apparition, [@Combi2011] found a value of 5.80$\times$10$^{28}$ molec/s from the H Ly-$\alpha$ emission observed by the SWAN/SOHO at r$_h$=1.08 au. Comparing these data, we found that the water production rates measured by [@Combi1992] in 1985 and [@Combi2011] in 2005 are systematically higher by a factor of about two with respect to our results in 2018. Such an offset between various techniques has been reported in previous studies and as early as comet 1P/Halley [@Schleicher1998]. The origin of this discrepancy is not clear but it is obvious that there is a good agreement when the same technique is used. This indicates that the level of activity of 21P was the same over the past four decades and did not decrease like comet 41P/Tuttle–Giacobini–Kresak which has been losing as much as 30% to 40% of its activity from one orbit to the next [@Moulane2018].

Active area of the nucleus {#active-area}
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To estimate the active area of the nucleus’ surface, we modelled the water production using the sublimation model of [@cowan1979]. Due to the low thermal inertia of cometary nuclei [@gulkis2015], the slow-rotator approach was adopted in a number of cases as the most appropriate way to compute the cometary outgassing [see 