i (imaginary unit number) Iapetus iatrogenic disease IAU IBEX (Interstellar Boundary Explorer) IBSS (Infrared Background Signature Survey) ibuprofen IC 10 IC 10 X-1 (stellar black hole) IC 349 (Barnard's Merope Nebula) IC 410 IC 434 (emission nebula) IC 59 (nebula near Gamma Cassiopeiae) Icarus (minor planet 1566) ICE (International Cometary Explorer) ice ice ages ice dwarf ice frost ice volcanism ice worm iceberg icebreaker IceCube Iceland spar ICESat (Ice, Cloud, and Land Elevation Satellite) ice-sheet ice-water interface station ichthyosaur Ichthyostega ICO icosahedron Icosian Game Ida (minor planet 243) IDCSP (Initial Defense Communications Satellite Program) ideal burning ideal gas ideal gas law ideal rocket idealism idempotent identity idiopathic idiopathic pulmonary fibrosis idiopathic thrombocytopenic purpura idocrase IEH (International Extreme Ultraviolet Hitchhiker) IEOS (International Earth Observing System) igneous igneous age ignition ignition lag ignitron IGY (International Geophysical Year) Ikeya-Seki, Comet (C/1965 S1) Ikonos iliac arteries ileum ilium illicit cargo illuminance illusion ILLUSIONS AND IMPOSSIBLE FIGURES ilmenite image IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) image intensifier image processing image tube imaginary number imaginary operator imaginary time imaging, of extrasolar planets imago IMEWS (Integrated Missile Early Warning Satellite) imide immune system immunity immunization immunoglobulin immunology immunosuppressant drug immunotherapy IMP (Interplanetary Monitoring Platform) impact basin impact crater impact feature impact melt impactor impedance impetigo implantable cardioverter defibrillator implication implosion impossibilities in mathematics impossible figure impossible motion impossible trident imprinting impulse impulse, total impulse-weight ratio in situ hybridization inbreeding incandescence incarnation and redemption incenter inch (in) inclination inclined plane inclinometer inclusion incomplete dominance incomputable number incubation incubation period incubator indehiscent independent assortment indeterminate equations index Index Catalogue (IC) index fossil INDIA IN SPACE Indian astronomy Indian launch vehicles Indian summer indicator indigo indium inductance induction (electromagnetic) induction (mathematical) induction coil induction hardening induction motor induction ring induction sail inductor Indus (constellation) industrial chemistry inelastic collision inequality inert gas inertia inertia wheel inertial force inertial guidance inertial platform inertial reference frame inertial stabilization inertial upper stage (IUS) infall velocity infectious disease inferior planet inferior thyroid artery inferior vena cava infinite dimensions infinite series infinitesimal infinity inflammation Inflatable Antenna Experiment (IAE) inflationary model inflection inflorescence influenza information storage and retrieval information technology information theory INFOSEC infrared infrared astronomy Infrared Astronomy Satellite (IRAS) infrared astronomy satellites infrared cirrus infrared excess infrared galaxy infrared radiometer Infrared Space Interferometer (IRSI) Infrared Space Observatory (ISO) infrared star Infra-Red Telescope Facility (IRTF) infrasound ingress inherited inhibitor initiator injection injection molding injector ink inlier Inmarsat inner ear Innes, Robert Thorburn Ayton (1861–1933) Innisfree meteorite innominate innominate artery innominate veins inoculation inorganic INORGANIC CHEMISTRY inorganic chemistry INPE (Instituto Nacional de Pesquisas Espacias) Insat inscribed angle inscribed circle inscription insect insect senses Insectivora inselberg insertion InSight (Mars spacecraft) insomnia Inspector instability instability strip instantaneous bandwidth instinct instruction set instrument landing system instrumentalism INSTRUMENTATION insulin INTA (Instituto Nacional de Tecnica Aeroespacial) integer integral INTEGRAL (International Gamma-ray Astrophysics Laboratory) integral equation Integral Sign Galaxy (UGC 3697) integrated magnitude Integrated Truss Structure integration integration time integumentary system intelligence, artificial intelligence, nature of