  This article proposes a new statistical numerical method to address gas
kinetics problems obeying the Boltzmann equation. This method is inspired from
some Monte-Carlo algorithms used in linear transport physics, where virtual
particles are followed backwards in time along their paths. The non-linear
character of gas kinetics translates, in the numerical simulations presented
here, in branchings of the virtual particle paths. The obtained algorithms have
displayed in the few tests presented here two noticeable qualities: (1) They
involve no mesh. (2) They allow to easily compute the gas density at rarefied
places of the phase space, for example at high kinetic energy.
