Glossary

ablation: removal of the outer layers of an object by erosion, melting, or vaporization
abort-to-orbit: emergency procedure planned for the space shuttle and other spacecraft if the spacecraft reaches a lower than planned orbit
accretion: the growth of a star or planet through the accumulation of material from a companion star or the surrounding interstellar matter
adaptive optics: the use of computers to adjust the shape of a telescope's optical system to compensate for gravity or temperature variations
aeroballistic: describes the combined aerodynamics and ballistics of an object, such as a spacecraft, in flight
aerobraking: the technique of using a planet's atmosphere to slow down an incoming spacecraft; its use requires the spacecraft to have a heat shield, because the friction that slows the craft is turned into intense heat
aerodynamic heating: heating of the exterior skin of a spacecraft, aircraft, or other object moving at high speed through the atmosphere
Agena: a multipurpose rocket designed to perform ascent, precision orbit injection, and missions from low Earth orbit to interplanetary space; also served as a docking target for the Gemini spacecraft
algae: simple photosynthetic organisms, often aquatic
alpha proton X-ray: analytical instrument that bombards a sample with alpha particles (consisting of two protons and two neutrons); the X rays are generated through the interaction of the alpha particles and the sample
altimeter: an instrument designed to measure altitude above sea level
amplitude: the height of a wave or other oscillation; the range or extent of a process or phenomenon
angular momentum: the angular equivalent of linear momentum; the product of angular velocity and moment of inertia (moment of inertia = mass × radius²)
angular velocity: the rotational speed of an object, usually measured in radians per second

anisotropy: a quantity that is different when measured in different directions or along different axes
annular: ring-like
anomalies: phenomena that are different from what is expected
anorthosite: a light-colored rock composed mainly of the mineral feldspar (an aluminum silicate); commonly occurs in the crusts of Earth and the Moon
anthropocentrism: valuing humans above all else
antimatter: matter composed of antiparticles, such as positrons and antiprotons
antipodal: at the opposite pole; two points on a planet that are diametrically opposite
aperture: an opening, door, or hatch
aphelion: the point in an object's orbit that is farthest from the Sun
Apollo: American program to land men on the Moon; Apollo 11, 12, 14, 15, 16, and 17 delivered twelve men to the lunar surface between 1969 and 1972 and returned them safely back to Earth
asthenosphere: the weaker portion of a planet's interior just below the rocky crust
astrometry: the measurement of the positions of stars on the sky
astronomical unit: the average distance between Earth and the Sun (152 million kilometers [93 million miles])
atmospheric probe: a separate piece of a spacecraft that is launched from it and separately enters the atmosphere of a planet on a one-way trip, making measurements until it hits a surface, burns up, or otherwise ends its mission
atmospheric refraction: the bending of sunlight or other light caused by the varying optical density of the atmosphere
atomic nucleus: the protons and neutrons that make up the core of an atom
atrophy: condition that involves withering, shrinking, or wasting away
auroras: atmospheric phenomena consisting of glowing bands or sheets of light in the sky caused by high-speed charged particles striking atoms in Earth's upper atmosphere
avionics: electronic equipment designed for use on aircraft, spacecraft, and missiles
azimuth: horizontal angular distance from true north measured clockwise from true north (e.g., if North = 0 degrees; East = 90 degrees; South = 180 degrees; West = 270 degrees)
ballast: heavy substance used to increase the stability of a vehicle
ballistic: the path of an object in unpowered flight; the path of a spacecraft after the engines have shut down

basalt: a dark, volcanic rock with abundant iron and magnesium and relatively low silica common on all of the terrestrial planets
base load: the minimum amount of energy needed for a power grid
beacon signal generator: a radio transmitter emitting signals for guidance or for showing location
berth space: the human accommodations needed by a space station, cargo ship, or other vessel
Big Bang: name given by astronomers to the event marking the beginning of the universe when all matter and energy came into being
biocentric: notion that all living organisms have intrinsic value
biogenic: resulting from the actions of living organisms; or, necessary for life
bioregenerative: referring to a life support system in which biological processes are used; physiochemical and/or nonregenerative processes may also be used
biosignatures: the unique traces left in the geological record by living organisms
biosphere: the interaction of living organisms on a global scale
bipolar outflow: jets of material (gas and dust) flowing away from a central object (e.g., a protostar) in opposite directions
bitumen: a thick, almost solid form of hydrocarbons, often mixed with other minerals
black holes: objects so massive for their size that their gravitational pull prevents everything, even light, from escaping
bone mineral density: the mass of minerals, mostly calcium, in a given volume of bone
breccia: mixed rock composed of fragments of different rock types; formed by the shock and heat of meteorite impacts
bright rays: lines of lighter material visible on the surface of a body and caused by relatively recent impacts
brown dwarf: star-like object less massive than 0.08 times the mass of the Sun, which cannot undergo thermonuclear process to generate its own luminosity
calderas: the bowl-shaped crater at the top of a volcano caused by the collapse of the central part of the volcano
Callisto: one of the four large moons of Jupiter; named for one of the Greek nymphs
Caloris basin: the largest (1,300 kilometers [806 miles] in diameter) well-preserved impact basin on Mercury viewed by Mariner 10
capsule: a closed compartment designed to hold and protect humans, instruments, and/or equipment, as in a spacecraft

