Aerospace Science and Technonlogy Dictionary M Section

m: Meter (U.S. spelling; elsewhere metre), the international standard of linear measurement..
M: Mega (million).
M: Mass.
Mögel-Dellinger effect: = fadeout.
M-curve: A plot of values of M-units (modified index of refraction) as a function of height in an atmosphere. M-curves are frequently used in ray tracing studies.
M-display: In radar, a display in which target distance is determined by moving an adjustable blip along the baseline until it coincides with the horizontal position of the target signal deflections. The control which moves the blip is calibrated in distance. Also called M-scan, M-scope, M-indicator.
M-indicator: = M-display
M-region: See M-region magnetic storm, note.
M-region magnetic storm: A magnetic storm that is independent of visible solar disk features; it begins gradually and shows a strong tendency to recur within a period of 27 days.
The hypothetical region on the solar disk assumed to be the source of the incident corpuscular radiation is called the M-region.
M-scan: = M-display.
M-scope: = M-display.
M-unit: See modified index of refraction.
M₀: A representation of the mass of an object in terms of Solar mass. The average mass of the Sun is about 2x10³³ grams. Astronomers often express units for other objects in terms of solar units-- it makes the resulting numbers smaller and easier to deal with.
Mach: = Mach number.
Some writers use Mach as a unit of speed equivalent to a Mach number of 1.00, as a speed of Mach 3.1.
Mach angle: The angle between a Mach line and the direction of movement of undisturbed flow. See Mach wave.
Mach cone: 1. The cone-shaped shock wave theoretically emanating from an infinitesimally small particle moving at supersonic speed through a fluid medium. It is the locus of the Mach Lines.
2. The cone-shaped shock wave generated by a sharp-pointed body, as at the nose of a high-speed aircraft. See Mach wave.
Mach indicator: = Machmeter.
Mach line: A line representing a Mach wave; a Mach wave.
Mach number: (symbols M, N_Ma)
(Pronounced mock, after Ernst Mach, 1838-1916, Austrian scientist). A number expressing the ratio of the speed of a body or of a point on a body with respect to the surrounding air or other fluid, or the speed of a flow, to the speed of sound in the medium; the speed represented by this number. See Cauchy number.
If the Mach number is less than 1, the flow is called subsonic and local disturbances can propagate ahead of the flow. If the Mach number is greater than 1, the flow is called supersonic and disturbances cannot propagate ahead of the flow with the result that shock waves form.
Some authorities use mach number but engineering practice is to use a capital M in all words and combinations employing Mach.
Mach reflection: The reflection of a shock wave from a rigid wall in which the shock strength of the reflected wave and the angle of reflection both have the smaller of the two values theoretically possible.
Mach wave: 1. A shock wave theoretically occurring along a common line of intersection of all the pressure disturbances emanating from an infinitesimally small particle moving at supersonic speed through a fluid medium, with such a wave considered to exert no changes in the condition of the fluid passing through it.
The concept of the Mach wave is used in defining and studying the realm of certain disturbances in a supersonic field of flow.
2. A very weak shock wave appearing, e.g., at the nose of a very sharp body, where the fluid undergoes no substantial change in direction.
Mach-Zehnder interferometers: This is a variation of the Michelson interferometer which is used mainly in measuring the spatial variation in the refractive index of a gas (or plasma). A Mach-Zender interferometer uses two semi-transparent mirrors and two fully reflective mirrors located at the corners of a rectangle. The incoming beam is split in two at the first semi-transparent mirror, and the two halves of the beam travel along separate paths around the edge of the rectangle, meeting at the opposite corner. Typically one beam is a control, and the other travels through the system under study. The two beams meet at the second semi-transparent mirror, after which they are mixed together and interfere.
machine error: See error, note.
machine language: 1. A language, occurring within a computer, ordinarily not perceptible or intelligible to persons without special equipment or training.
2. A translation or transliteration of sense 1 into more conventional characters but frequently still not intelligible to persons without special training.
machine word: For a given computer, the number of information characters handled in each transfer. This number is usually fixed, but may be variable in some computers.
Machmeter: An instrument that measures and indicates speed relative to the speed of sound, i.e., that indicates the Mach number. Also called Mach indicator.
MACHOs: See massive compact halo objects.
Maclaurin series: See Taylor theorem.
macroscopic: Large enough to be visible to the naked eye or under low order of magnification.
macrosonics: The technology of sound at signal amplitudes so large that linear approximations are not valid.
Magellan project (NASA): A Venus exploratory mission to acquire radar imagery and topographic profiles of the planet surface and determine the characteristics of the Venusian gravity field. (This term is used to designate general project reviews, chronologies, and project management and planning.) Used for Venus Radar Mapper Project.
Magellan spacecraft (NASA): A Venus probe incorporating Voyager and Galileo hardware designs equipped with a synthetic aperture radar system to acquire surface imagery, altimetric profiles, and surface radiothermal emissivities. Earth-based Doppler radio tracking of the spacecraft will be used to derive gravimetric data. (This term designates the spacecraft intrinsic and support hardware, instrumentation acquired data.) Used for Venus Radar Mapper.
Magellan ultraviolet astronomy satellite: This ESA mission will provide high resolution spectra of celestial sources down to sixteenth magnitude over the extreme ultraviolet wavelength range (between 50 and 150 nm). This mission is still in the study phase. Used for Magellan Mission (ESA).
magic tee: A compound waveguide or coaxial tee with four arms which exhibits directional characteristics, when properly matched, so that a signal entering one arm will be split between two of the other arms but not the third. A signal entering another arm is likewise split with half the energy entering one of the arms common to the other input but not its second arm and the other half of the energy entering the arm not used by the other input.
The magic tee is used in radar as a transmitter-receiver duplexer.
magma: Magma is molten or partially molten rock beneath the Earth's surface. When magma erupts onto the surface, it is called lava. Magma typically consists of (1) a liquid portion (often referred to as the melt); (2) a solid portion made of minerals that crystallized directly from the melt; (3) solid rocks incorporated into the magma from along the conduit or reservoir called xenoliths or inclusions; and (4) dissolved gases.
magma: Naturally occurring mobile rock materials, generated within the Earth and capable of intrusion and extrusion, from which igneous rocks are thought to have been derived by solidification and related processes.
magnesium cells: Primary cells with the negative electrodes made of magnesium or its alloy.
magnesyn: (A trade name, from magnetic + synchronous; often capitalized). An electromagnetic device that transmits the direction of a magnetic field from one coil to another, used to transmit measurements electrically from a point of measurement to an indicator in a remote-indicating system.
magnet: A body which produces a magnetic field around itself.
magnetars: Highly magnetized neutron stars believed to emit quasi-steady x-rays along with bursts of soft gamma rays-- emissions powered by their magnetic energy. According to the magnetar theory, these stars form in some fraction of all supernovae. When they are young (with ages less than about 10,000 years) magnetars may be observed as soft gamma repeaters (SGRs) or anomalous x-ray pulsars.
magnetic: 1. Of or pertaining to a magnet.
2. Of or pertaining to a material which is capable of being magnetized.
3. Related to or measured from magnetic north.
magnetic axis: the location of the innermost flux "surface" in a toroidal device, the one which encloses no volume and has therefore degenerated from a flux surface into a single field line. Roughly, the circle through the middle of the dough of the donut. Additionally, in systems with magnetic islands (see entry below), each island has a local magnetic axis, distinct from the overall magnetic axisof the torus.
magnetic bay: A small magnetic disturbance whose magnetograph resembles an indentation of a coastline.
