Comet Tsuchinshan-Atlas is ready to shine this fall

If predictions and forecasts come true, a beautiful comet could appear in the starry sky in late September or early October this year. We’re talking about Comet C/2023 A3 Tuchinshan-ATLAS, which was discovered early last year and is eagerly awaited by many amateur astronomers and astrophotographers. Early signs suggest that this could be the best comet in recent years… if it survives perihelion, the point closest to the Sun.

The comet was jointly discovered by the Zuochenshan (Purple Mountain) Observatory in China and the ATLAS (Asteroid Terrestrial Impact Last Alert System) sky survey on January 9, 2023. Subsequent “pre-discovery” images date back to 2022. The comet was discovered 7.2 astronomical units (AU) beyond the solar system, beyond the orbit of Jupiter. This is always a good sign of how a comet will behave during its approach. Initial estimates put the comet’s orbital period at 80,000 years. Subsequent orbital refinements now estimate it to be millions of years, probably due to post-perihelion ejection. This means that C/2023 A3 T suchinshan-ATLAS is likely a first-time visitor to the Oort Cloud and should be new. This is another feature that is always good for comet activity.

The comet’s path appears to remain steady in the constellation Sextant through September as it moves toward us from our vantage point in space. The comet is in a retrograde orbit around the Sun at a high inclination of 139 degrees. Perihelion falls within Mercury’s apogee of 0.39 AU from the Sun on September 27, and the comet will come closest to Earth at a distance of 0.472 AU (70.6 million km) on October 12. Observers in the Southern Hemisphere can see the comet briefly from mid-September. The best viewing for northerners is from the last week of September through the first week of October, when the comet is about 10 degrees above the eastern horizon at dawn. The waning crescent moon pairs up with the comet on the mornings of September 30 and October 1. We’ll also get a look at Comet Tuchinshan-ATLAS thanks to the SOHO joint Solar Observing Mission, which will pass through LASCO C3’s field of view from October 7 to 10, 2024. From there, the comet will be visible in the evening sky in late October, as it returns to the binocular range and leaves the solar system, perhaps never to return.

The graph above shows the current brightness of the comet (black and blue dots) based on observations.
The green line is a prediction of brightness – Illustration: astro.vanbuiten.nl

There has been a lot of buzz around this comet. How bright will comet Tsuchinshan-Atlas be at perihelion? This is undoubtedly the most important question every amateur astronomer is asking themselves right now. The comet has been showing a lackluster performance in terms of brightness in early 2024, suggesting that the comet’s disintegration or breakup may be imminent. At the time of writing (August 21, 2024), the comet has returned to its expected light curve of around magnitude +8. Comets often don’t match expectations and can’t live up to expectations… We all remember Comet ISON in 2013. On the other hand, remember F3 NEOWISE in 2020, which even exceeded expectations? One factor that will help with seeing comet Tsuchinshan-Atlas as it approaches Earth is that the comet’s brightness may be slightly boosted by an effect known as forward scatter. Of course, a well-placed rush can boost the brightness even more.

Keep in mind that, as with deep sky objects, all that precious comet volume is “smeared” onto the apparent surface. We usually think of the maximum number of stars visible to the naked eye in a good dark sky as +6, but a comet is usually only visible to the naked eye from around +3. This is also the point at which a comet becomes bright enough to be seen with the naked eye. This is also usually the point at which a particular comet becomes bright enough to be captured alongside foreground objects, always creating an attractive image.

Some important dates for the appearance of Comet Tsuchinshan-Atlas:

• September 7: Comet visible in morning sky
• September 27: The comet reaches the perihelion.
• September 28: The comet is visible in the constellation Leo (The Lion).
• October 4: The comet appears in the constellation Virgo (The Virgin).
• October 11: The comet becomes visible in the evening sky.
• October 15: The comet appears in the constellation Serpens Caput.
• October 16: The 1-magnitude comet can be seen near the globular star cluster Messier 5.
• October 19: The comet is visible in the constellation Ophiuchus.

Comet Tsochinshan-Atlas photographed on August 1, 2024 from Namibia – Photo: Bibi Shambhu