Using newly developed silicon micromachining technology that enables low-mass and highly integrated receivers, we are developing a state-of-the-art terahertz radiometer/spectrometer instrument for planetary orbiter missions to Mars, Venus, Titan, and the Galilean moons. Our flexible receiver architecture provides a powerful instrument capability in a light-weight, low-power consuming compact package which offer unprecedented sensitivity performance, spectral coverage, and scalability to meet the scientific requirements of multiple missions. The instrument will allow a large number of chemical species, such as water, NO2, N2O, NH3, SO2, H2S, CH4, and HCN, among others, in the atmospheres of Mars, Venus, and Titan to be detected at concentrations below a part per billion. It will also be able to pinpoint their location in latitude, longitude, and in altitude. The maturation of this terahertz instrument will have an immediate impact on other areas such as multi-pixel focal plane heterodyne arrays for astrophysics and terahertz imagers and radars for a variety of national security applications.