carbon-fiber composites: combinations of carbon fibers with other materials such as resins or ceramics; carbon fiber composites are strong and lightweight
carbonaceous meteorites: the rarest kind of meteorites, they contain a high percentage of carbon and carbon-rich compounds
carbonate: a class of minerals, such as chalk and limestone, formed by carbon dioxide reacting in water
cartographic: relating to the making of maps
Cassini mission: a robotic spacecraft mission to the planet Saturn scheduled to arrive in July 2004 when the Huygens probe will be dropped into Titan's atmosphere while the Cassini spacecraft studies the planet
catalyst: a chemical compound that accelerates a chemical reaction without itself being used up; any process that acts to accelerate change in a system
catalyze: to change by the use of a catalyst
cell culture: a means of growing mammalian (including human) cells in the research laboratory under defined experimental conditions
cellular array: the three-dimensional placement of cells within a tissue
centrifugal: directed away from the center through spinning
centrifuge: a device that uses centrifugal force caused by spinning to simulate gravity
Cepheid variables: a class of variable stars whose luminosity is related to their period. Their periods can range from a few hours to about 100 days and the longer the period, the brighter the star
Ĉerenkov light: light emitted by a charged particle moving through a medium, such as air or water, at a velocity greater than the phase velocity of light in that medium; usually a faint, eerie, bluish, optical glow
chassis: frame on which a vehicle is constructed
chondrite meteorites: a type of meteorite that contains spherical clumps of loosely consolidated minerals
cinder field: an area dominated by volcanic rock, especially the cinders ejected from explosive volcanoes
circadian rhythm: activities and bodily functions that recur every twenty-four hours, such as sleeping and eating
Clarke orbit: geostationary orbit; named after science fiction writer Arthur C. Clarke, who first realized the usefulness of this type of orbit for communication and weather satellites
coagulate: to cause to come together into a coherent mass
comet matrix material: the substances that form the nucleus of a comet; dust grains embedded in frozen methane, ammonia, carbon dioxide, and water

cometary outgassing: vaporization of the frozen gases that form a comet nucleus as the comet approaches the Sun and warms
communications infrastructure: the physical structures that support a network of telephone, Internet, mobile phones, and other communication systems
convection: the movement of heated fluid caused by a variation in density; hot fluid rises while cool fluid sinks
convection currents: mechanism by which thermal energy moves because its density differs from that of surrounding material. Convection current is the movement pattern of thermal energy transferring within a medium
convective processes: processes that are driven by the movement of heated fluids resulting from a variation in density
coronal holes: large, dark holes seen when the Sun is viewed in X-ray or ultraviolet wavelengths; solar wind emanates from the coronal holes
coronal mass ejections: large quantities of solar plasma and magnetic field launched from the Sun into space
cosmic microwave background: ubiquitous, diffuse, uniform, thermal radiation created during the earliest hot phases of the universe
cosmic radiation: high energy particles that enter Earth's atmosphere from outer space causing cascades of mesons and other particles
cosmocentric ethic: an ethical position that establishes the universe as the priority in a value system or appeals to something characteristic of the universe that provides justification of value
cover glass: a sheet of glass used to cover the solid state device in a solar cell
crash-landers: or hard-lander; a spacecraft that collides with the planet, making no—or little—attempt to slow down; after collision, the spacecraft ceases to function because of the (intentional) catastrophic failure
crawler transporter: large, tracked vehicles used to move the assembled Apollo/Saturn from the VAB to the launch pad
cryogenic: related to extremely low temperatures; the temperature of liquid nitrogen or lower
cryptocometary: another name for carbonaceous asteroids—asteroids that contain a high percentage of carbon compounds mixed with frozen gases
cryptoendolithic microbial: microbial ecosystems that live inside sandstone in extreme environments such as Antarctica
crystal lattice: the arrangement of atoms inside a crystal
crystallography: the study of the internal structure of crystals
dark matter: matter that interacts with ordinary matter by gravity but does not emit electromagnetic radiation; its composition is unknown
density-separation jigs: a form of gravity separation of materials with different densities that uses a pulsating fluid

desiccation: the process of drying up
detruents: microorganisms that act as decomposers in a controlled environmental life support system
diffuse: spread out; not concentrated
DNA: deoxyribonucleic acid; the molecule used by all living things on Earth to transmit genetic information
docking system: mechanical and electronic devices that work jointly to bring together and physically link two spacecraft in space
doped: semiconductor such as silicon with an addition of small amounts of an impurity such as phosphorous to generate more charge carriers (such as electrons)
dormant comet: a comet whose volatile gases have all been vaporized, leaving behind only the heavy materials
downlink: the radio dish and receiver through which a satellite or spacecraft transmits information back to Earth
drag: a force that opposes the motion of an aircraft or spacecraft through the atmosphere
dunites: rock type composed almost entirely of the mineral olivine, crystallized from magma beneath the Moon's surface
dynamic isotope power: the decay of isotopes such as plutonium-238, and polonium-210 produces heat, which can be transformed into electricity by radioisotopic thermoelectric generators
Earth-Moon LaGrange: five points in space relative to Earth and the Moon where the gravitational forces on an object balance; two points, 60 degrees from the Moon in orbit, are candidate points for a permanent space settlement due to their gravitational stability
eccentric: the term that describes how oval the orbit of a planet is
ecliptic: the plane of Earth's orbit
EH condrites: a rare form of meteorite containing a high concentration of the mineral enstatite (a type of pyroxene) and over 30 percent iron
ejecta: the pieces of material thrown off by a star when it explodes; or, material thrown out of an impact crater during its formation
ejector ramjet: engine design that uses a small rocket mounted in front of the ramjet to provide a flow of heated air, allowing the ramjet to provide thrust when stationary
electrodynamic: pertaining to the interaction of moving electric charges with magnetic and electric fields
electrolytes: a substance that when dissolved in water creates an electrically conducting solution
electromagnetic spectrum: the entire range of wavelengths of electromagnetic radiation

