geodesy. The science that deals mathematically with the size and shape of the earth, the earth's external gravity field, and surveys so precise that size and shape of the earth must be taken into consideration. See pp. 186-96.

geoid. The figure of the earth as defined by the level surface that over the oceans coincides with mean sea level. See pp. 189–96.

geomagnetic equator. The great circle of the earth lying midway between the north and south poles of the earth's dipole magnetic field.

geomagnetic latitude and longitude. Analogous to geographic latitude and longitude, but referred to the dipole magnetic poles and geomagnetic equator instead of to the geographic poles and equator.

geometrical geodesy. Study, by geometrical and astronomical measurements, of the precise size and shape of the earth and accurate location of points on the surface of the earth. See p. 186.

geopotential function. That function which at each point is equal to an arbitrary constant minus the energy that would be acquired by a unit mass in falling from rest at infinity to the point in question. The gradient of the geopotential function, which can be obtained by vector calculus, yields the earth's gravitational field.

geosyncline. Layers of rocks that have been tilted from their original horizontal stratification to form a huge basin which then fills with sediments.

gradient. The rate of change of a quantity with distance in a specified direction is the gradient of the quantity in that direction. When the term gradient is used without specifying the direction, it is taken to mean the rate of change of the quantity in question in the direction of greatest rate of change.

guidance. The process of directing the movements of an aircraft, spacecraft, missile, or other vehicle. In general such vehicles are equipped so that they can be controlled to follow the guidance supplied to them.

hydrated compound. A compound in which the molecules contain water (HO) or hydroxyl radicals and protons (OH and H).

integration. A mathematical process used in the calculus for deriving a function from its slope. The reverse process, which yields the slope of a given function, is differentiation. International Geophysical Year (IGY). An internationally agreed on period, July 1957 through December 1958, during which observation of worldwide geophysical phenomena was greatly increased by cooperative effort of participating nations. Activities were continued through December 1959 as International Geophysical Cooperation. See pp. 50-51.

ion. A charged atom or molecularly bound group of atoms; sometimes also a free electron or other charged subatomic particle. In the normal state atoms and molecules are electrically neutral. An atom or molecule that acquires one or more electrons becomes negatively charged and is called a negative ion. If the particle loses one or more electrons it becomes positively charged and is called a positive ion. ionization gauge. A gauge in which the effect of an ambient gas on the electric current flow from a hot filament is used to measure a property of the gas, such as charge density. ionosphere. Upper levels of the earth's atmosphere, extending outward from about 70 kilometers altitude and containing free electronically charged particles that reflect radio waves. See pp. 64-65.

isothermal. Of constant temperature.

isotropic radiation. Radiation that is of equal intensity in all directions.

law of sines. In trigonometry, the law which states that in a triangle the lengths of the sides are in proportion to the sines of the opposite angles. Thus, if a, b, c are the three sides of a triangle, and a, ẞ, y are the angles opposite to a, b, and c, respectively, then

a/sin a = b/sin B

=

c/sin y.

level surface. A surface to which the force of gravity is everywhere perpendicular. See pp. 190-92.

liftoff. The rising of a space launch vehicle from its launching stand immediately after firing.

light-year. The distance light travels in one year, equal to 9.46055 × 105 meters.

line in a spectrum. Light emitted or absorbed at a discrete wavelength by a radiating or absorbing atom.

Lyman alpha line. The hydrogen atom is capable of emitting (or absorbing) electromagnetic radiation in several different series of lines. One of these is known as the Lyman series. The longest wavelength of the Lyman series is at 1216 Å in the ultraviolet region, and is known as Lyman alpha. The next longest wavelength of the series is Lyman beta, then Lyman gamma, etc.

magnetic storm. A disturbance in the earth's magnetic field assumed to be caused by streams of particles and magnetic fields from the sun.

magnetosphere. The region of space surrounding the earth where the magnetic field plays a prominent, often controlling, role relative to particle radiations found there. See pp. 173-84.

main stage. That stage of a launch vehicle—usually the largest stage-that is used to lift the launch vehicle and its payload to a high enough altitude that the remaining stages of the vehicle can project the payload into orbit or into a space trajectory. man-machine relations. Relations between man and a machine-such as an aircraft or spacecraft. The matching of human characteristics to a machine so as to obtain maximum efficiency or optimum conditions for operation is called man-machine integration. The integrated combination of man and machine in an operating unit is called a man-machine system.

