2024 April 21

Comet C/2023 A3 (Tsuchinshan-ATLAS) Observing campaign

Nick James

C/2023 A3 (Tsuchinshan-ATLAS) was discovered at the start of 2023 when it was 7.5 au from the Sun. It has the potential to be a bright comet in the evening sky this October.  The comet has been brightening slowly over the last year and it is now approaching a total magnitude of around 10 as it comes within 3 au of the Sun. It also shows a short tail to the east. The tail is actually pointing almost directly away from us so this short tail is a projection of a much longer tail onto the plane of the sky.

The comet is a good imaging target in the evening sky for the next few months before it disappears into the evening twilight. This objective of this observing project is to track the near nucleus brightness of the comet in order to identify any outbursts. This project uses an identical technique to the one used for 12P/Pons-Brooks in that we are using a nominal 9 arcsec radius photometric aperture with Astrometrica, unfiltered imaging and Gaia G as a reference catalogue. Details of how to contribute to the C/2023 A3 observing programme and submit your observations are here. The current magnitude in the 9 arcsec aperture is around 12.

2024-04-21. So far the lightcurve is not very interesting but, as with all comets, we never quite know what to expect so please send in your observations. If possible please also send in observations made earlier this year or last year so that I can add the data to the lightcurve and fill in the big gap between 2023 June and 2024 March although for some of this time the comet was unobservable since it was in conjunction.

Update: 2024-05-06 – The recent lightcurve shows a clear dip in brightness. The comet was at a very low phase angle in mid April and so we were probably seeing a brightness enhancement due to the opposition effect and the inclusion of some contribution due to the tail. The comet does appear to be very dusty.

Update: 2024-05-19 – The recent decline continues although it does appear to be slowing down. The comet is now around a magnitude below the prediction using observations up to mid April. See the comets-ml discussion here.

Update: 2024-05-29 – The comet’s fade appears to have stopped and the magnitude in a 9 arcsec radius has been around 12.4 for the last couple of weeks.  At present the comet is approaching the Sun but is moving away from the Earth. There are lots of images in our section archive here. Many thanks to all of the observers who have been sending in data.

Update: 2024-06-06 – There are hints in the data that the comet is starting to brighten in a 9 arcsec radius aperture.  Due to the Earth’s motion around the Sun the comet is currently moving away from us and will continue to recede until the end of July. From 2024 May through to late August the distance from Earth stays in the range 1.75 – 2.05 au. The distance from the Sun in the same period drops from 2.8 to 0.8 au.

Update: 2024-07-10 – A new paper by Sekanina suggests that the comet is currently fragmenting. Our data shows no evidence of that. Astrometry shows no biases and the photometry is smooth.

Update: 2024-07-21: The comet is now brightening rapidly as it approaches the Sun. No sign of any fragmentation in our data.