A new instrument for in situ rotational spectroscopy of gases is presented. The design is based on the pulsed Fourier transform method of Balle-Flygare but operates at higher frequency than traditional microwave implementations. A semi-confocal cavity is an essential part of the new technology, which builds field strength for pumping rotational transitions. Details about the cavity quality factor and design are discussed. The cavity is combined with custom CMOS integrated circuits that synthesize, amplify, and mix the transmitter and receiver signals. Proof-of-concept laboratory measurements of molecular gases are presented. Incorporation in a comet surface sample return mission concept is discussed in detail. The sensor could be used in number of different planetary missions.