| Acronym | Meaning |
 | HRSR | High-Resolution Scanning Radiometer |
| HSP | High Speed Photometer |
| HST | Hubble Space Telescope |
| HTHL | Horizontal Takeoff Horizontal Landing |
| HTVL | Horizontal Takeoff Vertical Landing |
| HUT | Hopkins Ultraviolet Telescope |
| HV | High Voltage |
| IAPPP | International Amateur/Professional Photoelectric Photometry |
| IAU | International Astronomical Union |
| IAUC | IAU Circular |
| IBSS | Infrared Background Signature Survey |
| IC | Index Catalog |
| ICE | International Cometary Explorer |
| IDA | International Dark-sky Association |
| IDL | Interactive Data Language |
| IFEA | Integrated Furnace Experiment Assembly |
| IGBP | International Geosphere/Biosphere Programme |
| IGM | InterGalactic Medium |
| IGY | International Geophysical Year |
| IKI | Institut Kosmischeskikh Issledovaniya |
| IML | International Microgravity Lab |
| IMO | International Meteor Organization |
| IMU | Inertial Measurement Unit |
| INCO | Integrated Communications Officer |
| INMARSAT | International Maritime Satellite |
| INTELSAT | International Telecommunications Satellite |
| IOA | Institute Of Astronomy (Cambridge) |
| IOBM | In Ocean By Mistake |
| IOC | In-Orbit Checkout And Calibration |
| IOTA | Infrared-Optical Telescope Array |
| IOTA | International Occultation Timing Association |
| IPAC | Infrared Processing & Analysis Center |
| IPMP | Investigations Into Polymer Membrane Processing |
| IPS | Inertial Pointing System |
| IR | InfraRed |
| IRAF | Image Reduction And Analysis Facility |
| IRAS | InfraRed Astronomical Satellite |
| IRFNA | Inhibited Red Fuming Nitric Acid |
| ISAMS | Improved Stratospheric And Mesospheric Sounder |
| ISAS | Institute Of Space AND Astronautical Science |
| ISEP | Isolated/Stabilized Exercise Platform |
| ISM | InterStellar Medium |
| ISO | Imaging Spectrometic Observatory |
| ISO | Infrared Space Observatory |
| ISO | International Standards Organization |
| ISPM | International Solar Polar Mission |
| ISRO | Indian Space Research Organisation |
| ISTS | Institute For Space And Terrestrial Science |
| ISU | International Space University |
| ISY | International Space Year |
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