On earth, magnetic bays occur mainly in the polar regions and have durations on the order of a few hours.
magnetic bearings: Any application of the principle in which something capable of rotation and translation is held by the use of electromagnetic force without touching it. Applications range from small instruments to very large forces.
magnetic binary core: = binary magnetic core.
magnetic bottle: Colorful term used to describe a magnetic field structure which confines a plasma "like in a bottle".
magnetic character figure: = C-index.
magnetic compass: Compasses whose operation depends upon an element that senses the Earth's magnetic field, e.g., an instrument having a magnetic needle that turns freely on a pivot in a horizontal plane and that always swings to such a position that one end points to magnetic north.
magnetic compression: The force exerted by a magnetic field on an electrically conducting fluid or on a plasma.
magnetic confinement: Use of magnetic fields to confine a plasma. (Confinement involves restricting the volume of the plasma and/or restricting particle or energy transport from the center of the plasma to the edge.)
magnetic confinement fusion: Method of fusion which uses magnetic fields / magnetic bottles to confine a hot plasmauntil fusion occurs.
magnetic cooling: Keeping a substance cooled to about 0.2 K by using a working substance (paramagnetic salt) in a cycle of processes between a high-temperature reservoir (liquid helium) at 1.2 K and a low temperature reservoir containing the substance to be cooled.
magnetic core: = binary magnetic core.
magnetic crotchet: A sudden change in the earth's magnetic field due to an increase in the conductivity of the lower ionosphere. See sudden ionospheric disturbance.
magnetic current sheath: See plasma sheath.
magnetic declination: (symbol D, )
In terrestrial magnetism; at any given location, the angle between the geographical meridian and the magnetic meridian; that is, the angle between true north and magnetic north. Also called declination , and in navigation, variation.
Declination is either east or west according as the compass needle points to the east or west of the geographical meridian. Lines of constant declination are called isogonic lines and the one of zero declination is called the agonic line.
magnetic deviation: The angle between the magnetic meridian and the axis of a compass card, expressed in degrees east or west to indicate the direction in which the northern end of the compass card is offset from magnetic north. Also called deviation. Compare variation.
magnetic dip: (symbol i)
The angle between the horizontal and the direction of a line of force of the earth's magnetic field at any point. Also called magnetic inclination, magnetic latitude, inclination, dip.
magnetic dipole moment: = magnetic moment.
magnetic disturbance daily variation: (symbol S_D)
A periodic variation of the earth's magnetic field that is in phase with solar (local) time. It is the difference between the solar daily variation (or the disturbed-day solar daily variation) and the quiet-day solar daily variation. This variation is primarily an effect of enhanced electromagnetic radiation during increased solar activity.
magnetic disturbed-day solar daily variation: (symbol Sd)
The solar daily variation of the earth's magnetic field obtained from the 5 most disturbed days of the month.
magnetic domains: Small areas on the surface of the body of thin films of a magnetic medium, each of which maintains a descrete magnetic field orientation relative to the others around it.
magnetic double refraction: The splitting, into two components, of a radio wave traveling in a region of free electrons. This is due to the interaction of the earth's magnetic field and the alternating field of the radio wave. Except for waves near the gyrofrequency, the components of the split wave, the ordinary ray and the extraordinary ray, will travel with slightly different velocities and be reflected at different heights. See magneto-ionic theory.
magnetic drum: A memory device used in computers; a rotating cylinder on which information may be stored as magnetically polarized areas, usually along several parallel tracks around the periphery.
magnetic element: 1. In terrestrial magnetism, any of the following measurements: the vector magnetic field, also called total field (symbol ); the scalar intensity of the total field ( symbol F); declination, also called variation ( symbol D); the intensity of the horizontal component of the earth's field ( symbol H); the intensity of the vertical component ( symbol Z), taken as positive downward; the inclination or dip ( symbol I); the angle between and H; the intensity of the component of the horizontal field in the geographic north direction ( symbol X); and the intensity of the component of the horizontal field in the geographic east direction.
2. That part of an instrument producing or influenced by magnetism.
magnetic equator: That line on the surface of the earth connecting all points at which the magnetic dip is zero. Also called aclinic line. See geomagnetic equator.
magnetic equivalent amplitude indices: A linear measure of geomagnetic disturbance activity, based on the K-indices that gives an equivalent amplitude of the magnetic disturbance for the 3-hour period denoted by a_p. A daily index A_p is defined as the average of the a_p value over the 8 values of the day. See magnetic K-indices.
magnetic field coil: Coiled current-carrying wires used to generate magnetic fields.
magnetic field intensity: The magnetic force exerted on an imaginary unit magnetic pole placed at any specified point of space. It is a vector quantity. Its direction is taken as the direction toward which a north magnetic pole would tend to move under the influence of the field. If the force is measured in dynes and the unit pole is a cgs unit pole, the field intensity is given in oersteds. Also called magnetic intensity, magnetic field, magnetic field strength.
Prior to 1932 the oersted was called the gauss; but the latter term is now used to measure magnetic induction (within magnetic materials), whereas oersted is reserved for magnetic force. By definition, one magnetic line of force per square centimeter (in air) represents the field intensity of 1 oersted.
magnetic field lines: Lines in space, used for visually representing magnetic fields. At any point in space, the local field line points in the direction of the magnetic force which an isolated magnetic pole at that point would experience. In a plasma, magnetic field lines also guide the motion of an ion and electron, and direct the flow of some electric current.
magnetic field reconnection: A change in topology of the magnetic field configuration resulting from a localized breakdown of the requirement for 'connection' of fluid elements at one time on a common magnetic field line. Alternatively, it occurs when an electric field exists with a component parallel to a locally two-dimensional X-type magnetic neutral line which is equivalent to a breakdown in connection.
magnetic field strength: = magnetic field intensity.
magnetic fields: Fields of force that are generated by electric currents. The Sun's average large-scale magnetic field, like that of the Earth, exhibits a north and a south pole linked by lines of magnetic force.
magnetic fields: 1. Regions of space wherin any magnetic dipole would experience a magnetic force or torque; often represented as the geometric array of the imaginary magnetic lines of force that exist in relation to magnetic poles.
2. = magnetic field intensity.
magnetic flux: The magnetic force exerted on an imaginary unit magnetic pole placed at any specified point of space. It is a vector quantity. Its direction is taken as the direction toward which a north magnetic pole would tend to move under the influence of the field. If the force is measured in dynes and the unit pole is a cgs unit pole, the field intensity is given in oersteds. Used for magnetic field intensity.
magnetic giant pulsations: Magnetic micropulsations having large amplitudes.
magnetic inclinations: = magnetic dip.
magnetic induction: A measure of the strength of a magnetic field existing within a magnetic medium.
