Comet Tsuchinshan-ATLAS Set to Perform This Fall

Comet Tsuchinshan-ATLAS may be one to watch for at dawn late next month.

If predictions and prognostications hold true, a decent comet could grace dawn skies in late September into early October. We’re talking about Comet C/2023 A3 Tsuchinshan-ATLAS, discovered early last year. Early signs suggest it could be the best comet of 2024… if it survives until perihelion.

The Discovery

The comet was discovered jointly by the Tsuchinshan (Purple Mountain) observatory in China and the ATLAS (Asteroid Terrestrial-impact Last Alert System) sky survey on January 9^th, 2023. Later ‘pre-discovery’ images date back to 2022. The comet was discovered 7.2 Astronomical Units (AU) out in the outer solar system, beyond the orbit of Jupiter. This always a good sign in terms of how the comet will behave on approach.

Early estimates put the orbital period for the comet at 80,000 years. Later refinements on the orbit now puts it at millions of years, possibly due for ejection post perihelion. This means that C/2023 A3 Tsuchinshan-ATLAS is likely a first-time visitor from the Oort Cloud, and should be dynamically new. This another plus and always a good thing in terms of activity.

Following the Comet Through Fall

The path of the comet seems to hang stationary in the constellation Sextans through September, as it is headed towards us as seen from our vantage point in space. The comet is on a high 139º inclination retrograde orbit about the Sun. Perihelion 0.39 AU from the Sun interior to the orbital aphelion distance of Mercury occurs on September 27th, and the closest Earth approach for the comet occurs on October 12^th, at 0.472 AU (70.6 million kilometers) distant.

Southern hemisphere observers may get a short look at the comet starting in mid-September. The best show for folks up north begins in the last week of September into the first week of October, when the comet sits about 10 degrees above the eastern horizon in the dawn. The slim waning crescent Moon will pair with the comet on the mornings of September 30^th and October 1^st.

Said Moon is headed towards an annular solar eclipse on October 2^nd. Lucky observers along the path crossing the southern tip of South America might just see the comet along with Venus during annular phases, if (a big ‘if’ to be sure) it exceeds expectations and reaches negative magnitudes.

We’ll also get a look at Comet Tsuchinshan-ATLAS courtesy of the joint solar observing SOHO mission, as it transits the LASCO C3 field of view from October 7^th to the 10^th. From there, the comet will transition to the evening sky in late October, as it fades back down into binocular visibility range and heads back out of the solar system, perhaps to never return again.

Dazzle, or Fizzle?

It has been a roller coaster ride for the comet in 2024. How bright will Comet Tsuchinshan-ATLAS be at perihelion? The comet seemed to be under-performing in terms of brightness in early 2024, suggesting a breakup and a ‘fizzle’ could be imminent. As of writing this, the comet is back up on the expected light curve at magnitude +8. Comets often neglect to read predictions, and can fail to measure up to expectations… we all remember ISON in 2013. On the plus side, remember F3 NEOWISE in 2020, which actually exceeded anticipations? One factor will aid the visibility of comet Tsuchinshan-ATLAS right around Earth approach: its brightness may be helped a bit by an effect known as forward scattering. And of course, all bets are off in terms of brightness in the event of a well-placed outburst.

“A3 (Tsuchinshan-ATLAS) is now sort of on the brightening line to get mostly as bright like predicted, although my own projection shows it peaking more like magnitude +4 to +5 because the trend line is a bit south of the optimum line…” astrophotographer Eliot Herman told Universe Today. “So it will not be (naked eye) visible at brightest, but…it will be a fine telescopic comet (or ‘camera comet’) kind of like P1 Nishimura. That is what I am thinking unless it shatters or splits. I think it is going to fall short of imagination, which is about the norm for comets.”

Here’s a Month-by-Month look at Comet Tsuchinshan-ATLAS:

September

7-Flips over to the dawn sky

27-Reaches perihelion

28-Crosses into the constellation Leo

October

1-The waning crescent Moon sits 11º from the comet

2-The Comet is 18º from the Sun during an annular solar eclipse

4-Crosses into the constellation Virgo

7-Crosses into SOHO’s LASCO C3 field of view

8-Crosses the ecliptic plane northward

9-Passes 3º from the Sun

10-Exits SOHO’s LASCO C3 field of view

11-Flips over to the dusk sky

12-Passes closest to the Earth

14-Crosses the celestial equator northward, and makes an edge-on, orbital plane crossing

15-Photo-Op: Passes 1º from +9th magnitude Comet 13 Olbers

15-Crosses into the constellation Serpens Caput

16-Passes just over 1º from the globular cluster Messier 5

19-Crosses into the constellation Ophiuchus

20-Passes near the +3.8 magnitude star Lambda Ophiuchi (Marfik)

29-Passes between the +3^rd magnitude stars Cebalrai and Gamma Ophiuchi

November

1- Passes near the +3.9 magnitude star 67 Ophiuchi

10-Passes into the constellation Serpens Cauda

20-Passes very near (occults) the +4.6 magnitude star Alya

25-Passes into the constellation Aquila

26-Crosses the Celestial Equator southward

In December into early 2025, the comet drops back below +10^th magnitude.

Keep in mind, like deep-sky targets, all of that precious quoted magnitude for a comet is ‘smeared out’ over an apparent surface area. We usually think of the naked eye cutoff for stars under a good dark sky is +6, but a comet generally won’t reach naked eye visibility until about +3 magnitude or so. This is also usually the point at which a given comet becomes bright enough to capture along with foreground objects, always a photogenic sight.

Good luck and clear skies on your quest to see comet Tsuchinshan-ATLAS, on what is very probably its one-time only visit to the inner solar system.