| Acronym | Meaning |
 | MACHO | MAssive Compact Halo Object |
| MACRO | Monopoles |
| MACS | Modular Attitude Control System |
| MARS | Series Of Soviet Mars Spacecraft |
| MAS | Millimeter-wave Atmospheric Sounder |
| MB | Manned Base |
| MBB | Messerschmitt-Belkow-Blohm |
| MCC | Mission Control Center |
| MCCC | Maragone Convection On Closed Containers |
| MDSSC | McDonnell Douglas Space Systems Corp |
| MECO | Main Engine CutOff |
| MET | Mission Elapsed Time |
| MEV | Mars Excursion Vehicle |
| MFPE | Misson From Planet Earth |
| MFV | Main Fuel Valve |
| MIMR | Multifrequency Imaging Microwave Radiometer |
| MISR | Multiangle Imaging Spectro-Radiometer |
| MITI | Ministry Of International Trade And Industry |
| MJD | Modified Julian Date |
| ML | Mobile Launcher |
| MLI | Multi-Layer Insulation |
| MLP | Mobile Launch Platform |
| MLS | Microwave Limb Sounder |
| MMCC | Mission Management And Control Centre |
| MMH | MonoMethyl Hydrazine |
| MMPO | Mission Management And Planning Office |
| MMS | Multi-mission Modular Spacecraft |
| MMT | Multiple Mirror Telescope |
| MMU | Manned Maneuvering Unit |
| MNRAS | Monthly Notices Of The Royal Astronomical Society |
| MOC | Mars Observer Camera |
| MODE | Middeck Zero-Gravity Dynamics Experiment |
| MODIS | Moderate Resolution Imaging Spectrometer |
| MODIS-N | MODIS-Nadir |
| MODIS-T | MODIS-Tilt |
| MOL | Manned Orbiting Laboratory |
| MOLA | Mars Observer Laser Altimeter |
| MOMS | Modular Optoelectronic Multispectral/Stereo Scanner |
| MOMV | Manned Orbital Maneuvering Vehicle |
| MOPITT | Measurements Of Pollution In The Troposphere |
| MOTV | Manned Orbital Transfer Vehicle |
| MPC | Minor Planets Circular |
| MPEC | Multi-Purpose Experiment Canister |
| MPESS | Multiple Purpose Experiment Support Structure |
| MRIR | Medium Resolution Infrared Radiometer |
| MRO | Memory Read-Out |
| MRSR | Mars Rover And Sample Return |
| MRSRM | Mars Rover And Sample Return Mission |
| MS | Main Sequence |
| MSFC | Marshall Space Flight Center |
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