The relation between the magnetic induction and magnetic field intensity is such that the magnetic induction within a small mass of material of magnetic permeability µ is, except for possible hysteresis effects, µ times greater than the external magnetic field intenstiy. Whereas magnetic field intensity is measured in oersteds, magnetic induction is measured in gausses.
magnetic intensity: = magnetic field intensity.
magnetic islands: A magnetic topology near a "rational surface" (see entry) where the flux surface is broken up into tubes which are not connected with each other poloidally. Islands may develop in non-ideal magnetohydrodynamic fluids, where electrical resistance becomes important and magnetic field lines are no longer "frozen-in" to the fluid. Then magnetic tearing and reconnection may allow field lines to link up and form "islands" with a local magnetic axis (see entry) in a narrow region near a rational surface (see entry).
magnetic K-indices: An approximately logarithmic measure of geomagnetic disturbance activity based on the range of the most disturbed magnetic element during each 3-hour interval of the day. The K-indices are assigned integers from 0 to 9.
The K-indices averaged over the observatories of the earth are called planetary indices K_p and divided into 28 grades.
magnetic latitude: Geographic latitude of a location, in a system of latitudes and longitudes whose axis is not the rotation axis of the Earth but the magnetic axis, i.e. the axis of the dipole at the Earth´s center which best fits the internal magnetic field. The auroral zone, for instance, is near magnetic latitude 65 degrees.
magnetic latitude: = magnetic dip
See geomagnetic latitude.
magnetic lines of force: Imaginary lines so drawn in a region containing a magnetic field to be everywhere tangent to the magnetic field intensity vector if in vacuum or nonmagnetic material, or parallel to the magnetic induction vector if in a magnetic medium. See electric lines of force.
As so defined, these lines of force are merely convenient artifices for delineating the geometry of a magnetic field. They are given quantitative significance in magnetic theory by associating one line of force per square centimeter normal to the force for every oersted of field intensity (in vacuum), for every gauss of magnetic induction (in magnetic media).
magnetic local time: (MLT)--in the a system of latitude and longitude whose axis is the dipole axis, magnetic local time is the longitude, measured not in degrees but in hours (1 hour = 15 degrees).
magnetic lunar daily variation: (symbol L)
A periodic variation of the earth's magnetic field that is in phase with the transit of the moon.
This variation is essentially a tidal effect. The amplitude of this variation changes with the phase of the moon, the seasons, and the sunspot cycle.
magnetic mach number: A dimensionless number equal to the ratio of the velocity of a fluid to the velocity of Alfvén waves in that fluid.
magnetic memory: 1. The ability of a material to retain magnetism after the magnetizing force is removed.
2. = magnetic storage.
magnetic meridian: The horizontal line which is oriented, at any specified point on the earth's surface, along the direction of the horizontal component of the earth's magnetic field at that point; not to be confused with isogonic line. Also called geomagnetic meridian. Compare isoclinic line, magnetic equator.
magnetic micropulsations: Oscillations in magnetic records having periods of between a fraction of a minute and a few minutes, lasting for an hour or so.
magnetic mirror: A magnetic field so arranged that it will theoretically confine a hot plasma.
magnetic moment: 1. The quantity obtained by multiplying the distance between two magnetic poles by the average strength of the poles.
2. A measure of the magnetic flux set up by the gyration of an electric charge in a magnetic field. The moment is negative, indicating it is diamagnetic, and equal to the energy of rotation divided by the magnetic field.
3. (symbol m). In atomic and nuclear physics, a moment, measured in Bohr magnetons, associated with the intrinsic spin of the particle and with the orbital motion of the particle in a system. Also called magnetic dipole moment.
magnetic monopoles: A hypothetical particle which constitutes sources and sinks of the magnetic field. Magnetic monopoles have never been found, but would only cause fairly minor modifications to Maxwell's equations. They also seem to be predicted by some grand-unified theories. If magnetic monopoles do exist, they do not seem to be very common in our Universe.
magnetic north: The direction north at any point as determined by the earth's magnetic lines of force; the reference direction for measurement of magnetic directions.
magnetic nozzles: Nozzle devices used in some nuclear and plasma propulsion systems that utilize magnetic fields to direct and accelerate plasma flows, thereby providing thrust for propulsion.
magnetic number: A dimensionless number equal to the squareroot of the magnetic force parameter.
magnetic pole: 1. Either of the two places on the surface of the earth where the magnetic dip is 90 °, that in the Northern Hemisphere (at, approximately, latitude 73° 8 N, longitude, 101° W in 1955) being designated north magnetic pole , and that in the Southern Hemisphere (at, approximately, latitude, 68° S, longitude, 144° E in 1955) being designated south magnetic pole. Also called dip pole. See geomagnetic latitude, geomagnetic pole, magnetic latitude.
2. Either of those two points of a magnet where the magnetic force is greatest.
3. In magnetic theory, a fictitious entity analogous to a unit electric charge of electrostatic theory. In nature only dipoles, not isolated magnetic poles, exist.
magnetic pressure: Pressure which a magnetic field is capable of exerting on a plasma; equal to the magnetic energy density.
magnetic pressure: The energy density associated with a magnetic field.
In a very real sense, there is energy stored in a magnetic field, and since energy per unit volume is equivalent to force per unit area or pressure, one may speak of the pressure exerted by a magnetic field. For plasma containment in a thermonuclear device, the magnetic pressure must be greater than the kinetic pressure of the plasma. See beta factor. A pressure of 1 atmosphere corresponds approximately to 5,000 gausses, and the pressure is proportional to the square of the field.
magnetic probes: A conducting coil (sometimes insulated and inserted into the plasma) will have an induced voltage due to changes in the magnetic flux through the coil, and can therefore be used to measure changes in magnetic field strength. Small coils used to measure the local field strength are known as probes. Magnetic probes placed outside a toroidal plasma which are used to measure the poloidal magnetic field are also called Mirnov coils.
magnetic properties: The property of a material to attract iron, cobalt, or nickel.
magnetic pumping: Form of plasma heating where the plasma is successively compressed and expanded by means of a fluctuating external magnetic field.
magnetic quiet-day solar daily variation: (symbol S_q)
The magnetic solar daily variation obtained from the 5 most quiet days of the month.
magnetic reconnection: In a plasma, the process by which plasma particles riding along two different field lines can be made to share the same field line For instance, following reconnection, solar wind particles on an interplanetary field line, and magnetospheric ones on a field line attached to Earth, may find themselves sharing the same "open" field line, which has one end anchored on Earth and the other extending to distant space.
magnetic sails: Devices that provide low thrust spacecraft propulsion by deflecting plamsa winds with a superconducting cable-generated magnetic field.
magnetic solar daily variation: (symbol S)
A periodic variation of the earth's magnetic field that is in phase with solar (local) time.
The primary source of this variation is the ionizing effect of solar electromagnetic radiation on the atmosphere coupled with the earth's rotation. The amplitude of this variation changes with the seasons and the sunspot cycle.
magnetic storage: In computer terminology, any device which makes use of the magnetic properties of materials for the storage of information.
magnetic storm-time variation: (symbol D_st)
A nonperiodic variation determined from the onset of a magnetic storm. This variation is characterized by a rapid increase of the magnetic horizontal intensity above the normal value, remaining so for a few hours and then rapidly decreasing to below the normal value and remaining so for periods up to several days. The intensity then returns slowly to the normal value.
magnetic storms: Worldwide disturbances of the earth's magnetic field. See M-region.
Magnetic storms are frequently characterized by a sudden onset, in which the magnetic field undergoes marked changes in the course of an hour or less, followed by a very gradual return to normality, which may take several days. Magnetic storms are caused by solar disturbances, though the exact nature of the link between the solar and terrestrial disturbances is not understood. They are more frequent during years of high sunspot number. Sometimes a magnetic storm can be linked to a particular solar disturbance. In these cases, the time between solar flare and onset of the magnetic storm is about 1 or 2 days, suggesting that the disturbance is carried to the earth by a cloud of particles thrown out by the sun.
