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The near‐Earth asteroid thermal model (NEATM) has proven to be a reliable simple thermal model for radiometric diameter determination. However, NEATM assumes zero thermal emission on the night side of an asteroid. We investigate how this assumption affects the best‐fitting beaming parameter η, overestimates the effective diameter  D  eff  and underestimates the albedo  p  v  at large phase angles, by testing NEATM on thermal infrared fluxes generated from simulated asteroid surfaces with different thermal inertia Γ. We compare NEATM to radar diameters and find that NEATM overestimates the diameter when η is fitted to multiwavelength observations and underestimates the diameter when default η is used. The night emission simulated thermal model (NESTM) is introduced. NESTM models the night side temperature  ( T  night )  as an iso‐latitudinal fraction  ( f )  of the maximum day side temperature ( T  max  calculated for NEATM with  η= 1 ):   T  night = fT  max  cos 1/4 φ , where φ is the latitude. A range of  f  is found for different thermal parameters, which depend on Γ. NESTM diameters are compared with NEATM and radar diameters, and it is shown that NESTM may reduce the systematic bias in overestimating diameters. It is suggested that a version of the NESTM which assumes  Γ= 200 J m −2  s −1/2  K −1   is adopted as a default model when the solar phase angle is greater than 45°.

Investigation of systematic bias in radiometric diameter determination of near‐Earth asteroids: the night emission simulated thermal model (NESTM)