A simple experiment has been conceived for providing highly accurate knowledge on the density/ and pres sur e/altitude profiles of planetary atmospheres, notably those of Mars and Venus. The experiment consists of a small, lightweight spherical probe injected into the planetary atmosphere either from a flyby spacecraft prior to planetary encounter or from a planetary orbiter prior to orbit injection. Information on the probe's entry flight-path angle and its velocity history before and after communications blackout will be obtained through a two-way doppler measurement by the Deep Space Instrumentation Facility (DSIF).