When these disturbances are observable only in the auroral zones, they may be termed polar magnetic storms.
magnetic stress tensor: A second-rank tensor, proportional to the dyadic product of the magnetic field (B) with itself. The divergence of the magnetic stress tensor gives that part of the force which a magnetic field exerts on a unit volume of conducting fluid due to the curvature of the magnetic field lines.
magnetic switching: The use as switches of saturable inductors for producing high power pulses without electrical arcs. This is a principal technology for extending single-shot accelerators in light-ion-beam-driven inertial confinement fusion to repetitively pulsed devices for possible reactors. Three terawatt, 200 KJ magnetic switches have been developed for fusion drivers at Sandia National Laboratories. (Info from the 1985 OSTI Glossary of Fusion Energy; may be out of date.)
magnetic tape: A ribbon of paper, metal, or plastic, coated or impregnated with magnetic material on which information may be stored in the form of magnetically polarized areas.
magnetic variation: 1. Variation, definition 1.
2. Change in a magnetic element.
magnetic variations: Changes in magnetic fields in time or space.
magnetic viscosity: A magnetic field in a conducting fluid will damp fluid motions perpendicular to the field lines, similar to ordinary viscosity, even in the absence of sizeable mechanical forces or electric fields.
magnetic wire: Wire made of magnetic material on which information may be stored in the form of magnetically polarized areas.
magnetically insulated transmission line: Used to transport power efficiently in vacuum lines at very high power densities. Although the cathode is a space-charge limited electron emitter, the electron flow is confined by self-generated or applied magnetic fields. MITL's are used extensively in light-ion-driven inertial confinement fusion.
magnetoelectric: Of or pertaining to electricity produced by or associated with magnetism.
Electromagnetic pertains to magnetism produced by or associated with electricity.
magnetoelectric transducer: A transducer which measures the electromotive force generated by the movement of a conductor relative to a magnetic field.
magnetofluiddynamics: = magnetohydrodynamics.
magnetogasdynamics: = magnetohydrodynamics.
magnetogram: A map showing the strength of the magnetic field in different locations.
magnetograph: The trace of an instrument recording variations in the geomagnetic field.
magnetohydrodynamic generators: A device that extracts kinetic energy from a jet of plasma and generates electricity.
magnetohydrodynamic instability: Class of unstable (growing, not damped) waves and other modes of oscillation which are described by MHD theory.
magnetohydrodynamic wave: = Alfvén wave.
magnetohydrodynamic waves: Low frequency waves in an electrically highly conducting fluid (such as a plasma) permeated by static magnetic fields. The restoring forces of the waves are, in general, the combination of a magnetic tensile stress along the magnetic field lines and the comprehensive stress between the field lines and the fluid pressure. Used for Alfvén waves, hydromagnetic waves, and plasma sound waves.
magnetohydrodynamics: (abbr MHD)
The study of the interaction that exists between a magnetic field and an electrically conducting fluid. Also called magnetoplasmadynamics, magnetogasdynamics, hydromagnetics.
magnetoionic theory: The theory of propagation of electromagnetic radiation through a medium containing ions in the presence of an external magnetic field.
It applies to the propagation of radio waves in the ionosphere, and provides theoretical relationships among such aspects of the subject as the index of refraction, radiofrequency, free-electron density, electron collision frequency, the earth's magnetic field (components relative to the direction of propagation), the nature of polarization, etc. See magnetic double refraction.
magnetoionic wave component: Either of the two elliptically polarized wave components into which a linearly polarized electromagnetic wave, incident of the ionosphere, is separated because of the earth's magnetic field.
magnetomechanics (physics): Study of the effects which the magnetization of a material and its strain have on each other.
magnetometer: An instrument used in the study of geomagnetism for measuring a magnetic element.
magneton: See Bohr magneton.
magnetopause: The boundary of the magnetosphere, separating plasma attached to Earth from the one flowing with the solar wind.
magnetoplasmadynamics: The study of the dynamics of generating electricity by passing a beam of ionized gas through a magnetic field.
magnetoplasmadynamics: = magnetohydrodynamics.
magnetosheath: The region between the magnetopause and the bow shock, containing solar wind which has been slowed down by passage through the bow shock. As the magnetosheath plasma streams away from the bow shock, it gradually regains its former velocity.
magnetosphere: The region of the earth's atmosphere where ionized gas plays an important part in the dynamics of the atmosphere and where the geomagnetic field, therefore, plays an important role. The magnetosphere begins, by convention, at the maximum of the F layer at about 350 kilometers and extends to 10 or 15 earth radii to the boundary between the atmosphere and the interplanetary plasma.
magnetospheres: Region surrounding a celestial body where its magnetic field controls the motions of charged particles.
magnetostriction: 1. The phenomenon wherein ferromagnetic materials experience an elastic strain when subjected to an external magnetic field.
2. The converse of sense 1 in which mechanical stresses cause a change in the magnetic induction of a ferromagnetic material.
magnetostrictive delay line: In electronic computers, a device in which a wave is induced by the characteristic, possessed by nickel and certain other materials, of shortening in length when placed in a magnetic field. The wave travels at the speed of sound through the material. See delay line.
magnetotails: The portion of a planetary magnetosphere which is pushed in the direction of the solar wind; The long stretched-out nightside of the magnetosphere, the region in which substorms begin.
magnetron: An electron tube characterized by the interaction of electrons with the electric field of a circuit element in crossed steady electric and magnetic fields to produce alternating-current power output.
magnetron sputtering: A deposition method in which a microwave tube is utilized to confine a plasma magnetically to produce high deposition rates and a low working-gas partial pressure.
magnets: Bodies which produce magnetic fields around themselves.
magnification: A ratio of the size of an image to its corresponding object. This is usually determined by linear measurement. Used for magnifiers.
magnitude: The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude.
magnitude: (symbol m)
1. The relative luminance of a celestial body. The smaller (algebraically) the number indicating magnitude, the more luminous the body. Also called stellar magnitude. See absolute magnitude.
The ratio of relative luminosity of two celestial bodies differing in magnitude by 1.0 is 2.512, the fifth root of 100.
Decrease of light by a factor of 100 increases the stellar magnitude by 5.00; hence, the brightness objects have negative magnitudes (Sun: -26.8; mean full moon: -12.5; Venus at brightest: -4.3; Jupiter at opposition: -2.3; Sirius: -1.6; Vega: 0.2; Polaris: 2.1). The faintest stars visible to the naked eye on a clear dark night are of about the sixth magnitude (though on a perfectly black background the limit for a single luminous point approaches the eighth magnitude). The faintest stars visible with a telescope of aperture a (in inches) is one approximately of magnitude $9 + 5 log$ ₁₀ a. The magnitude of the faintest stars which can be photographed with the 200-inch telescope is about +22.7.
The expression first magnitude is often used somewhat loosely to refer to all bodies of magnitude 1.5 or brighter, including negative magnitudes.
2. Amount; size; greatness. See order of magnitude.
MagSat 1 satellite: A scientific satellite launched by NASA for surveying the Earth's magnetic field. It was launched in October 1979 and reentered in June 1980.