electron: a negatively charged subatomic particle
electron volts: units of energy equal to the energy gained by an electron when it passes through a potential difference of 1 volt in a vacuum
electrostatic separation: separation of substances by the use of electrically charged plates
elliptical: having an oval shape
encapsulation: enclosing within a capsule
endocrine: system in the body that creates and secretes substances called hormones into the blood
equatorial orbit: an orbit parallel to a body's geographic equator
equilibruim point: the point where forces are in balance
Europa: one of the large satellites of Jupiter
eV: an electron volt is the energy gained by an electron when moved across a potential of one volt. Ordinary molecules, such as air, have an energy of about 3x10^-2 eV
event horizon: the imaginary spherical shell surrounding a black hole that marks the boundary where no light or any other information can escape
excavation: a hole formed by mining or digging
expendable launch vehicles: launch vehicles, such as a rocket, not intended to be reused
extrasolar planets: planets orbiting stars other than the Sun
extravehicular activity: a space walk conducted outside a spacecraft cabin, with the crew member protected from the environment by a pressurized space suit
extremophiles: microorganisms that can survive in extreme environments such as high salinity or near boiling water
extruded: forced through an opening
failsafe: a system designed to be failure resistant through robust construction and redundant functions
fairing: a structure designed to provide low aerodynamic drag for an aircraft or spacecraft in flight
fault: a fracture in rock in the upper crust of a planet along which there has been movement
feedstock: the raw materials introduced into an industrial process from which a finished product is made
feldspathic: rock containing a high proportion of the mineral feldspar
fiber-optic cable: a thin strand of ultrapure glass that carries information in the form of light, with the light turned on and off rapidly to represent the information sent

fission: act of splitting a heavy atomic nucleus into two lighter ones, releasing tremendous energy
flares: intense, sudden releases of energy
flybys: flight path that takes the spacecraft close enough to a planet to obtain good observations; the spacecraft then continues on a path away from the planet but may make multiple passes
fracture: any break in rock, from small "joints" that divide rocks into planar blocks (such as that seen in road cuts) to vast breaks in the crusts of unspecified movement
freefall: the motion of a body acted on by no forces other than gravity, usually in orbit around Earth or another celestial body
free radical: a molecule with a high degree of chemical reactivity due to the presence of an unpaired electron
frequencies: the number of oscillations or vibrations per second of an electromagnetic wave or any wave
fuel cells: cells that react a fuel (such as hydrogen) and an oxidizer (such as oxygen) together; the chemical energy of the initial reactants is released by the fuel cell in the form of electricity
fusion: the act of releasing nuclear energy by combining lighter elements such as hydrogen into heavier elements
fusion fuel: fuel suitable for use in a nuclear fusion reactor
G force: the force an astronaut or pilot experiences when undergoing large accelerations
galaxy: a system of as many as hundreds of billions of stars that have a common gravitational attraction
Galilean satellite: one of the four large moons of Jupiter first discovered by Galileo
Galileo mission: succesful robot exploration of the outer solar system; this mission used gravity assists from Venus and Earth to reach Jupiter, where it dropped a probe into the atmosphere and studied the planet for nearly seven years
gamma rays: a form of radiation with a shorter wavelength and more energy than X rays
Ganymede: one of the four large moons of Jupiter; the largest moon in the solar system
Gemini: the second series of American-piloted spacecraft, crewed by two astronauts; the Gemini missions were rehearsals of the spaceflight techniques needed to go to the Moon
general relativity: a branch of science first described by Albert Einstein showing the relationship between gravity and acceleration
geocentric: a model that places Earth at the center of the universe

geodetic survey: determination of the exact position of points on Earth's surface and measurement of the size and shape of Earth and of Earth's gravitational and magnetic fields
geomagnetic field: Earth's magnetic field; under the influence of solar wind, the magnetic field is compressed in the Sunward direction and stretched out in the downwind direction, creating the magnetosphere, a complex, teardrop-shaped cavity around Earth
geospatial: relating to measurement of Earth's surface as well as positions on its surface
geostationary: remaining above a fixed point above Earth's equator
geostationary orbit: a specific altitude of an equatorial orbit where the time required to circle the planet matches the time it takes the planet to rotate on its axis. An object in geostationary orbit will always remain over the same geographic location on the equator of the planet it orbits
geosynchronous: remaining fixed in an orbit 35,786 kilometers (22,300 miles) above Earth's surface
geosynchronous orbit: a specific altitude of an equatorial orbit where the time required to circle the planet matches the time it takes the planet to rotate on its axis. An object in geostationary orbit will always remain over the same geographic location on the equator of the planet it orbits
gimbal motors: motors that direct the nozzle of a rocket engine to provide steering
global change: a change, such as average ocean temperature, affecting the entire planet
global positioning systems: a system of satellites and receivers that provide direct determination of the geographical location of the receiver
globular clusters: roughly spherical collections of hundreds of thousands of old stars found in galactic haloes
grand unified theory: (GUT) states that, at a high enough energy level (about 10²⁵ eV), the electromagnetic force, strong force, and weak force all merge into a single force
gravitational assist: the technique of flying by a planet to use its energy to "catapult" a spacecraft on its way—this saves fuel and thus mass and cost of a mission; gravitational assists typically make the total mission duration longer, but they also make things possible that otherwise would not be possible
gravitational contraction: the collapse of a cloud of gas and dust due to the mutual gravitational attraction of the parts of the cloud; a possible source of excess heat radiated by some Jovian planets
gravitational lenses: two or more images of a distant object formed by the bending of light around an intervening massive object
gravity assist: using the gravity of a planet during a close encounter to add energy to the motion of a spacecraft

