Will Comet A3 Tsuchinshan-ATLAS Shine Brighter Than Expected?

Now is the time to catch Comet A3-Tsuchinshan-ATLAS at dawn.

The window is now open. If skies are clear, set your alarm heading into this weekend to see Comet C/2023 A3 Tsuchinshan-ATLAS at dawn. We’re already seeing great views of the comet this week from southern observers and astronauts aboard the International Space Station. The visibility window is now even creeping up to the southern tier latitudes of the contiguous United States (CONUS). If fortune favors us, the comet could hit an easy naked eye magnitude +2 by next week, and forward scattering could even boost this into negative magnitudes… the rare term ‘daytime comet’ is even getting kicked around a bit in cometwatching circles.

But the span to see this comet will be brief indeed. For most northern hemisphere observers, the comet will be a bashful one, never reaching much more than 10 degrees above the eastern horizon about 45 minutes before sunrise on the week centered around September 29th.

The Story of Comet A3 Tsuchinshan-ATLAS Thus Far

We wrote about prospects for this comet for Universe Today previously just last month. China’s Tsuchinshan (Purple Mountain) observatory and the automated ATLAS (Asteroid Terrestrial impact Last Alert System) survey discovered the comet on January 9^th, 2023. I’ve seen the name abbreviated to simply ‘Comet A3’ or ‘Comet T-ATLAS’ in discussions on keystroke-conservative social media.

Likely a first-time visitor to the inner solar system from the distant Oort Cloud, the comet is on an orbit measured in millions of years. This may also be the one and only appearance of the comet in the inner solar system. That’s a good thing, in terms of dynamics and activity, as the comet may have never experienced the heat of the inner solar system in the past. The comet could well head towards permanent ejection from the solar system after perihelion.

Key dates coming right up include when the comet reaches perihelion this coming Friday on September 27^th at 0.391 Astronomical Units (AU, 36.4 million miles or 58.6 million kilometers) from the Sun, just interior to Mercury’s aphelion point. The comet then makes its closest Earth approach on October 12^th, at 0.556 AU distant.

Comet A3 Tsuchinshan-ATLAS will become more difficult to catch after October 7^th, as it heads in to the Solar Heliospheric Observatory’s (SOHO) LASCO C3 field of view and approaches less than 15 degrees elongation from the Sun. The comet makes a second evening reappearance mid-month, which will most likely be less than favorable as it heads away from us and back out of the inner solar system. We could, however, see something interesting in late October (if the comet survives perihelion) as the tail precedes ahead of the outbound comet. Simulations also suggest that the comet could sport a spiky sunward ‘anti-tail’ around this time, reminiscent of Comet C/1956 R1 Arend-Roland.

How the Comet is Performing Now

The comet seemed to be headed towards the long rolls of ‘great comets that weren’t’ this past summer, as it stalled at +10^th magnitude. Now, the trend seems to have shifted, as the comet is over-performing versus expectations. As of writing this, the comet stands at +3^rd magnitude and is rapidly brightening.

We’re already seeing signs of two tails (a dust fan, and a much fainter ion tail) forming in this week’s images of the comet. Forward scattering may help boost the visibility of the comet next week, as all those dust particles reach a maximum illumination angle as seen from our Earthly vantage point in early October. The comet’s orbit passes edge-on from our vantage point on October 14^th. The comet will seem to hang stationary low in the dawn next week, as it loops towards us, and then crosses between the Earth and the Sun.

How to See the Comet

The October apparition will be a tricky one for sure. A good strategy is to use binoculars in your dawn comet patrol. Start sweeping low to the eastern horizon about an hour before local sunrise. The +1^st magnitude star Regulus (Alpha Leonis) will make a good ‘guide star’ to find the comet. The star will be about an outstretched hand’s width to the observer’s lower right. The comet pairs with the slim waning crescent Moon on the morning of September 30^th. This will make a grand photo-op. That same Moon is headed towards an annular solar eclipse on October 2^nd.

Clouded out? We feel your frustration here in eastern Tennessee, as clouds from approaching hurricane Helene move inland this coming weekend. Astronomer Gianluca Masi will also carry the comet live on the evening of October 9^th.

“It (Comet T-ATLAS) survived and so far, it looks brighter than expected.” Astrophotographer Eliot Herman told Universe Today. “I still don’t think it will be amazing when it can be seen when dark enough… I am thinking maybe March 2013 Comet (C/2011 L4) PanSTARRS level – which was visible to the eye and pretty nice with a camera.”

We can only hope for a bright comet as depicted by astronomer Charles Piazzi Smyth’s painting of the Great Daytime Comet of 1843:

The Comet From the ISS

Astronauts aboard the International Space Station already caught the comet from their vantage point in low Earth orbit this week. NASA astronaut Matthew Dominick produced this fine animation:

Comet A3 Tsuchinshan-ATLAS is teasing us with the recent memories of two other dawn comets. Remember P1 McNaught in 2006-2007 and W3 Lovejoy in 2011-2012? Both beat the odds, and went on to become fine comets, ahead of expectations.

As always with comets, a caveat is in order: several factors will conspire against your cometary quest. First: as noted, the comet will appear very low to the horizon. This means it will fight against the thick murk of the atmosphere and the brightening twilight sky. Secondly, comets are intrinsically dark objects, with a low surface brightness or albedo… remember Rosetta’s views of Comet 67P Churumov-Gerasimenko? Lastly, like deep sky objects, all of that precious magnitude gets dispersed over an apparent surface area. This makes a +2 magnitude comet much fainter looking versus a +2^nd magnitude star. During F3 NEOWISE’s 2020 apparition, I could juuuust start to convince myself that it was naked eye when it reached around +1^st magnitude.

NEOWISE… or Nishimura?

We had two recent comets perform very similar to Comet A3 Tsuchinshan-ATLAS. In 2020, Comet F3 NEOWISE became a fine naked eye comet at dawn, wowing early morning observers. On the flip side, 2023’s Comet P1 Nishimura flirted with naked eye brightness, but never really became a general crowd pleaser.

Clear skies on your hunt this coming week, to see what’s most likely to be the best comet of 2024.