MagSat B satellite: The second in a series of satellites for measuring the Earth's magnetic field. Similar magnetic measurements are proposed as part of the Geopotential Research Mission.
MagSat satellites: A series of satellites used to study the magnetic field.
main bang: The transmitted pulse, within a radar system.
main sequence stars: An area on the Hertzsprung-Russel Diagram containing "middle aged" stars like the Sun.
main stage: 1. In a multistage rocket, the stage that develops the greatest amount of thrust, with or without booster engines.
2. In a single-stage rocket vehicle powered by one or more engines, the period when full thrust (at or above 90 percent) is attained.
3. A sustainer engine, considered as a stage after booster engines have fallen away, as in the main stage of the Atlas.
maintainability: A measure of the ease and rapidity with which a system or equipment can be retained in operational status through preventive maintenance or restored to operational status following a failure. It is characteristic of equipment design and installation, personnel availability in the required skill levels, adequacy of maintenance procedures and test equipment, and the physical environment under which maintenance is performed.
major axis: The longest diameter of an ellipse or ellipsoid.
major lobe: See lobe.
major planets: The four largest planets: Jupiter, Saturn, Uranus, and Neptune.
malfunction: Improper functioning of a component, causing improper operation of a system.
mammatus clouds: Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm cloud). anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well.
man powered aircraft: Aircraft powered by human energy.
man tended free flyers: Intermittently manned spacecraft or platforms designed primarily to carry out experiments in reduced gravity and life science research. They also serve as annexes or components of space stations. Used for MTTF (space station).
man-machine integration: The matching of the characteristics and capabilities of man and machine in order to obtain optimum conditions and maximum efficiency of the combined system. See man-machine system.
man-machine system: A system in which the functions of the man and the machine are interrelated and necessary for the operation of the system.
manatees: Large plant eating aquatic mammals living in shallow tropical waters near the coasts of North and South America.
maneuverable reentry bodies: (1) Reentry vehicles capable of performing preplanned flight maneuvers during the reentry phase. (2) Ballistic missil reentry vehicles whose ballistic trajectory can be adjusted by internal or external mechanisms, enabling them to evade antiballistic defenses and.or strike their target with a high degree of accuracy.
manned: Of a vehicle occupied by one or more persons who normally have control over the movements of the vehicle, as in a manned aircraft or spacecraft, or who perform some useful function while in the vehicle.
manned maneuvering units: A propulsive backpack device for extravehicular activity. It uses a low thrust, dry, cold nitrogen propellant.
manned Mars missions: Any of several options for manned missions to Mars in which spacecraft are built for a particular mission. A mission is estimated by around 2020 and may last from one year to three years depending on speed and design.
manometers: Instruments used for measuring the pressure of gases and vapors both above and below atmospheric pressure. See vacuum gage.
manometric equivalent: The length in millimeters of a vertical column of a given liquid at standard room temperature equivalent to 1 millimeter of mercury at 0° C.
manures: Materials that fertilize land. Refuse of stables and barnyards consisting of mammal and bird excreta with or without litter.
many-to-few matrix: = decoder.
map-matching guidance: 1. The guidance of a rocket or aerodynamic vehicle by means of a radarscope film previously obtained by a reconnaissance flight over the terrain of the route, and used to direct the vehicle by aligning itself with radar echoes received during flight from the terrain below.
2. Guidance by stellar map matching.
Mapsat: A proposed stereoscopic system for mapping the Earth from space to replace Landsat D as defined by the US Geological Survey.
Marangoni convection: Convective flow induced by surface tension gradients. This is important in both ground and space processing where a free surface is present.
March equinox: = vernal equinox.
mare (pl. maria): Latin for sea. The large, dark, flat areas on the lunar surface, thought by early astronomers to be bodies of water. The term is also applied to less well-defined areas on Mars.
Marecs maritime satellites: The European Space Agency's system of two satellites provides maritime communications links between ships and coast Earth stations. Originally known as Marots, the system operates with one satellite over the Atlantic Ocean and one over the Pacific Ocean. It was leased to the International Maritime Satellite Organization for five years. Also known as the maritime European communications satellite.
marine biology: The study of marine fauna and flora and related topics.
marine chemistry: The study of the chemical processes in oceanic environments.
Mariner Mark 2 Spacecraft: A NASA concept of a basic planetary spacecraft for studying the outer planets, comets, and asteroids. The first of the series will be a comet rendezvous mission to be launched in 1994.
Mariotte law: = Boyle-Mariotte law.
Marisat 1 satellite: The first commercial maritime communication satellite.
Marisat satellites: A class of maritime commercial communication service satellites designed to provide telephone, telegraph, radio, distress messages and facsimile services to merchant ships, etc.
marmon clamp: A ring-shaped clamp, consisting of three equal length segments held together by explosive bolts, used to couple the main subsections of a rocket vehicle.
Marots (ESA): Earlier name for the Marecs maritime satellites. Used for Maritime Communication Satellite (ESA) and Maritime Orbital Test Satellite.
marriage: = mating.
Mars: Fourth planet from the sun, a terrestrial planet.
Mars: See planet, table.
Mars 4 Spacecraft: One of a series of Soviet unmanned spacecraft designed for Mars exploration.
Mars Climate Orbiter: One of two spacecraft comprising the Mars Surveyor 98 program; launched December 1998. After obtaining a polar, nearly circular orbit around Mars, the Orbiter will serve as a radio relay during the Lander surface mission, then begin monitoring the atmosphere, surface, and polar caps for a complete Martian year. The Orbiter carries two science instruments: the Pressure Modulated Infrared Radiometer and the Mars Color Imager.
Mars craters: Craters from meteoritic impact on the surface of Mars.
Mars Global Surveyor: Spacecraft and related mission designed to orbit Mars over a two year period and collect data on the surface morphology, topography, composition, gravity, atmospheric dynamics, and magnetic field. Launched November 1996.
Mars Observer: Spacecraft and related mission to study the geoscience and climate of Mars. Launched September 1992. Contact with the spacecraft was lost in August 1993, three days before the scheduled Mars orbit insertion.
Mars Polar Lander: One of two spacecraft comprising the Mars Surveyor 98 program; launched January 1999. After a soft landing near the Martian south pole, the Lander will search for near-surface ice and possible surface records of cyclic climate change, and characterize physical processes key to the seasonal cycles of water, carbon dioxide and dust on Mars. Prior to landing, the Deep Space 2 microprobes will be released as part of a technology-validation mission related to multiple-lander spacecraft.
Mars Surveyor 98 Program: Mars exploration program consisting of two mission spacecraft-- the Mars Climate Orbiter and the Mars Polar Lander. Two surface penetrating microprobes (part of the associated Deep Space 2 mission) for detecting water ice are also piggybacking on the Lander.
Mars volcanoes: Volcanoes on the planet Mars.
marshlands: Transitional land-water areas, covered at least part of the time by estuarine or coastal waters and characterized by aquatic and grasslike vegetation. Used for bogs, coastal marshlands, marshes, and swamps.
martensitic transformation: A phase transformation occurring in some metals and resulting in formation of martensite.
martingales: In game theory, a procedure for recouping one's losses in previous wagers by doubling or otherwise increasing the amount bet.
