Fitting the global water production rate of 21P as a function of days from perihelion by applying ThermoONet to different shapes. The black line corresponds to a sphere model, the blue line is a ellipsoid model with a_x = 2a_y = 2a_z and the red line is a ellipsoid with a_x = 2a_y = 4a_z. Other parameters are as stated in Table 3. The scatter points with different colors and shapes represent observational data at different apparitions (Combi et al. 2019), where purple circles from 1998, blue triangles from 2005, green squares from 2012, and black asterisks from 2018. The detailed description about the observation data and sources thereof can be found in Combi et al. (2019).