gravity gradient: the difference in the acceleration of gravity at different points on Earth and at different distances from Earth
gravity waves: waves that propagate through space and are caused by the movement of large massive bodies, such as black holes and exploding stars
greenhouse effect: process by which short wavelength energy (e.g., visible light) penetrates an object's atmosphere and is absorbed by the surface, which reradiates this energy as longer wavelength infrared (thermal) energy; this energy is blocked from escaping to space by molecules (e.g., H₂O and CO₂) in the atmosphere; and as a result, the surface warms
gyroscope: a spinning disk mounted so that its axis can turn freely and maintain a constant orientation in space
hard-lander: spacecraft that collides with the planet or satellite, making no attempt to slow its descent; also called crash-landers
heliosphere: the volume of space extending outward from the Sun that is dominated by solar wind; it ends where the solar wind transitions into the interstellar medium, somewhere between 40 and 100 astronomical units from the Sun
helium-3: a stable isotope of helium whose nucleus contains two protons and one neutron
hertz: unit of frequency equal to one cycle per second
high-power klystron tubes: a type of electron tube used to generate high frequency electromagnetic waves
hilly and lineated terrain: the broken-up surface of Mercury at the antipode of the Caloris impact basin
hydrazine: a dangerous and corrosive compound of nitrogen and hydrogen commonly used in high powered rockets and jet engines
hydroponics: growing plants using water and nutrients in solution instead of soil as the root medium
hydrothermal: relating to high temperature water
hyperbaric chamber: compartment where air pressure can be carefully controlled; used to gradually acclimate divers, astronauts, and others to changes in pressure and air composition
hypergolic: fuels and oxidizers that ignite on contact with each other and need no ignition source
hypersonic: capable of speeds over five times the speed of sound
hyperspectral: imaging technique in remote sensing that uses at least sixteen contiguous bands of high spectral resolution over a region of the electromagnetic spectrum; used in NASA spacecraft Lewis' payload
ilmenite: an important ore of titanium
Imbrium Basin impact: largest and latest of the giant impact events that formed the mare-filled basins on the lunar near side

impact craters: bowl-shaped depressions on the surfaces of planets or satellites that result from the impact of space debris moving at high speeds
impact winter: the period following a large asteroidal or cometary impact when the Sun is dimmed by stratospheric dust and the climate becomes cold worldwide
impact-melt: molten material produced by the shock and heat transfer from an impacting asteroid or meteorite
in situ: in the natural or original location
incandescence: glowing due to high temperature
indurated rocks: rocks that have been hardened by natural processes
information age: the era of our time when many businesses and persons are involved in creating, transmitting, sharing, using, and selling information, particularly through the use of computers
infrared: portion of the electromagnetic spectrum with waves slightly longer than visible light
infrared radiation: radiation whose wavelength is slightly longer than the wavelength of light
infrastructure: the physical structures, such as roads and bridges, necessary to the functioning of a complex system
intercrater plains: the oldest plains on Mercury that occur in the highlands and that formed during the period of heavy meteoroid bombardment
interferometers: devices that use two or more telescopes to observe the same object at the same time in the same wavelength to increase angular resolution
interplanetary trajectories: the solar orbits followed by spacecraft moving from one planet in the solar system to another
interstellar: between the stars
interstellar medium: the gas and dust found in the space between the stars
ion propulsion: a propulsion system that uses charged particles accelerated by electric fields to provide thrust
ionization: removing one or more electrons from an atom or molecule
ionosphere: a charged particle region of several layers in the upper atmosphere created by radiation interacting with upper atmospheric gases
isotopic ratios: the naturally occurring ratios between different isotopes of an element
jettison: to eject, throw overboard, or get rid of
Jovian: relating to the planet Jupiter
Kevlar®: a tough aramid fiber resistant to penetration
kinetic energy: the energy an object has due to its motion

