Get Ready for Comet Tsuchinshan-ATLAS — The Best Is Yet to Come!

I'm mostly a nighttime observer, but Comet Tsuchinshan-ATLAS (C/2023 A3) has turned my schedule upside down. For the past 10 days, I've been up at dawn, peering through the horizon murk for brief views of the comet, then sticking around for an extra hour to see sunup. In that time I've met a whole new group of people — runners, birders and of course, sunrise-watchers.

From the northern U.S., the comet has primarily been a photographic object too faint to see without binoculars. But those in the southern U.S. and southward, where the comet stands higher and suffers less from atmospheric absorption, have seen a bright, strongly condensed coma and a tail more than 15° long. Observers there simply have to look up, and there it is. Even smartphones suffice for capturing the comet with a favorite twilight scene.

Groups devoted to comet-observation on Facebook such as Comet Watch and ICQ Comet Observations are brimming with images of the Oort Cloud emissary. With a current magnitude of about 1.5, it's the brightest comet since NEOWISE (C/2020 F3) in 2020.

And we're only getting started. Tsuchinshan-ATLAS is right in line with the most optimistic brightness models described in our earlier post as it prepares to transition to the evening sky on October 9th. Closest approach to Earth — it'll come within 70.5 million kilometers — occurs three days later. Given October's early sunsets, the comet's timing couldn't be better both for amateurs and the viewing public with its armada of smartphone cameras. I suspect this icy dirtball will become the most photographed comet in history.

Still a Few Mornings Left

The comet remains visible low in the dawn sky, approximately 45 minutes to an hour before sunrise — that will be true through about October 4th for observers at latitude 45°N, Oct. 5th for those at 40°N, and Oct. 6th for those southward of latitudes 25°-30°. The comet should rapidly brighten as its dust-rich coma and tail strongly forward-scatter the sunlight. On the last date of visibility, the comet should gleam around magnitude –1.

Then the comet slides into the daytime sky; predictions suggest it will rocket to magnitude –4.5 by October 9th. Even when its head is doused by the solar glare, photographers may still be able to photograph the tail standing up from the horizon in the relatively dark skies of early dawn. Time-exposures will capture its extreme limits.

Given those forecasted magnitudes, I won't be the only one attempting to see the comet in daylight, whether through a carefully positioned telescope or by hiding the Sun behind a post or roofline and searching for the comet's position with binoculars. Whichever way you choose, be extremely cautious in two regards — never look directly at the Sun even for a moment or you'll put your vision at risk. Second, be sure your instrument is protected from direct solar rays that might damage a component of the optical system.

A less perilous alternative is to watch the comet pass through the field of view of the LASCO C3 coronagraph of the ESA-NASA Solar and Heliospheric Observatory (SOHO) from Oct. 7th through 10th.

Act II: Spectacle at Dusk

When C/2023 A3 first appears at dusk on or about Oct. 11, the dust tail will point downward toward the horizon but then quickly flip up in the anti-solar direction by Oct. 13. In the same way we might photograph the comet's lengthening tail at dawn we can do the same in the evening sky beginning at that time, albeit diluted by a modicum of moonlight.

The tail will likely exceed 20-25° during early-mid October as the comet's overall magnitude fades from around –1.6 on Oct. 11 to 0 on Oct. 13. My simple attempt at illustrating the comet's appearance at the start of the evening apparition (above) shows how the tail angle rapidly changes in just two days time. To see the "full story" from dawn through solar conjunction to dusk take a look at Nicolaus Lefaudeux's excellent simulation below.

Keeping Tabs on the Weather

Although Comet Tsuchinshan-ATLAS will be visible in both hemispheres, the northern one is favored because the comet tracks north. Also, sunsets are getting earlier and twilights shorter, while the opposite is happening in southern latitudes.

Observers should be aware that the Moon will interfere for several nights, from about Oct. 15-20 (full Moon is on Oct. 17th), around the same time the comet climbs out of twilight. Something tells me that won't stop any of us from taking advantage of every opportunity to look at and photograph the comet!

To aid in finding a clear sky to do so I suggest using a weather app. I'm sure most of you already have a favorite, but in case you don't I recommend the free Windy app. It offers easy-to-read nighttime cloud maps and a handy two-hour time-lapse feature that reveals weather trends, so you can see at a glance whether clouds are coming or going.

What to Watch For

C/2023 A3's path takes it across Serpens and Ophiuchus this month. On the night of Oct. 15th, it will pass just 1.1° south of the glorious 6th-magnitude globular cluster M5 — a not-to-miss opportunity for photographers.

Besides a prominent dust tail, the comet will also exhibit a fainter ion (gas) tail, best viewed in binoculars and telescopes. Time-exposures clearly reveal its vivid blue color due to carbon monoxide emission stimulated by ultraviolet sunlight. Watch for kinks and blobs within the tail, which are caused by its interaction with the variable solar wind. Telescopes and especially cameras are excellent for detecting synchronic bands in the dust tail. These occur when chunks of comet material break off, crumble into dust and get spread into linear streaks by solar radiation pressure.

One of my favorite things to look for are jets and fountains of dust emanating from the comet's pseudo-nucleus, the dense ball of dust and gases surrounding C/2023 A3's true nucleus, estimated to be about 2 kilometers (1.2 miles) across. Use high magnifications — 250× and up — to discern these low-contrast features.

Earth crosses the comet's orbital plane on Oct. 14th, when we'll view its orbit edge-on. This special perspective is expected to create a prominent anti-tail and other enhancements:

"The orbital plane crossing won't just cause an overall brightening of the normal dust tail," writes Qicheng Zhang, a postdoctoral fellow working on small-body astronomy at Lowell Observatory in Arizona), "but will create the appearance of a sharp, bright line (from the dust in the orbital plane, viewed edge-on), superimposed on top of that normal dust tail, that should also extend far in the opposite direction (likely many degrees) toward the Sun. This effect will give the comet a distinctive appearance different from most other recent comets." Comet Arend-Roland (C/1956 R1) sported a similar dusty goatee back in 1957.

Another Bright Comet Coming?

The Asteroid Terrestrial-impact Last Alert System (ATLAS) spotted a new comet on Sept. 27th, now confirmed as a Kreutz sungrazer. Kreutz comets are thought to be fragments of a once much-larger object that broke apart. The most famous of them is Comet Ikeya-Seki from 1965. All have perihelia that bring them searingly close to the Sun.

This one passes perihelion on Oct. 28th at a distance of only 1.2 million kilometers (745,650 miles). Should it survive the heat and tidal forces, we could be in for a spectacular show. Designed Comet ATLAS (C/2024 S1), the object is currently around 12th magnitude in the morning sky in Hydra. As the orbit and predictions are refined, we'll post them here. Wishing clear skies and many happy (comet) trails!