Marx generator: A pulsed-power device invented by Erwin Marx. Capacitors are charged in parallel and then quickly discharged in series to produce high voltage, high current (and thus high power) pulses. Used in light-ion-driven and some laser-driven inertial confinement fusion systems.
mascons: Large scale, high density lunar mass concentrations below ringed mare.
maser: An amplifier utilizing the principle of microwave amplification by stimulated emission of radiation. Emission of energy stored in a molecular or atomic system by a microwave power supply is stimulated by the input signal.
mass: (symbol m)
A quantity characteristic of a body, which relates the attraction of this body toward another body. Since the mass of a body is not fixed in magnitude, all masses are referred to the standard kilogram, which is a lump of platinum.
Mass of a body always has the same value; weight changes with change in the acceleration of gravity.
mass drivers: Electromagnetic devices for the linear acceleration of projectiles or payloads. Applications include orbital insertion and transfer, propulsion systems, and hypervelocity accelerators.
mass flow rate per unit area: (symbol G)
In aerodynamics, the product of fluid density and the linear velocity of the fluid v or
G = pv _{p = lower case Rho}
mass number: The whole number nearest the value of the atomic mass of an element as expressed in atomic mass units.
The mass number is assumed to represent the total number of protons and neutrons in the atomic nucleus of the element and is therefore equal to the atomic number plus the number of the neutrons. The mass number of an atom is usually written as a superscript to the element symbol, as in O¹⁸, an isotope of oxygen with mass number 18.
mass ratio: The ratio of the mass of the propellant charge of a rocket to the total mass of the rocket when charged with the propellant.
mass spectrometers: Instruments that are capable of separating ionized molecules of different mass to charge ratio and measuring the respective ion currents. Used for ion spectrometers and retarding ion mass spectrometers.
mass to light ratios: The ratio of the mass of celestial body to its luminosity.
mass wasting: The downslope movement of rock, regolith, and/or soil under the influence of gravity.
mass-charge ratio: The ratio of the mass number of an element to the number of electronic charges gained or lost in ionization.
mass-density: (symbol )
Mass per unit volume.
mass-energy equivalence: The equivalence of a quantity of mass m and a quantity of energy E, the two quantities being related by the mass-energy relation, E = mc².
This relation was proposed by Einstein as a consequence of his restricted (or special) theory of relativity; it has subsequently received abundant experimental confirmation and is regarded as the conversion factor relating units of energy and mass; various useful forms of this factor are: $c$ ² = (2.998 X 10¹⁰)² centimeters per second = 8.987 X 10²² ergs per gram = 931.1 million electron volts per atomic mass unit.
See relativity.
mass-velocity ratio: A quantity m_v /m_r expressing the relativistic variation of mass with velocity. where m_v is moving mass, m_r is rest mass, v is velocity, and c is the velocity of light.
This ratio becomes important only at speeds approaching the speed of light.
massifs: Massive topographic and structural features, especially in orogenic belts, commonly formed of rocks more rigid than those of their surroundings. These rocks may be protruding bodies of basement rocks, consolidated during earlier orogenies, or younger plutonic bodies.
massive compact halo objects: Objects, such as brown dwarfs, black holes, and massive planets, hypothesized to account for the dark matter in the halo of the Milky Way. The signature of these objects is the occasional amplification of the light from extragalactic stars by the gravitational lens effect.
master station: In a hyperbolic navigation system, such as loran, that transmitting station which controls the transmissions of another station or of other stations. See hyperbolic navigation, slave station.
mate: To fit together two major components of a system. Also called marry.
Mate (abbr): Modular automatic test equipment.
material coordinates: = Lagrangian coordinates.
materials: In general, the substances of which rockets and space vehicles are composed; specifically, the metals, alloys, ceramics, and plastics used in structural, protective, and electronic functions.
materials recovery: The treatment of a material to reclaim one or more of its components.
mathematics: The study of the logical relationships among abstract entities. These relationships are expressed in numbers, symbols, and signs and may also be applied to concrete instances such as measures and properties of shapes. The main subdivisions include algebra, geometry, and analysis.
mating: 1. The act of fitting together two major components of a system as mating of a launch vehicle and a spacecraft. Also called marriage.
2. = interface.
matrix: 1. Any rectangular array of elements composed of rows and columns; specifically, such an array consisting of numbers or mathematical symbols which can be manipulated according to certain rules.
2. In electronic computers, any logical network whose configuration is a rectangular array of intersections of its input-output leads, with elements connected at some of these intersections. The network usually functions as an encoder or decoder . Loosely, any encoder, decoder, or translator.
matrix management: An organized approach to administration of a program by defining and structuring all elements to form a single system with components united by interaction.
matrix materials: The ingredients used as binding agents to produce composite materials.
matter-antimatter propulsion: Spacecraft propulsion by use of matter-antimatter annihilation reactions.
Matts (abbr): = multiple airborne target trajectory system.
maximum energy density: See sound energy density.
maximum entropy method: Procedure used in estimating high resolution power spectra from short data lengths.
maximum evaporation rate: The maximum rate at which molecules could emerge from a surface, deduced from measurements of saturated vapor pressure at the same temperature. Also called Knudsen rate of evaporation or Langmuir rate of evaporation.
maximum sound pressure: For any given cycle of a periodic wave, the maximum absolute value of the instantaneous sound pressure, without regard to sign, occurring during that cycle. The unit is the microbar.
In the case of a sinusoidal sound wave, the maximum sound pressure is also called the pressure amplitude.
maximum usable frequency: (abbr MUF)
For a given distance from a transmitter, the highest frequency at which sky waves can be received.
Maxwell equation: The key equations governing electrical and magnetic phenomena. These are a set of four vector partial differential equations relating electric and magnetic fields to each other and to electric charges andcurrents.
Maxwell-Boltzmann distribution: Distribution function of particle velocities (or energies) corresponding to a system in thermal equilibrium with a temperature value of T.
Maxwellian distribution: The velocity distribution, as computed in the kinetic theory of gases, of the molecules of a gas in thermal equilibrium.
This distribution is often assumed to hold for neutrons in thermal equilibrium with the moderator (thermal neutrons).
maypole antennas: A class of antennas which use the deployable reflector concept for large space systems applications.
MBM junctions: Diode devices using metal-barrier-metal layers. Used for metal-barrier-metal junctions.
MC-cubed: Mission Control and Computing Center.
MCCC: Mission Control and Computing Center.
McLeod gage: A liquid-level vacuum gage in which a known volume of gas, at the pressure to be measured, is compressed by the movement of a liquid column to a much smaller known volume, at which the resulting higher pressure is measured.
Particular designs are named after the inventors or by various trade names.
MCT: Mission Control Team, Section 391 project operations.
MCW (abbr): = modulated continuous wave.
MDI/SOI: See Michelson Doppler Imager/Solar Oscillations Investigation
mean: = arithmetic mean.
mean anomaly: See anomaly.
mean center of moon: A central point for a lunar coordinate system; the point on the lunar surface intersected by the lunar radius that is directed toward the earth's center when the moon is at the mean ascending node and when the node coincides with the mean perigee or mean apogee.
mean deviation: = average deviation.
mean distance: = semimajor axis.
mean Earth-Sun distance: Mean Earth-Sun distance is the arithmetical mean of the maximum and minimum distances between a planet (Earth) and the object about which it revolves (Sun).
mean equinox: A fictitious equinox whose position is that of the vernal equinox at a particular date with the effect of nutation removed. Also called mean equinox of date.
mean equinox of date: = mean equinox.
mean error: = root-mean-square error.
mean free path: (symbol l, , L)
1. Of any particle, the average distance that a particle travels between successive collisions with the other particles of an ensemble.