KREEP: acronym for material rich in potassium (K), rare earth elements (REE), and phosphorus (P)
L-4: the gravitationally stable Lagrange point 60 degrees ahead of the orbiting planet
L-5: the gravitationally stable Lagrange point 60 degrees behind the orbiting planet
Lagrangian point: one of five gravitationally stable points related to two orbiting masses; three points are metastable, but L4 and L5 are stable
laser-pulsing: firing periodic pulses from a powerful laser at a surface and measuring the length of time for return in order to determine topography
libration point: one of five gravitationally stable points related to two orbiting masses; three points are metastable, but L4 and L5 are stable
lichen: fungus that grows symbiotically with algae
light year: the distance that light in a vacuum would travel in one year, or about 9.5 trillion kilometers (5.9 trillion miles)
lithosphere: the rocky outer crust of a body
littoral: the region along a coast or beach between high and low tides
lobate scarps: a long sinuous cliff
low Earth orbit: an orbit between 300 and 800 kilometers above Earth's surface
lunar maria: the large, dark, lava-filled impact basins on the Moon thought by early astronomers to resemble seas
Lunar Orbiter: a series of five unmanned missions in 1966 and 1967 that photographed much of the Moon at medium to high resolution from orbit
macromolecules: large molecules such as proteins or DNA containing thousands or millions of individual atoms
magnetohydrodynamic waves: a low frequency oscillation in a plasma in the presence of a magnetic field
magnetometer: an instrument used to measure the strength and direction of a magnetic field
magnetosphere: the magnetic cavity that surrounds Earth or any other planet with a magnetic field. It is formed by the interaction of the solar wind with the planet's magnetic field
majority carriers: the more abundant charge carriers in semiconductors; the less abundant are called minority carriers; for n-type semiconductors, electrons are the majority carriers
malady: a disorder or disease of the body
many-bodied problem: in celestial mechanics, the problem of finding solutions to the equations for more than two orbiting bodies
mare: dark-colored plains of solidified lava that mainly fill the large impact basins and other low-lying regions on the Moon

Mercury: the first American piloted spacecraft, which carried a single astronaut into space; six Mercury missions took place between 1961 and 1963
mesons: any of a family of subatomic particle that have masses between electrons and protons and that respond to the strong nuclear force; produced in the upper atmosphere by cosmic rays
meteor: the physical manifestation of a meteoroid interacting with Earth's atmosphere; this includes visible light and radio frequency generation, and an ionized trail from which radar signals can be reflected. Also called a "shooting star"
meteorites: any part of a meteoroid that survives passage through Earth's atmosphere
meteoroid: a piece of interplanetary material smaller than an asteroid or comet
meteorology: the study of atmospheric phenomena or weather
meteorology satellites: satellites designed to take measurements of the atmosphere for determining weather and climate change
microgravity: the condition experienced in freefall as a spacecraft orbits Earth or another body; commonly called weightlessness; only very small forces are perceived in freefall, on the order of one-millionth the force of gravity on Earth's surface
micrometeoroid flux: the total mass of micrometeoroids falling into an atmosphere or on a surface per unit of time
micrometeoroid: any meteoroid ranging in size from a speck of dust to a pebble
microwave link: a connection between two radio towers that each transmit and receive microwave (radio) signals as a method of carrying information (similar to radio communications)
minerals: crystalline arrangements of atoms and molecules of specified proportions that make up rocks
missing matter: the mass of the universe that cannot be accounted for but is necessary to produce a universe whose overall curvature is "flat"
monolithic: massive, solid, and uniform; an asteroid that is formed of one kind of material fused or melted into a single mass
multi-bandgap photovoltaic: photovoltaic cells designed to respond to several different wavelengths of electromagnetic radiation
multispectral: referring to several different parts of the electromagnetic spectrum, such as visible, infrared, and radar
muons: the decay product of the mesons produced by cosmic rays; muons are about 100 times more massive than electrons but are still considered leptons that do not respond to the strong nuclear force
near-Earth asteroids: asteroids whose orbits cross the orbit of Earth; collisions between Earth and near Earth asteroids happen a few times every million years

nebulae: clouds of interstellar gas and/or dust
neutron: a subatomic particle with no electrical charge
neutron star: the dense core of matter composed almost entirely of neutrons that remain after a supernova explosion has ended the life of a massive star
New Millennium: a NASA program to identify, develop and validate key instrument and spacecraft technologies that can lower cost and increase performance of science missions in the twenty-first century
Next Generation Space Telescope: the telescope scheduled to be launched in 2009 that will replace the Hubble Space Telescope
nuclear black holes: black holes that are in the centers of galaxies; they range in mass from a thousand to a billion times the mass of the Sun
nuclear fusion: the combining of low-mass atoms to create heavier ones; the heavier atom's mass is slightly less than the sum of the mass of its constituents, with the remaining mass converted to energy
nucleon: a proton or a neutron; one of the two particles found in a nucleus
occultations: a phenomena that occurs when one astronomical object passes in front of another
optical interferometry: a branch of optical physics that uses the wavelength of visible light to measure very small changes within the environment
optical-interferometry based: the use of two or more telescopes observing the same object at the same time at the same visible wavelength to increase angular resolution
optical radar: a method of determining the speed of moving bodies by sending a pulse of light and measuring how long it takes for the reflected light to return to the sender
orbit: the circular or elliptical path of an object around a much larger object, governed by the gravitational field of the larger object
orbital dynamics: the mathematical study of the nature of the forces governing the movement of one object in the gravitational field of another object
orbital velocity: velocity at which an object needs to travel so that its flight path matches the curve of the planet it is circling; approximately 8 kilometers (5 miles) per second for low-altitude orbit around Earth
orbiter: spacecraft that uses engines and/or aerobraking, and is captured into circling a planet indefinitely
orthogonal: composed of right angles or relating to right angles
oscillation: energy that varies between alternate extremes with a definable period
osteoporosis: the loss of bone density; can occur after extended stays in space