mantle of the earth. The interior of the earth between about 3400 kilometers-the outer boundary of the core-and the crust.

mare (pl. maria). The maria are the dark regions of the moon, once thought to be seas, hence the term mare from the Latin for sea. The term is also applied to dark regions on Mars.

mean free path. The average distance a gas particle travels between successive collisions with other particles.

meson. An elementary particle of mass intermediate between the masses of the electron and proton.

mesosphere. Sometimes used to denote the middle atmosphere, between the stratosphere and the ionosphere.

metagalaxy. The system of all galaxies. The physical universe.

meteor. A fast moving mass from space traversing the atmosphere.

meteorite. A meteor that survived passage through the atmosphere to strike the ground. micrometeor; micrometeorite. A meteor or meteorite that is a few hundreds of micrometers or less in diameter.

mission-oriented science. Applied science carried out in support of a specifically stated mission, such as providing the necessary weaponry for naval warfare, maintaining adequate air power, improving agriculture, developing natural resources, or predicting the weather.

monochromatic beam. A beam of radiation of a single wavelength. nebula. A huge cloud of gas and dust in space.

neutron. An electrically neutral particle of mass essentially equal to that of the proton. A free neutron is unstable, decaying (with a half life of about 12 minutes) to an electron and a proton. (To say that the half life of a neutron is 12 minutes means that of a group of N neutrons, N/2 of them will decay before 12 minutes, and N/2 of them after 12 minutes.)

oblate ellipsoid of revolution. The surface (or solid) obtained by revolving an ellipse about its minor axis. Revolving the ellipse about its major axis produces a prolate ellipsoid of revolution.

orbit. The closed path of a moving body or particle around another object. See also trajectory.

ozone. The gas whose molecules consist of three oxygen atoms combined: 03.

ozonosphere. The region of the atmosphere in the upper stratosphere and lower mesosphere where there are appreciable quantities of ozone. See pp. 61, 63, fig. 1. perigee. For a satellite orbit, the nearest point to the center of the earth. photosphere of the sun. The visible disk of the sun. See pp. 364-65.

physical geodesy. The study of the gravitational field of the earth and its relationship to the solid structure of the planet. Also called dynamical geodesy. See p. 186.

Pirani gauge. A pressure gauge in which the rate of cooling of a heated filament by the ambient gas is used to measure the pressure of the gas. plasma. A hot gas consisting of equal numbers of positive and negative ions. polarized light. The wave theory of light pictures light as electromagnetic vibrations in space, with electric and magnetic vectors vibrating perpendicularly to each other. Normally in a beam of light the orientations of the electric and magnetic vectors are random. But if the electric (or magnetic) vectors all vibrate parallel to a common plane, the light in the beam is said to be plane polarized.

programmatic science. Science carried out as part of a specific program—say, the atomic energy program, or the NASA space science program as illustrations—and constrained (loosely perhaps, but constrained nevertheless) to fit within the general confines of the program.

proton. The nucleus of the hydrogen atom. The proton carries a single positive electric charge, and has a mass of 1.673 × 10-27 kilograms.

pulsar. A star that emits radiation in equally spaced pulses.

pure science. See uncommitted science.

quantum theory. The theory that all electromagnetic radiation is emitted and absorbed in quanta of energy equal to hy, where h is a constant called the Planck constant after the propounder of quantum theory, and v is the frequency of the radiation.

quasar. An exceedingly remote astronomical object which appears like a blue star but which because of the prodigious quantities of energy it radiates may turn out to be a galaxy.

radiation belt. The portion of a planetary magnetosphere that contains charged particle radiations unable to escape because of the ambient magnetic field.

radio galaxy. A galactic system that emits prominently in radio wavelengths. recombination. The reverse of dissociation, with the separated parts of a molecule rejoining to reproduce the original molecule from which they came. For example, an electron may recombine with a positive ion to form a neutral atom or molecule; a positive ion may join with a negative ion to form a neutral molecule; or two neutral atoms might recombine to form a molecule.

recombination coefficient. A quantitative measure of the affinity of particles for recombining. reflection coefficient. A numerical quantity that gives the proportions and manner in which a medium reflects incident radiation.

