Evolution of the production rates of comet 21P. Empty symbols (left side) are production rates measured in October–November 1998 a few days before perihelion (r_h = 1.034 on 21.3 November 1998). Filled symbols correspond to measurements of 2018, shortly after perihelion (r_h = 1.013 on 10.3 September 2018). Downward pointing empty triangles are upper limits (for CO in dark blue, H₂ S in red and HNC in blue) for the corresponding period. The vertical scale is the production rate in molec. s⁻¹, and the horizontal scale the date of observation relative to the relevant perihelion date.