oxidizer: a substance mixed with fuel to provide the oxygen needed for combustion
paleolake: depression that shows geologic evidence of having contained a lake at some previous time
Paleozoic: relating to the first appearance of animal life on Earth
parabolic trajectory: trajectory followed by an object with velocity equal to escape velocity
parking orbit: placing a spacecraft temporarily into Earth orbit, with the engines shut down, until it has been checked out or is in the correct location for the main burn that sends it away from Earth
payload: any cargo launched aboard a rocket that is destined for space, including communications satellites or modules, supplies, equipment, and astronauts; does not include the vehicle used to move the cargo or the propellant that powers the vehicle
payload bay: the area in the shuttle or other spacecraft designed to carry cargo
payload fairing: structure surrounding a payload; it is designed to reduce drag
payload operations: experiments or procedures involving cargo or "payload" carried into orbit
payload specialists: scientists or engineers selected by a company or a government employer for their expertise in conducting a specific experiment or commercial venture on a space shuttle mission
perihelion: the point in an object's orbit that is closest to the Sun
period of heavy meteoroid: the earliest period in solar system history (more than 3.8 billion years ago) when the rate of meteoroid impact was very high compared to the present
perturbations: term used in orbital mechanics to refer to changes in orbits due to "perturbing" forces, such as gravity
phased array: a radar antenna design that allows rapid scanning of an area without the need to move the antenna; a computer controls the phase of each dipole in the antenna array
phased-array antennas: radar antenna designs that allow rapid scanning of an area without the need to move the antenna; a computer controls the phase of each dipole in the antenna array
photolithography: printing that uses a photographic process to create the printing plates
photometer: instrument to measure intensity of light
photosynthesis: a process performed by plants and algae whereby light is transformed into energy and sugars
photovoltaic: pertaining to the direct generation of electricity from electromagnetic radiation (light)

photovoltaic arrays: sets of solar panels grouped together in big sheets; these arrays collect light from the Sun and use it to make electricity to power the equipment and machines
photovoltaic cells: cells consisting of a thin wafer of a semiconductor material that incorporates a p-n junction, which converts incident light into electrical power; a number of photovoltaic cells connected in series makes a solar array
plagioclase: most common mineral of the light-colored lunar highlands
planetesimals: objects in the early solar system that were the size of large asteroids or small moons, large enough to begin to gravitationally influence each other
pn single junction: in a transistor or other solid state device, the boundary between the two different kinds of semiconductor material
point of presence: an access point to the Internet with a unique Internet Protocol (IP) address; Internet service providers (ISP) like AOL generally have multiple POPs on the Internet
polar orbits: orbits that carry a satellite over the poles of a planet
polarization state: degree to which a beam of electromagnetic radiation has all of the vibrations in the same plane or direction
porous: allowing the passage of a fluid or gas through holes or passages in the substance
power law energy spectrum: spectrum in which the distribution of energies appears to follow a power law
primary: the body (planet) about which a satellite orbits
primordial swamp: warm, wet conditions postulated to have occurred early in Earth's history as life was beginning to develop
procurement: the process of obtaining
progenitor star: the star that existed before a dramatic change, such as a supernova, occurred
prograde: having the same general sense of motion or rotation as the rest of the solar system, that is, counterclockwise as seen from above Earth's north pole
prominences: inactive "clouds" of solar material held above the solar surface by magnetic fields
propagate: to cause to move, to multiply, or to extend to a broader area
proton: a positively charged subatomic particle
pseudoscience: a system of theories that assumes the form of science but fails to give reproducible results under conditions of controlled experiments
pyroclastic: pertaining to clastic (broken) rock material expelled from a volcanic vent

pyrotechnics: fireworks display; the art of building fireworks
quantum foam: the notion that there is a smallest distance scale at which space itself is not a continuous medium, but breaks up into a seething foam of wormholes and tiny black holes far smaller than a proton
quantum gravity: an attempt to replace the inherently incompatible theories of quantum physics and Einstein gravity with some deeper theory that would have features of both, but be identical to neither
quantum physics: branch of physics that uses quantum mechanics to explain physical systems
quantum vacuum: consistent with the Heisenberg uncertainty principle, vacuum is not empty but is filled with zero-point energy and particle-antiparticle pairs constantly being created and then mutually annihilating each other
quasars: luminous objects that appear star-like but are highly redshifted and radiate more energy than an entire ordinary galaxy; likely powered by black holes in the centers of distant galaxies
quiescent: inactive
radar: a technique for detecting distant objects by emitting a pulse of radio-wavelength radiation and then recording echoes of the pulse off the distant objects
radar altimetry: using radar signals bounced off the surface of a planet to map its variations in elevation
radar images: images made with radar illumination instead of visible light that show differences in radar brightness of the surface material or differences in brightness associated with surface slopes
radiation belts: two wide bands of charged particles trapped in a planet's magnetic field
radio lobes: active galaxies show two regions of radio emission above and below the plane of the galaxy, and are thought to originate from powerful jets being emitted from the accretion disk surrounding the massive black hole at the center of active galaxies
radiogenic isotope techniques: use of the ratio between various isotopes produced by radioactive decay to determine age or place of origin of an object in geology, archaeology, and other areas
radioisotope: a naturally or artificially produced radioactive isotope of an element
radioisotope thermoelectric: device using solid state electronics and the heat produced by radioactive decay to generate electricity
range safety destruct systems: system of procedures and equipment designed to safely abort a mission when a spacecraft malfunctions, and destroy the rocket in such a way as to create no risk of injury or property damage
Ranger: series of spacecraft sent to the Moon to investigate lunar landing sites; designed to hard-land on the lunar surface after sending back television pictures of the lunar surface; Rangers 7, 8, and 9 (1964-1965) returned data

