Glossary of Space Exploration Terminology

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M
Mach
The ratio of the speed of a vehicle (or of a liquid or gas) to the local speed of sound.
Magnetic field
A region of space near a magnetized body where magnetic forces can be detected.
Magnetic field line
Lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.
Magnetic pole
Two meanings: (1) the points on Earth towards which the compass needle points. (2) A concentrated source of magnetic force, e.g. a bar magnet has two magnetic poles near its end.
Magnetic storm
A disturbance of the Earth's magnetic field initiated by a solar flare or sunspot.
MagnetoHydroDynamics
The study of plasma motion and dynamics in the presence of a magnetic field.
Magnetometer
A device for measuring the strength and direction of the interplanetary and solar magnetic fields.
Magnetopause
The boundary of the magnetosphere, lying inside the bow shock. The location in space where Earth's magnetic field balances the pressure of the solar wind. It is located about 63,000 km from Earth in the direction of the Sun.
Magnetosphere
That region of space surrounding the Earth which is dominated by the magnetic field.
Magnetron
A vacuum tube in which the flow of electrons is subject to the control of an external magnetic field.
Major axis
The maximum diameter of an ellipse.
Manned maneuvering unit
A portable jet-pack device used by astronauts to propel themselves through space independent of a spacecraft.
Mantle
Middle layer of the Earth; between the crust and the core.
Maria
Dark areas on the Moon, actually lava plains, once believed to be seas.
Mars
Fourth planet from the Sun, a terrestrial planet.
Mass
The quantity of matter in a body. It can be determined by measuring the force of gravity (weight) acting on it and dividing this by the gravitational acceleration at that point. Thus, the mass of a given body remains the same everywhere, while its weight changes with the gravitational attraction.
Mass ratio
Ratio of the total mass of a rocket vehicle to the mass remaining when all the propellant is consumed.
Max Q
Maximum dynamic pressure; the point during launch when the vehicle is subjected to its greatest aerodynamic stress.
Mean
Synonym for average.
Mean solar time
Time based on an average of the variations caused by Earth's non-circular orbit.
Medium Earth Orbit
An orbit in the region of space above low Earth orbit (2,000 kilometers) and below geosynchronous orbit (35,786 kilometers). Sometimes called Intermediate Circular Orbit.
Medium-gain antenna
A spacecraft antenna that provides greater data rates than a low-gain antenna, with wider angles of coverage than a high gain antenna, about 20-30 degrees.
Memory
The faculty of an electronic device to record and store data and/or instructions for future action on a command.
MEO
Medium Earth Orbit.
Mercury
First planet from the Sun, a terrestrial planet.
Meridian
Great circle that passes through both the north and south poles, also called line of longitude.
Mesosphere
A division of the Earth's atmosphere extending from altitudes ranging 18-30 miles to 48-55 miles.
Meteor
The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
Meteorite
A part of a meteoroid that survives through the Earth's atmosphere.
Meteoroid
A solid body, moving in space, that is smaller than an asteroid and at least as large as a speck of dust. Nearly all meteoroids originate from asteroids or comets.
MeV
One million electron volts.
MHz
Megahertz, equal to one million hertz.
Microgravity
An environment of very weak gravitational forces, such as those within an orbiting spacecraft. Microgravity conditions in space stations may allow experiments or manufacturing processes that are not possible on Earth.
Micrometeoroid
Meteoroid less than 1/250th of an inch in diameter.
Micrometeoroid protection
Shielding used to protect spacecraft components from micrometeroid impacts. Interplanetary spacecraft typically use tough blankets of Kevlar or other strong fabrics to absorb the energy from high-velocity particles.
Microwaves
Radio waves having wavelengths of less than 20 centimeters.
Milky Way
The galaxy which includes the Sun and Earth.
Minor planet
An asteroid.
Missile
An object or a weapon that is fired, thrown, dropped, or otherwise projected at a target; a projectile.
Mixture ratio
Ratio of the masses of the fuel to the oxidizer at any given time.
MMH
Monomethyl Hydrazine. A liquid hypergolic fuel.
MMU
Manned Maneuvering Unit.
Mock-up
A full-size replica or dummy of a vehicle, e.g. a spacecraft, often made of some substitute material such as wood to assess design features.
Modulation
The variation of a property of an electromagnetic wave or signal, such as its amplitude, frequency, or phase.
Module
A self-contained unit of a spacecraft or space station which serves as a building block for the total structure.
Momentum
The product of the mass of a body and its velocity.
Mono-propellant
A rocket propellant consisting of a single substance, especially a liquid containing both fuel and oxidizer, either combined or mixed together.
Moon
A small natural body which orbits a larger one. A natural satellite.
Motor
In spacecraft, a rocket that burns solid propellants.
MRBM
Medium Range Ballistic Missile (range 800-2,400 km).
MSFC
Marshall Space Flight Center (Huntsville, Alabama).
MT
Megatonne, equal to 1 million tonnes.
MT
Moscow Time.
Multiplexer
A mechanical or electrical device for sharing a circuit by two or more coincident signals.
Multistage rocket
A rocket having two or more stages which operate in succession each being discarded as its job is done.

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Rocketry & Space Technology

Glossary courtesy of Robert Braeunig's
Rocketry & Space Technology website.