TPM χ²-optimization process to find robust solutions for diameter, albedo and thermal inertia simultaneously. Each line represents the reduced χ² values for an individual shape/spin-vector solution as a function of thermal inertia. The surface roughness was kept constant using the “default values” of ρ = 0.7 and f = 0.6 as proposed by Müller et al. (1999). Model 7_1 solutions marked with squares on the solid line correspond to the two cases with thermal inertias of 20 and 1000 J m^-2 s^-0.5 K^-1 shown in Fig. 3 (left and middle).