In vacuum technology, the ensemble of particles of interest comprises only the molecules in the gas phase.
2. Specifically, the average distance traveled by the molecules of a perfect gas between consecutive collisions with one another. It may be determined roughly from either of the formulas or where l is the mean free path; u (lower case Mu) is the dynamic viscosity; v is the kinematic viscosity; p (lower case Rho) is the density; c is the molecular speed (a function of the gas temperature); n is the number of molecules per unit volume; and d is the molecule diameter.
Given the mean free path l₀ at a level where the pressure is p₀ , the temperature is T₀ (°K), and the acceleration of gravity is g₀, then its value at any other level is l = l₀ p₀ Tg / pT₀ g₀ where p, T, and g are the pressure, temperature, and acceleration of gravity, respectively, at the new level. See mixing length.
3. For any process the reciprocal of the cross section per unit volume for that process.
mean motion: (symbol n, µ)
Of an object in orbit, a measure of angular velocity, n = / P where P = period.
mean noon: The instant the mean sun is over the upper branch of the reference meridian; twelve o'clock mean time.
mean position: Of a star, the position on the celestial sphere computed from past observations plus known proper motion but not corrected for short term variations. See Besselian star numbers.
mean sea level: The average height of the surface of the sea for all stages of the tide over a 19-year period, usually determined from hourly height readings.
Mean sea level is the datum from which heights are measured. In this sense sometimes shortened to sea level. See geoid.
mean sidereal time: Sidereal time adjusted for nutation to eliminate slight irregularities in the rate.
mean solar day: The duration of one rotation of the earth on its axis, with respect to the mean sun.
The length of the mean solar days is 24 hours of mean solar time or 24 hours 3 minutes 56.555 seconds of mean sidereal time. A mean solar day beginning at midnight is called a civil day; and one beginning at noon, 12 hours later, is called an astronomical day. See calendar day.
mean solar second: Prior to 1960 the fundamental unit of time, equal to 1/86,400 of the mean solar day. Now replaced by the ephemeris second.
mean solar time: Time based on an average of the variations caused by Earth's non-circular orbit.
mean solar time: See solar time.
mean square: Referring to the arithmetic mean of the squares of the values under consideration, as mean-square amplitude, mean-square error.
mean square values: In statistics, values representing the average of the sum of the squares of the deviations from the mean value.
mean sun: A fictitious sun conceived to move eastward along the celestial equator at a rate that provides a uniform measure of time equal to the average apparent time; the reference for reckoning mean time, zone time, etc. See dynamical mean sun.
mean time: Time based upon the rotation of the earth relative to the mean sun.
Mean time may be designated as local or Greenwich as the local or Greenwich meridian is the reference. Greenwich mean time is also called universal time. Zone, standard, daylight saving or summer, and war time are also variations of mean time, specified meridians being used as the reference. Mean time reckoned from the upper branch of the meridian is called astronomical time. Mean time was called civil time in U.S. terminology from 1925 through 1952. See equation of time, mean sidereal time.
mean-square error: The quantity whose square is equal to the sum of the squares of the individual errors divided by the number of those errors.
meanders: Freely developing sinuous curves, bends, loops, turns, or windings in the courses of streams. They are produced by mature streams swinging from side to side as they flow across flood plains or shift course laterally toward the convex side of an original curve.
measurand: A physical quantity, force, property or condition which is to be measured. Also called stimulus.
measurement: The technical action required to assign values (numbers) to represent certain properties or attributes, using rules based on scientific laws. Used for determination, measuring, and quantization.
mechanical engineering: Branch of engineering dealing with the design, development and operation of machines including mechanical devices and prime movers, vehicles, machine tools, and manufacturing machinery.
mechanical equivalent of heat: (symbol J) = Joule constant.
mechanical system: In the study of vibration, an aggregate of matter comprising a defined configuration of mass, mechanical stiffness, and mechanical resistance.
mechanoreceptor: A nerve ending that reacts to mechanical stimuli, as touch,, tension, and acceleration.
median: The middle term of a series, or the interpolated value of the two middle terms if the number of terms is even. Compare mean.
median lethal dose: The amount of radiation required to kill, within a specified period, 50 percent of the individuals of a group of animals or organisms.
medium frequency: (abbr MF)
See frequency band.
mega: (abbr M)
A prefix meaning multiplied by 10⁶.
megacycle: (abbr Mc, mc)
One million cycles; one thousand kilocycles.
The term is often used as the equivalent of one million cycles per second.
megaparsec: One million parsecs. See parsec.
mel: A unit of acoustic pitch. By definition, a simple tone of frequency 1000 cycles per second, 40 decibels above a listener's threshold, produces a pitch of 1000 mels. The pitch of any sound that is judged by the listener to be n times that of a 1-mel tone is n mels.
melt spinning: A material process by which polymers such as nylon and polyesters and glass are melted to permit extrusion into fibers through spinnerets.
meltdown: In a fission reactor, if there is insufficient coolant or the fission chain reaction proceeds too rapidly, heat can build up in the reactor fuel, causing it to melt. In extreme cases the whole fission core can melt down to (or even through) the reactor floor. Fusion reactors are not vulnerable to this.
melting points: liquidus and solidus coincide at an invariant point. In a phase diagram, the temperature at which the
melts (crystal growth): Molten substances from which crystals are formed during the cooling or solidifying process.
membrane structure: A shell structure, often pressurized, that does not take wall bending or compression loads.
memory: The component of a computer, control system, guidance system, instrumented satellite, or the like, designed to provide ready access to data or instructions previously recorded so as to make them bear upon an immediate problem, such as the guidance of a physical object, or the analysis and reduction of data.
memory (computers): The component of a computer, control system, guidance system, instrumented satellite, or the like, designed to provide ready access to data or instructions previously recorded so as to make them bear upon an immediate problem, such as the guidance of a physical object, or the analysis and reduction of data.
memory capacity: See storage capacity.
memory device: See storage.
Men, Mens: International Astronomical Union abbreviations for Mensa. See constellation.
Mensa: (abbr Men, Mens)
See constellation.
Mercator projection: An equatorial, cylindrical, conformal map projection derived by mathematical analysis (not geometrically) in which the equator is represented by a straight line true to scale. The meridians are represented by parallel straight lines perpendicular to the equator and equally spaced according to their distance apart at the equator. The parallels are represented by straight lines perpendicular to the meridians and parallel with (and the same length as) the equator. The parallels are spaced so as to achieve conformality, their spacing increasing rapidly with their distance from the equator so that at all places the degrees of latitude and longitude have the same ratio to each other as to the sphere itself. This results in greater distortion of distances, areas, and shapes in the polar regions (above 80 deg. latitude). The scale is increasingly poleward as the secant of the latitude. Because any line of constant direction (azimuth) on the sphere is truly represented on the projection by a straight line, the Mercator projection is of great value in navigation. It is used for hydrographic charts, and also to show geographic variations of some physical property (such as magnetic declination) or to plot trajectories of Earth satellites in oblique orbits. It is named after Gerhardus Mercator (1512-1594), a Flemish mathematician and geographer, whose world map of 1569 used this projection.
Mercury: See planet, table.