rarefaction: decreased pressure and density in a material caused by the passage of a sound wave
reconnaissance: a survey or preliminary exploration of a region of interest
reflex motion: the orbital motion of one body, such as a star, in reaction to the gravitational tug of a second orbiting body, such as a planet
regolith: upper few meters of a body's surface, composed of inorganic matter, such as unconsolidated rocks and fine soil
relative zero velocity: two objects having the same speed and direction of movement, usually so that spacecraft can rendezvous
relativistic time dilation: effect predicted by the theory of relativity that causes clocks on objects in strong gravitational fields or moving near the speed of light to run slower when viewed by a stationary observer
remote manipulator system: a system, such as the external Canada2 arm on the International Space Station, designed to be operated from a remote location inside the space station
remote sensing: the act of observing from orbit what may be seen or sensed below on Earth
retrograde: having the opposite general sense of motion or rotation as the rest of the solar system, clockwise as seen from above Earth's north pole
reusable launch vehicles: launch vehicles, such as the space shuttle, designed to be recovered and reused many times
reusables: launches that can be used many times before discarding
rift valley: a linear depression in the surface, several hundred to thousand kilometers long, along which part of the surface has been stretched, faulted, and dropped down along many normal faults
rille: lava channels in regions of maria, typically beginning at a volcanic vent and extending downslope into a smooth mare surface
rocket: vehicle or device that is especially designed to travel through space, and is propelled by one or more engines
"rocky" planets: nickname given to inner or solid-surface planets of the solar system, including Mercury, Venus, Mars, and Earth
rover: vehicle used to move about on a surface
rutile: a red, brown, or black mineral, primarily titanium dioxide, used as a gemstone and also a commercially important ore of titanium
satellite: any object launched by a rocket for the purpose of orbiting the Earth or another celestial body
scoria: fragments of lava resembling cinders
secondary crater: crater formed by the impact of blocks of rock blasted out of the initial crater formed by an asteroid or large meteorite
sedentary lifestyle: a lifestyle characterized by little movement or exercise

sedimentation: process of depositing sediments, which result in a thick accumulation of rock debris eroded from high areas and deposited in low areas
semiconductor: one of the groups of elements with properties intermediate between the metals and nonmetals
semimajor axis: one half of the major axis of an ellipse, equal to the average distance of a planet from the Sun
shepherding: small satellites exerting their gravitational influence to cause or maintain structure in the rings of the outer planets
shield volcanoes: volcanoes that form broad, low-relief cones, characterized by lava that flows freely
shielding: providing protection for humans and electronic equipment from cosmic rays, energetic particles from the Sun, and other radioactive materials
sine wave: a wave whose amplitude smoothly varies with time; a wave form that can be mathematically described by a sine function
smooth plains: the youngest plains on Mercury with a relatively low impact crater abundance
soft-landers: spacecraft that uses braking by engines or other techniques (e.g., parachutes, airbags) such that its landing is gentle enough that the spacecraft and its instruments are not damaged, and observations at the surface can be made
solar arrays: groups of solar cells or other solar power collectors arranged to capture energy from the Sun and use it to generate electrical power
solar corona: the thin outer atmosphere of the Sun that gradually transitions into the solar wind
solar flares: explosions on the Sun that release bursts of electromagnetic radiation, such as light, ultraviolet waves, and X rays, along with high speed protons and other particles
solar nebula: the cloud of gas and dust out of which the solar system formed
solar prominence: cool material with temperatures typical of the solar photosphere or chromosphere suspended in the corona above the visible surface layers
solar radiation: total energy of any wavelength and all charged particles emitted by the Sun
solar wind: a continuous, but varying, stream of charged particles (mostly electrons and protons) generated by the Sun; it establishes and affects the interplanetary magnetic field; it also deforms the magnetic field about Earth and sends particles streaming toward Earth at its poles
sounding rocket: a vehicle designed to fly straight up and then parachute back to Earth, usually designed to take measurements of the upper atmosphere

space station: large orbital outpost equipped to support a human crew and designed to remain in orbit for an extended period; to date, only Earth-orbiting space stations have been launched
space-time: in relativity, the four-dimensional space through which objects move and in which events happen
spacecraft bus: the primary structure and subsystems of a spacecraft
spacewalking: moving around outside a spaceship or space station, also known as extravehicular activity
special theory of relativity: the fundamental idea of Einstein's theories, which demonstrated that measurements of certain physical quantities such as mass, length, and time depended on the relative motion of the object and observer
specific power: amount of electric power generated by a solar cell per unit mass; for example watts per kilogram
spectra: representations of the brightness of objects as a function of the wavelength of the emitted radiation
spectral lines: the unique pattern of radiation at discrete wavelengths that many materials produce
spectrograph: an instrument that can permanently record a spectra
spectrographic studies: studies of the nature of matter and composition of substances by examining the light they emit
spectrometers: an instrument with a scale for measuring the wavelength of light
spherules: tiny glass spheres found in and among lunar rocks
spot beam technology: narrow, pencil-like satellite beam that focuses highly radiated energy on a limited area of Earth's surface (about 100 to 500 miles in diameter) using steerable or directed antennas
stratigraphy: the study of rock layers known as strata, especially the age and distribution of various kinds of sedimentary rocks
stratosphere: a middle portion of a planet's atmosphere above the tropopause (the highest place where convection and "weather" occurs)
subduction: the process by which one edge of a crustal plate is forced to move under another plate
sublimate: to pass directly from a solid phase to a gas phase
suborbital trajectory: the trajectory of a rocket or ballistic missile that has insufficient energy to reach orbit
subsolar point: the point on a planet that receives direct rays from the Sun
substrate: the surface, such as glass, metallic foil, or plastic sheet, on which a thin film of photovoltaic material is deposited
sunspots: dark, cooler areas on the solar surface consisting of transient, concentrated magnetic fields

