  We report results of a recently-completed pre-Formulation Phase study of
SPIRIT, a candidate NASA Origins Probe mission. SPIRIT is a spatial and
spectral interferometer with an operating wavelength range 25 - 400 microns.
SPIRIT will provide sub-arcsecond resolution images and spectra with resolution
R = 3000 in a 1 arcmin field of view to accomplish three primary scientific
objectives: (1) Learn how planetary systems form from protostellar disks, and
how they acquire their inhomogeneous composition; (2) characterize the family
of extrasolar planetary systems by imaging the structure in debris disks to
understand how and where planets of different types form; and (3) learn how
high-redshift galaxies formed and merged to form the present-day population of
galaxies. Observations with SPIRIT will be complementary to those of the James
Webb Space Telescope and the ground-based Atacama Large Millimeter Array. All
three observatories could be operational contemporaneously.
