again for the 2018 return using various state of the art instrumentation (IR and optical spectrographs on large telescopes) in order to better understand these peculiarities.

This work is organized as follows: after the introduction and historical background given in section \[intro\], we describe the observing circumstances and the reduction process of TRAPPIST images and UVES/VLT spectra in section \[data\_reduction\]. In section \[activity\], we compute the production rates and we discuss the gas and dust activity pre- and post-perihelion as well as the properties of the dust. The relative molecular abundances and their evolution with respect to the heliocentric distance are discussed in section \[abundances\] and compared to the IR mother species abundances. In section \[sec\_ratios\], we present the nitrogen and carbon isotopic ratios and the NH$_2$ (and NH$_3$) ortho-para ratio derived from the UVES high resolution spectrum. In section \[sec\_dynamical\], we investigate the dynamical evolution of comet 21P within the last 10$^5$ years. Discussion of the chemical composition of 21P and the possible scenarios of its depletion in carbon species are given in section \[discussion\]. The summary and conclusions of this work are given in the last section \[sec\_conclusion\].

Observation and data reduction {#data_reduction}
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Photometry (TRAPPIST) {#Photometry_TRAPPIST}
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