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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 led to its discovery. Of the two, 29P is much more active in that it exhibits about 20 outbursts per year ( strong events and mini-outbursts combined). 29P is much more observable than 17P occupying a near-circular orbit almost 1 au further from the Sun than Jupiter.

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 2014 with Jean-Francois Soulier, Kevin Hills, and Richard Miles (as part of the Faulkes Telescope Project), targeting it photometrically. Since then many amateurs have contributed to the campaign.

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

One likelihood is that they are both very slow rotators as there is evidence 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 the longer the surface cools during the night-time, 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. More on that later.

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)