The brightest comet in nearly 20 years — comet G3 ATLAS (C/2024) — is expected to reach its peak brilliance later this week into early next week.
The only question is: "Will you actually be able to see it?"
This celestial paradox belongs to Comet 2024 G3 (ATLAS), which was first sighted on April 5 of last year by the
Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, in images obtained with a 0.5-meter (19.7-inch) reflector telescope located in Rio Hurtado, Chile. At the time of discovery, the comet was 407 million miles (655 million km) from
Earth and shining at an exceedingly faint
magnitude of +19. That's roughly 158,000 times dimmer than the faintest star visible to the naked eye.
Returning after 160 millennia
A preliminary orbit for this comet indicated that it was going to pass exceptionally close — less than 9 million miles (14 million km) from
the sun in mid-January 2025; only about one-quarter the distance of
Mercury, the closest planet to the sun. However, these initial calculations also suggested that G3 ATLAS was a new comet coming directly from out of the
Oort cloud, a vast bubble composed of countless billions of icy objects encircling our
solar system, located perhaps 10 trillion miles (16 trillion km) from the sun. Small sun-skirting
comets originating from the Oort Cloud, making their first approach to the sun, often disintegrate before reaching perihelion (their closest approach to the sun).
But after G3 ATLAS’s orbit was refined, it was determined that it was actually a dynamically old comet, having made at least one previous close approach to the sun, with an orbital period of roughly 160,000 years. This provided hope that — having survived a prior close encounter with the sun — this comet could survive its impending close brush and possibly put on a bright display after the start of the new year.
Unfortunately, to date, the predicted path of G3 ATLAS has only favored those in the Southern Hemisphere. Practically the entire predicted track of this comet lies through
constellations well south of the celestial equator. For those in the Northern Hemisphere, the comet’s path over the past month has taken it through the southern constellations of Lupus and
Scorpius, and it currently resides in Sagittarius.
The path of Comet G3 ATLAS across constellations -- primarily of the southern skies --is depicted by Japanese comet expert Seiichi Yoshida. (Image credit: Seiichi Yoshida)
This means that for observers north of the equator, the comet has been positioned much too low on the southeastern horizon and perpetually obscured by the glow of bright morning twilight.
However, as the comet is now coming down the “home stretch” before it whips around the sun, it has begun to brighten dramatically. For a relatively short time, its path will curve just far enough north to give those north of the equator a fighting chance to glimpse it.
Breakup avoided?
Recent observations collected on the
Comet Observation database (COBS) suggest that, on Jan. 2, 2025, G3 ATLAS experienced a sudden outburst, showing a nearly four-fold increase in apparent brightness. However, John Bortle, a well-known and highly reputable comet observer, expressed ambivalence about this unexpected brightness spike. In an email to Space.com, Bortle noted:
"The apparent instability exhibited in the recent magnitude increase usually can be interpreted as a bad sign that the comet is showing evidence of breaking up, but some other unusual features that have been reported such as the 'shadow of the nucleus' is a classic feature of any large comet near perihelion, and not a sign of disruption."
Still, Bortle is uncertain about G3 ATLAS's ultimate fate, "I think chances of its survival are about 50:50," Bortle noted.
Another noted comet observer, Nick James of the British Astronomical Association, also commented on the comet’s latest surge in brightness.
"The sudden jump in brightness was significant. This could have been an early sign of disintegration, but since then, the comet is still brightening. It looks as if this outburst has not been fatal," James
told Spaceweather.com.
How bright will it be?
G3 ATLAS will always be positioned very near the sun, raising the main question: Will it get bright enough to be readily seen against the twilight sky? Unfortunately, that is difficult to predict. Like snowflakes, no two comets are exactly alike. Astronomers have developed mathematical equations to forecast brightness, but it’s especially challenging for new or long-period comets like G3 ATLAS, where no prior visits are recorded.
On January 4, Daniel Green of the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts,
issued an updated circular on G3 ATLAS, forecasting its brightness based on its development in recent weeks.
The comet is expected to reach perihelion on Jan. 13 at 10:17 Universal Time (UT), at a distance of 8,695,216 miles (13,993,594 km) from the sun. Later that same day, it will come closest to Earth at a distance of 87.1 million miles (140.1 million km).
Green predicts the comet could reach a maximum magnitude of -3.2, placing it between the brightness of
Jupiter and
Venus, which are prominent in our evening sky. "The comet will be close to the sun when brightest, but it’s becoming difficult now even for Southern Hemisphere observers to see," Green told Space.com. "It’s really hard to call these comets when close to the sun like this, in terms of visibility."
Indeed, although Green's forecast suggests G3 ATLAS will get quite brilliant, its close proximity to the sun makes it rather debatable as to whether a casual observer will be able to pick it out against the bright twilight sky. If the comet were to become brighter, however, it might be easier to see.
For that to happen, we would need to consider forward scattering.
Forward scattering: The 'dusty windshield effect'
Forward scattering in comets is a phenomenon that occurs when sunlight is scattered by dust particles in a comet's tail and coma, causing the comet to appear brighter. This happens when a comet is between the Earth and the sun, or when the Earth-comet-sun angle is high. Forward scattering can make a comet appear significantly brighter, especially when the phase angle is high. It is similar to the effect of driving toward the sun with a dusty windshield.
Forward scattering in comets occurs when sunlight is scattered by dust particles in a comet’s tail and coma, causing the comet to appear brighter. This phenomenon happens when a comet is between the Earth and the sun, or when the Earth-comet-sun angle is high. Forward scattering can significantly enhance a comet’s brightness, especially when the phase angle (Earth-comet-sun angle) is high. It’s similar to the effect of driving toward the sun with a dusty windshield.
In the case of G3 ATLAS, its phase angle is predicted to reach a maximum of 115 degrees late on Jan. 13. How much this will enhance the comet’s brightness is debatable. From my own calculations, I estimate the comet could brighten to a magnitude of -4, putting it on par with Venus. However, others predict an even greater enhancement. The most extreme prediction comes from Dutch amateur
astronomer Gideon van Buitenen, who forecasts a peak magnitude of -6, nearly three times brighter than Venus!
Still, the odds of this extreme brightening happening are low.
When to look for comet G3 ATLAS
As of now, there appears to be a three-day window — Jan. 12 through 14 — for observers in the Northern Hemisphere to glimpse G3 ATLAS.
Your first opportunity will come early on Jan. 12. From mid-northern latitudes, the comet will rise about 35 minutes before sunrise and will be positioned 5 degrees directly above the sun at local sunrise. So, beginning about 25 minutes before sunrise, slowly scan/sweep along the east-southeast horizon with binoculars to try and sight the comet.
On the evening of Jan.14 , the comet will set 35 minutes after the sun and at local sunset will be positioned about 7 degrees almost directly above the sun. So, beginning about 10 minutes after sunset, slowly scan/sweep along the west-southwest horizon with binoculars to try and sight the comet. Continued.......
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@kofi_Davies1 Friday would be very interesting
