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Introduction

The mega-outburst of  Comet 17P/Holmes on 2007 October 24 could be considered to be the origin of this observing campaign. The intensity of the 17P event (amplitude 13-14 mag) was entirely unprecedented and has left astronomers questioning our understanding of the nature of comets in general. Comet 17P was discovered in 1892 following an outburst similar in intensity to that of 29P in 1927, which also led to its discovery. Of the two, 29P is much more active in that it exhibits about 25 outbursts per year ( 40% are strong events, the remainder are termed ‘mini-outbursts’). 29P is much more observable than 17P occupying a near-circular orbit almost 1 au further from the Sun than Jupiter. It’s synodic orbital year is about 393 days and we usually manage to cover all but about 70 of these days when it is in conjunction with the Sun.

Further in-depth studies should help to reveal the underlying nature and mechanism of its outbursts. Amateurs are ideally placed to continually monitor its behaviour. The first intensive observations began in March 2014 with Jean-Francois Soulier, Kevin Hills, and Richard Miles (as part of the Faulkes Telescope Project), targeting it photometrically. Since then we have obtained >10 years coverage thanks to the many dedicated amateurs who have contributed to the campaign over the years, detecting 236 events (as of 2024 Nov 22). Of these, some 97 were strong events.

What do Comets 29P and 17P have in common?

One likelihood is that they are both very slow rotators as there is evidence from Spanish observers that 17P’s rotation period is about 44 days. In the case of 29P, our observations have confirmed a 57-day periodicity in its strong outbursts. Heating of the nucleus by the Sun has to be the trigger for outbursts since this must indirectly provide the thermal energy to power the process. If there is a preference for outbursts to occur near the morning terminator on the nucleus, with dust and gas ejected in the sunwards direction, then the transfer of momentum to the ejecta will have a strong tendency to slow the rotation rate of the nucleus. One hypothesis is that the longer the nucleus is exposed to sunlight, the more energy it absorbs. And once a very slow rotation is established, the more time the surface has to cool during the night-time (temperatures can approach 20 K), the more likely a crust forms by freezing out of volatiles in the near-surface. A consolidated crust holds pressure allowing thermal transfer to proceed via solid-, liquid- and gas-phases. The two comets differ in their prehistory with 17P likely to be much more thermally evolved given that it is in an eccentric Jupiter-family orbit that approaches to within about 2 au of the Sun. The big difference chemically is that 29P’s eruptions are dominated by the release of CO gas, quite unlike 17P. Hence we are almost certainly looking at a marked difference in their chemical constituents. Richard Miles and the late George Faillace wrote a paper on the possibility of melting in cometary nuclei published in the journal, Icarus in 2012.

Astrometrica and the rise in observations starting in 2002

Herbert Raab’s software for astrometry of asteroids and comets was a game-changer for studies of 29P. The method uses a small circular aperture, the size of which is defined by the user, superimposed on the pseudonucleus. The flux within the aperture is summed and the background sky flux is subtracted to obtain what is in effect the m2 nuclear magnitude. It first began to be used widely in 2002 from which time the number of observations reported to the Minor Planet Center (MPC) increased rapidly.

One issue with the early MPC reports is that the aperture size projected on the sky was not a standard size so when the m2 magnitude was calculated it varied by about ±0.5 mag. This meant that only the stronger outbursts were detected. Around about 2010, Richard Miles worked with Herbert to improve the photometric accuracy of his software. It turns out that the configuration settings need to be slightly different for photometry compared with astrometry.

(to be continued)