The comparative approach to planetary problems is becoming increasingly fruitful as new information from various planet atmospheres is assimilated. In particular, it is clear that the important problem of CO2 cooling in the Earth's lower thermosphere is closely tied to the thermospheric heat budgets of Venus and Mars. CO2 cooling in each of these thermospheres is strongly impacted by collisions of CO2 and O, yielding vibrationally excited CO2 and enhanced 15-μm emissions in regions where non--local thermodynamic equilibrium conditions prevail. Both the relative abundance of atomic O and CO2-O relaxation rate affect the magnitude of this enhanced cooling process. We examine the recent progress in the debate on the CO2-O relaxation rate, its temperature dependence, and its corresponding impact on the thermospheric heat budgets of Venus, Earth, and Mars. This comparative approach provides the broadest range of conditions under which a common CO2-O relaxation rate should provide consistent results. New global mean calculations are presented for the heat budgets of these three planets using large CO2-O relaxation rates that have been inferred recently from Earth CO2 radiance measurements and laboratory studies. Results indicate that available Venus and Mars data constrain the CO2-O relaxation rate to be 2--4¿10-12 cm3/s at 300 K. For Venus, this strong cooling serves as an effective thermostat that gives rise to a small variation of thermospheric temperatures over the solar cycle, just as observed. Conversely, CO2 cooling does not appear to be dominant in the dayside heat budget of the Mars thermosphere over most of the solar cycle. For the Earth, this strong cooling implies that the lower thermosphere does not typically require significant eddy diffusion or heat conduction. However, global-scale dynamics or an additional heating mechanism may be needed to restore calculated temperatures to observed values when relaxation rates exceeding 2¿10-12 cm3/s are employed. ¿ American Geophysical Union 1994