Mercury (planet): First planet from the sun, a terrestrial planet.
mercury cadmium tellurides: Compounds of tellurium exhibiting photovoltaic characteristics and used for photodiodes and photodetectors in the 3 to 12 micrometer wavelengths at cryogenic temperatures. Used for cadmium mercury tellurides.
mercury ion engines: Machines providing thrust by expelling accelerated or high velocity mercury ions and often using energy provided by nuclear reactors.
mercury memory: The retention of information by the propagation of a sound wave in liquid memory.
Mercury surface: The surface of the planet Mercury.
meridian: A north-south reference line, particularly a great circle through the geographical poles of the earth. The term usually refers to the upper branch, that half, from pole to pole, which passes through a given place, the other half being called the lower branch. See coordinate, table.
A terrestrial meridian is a meridian of the earth. Sometimes designated true meridian to distinguish it from magnetic meridian, compass meridian, or grid meridian, the north-south lines relative to magnetic, compass, or grid direction, respectively. An astronomical meridian is a line connecting points having the same astronomical longitude. A geodetic meridian is a line connecting points of equal geodetic longitude. Geodetic and sometimes astronomical meridians are also called geographic meridians. Geodetic meridians are shown on charts. The prime meridian passes through longitude 0°. A fictitious meridian is one of a series of great circles or lines used in place of a meridian for certain purposes. A transverse or inverse meridian is a great circle perpendicular to a transverse equator. An oblique meridian is a great circle perpendicular to an oblique equator. Any meridian used as a reference for reckoning time is called a time meridian. The meridian through any particular place or observer, serving as the reference for local time, is called local meridian, in contrast with the Greenwich meridian, the reference for Greenwich time. A celestial sphere, through the celestial poles and the zenith.
meridian angle: Angular distance east or west of the local celestial meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the local celestial meridian and the hour circle of a celestial body, measured eastward or westward from the local celestial meridian through 180°, and labeled E or W to indicate the direction of measurement. See hour angle.
meridian transit: See transit.
meridians: Great circles that pass through both the north and south poles, also called lines of longitude.
meridional: Referring to a meridian.
meridional flow: Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.
mesas: Isolated, nearly level landmasses standing distinctly above the surrounding country, bounded by abrupt or steeply sloping erosion scarps on all sides, and capped by layers of resistant, nearly horizontal rock (often lava). Less strictly, very broad, flat topped, usually isolated hills or mountains of moderate height bounded on at least one side by a steep cliff or slope and representing an erosion remnant. Mesas are similar to, but have more summit area than buttes and are common topographical features in arid and semiarid regions of the United States. Mesas are often considered broad terraces or comparatively flat plateaus along river valleys. They are marked by an abrupt slope or escarpment on one side.
mesh: A set of branches forming a closed path in a network, provided that if any one branch is omitted from the set, the remaining branches of the set do not form a closed path.
The term loop is sometimes used in the sense of mesh.
meson: In the classification of a subatomic particles by mass, the second lightest of such particles. Its mass is intermediate between that of the lepton and the nucleon (see hyperon).
Mesons are highly unstable, very short-lived particles; they carry positive, negative, or no charge, and, in a vacuum, move with velocities approaching the speed of light. All of these particles have extremely short lifetimes and the heavier more unstable mesons tend to decay into lighter ones.
mesopause: The base of the inversion at the top of the mesosphere, usually found at 80 to 85 kilometers. See atmospheric shell.
mesopeak: The temperature maximum at about 50 kilometers in the mesosphere. See atmospheric shell.
mesoscale phenomena: Meteorological phenomena extending approximately one to a hundred kilometers (mesoscale cloud patterns, for example).
mesosphere: 1. The atmospheric shell, in which temperature generally decreases with heights, extending from the stratopause at about 50 to 55 kilometers to the mesopause at about 80 to 85 kilometers.
2. The atmospheric shell between the top of the ionosphere (the top of this region has never been clearly defined) and the bottom of the exosphere. (This definition has not gained general acceptance.)
Mesozoic Era: An era of geologic time, from the end of the Paleozoic Era to the beginning of the Cenozoic Era, or from about 225 to about 65 million years ago.
message: 1. An ordered selection from an agreed set of symbols, intended to communicate information.
2. The original modulating wave in a communication system.
The term in sense 1 is used in communication theory; the term in sense 2 is often used in engineering practice.
message processing: In communication operations, the acceptance, preparation for transmission, receipt and/or delivery of a series of words or symbols intended for conveying information.
MESUR: The Mars Environmental Survey project at JPL, the engineering prototype of which is called MESUR Pathfinder.
metabolic reserves: The energy source stored in chemical form, such as carbohydrates, that can be efficiently mobilized and utilized by the body, particularly for muscular activity and work beyond the normal level of activity of an individual.
metabolism: The sum of all physical and chemical processes by which living organized substance is produced and maintained and by which energy is made available for the use of the organism.
metabolites: Products of biological synthesis and/or metabolism.
metachemical: Pertaining to the chemistry of subatomic particles.
metagalaxy: The entire system of galaxies including the Milky Way.
metal clusters: Bonded aggregations of like metal atoms.
metal foams: Foamed materials formed under low gravity conditions in space from sputtered metal deposits. This experimental space processing was completed in the second NASA SPAR flight.
metal vapor lasers: Stimulated emission devices, the active materials of which are vaporized metals.
metal-nitride-oxide-semiconductors: Class of semiconductors utilizing silicon nitride and silicon oxide dielectrics.
metallic fuels: Of or pertaining to nuclear fuels which are a mixture, a pressed powder, or an alloy of a fissionable material, such as uranium-235 or plutonium-239, and a metal such as aluminum, zirconium, or stainless steel.
metallic glasses: Amorphous alloys (glassy metals) produced by extremely rapid quenching of molten transition-metal alloys (e.g., iron, nickel, and/or cobalt). These metallic glasses exhibit unique mechanical, magnetic, and electrical properties, superconductive behavior, and anticorrosion resistance, depending on the alloys, their formation and quenching techniques.
metallicity: The abundance index of a metal or metals for a celestial body.
metamorphic rocks: Rocks derived from pre-existing rocks by mineralogical, chemical and/or structural changes, essentially in the solid state. These changes are in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at the depth of the Earth's crust. Metamorphic rocks constitute one of the three main classes into which rocks are divided, the others being igneous rocks and sedimentary rocks.
metamorphism (geology): The mineralogical and structural adjustment of solid rocks to physical and chemical conditions which have been imposed at depth below the surface zones of weathering and cementation, which differ from the conditions under which the rocks in question originated.
metastable atom: An atom with an electron excited to an energy level where simple radiation is forbidden and thus the atom is momentarily stable. See forbidden line.
The presence of these metastable atoms in a discharge is the cause of several anomalous effects since in essence they are storing energy which can be released to other particles upon collision. The Penning effect is a result of the presence of metastable atoms.
metastable compound: A chemical compound of comparative stability which, however, becomes unstable under a particular set of conditions.
metastable propellant: A metastable compound used as a propellant.
Nitromethane (CH₃NO₂), for example, may be used as a monopropellant at chamber pressure above 500 pounds per square inch. At lower pressure, it requires an oxidizer for stable combustion.
metastable state: An excited stationary energy state, as of an atom, whose lifetime is unusually long.
meteor: The bright streak of light that appears in the sky as a result of