Iapetus IAU IC Icarus ICE ice dwarf ICM Ida IDP igneous rock IGY Ikeya-Seki, Comet image intensifier Image Photon Counting System image tube Imbrium Basin IMF immersion inclination Index Catalogue Indian Indus inequality inferior conjunction inferior culmination inferior planet inflationary universe infrared array Infrared Astronomical Satellite infrared astronomy infrared cirrus infrared galaxy Infrared Space Observatory infrared star infrared telescope Infrared Telescope Facility initial mass function inner planets insolation instability strip Institute de Radio Astronomie Millimetrique INTEGRAL intensity interferometer interacting binary interacting galaxies Interamnia interference filter interferometer intergalactic medium intermediate polar International Astronomical Union International Atomic Time International Cometary Explorer International Dark Sky Association International Gamma Ray Laboratory International Geophysical Year International Sun-Earth Explorer International Ultraviolet Explorer International Years of the Quiet Sun interplanetary dust interplanetary medium interplanetary scintillation interstellar dust interstellar extinction interstellar medium interstellar molecules interstellar reddening intracluster medium invariable plane inverse Compton effect inverse square law Io ion ionization ionization front ionized hydrogen ionosphere ion tail IPCS IQSY IR IRAM IRAS Iris iron meteorite irradiation(1) irradiation(2) irradiation(3) irregular galaxy irregular satellite irregular variable IRTF Isaac Newton Telescope ISAS ISEE Ishtar Terra island universe ISM ISO isophote isotropy IUE Ivar