Figure 1: Comet 12P on 2023-07-26 observed at ASERO. Note the “notch” in the cloud expanding from the outburst on July 20th

by Douglas Heggie

Comet 12P/Pons-Brooks is a historic, long-period comet which is returning to perihelion next spring, after a gap of 71 years. As it approaches the sun it warms and brightens by occasional explosive outbursts. After the first of these, in late July, ASE observers started to monitor the comet, and were quick enough to catch the rapidly expanding explosion cloud, with its characteristic “notch”.

Fortunately ASERO came on stream just in time, and in subsequent weeks it frequently imaged the now more slowly expanding cloud or “coma”, still showing its notch, and surrounding the small, bright “nucleus”. We accumulated 21 observations up to October 2, measuring the magnitude of the comet and nucleus, and the diameter of the coma, and noting even the beginnings of a faint tail.

But the biggest surprise was to come just 3 nights later, when we suspected that the nucleus was unusually bright, and measurement showed it was three magnitudes brighter than it had been on October 2 (about the difference between Sirius and Polaris). As it turned out, we had been fortunate to image it within about 24 hours of the time when the second outburst happened. Other observers also noticed the change, and showed that it had occurred between 4th-5th October.

We’ll continue to monitor the comet in the coming weeks, hoping once again to observe the emergence of a new expanding component of the coma, and perhaps further outbursts. The comet is still quite distant, about 3 AU from the Earth and Sun (about the distance of the asteroid belt). The tail will strengthen and the magnitude is expected to brighten to maybe 5 by the time of perihelion. Around then the comet will become difficult to observe for us, but up until the first few weeks of 2024 it will remain well placed in our evening sky.

As they come in, images and measurements are posted on the 12P Project web page of the imaging group, and all ASE members are strongly encouraged to add their own images and data, whether from ASERO or other instruments, and, in due course, perhaps even the naked eye!

12P 2023-08-10

Figure 2: The comet on 2023-08-10. Note the “notch” and the “nucleus”. The latter is simply the brightest central part of the coma (just below a slightly brighter star here), where the icy and dusty true nucleus of the comet lies, but the true nucleus is much too small to be resolved in ground-based images.

12P 2023-10-05

Figure 3: On October 5 the nucleus, just after outburst, outshines the coma, and the "notch" is no longer distinguishable. The sky background is slightly brighter to the left, which is where the faint tail is emerging.

Main banner image by Ramsay McIver, 2023-10-03 using ASERO data