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We report the microwave spectrum of $HNO3-NH3$ obtained by pulsed nozzle Fourier transform microwave spectroscopy. The complex is the weakly bound precursor to ammonium nitrate, which is an important aerosol in the earth’s troposphere The a-type spectrum has been recorded up to J=3 and the hyperfine structure fully analyzed. The a-type transitions follow a rigid rotor pattern, but the b-type spectra apparently do not. Preliminary analysis of the rotational constants indicates that the nitric acid forms a near- linear hydrogen bond to the ammonia with an intermolacular N-H separation of about 1.8 A. This rather short distance is distance in the context of incipient proton transfer.