intelligent dinosaurs Intelligent Life in the Universe Intelsat (International Telecommunications Satellite Organization) intensity interacting binary Interamnia (minor planet 704) Interbol intercalation compound intercept Intercosmos intercostal muscles interesting number interference interference pattern interferometer interferon intergalactic absorption intergalactic magnetic field intergalactic medium (IGM) intergalactic travel Intergalactic Wanderer (NGC 2419) intergranular lane interhalogen interior angle intermediate filament intermediate population star intermediate range ballistic missile (IRBM) internal carotid artery internal combustion engine internal conversion internal energy internal jugular vein internal mammary artery internal oblique muscle International Astronautical Federation (IAF) International Astronomical Union (IAU) International Atomic Time (TAI) International Celestial Reference Frame (ICFR) International Dark-Sky Association (IDA) International Date Line International Geophysical Year (IGY) International Launch Services (ILS) International Quiet Sun Years (IQSY) International Solar Polar Mission International Space Science Institute International Space Station (ISS) International Ultraviolet Explorer (IUE) interphase interplanetary communication interplanetary magnetic field (IMF) interplanetary medium interplanetary space interpolate interpreter intersatellite link intersection interstage interstellar absorption INTERSTELLAR AND INTERPLANETARY MATTER interstellar ark Interstellar Boundary Explorer (IBEX) interstellar cloud interstellar colonization interstellar communication interstellar distances interstellar dust interstellar erosion interstellar extinction interstellar maser interstellar medium (ISM) interstellar molecules interstellar polarization Interstellar Precursor Mission interstellar precursor missions Interstellar Probe interstellar probes interstellar propulsion interstellar radio messages interstellar ramjet interstellar reddening interstellar scintillation interstellar space interstellar travel interstellar wind interstitial cells intervertebral disk intestine intracellular intra-cluster medium intravenous drip intrinsic color index intrinsic variable intron intrusive intubation Invaders From Mars invar invariant invariant plane invariant theory Invasion of the Body Snatchers inverse inverse P Cygni profile inverse variation inversion invertebrate in-vitro fertilization invisible ink involute inward orbital migration Io iodine ion ion exchange ion propulsion ionic bond ionic crystal ionic equilibrium ionic radius ionization ionization chamber ionization equilibrium ionization front ionization potential ionization temperature ionized ionized layers ionizing radiation ionosphere Iota Horologii Iota Orionis IP Pegasi ipecac iPhone iPod iPod nano IRAS IRAS-Araki-Alcock, Comet (C/1983 H1) IRFNA (inhibited red fuming nitric acid) iridescence iridium (element) Iridium (satellites) iris (eye) Iris (minor planet 7) IRIS (Infrared Imaging Surveyor) Iris (sounding rocket) iron iron meteorite iron-deficiency anemia ironstone irradiance irradiation irradiation illusion irrational number irregular galaxy irregular moon irregular variable irritability IRS (Indian Remote Sensing satellite) IRSI IRTS (Infrared Telescope in Space) Irwin, James Benson (1930–1991) ISAS (Institute of Space and Astronautical Sciences) ischium ISEE (International Sun-Earth Explorer) Ishango bone Isidis Planitia ISIS (International Satellites for Ionospheric Studies) island island arc Island One islets of Langerhans ISM ISO isobar (meteorology) isobars (nuclear physics) isochrone isoelectronic isogamy isogonal conjugate isohyet isomeric transition isomerism isomers isometry isomorphism isoperimetric inequality isopod isoprene isosceles isostasy isotherm isothermal process isotomic conjugate isotones isotopes isotopic spin isotropic isotropic signal isovolume problem Israel in space ISS (International Space Station) ISS (Ionospheric Sounding Satellite) isthmus It Came From Outer Space iterate iteration Itokawa (minor planet 25143) ITOS (Improved TIROS Operating System) IUS (inertial upper stage) Ivuna meteorite Izar (Epsilon Bootis)