the nucleus shape, the spin axis orientation, and the distribution of activity on the comet’s surface can explain the water production rate light-curve as a function of the heliocentric distance.

Around the maximum of the activity, the production rates are almost the same as those measured in the previous apparitions showing that there is no decrease of the activity level of 21P over the last five orbits. Our production rates agree usually very well with those derived by [@Schleicher2018] who used the same technique, while we noticed a discrepancy at large heliocentric distance and post-perihelion in the 1985 and 1998 apparitions data [@Schleicher1987; @Schleicher2018]. This could be due to a sensitivity issue in their data as the production rates seem to level off on both sides of perihelion while the distance increase or to a higher activity level of the comet after perihelion in the past. It has been also found that there is no significant change in the production rates of hypervolatile molecules (CO, CH$_4$, and C$_2$H$_6$) in comet 21P over the three different apparitions, 1998 [@Weaver1999; @Mumma2000], 2005 [@DiSanti2012] and 2018 [@Faggi2019; @Roth2020].

 H$_2$O production rate {#water-production-rate}
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The water production rate is the most significant indicator of 