Williams et al. (1997) have suggested that a population of hot hydrogen ato
ms is created in the heliosphere through elastic H-H collisions between ene
rgetic 'solar' atoms (neutralized solar wind) and interstellar atoms. They
used a BGK-like approximation (Bhatnagar et al., 1954) for the Boltzmann co
llision term and the collision cross sections suggested by Dalgarno (1960).
We show that both assumptions result in a significant overestimation of th
e the H-H collision effect. On the basis of calculated momentum transfer cr
oss-sections for elastic H-H collisions, we argue that elastic H-H and H-p
collisions cannot produce hot H atoms in the heliosphere.