2 SWAN on September 13th, using the AITP (Alsace Internet Telescope Project) instruments based in Chile:
STEREO-A may have also been tracking the same object crossing its field of view in August. This gives observers a 1-month observation arc to deal with, as the comet’s orbit is further refined this week.
As of writing this, the orbit of the comet is very preliminary. The more observations are made, the better we’ll understand the comet’s past and future trajectory. The comet has just now received a formal IAU designation today on Monday, September 15th as Comet C/2025 R2 ATLAS, as it was discovered in the early half of September.
It appears that the comet reached perihelion 0.5 AU from the Sun on September 12th, just exterior to Mercury’s aphelion and about 75 million kilometers from the Sun. Early calculations suggest that Comet R2 SWAN is on a 20,000+ year orbit. This makes it a long period comet, but not a first time visitor to the inner solar system. It also seems that it was caught during a post-perihelion outburst, another plus. The comet already seems to be unfurling a long tail. Remember, comet tails actually precede ahead of a comet on its outbound leg, as the solar wind streams past. Comet R2 SWAN will pass about 0.26 AU (39 million kilometers) from Earth on October 19th.
If the comet brightens as expected (always a big ‘if’ when it comes to comets) it could top out at magnitude +4.
“If the comet continues to behave as it does now, we will have a great comet in October, also in the northern hemisphere,” astrophotographer Gerald Rhemann told Universe Today.
Right now, the comet is lost low in the murk to the west at dusk for northern hemisphere observers, but that will swiftly change in the coming weeks, as it vaults up along the ecliptic into nighttime skies.
Spica and Mars currently provide good guideposts to spot the elusive comet. The Minor Planet Center has preliminary Right Ascension and Declination coordinates by date to help you in your cometary quest. If this one holds up, I’d expect it to become a decent binocular comet in the west at dusk starting into the last half of September.
Here’s a tentative list of celestial dates with destiny for Comet R2 SWAN through early November. ‘Passes near’ denotes a within one degree pass, unless otherwise noted, Also, these are based on early orbital elements courtesy of Project Pluto:
September
15-Passes 20’ from the bright +1st magnitude star Spica.
20-Passes just over a degree from the planet Mars.
28-Crosses into the constellation Libra, and crosses the ecliptic plane northward.
October
2-Passes near the +2.8 magnitude star Zubenelgenubi (Alpha Librae).
9-Passes near the +4.1 magnitude star Theta Librae.
10-Crosses into the constellation Scorpius.
12-Crosses into the constellation Ophiuchus.
14-Passes near the +2.5 magnitude star Sabik (Eta Ophiuchi).
15-Crosses into the constellation Serpens Cauda.
18-Photo-op: Passes between the deep-sky objects Messier 16 and Messier 17, and crosses the galactic plane.
20-Crosses into the constellation Sagittarius.
21-Crosses into the constellation Aquila.
24-Nicks the corner of the constellation Capricornus.
26-Crosses into the constellation Aquarius.
November
3-Crosses the celestial equator northward.
From there, the comet continues its centuries-long journey out to an aphelion thousands of Astronomical Units from the Sun, far beyond the orbit of the dwarf planet Eris. There’s also the interesting possibility that the Earth will cross the point were the comet crossed the ecliptic plane on or around October 5th, and this ‘may’ spark a meteor shower as seen from the Earth.
Of course, this is all very preliminary, as the comet’s orbit is worked out. There’s lots more to come from comet R2 SWAN, as it takes the celestial center stage at dusk. Great work and congrats to all who were involved in the find… amateur comet-hunting isn’t dead, it has just moved online.