supercarbonaceous: term given to P-and D-type meteorites that are richer in carbon than any other meteorites and are thought to come from the primitive asteroids in the outer part of the asteroid belt
supernova: an explosion ending the life of a massive star
supernovae ejecta: the mix of gas enriched by heavy metals that is launched into space by a supernova explosion
superstring theory: the best candidate for a "theory of everything" unifying quantum mechanics and gravity, proposes that all particles are oscillations in tiny loops of matter only 10^-35 meters long and moving in a space of ten dimensions
superstrings: supersymmetric strings are tiny, one dimensional objects that are about 10^-33cm long, in a 10-dimensional spacetime. Their different vibration modes and shapes account for the elementary particles we see in our 4-dimensional spacetime
Surveyor: a series of spacecraft designed to soft-land robotic laboratories to analyze and photograph the lunar surface; Surveyors 1, 3, and 5-7 landed between May 1966 and January 1968
synchrotron radiation: the radiation from electrons moving at almost the speed of light inside giant magnetic accelerators of particles, called synchrotrons, either on Earth or in space
synthesis: the act of combining different things so as to form new and different products or ideas
technology transfer: the acquisition by one country or firm of the capability to develop a particular technology through its interactions with the existing technological capability of another country or firm, rather than through its own research efforts
tectonism: process of deformation in a planetary surface as a result of geological forces acting on the crust; includes faulting, folding, uplift, and down-warping of the surface and crust
telescience: the act of operation and monitoring of research equipment located in space by a scientist or engineer from their offices or laboratories on Earth
terrestrial planet: a small rocky planet with high density orbiting close to the Sun; Mercury, Venus, Earth, and Mars
thermodynamically: referring to the behavior of energy
thermostabilized: designed to maintain a constant temperature
thrust fault: a fault where the block on one side of the fault plane has been thrust up and over the opposite block by horizontal compressive forces
toxicological: related to the study of the nature and effects on humans of poisons and the treatment of victims of poisoning
trajectories: paths followed through space by missiles and spacecraft moving under the influence of gravity

transonic barrier: the aerodynamic behavior of an aircraft moving near the speed of sound changes dramatically and, for early pioneers of transonic flight, dangerously, leading some to hypothesize there was a "sound barrier" where drag became infinite
transpiration: process whereby water evaporates from the surface of leaves, allowing the plant to lose heat and to draw water up through the roots
transponder: bandwidth-specific transmitter-receiver units
troctolite: rock type composed of the minerals plagioclase and olivine, crystallized from magma
tunnelborer: a mining machine designed to dig a tunnel using rotating cutting disks
Tycho event: the impact of a large meteoroid into the lunar surface as recently as 100 million years ago, leaving a distinct set of bright rays across the lunar surface including a ray through the Apollo 17 landing site
ultramafic lavas: dark, heavy lavas with a high percentage of magnesium and iron; usually found as boulders mixed in other lava rocks
ultraviolet: the portion of the electromagnetic spectrum just beyond (having shorter wavelengths than) violet
ultraviolet radiation: electromagnetic radiation with a shorter wavelength and higher energy than light
uncompressed density: the lower density a planet would have if it did not have the force of gravity compressing it
Universal time: current time in Greenwich, England, which is recognized as the standard time that Earth's time zones are based
vacuum: an environment where air and all other molecules and atoms of matter have been removed
vacuum conditions: the almost complete lack of atmosphere found on the surface of the Moon and in space
Van Allen radiation belts: two belts of high energy charged particles captured from the solar wind by Earth's magnetic field
variable star: a star whose light output varies over time
vector sum: sum of two vector quantities taking both size and direction into consideration
velocity: speed and direction of a moving object; a vector quantity
virtual-reality simulations: a simulation used in training by pilots and astronauts to safely reproduce various conditions that can occur on board a real aircraft or spacecraft
visible spectrum: the part of the electromagnetic spectrum with wavelengths between 400 nanometers and 700 nanometers; the part of the electromagnetic spectrum to which human eyes are sensitive

volatile: ices (e.g., H₂O and CO₂) that are solids inside a comet nucleus but turn into gases when heated by sunlight
volatile materials: materials that easily pass into the vapor phase when heated
wavelength: the distance from crest to crest on a wave at an instant in time
X ray: form of high-energy radiation just beyond the ultraviolet portion of the spectrum
X-ray diffraction analysis: a method to determine the three-dimensional structure of molecules