The kinetic theory of a drifting Maxwellian gas has been applied to the gas sampling problem in atmospheric density measurements aboard satellites and high velocity probes; and a molecular beam mass spectrometer has been developed for gas density measurements in the upper atmosphere which substantially eliminates gas‐surface scattering and gas‐surface reactions in the sample, and preserves the integrity of the gas sample during the analysis process. An ion source lens system has been developed and experimentally evaluated over the pressure range 10−7−10−4Torr. The extracted ion beam was analyzed for energy distribution and angular distribution. For ion energy sufficiently high that the thermal energy of the gas can be neglected, the ion energy spread is less than 4 eV on‐axis and 90% of the ions are within 2.5° of the beam axis. It is concluded that the application of this technique to atmospheric composition studies will substantially improve our knowledge of the upper atmosphere of the earth and other planets.