| Acronym | Meaning |
 | A&A | Astronomy And Astrophysics |
| AACS | Attitude And Articulation Control Subsystem |
| AAO | Anglo-Australian Observatory |
| AAS | American Astronomical Society |
| AAS | American Astronautical Society |
| AAVSO | American Association Of Variable Star Observers |
| AC | Atlas Centaur |
| ACE | Advanced Composition Explorer |
| ACF | Automated Control Function |
| ACR | Active Cavity Radiometer |
| ACRIM | ACR Irradiance Monitor |
| ACRV | Assured Crew Return Vehicle |
| ACRV | Astronaut Crew Rescue Vehicle |
| ACTS | Advanced Communications Technology Satellite |
| ADEOS | Advanced Earth Observing Satellite |
| ADFRF | Ames-Dryden Flight Research Facility |
| ADS | Astrophysical Data System (at Havard-Smithsonian CfA) |
| ADS | Aitken Double Star Catalog |
| AE | Arianespace SA |
| AEPI | Atmospheric Emissions Photometric Imaging |
| AF | Air Force |
| AFETR | USAF's ETR |
| AFGWC | Air Force Global Weather Center |
| AFP | Air Force Program |
| AFPRO | Air Force Plan Representative Office |
| AFRSI | Advanced FRSI |
| AFWTR | USAF's WTR |
| AGB | Asymptotic Giant Branch |
| AGE | Auxiliary Ground Equipment |
| AGN | Active Galactic Nucleus |
| AGU | American Geophysical Union |
| AIAA | American Institute Of Aeronautics And Astronautics |
| AIPS | Astronomical Image Processing System |
| AIRS | Atmospheric Infrared Sounder |
| AJ | Astronomical Journal |
| ALAE | Atmospheric Lyman-Alpha Emissions |
| ALDP | Advanced Launch Development Program |
| ALEXIS | Array Of Low Energy X-ray Imaging Sensors |
| ALPO | Association Of Lunar And Planetary Observers |
| ALS | Advanced Launch System |
| AMOS | Air Force Maui Optical System |
| AMSR | Advanced Microwave Sounding Radiometer |
| ANSI | American National Standards Institute |
| AOA | Abort Once Around (shuttle Abort Plan) |
| AOA | Angle Of Attack |
| AOCS | Attitude And Orbit Control System |
| APE | Auroral Photography Experiment |
| APJ | Astrophysical Journal |
| APL | Applied Physics Laboratory (at Johns Hopkins) |
| APM | Attached Pressurized Module (a.k.a. Columbus) |
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