  We present a rigorous theoretical study of low-temperature collisions of
polyatomic molecular radicals with ^1S_0 atoms in the presence of an external
magnetic field. Accurate quantum scattering calculations based on ab initio and
scaled interaction potentials show that collision-induced spin relaxation of
the prototypical organic molecule CH_2(X^3B_1) (methylene) and nine other
triatomic radicals in cold 3He gas occurs at a slow rate, demonstrating that
cryogenic buffer-gas cooling and magnetic trapping of these molecules is
feasible with current technology. Our calculations further suggest that it may
be possible to create ultracold gases of polyatomic molecules by sympathetic
cooling with alkaline-earth atoms in a magnetic trap.
