PAET (Planetary Atmospheric Entry Test) is a NASA experiment to study the feasibility of using measurements (include mass spectrometer measurements) made by high-speed entry probes to determine the structure and composition of unknown planetary atmospheres. The concept, developed by Alvin Seiff of the Ames Research Laboratory, was demonstrated by PAET in Earth's atmosphere and later used to investigate the atmospheric structures, of Mars, Venus, and Jupiter. PAET was in fact the world's first reentry atmospheric reconstruction experiment.