refraction. The bending of electromagnetic rays by properties of the medium traversed. relativistic particle. A particle moving sufficiently close to the speed of light that effects of relativity, such as an increase in mass or a slowing of time, become significant. resonance line. The longest wavelength that an atom can emit or absorb. retrorocket. A rocket so mounted on a vehicle or spacecraft that it fires in the direction opposite to the motion of the vehicle or spacecraft. Retrorockets are used to slow down the vehicles on which they are mounted.

rocket stage. A self-propelled separable element of a rocket vehicle. In a multistage rocket, each rocket unit fires after the one behind it has used up its propellant and (normally) been discarded. See page. p. 135.

satellite. An attendant body that revolves around another body called the satellite's primary. By custom, the bodies revolving around the sun are called planets, not satellites, of the

sun.

scientific discipline. An area of scientific investigation in which the investigators share a common paradigm or group of paradigms-embracing a common body of theory-and techniques and often instrumentation that stem from the underlying theoretical basis of the discipline.

sectorial harmonic. One of a series of terms representing the gravitational potential of the earth, in which the terms exhibit pronounced variation with longitude. See spherical harmonic and p. 192.

Seyfert galaxy. A spiral galaxy with a very compact, highly luminous nucleus.

solar wind. A continuous wind of charged particles from the sun, blowing through inter

planetary space.

spacecraft. Devices, manned or unmanned, that are intended to be placed in an orbit about the earth in space, or on a trajectory to another celestial body.

space launch vehicle. A combination of rocket stages, with the necessary guidance and control equipment, used to project a spacecraft into space.

space probe. Spacecraft sent away from earth into space. See. p. 153.

space science. Scientific investigations made possible or significantly aided by rockets, satellites, and space probes. See chap. 1, especially, pp. 11-15.

spectrum. Electromagnetic radiation displayed or visualized as a function of wavelength. Thus, the rainbow is part of the spectrum of sunlight.

spherical harmonic. One of a series of terms expressing the geopotential function in sines and cosines of latitude and longitude. See pp. 192–93.

stage. See rocket stage.

stratosphere. The layer of the earth's atmosphere between about 16 and 50-55 kilometers altitude, lying above the troposphere and extending to the stratopause; temperature generally increases with altitude in the stratosphere. See pp. 60–62.

sunspot cycle. A cycle of variation in the total number and area of spots on the sun's surface, from a maximum to a minimum with an average period of 11 years. Since magnetic fields on the sun reverse with each such cycle, the period of a complete cycle of spottedness-plus-magnetic-condition is 22 years. superconducting cavity. When certain materials are brought sufficiently close to absolute zero temperature, they become perfect electrical conductors capable of sustaining circulating electrical currents indefinitely without resistance losses. A cavity in which such conditions of superconductivity are maintained is a superconducting cavity. The resonance properties of such superconducting cavities permit the construction of highly accurate oscillators, or clocks.

synchronous orbit. The orbit over the earth's equator at an altitude of 36 000 km, in which the rate of revolution of the satellite around the earth equals the rate of rotation of the earth, so that the satellite always stays over the same spot on the ground.

technology. Technical know-how; the knowledge and ability to do things of a technical or engineering nature, including the field of industrial arts.

telemetering. Measuring an object or phenomenon at a distance. Radio is often used to

transmit the measured data from the point of measurement to a remote observer. trajectory. In general, the path in three dimensions (i.e., space) of a moving body. The word trajectory is often used to mean flight path. If the trajectory is a closed path around another object like the earth, the trajectory is called an orbit. When the path is not closed, the word trajectory is usually used.

troposphere. The lowest portion of the atmosphere, extending from the ground to the base of the stratosphere.

uncommitted or pure science. Science carried out simply to pursue what the researcher considers to be important problems, the solutions to which show promise of revealing significant information about nature and the universe. Uncommitted or pure science is not constrained by programmatic, applications, or mission objectives.

vorticity. The amount of rotational motion possessed by a fluid is called its vorticity. zonal harmonic. One of a series of terms representing the gravitational potential of the earth. Zonal harmonics correspond to coefficients Jam for which m depend only on latitude. See spherical harmonic and also pp. 192-93.

= 0 and which