with respect to the Earth. More details about the UVES data reduction are given in the UVES manual[^4]. The dust-reflected sunlight was finally removed using a reference solar spectrum BASS2000[^5]. More in depth description of the steps for UVES data reduction and the solar spectrum subtraction is given in [@Manfroid2009] and references therein.

Activity and composition {#activity}
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Gas production rates
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Along with our monitoring of comet 21P with the TRAPPIST telescopes, we derived the OH, NH, CN, C$_3$, and C$_2$ production rates. They are summarized in Table \[tab:rates\] and their evolution as a function of the heliocentric distance are compared with two previous passages in Figure \[fig:gas\_rate\].

We started to detect most of the radicals in the coma by the end of June 2018 (except NH one month later). The various production rates and the dust activity have been increasing slowly as the comet was getting closer to the Sun (from 1.52 to 1.07 au). The maximum of the activity was reached at 1.07 au from the Sun, on August 17, 24 days before perihelion. It then started to decrease rapidly after perihelion. CN was detected in our data until the end of 2018 at 1.66 au, while 