A

absolute zero - The temperature limit at which no further cooling of a substance can occur, and at which the motion of the molecules is at a minimum (or, more accurately, at the minimum allowed by Quantum Mechanics). The energy associated with this minimum motion is called "zero point energy" by physicists.

absorption lines - Dark lines in a spectrum, produced when light or other electromagnetic radiation coming from a distant source passes through a gas or similar object closer to the observer.

absorption spectrum - A graph or display relating how a substance absorbs electromagnetic radiation as a function of wavelength.

accelerator - A machine for speeding subatomic particles to a high velocity, then colliding them with a stationary target or with another beam of particles moving in the opposite direction, producing exotic particles.

accretion - The process in which small rocks, dust particles or gases accumulate under the influence of gravity.

aerodynamic - Something shaped in a way in which air can easily move around it. Noun: A branch of dynamics that deals with the motion of air or other gases in motion.

alkali metals - A group of soft, very reactive elements including lithium, sodium and potassium.

alkaline - Non-acidic; basic, having a pH greater than 7.

alkaline earth metals - A group of metallic elements including calcium, strontium and barium.

alloy - A solid solution of two or more metals.

amu - An acronym for atomic mass unit.

Andromeda galaxy - A major spiral galaxy, 2.2 million light-years from the Earth, which is gravitationally bound to the Milky Way galaxy.

anemopause - The boundary where the pressure of the solar wind equals the pressure of a planet's upper atmosphere; also known as ionopause.

angstrom - A unit of distance measure that equals 10-10 meters.

anion - A negatively charged chemical substance.

anomaly - A departure from the expected; an abnormality.

anti-matter - Material composed of antiparticles, which are just like ordinary protons, electrons, and neutrons except they have opposite electrical charges and magnetic moments. When antimatter and matter collide, both are annihilated with the production of other elementary particles, such as photons.

antiquarks - Particles that have the same mass as ordinary quarks but have opposite charge.

aphelion - The farthest point from the sun in an orbit.

array - A group of individual pieces arranged in a repeating order to form a complete unit.

asteroid - A small solar system body ranging in size from a few hundred kilometers to less than one kilometer in size.

asteroid belt - The region between the orbits of Mars and Jupiter where most of the Solar System asteroids are found.

astronomers - Scientists that study the natural world beyond the Earth.

astronomical unit (AU) - The average distance between the Earth and the sun,
1.5 x 10ØØ meters.

astronomy - The science that studies the natural world beyond the Earth.

atom - The smallest unit of an element.

atomic mass unit - A unit of mass equal to 1/12 the mass of an atom of the carbon isotope with mass number 12; approximately 1.66 x 10-27 kilogram.

atomic number - The number of protons in an atom’s nucleus, used to identify the element.

atomic radius - A measure of the size of an atom, assuming the atom has the shape of a sphere.

atomic spectroscopy - The study of the absorption and emission of light by atoms.

average - (see mean)

average atomic mass - A value for an element's atomic mass based on the relative abundances of different isotopic masses.

avionics - The development and production of electrical or electronic devices for use in aviation, missiles, or astronautics.

axion - A theoretical elementary particle with a mass less than one millionth or an electron that has never been detected. If axions exist, dark matter could consist of large collections of axions.

axis - The straight line, through the poles about which a body rotates.

Azimuth - The angular position along the horizon, measured clockwise from the north.

 B

baryonic matter - (see MACHOs and baryons)

baryons - Elementary particles with a large mass that are influenced by the strong nuclear force and are held together by a combination of three quarks. Protons and neutrons are baryons. Baryons have half-integral spins.

Big Bang - A theory of cosmology that the universe originated billions of years ago as a violent eruption of a single point.

binary compounds
- Chemical compounds made of only two elements.

bipolar jets - Material emitted in streams from opposites sides of an object.

black hole - The highly compressed remains of a star which has mass but takes up no space.

blackbody - A body that absorbs and reemits all radiation falling on it.

blackbody spectrum - The spectrum of energy level against wavelength for electromagnetic radiation emitted by an object capable of absorbing all the energy that strikes it.

Bohr atomic model - A view of the atom as consisting of a central nucleus with electrons orbiting around the nucleus.

bosons - Force-carrying particles including photons, the quanta of electromagnetic force and gluons, quanta that carry the strong nuclear force.

bow shock - (see shock wave)

bright matter - (see luminous matter)

brown dwarf stars - Low-mass stellar objects too small and too cool for hydrogen fusion to occur.

bubbles - (see voids)

burner or combustor - Part of a propulsion system where high-pressure air is mixed with a highly flammable fuel and ignited.

 C

calorie - The unit of heat defined as the amount of heat required to cause a one degree temperature centigrade increase in one gram of water.

cation - A positively charged chemical substance.

center of gravity (CG) - The (imaginary) single point at which the mass of an object may be considered to be concentrated; used for calculations.

Cepheid variables - A group of variable stars with exceptionally regular periods of pulsation.

CERN - The Center for European Nuclear Research, Geneva, Switzerland.

charge-coupled devices (CCDs) - Technological instruments that contain light-sensitive silicon chips with an arrangement of light-sensitive spots called pixels.

chemical families - Groups of elements sharing similar physical and chemical characteristics.

chromosphere - An incandescent, transparent layer of gas lying above and surrounding the photosphere of a star, but clearly separated from the corona; the outermost layer of the solar atmosphere.

closed universe - A universe that will continue to expand slowly enough that the gravitational attraction between the different galaxies will cause the expansion to slow down until the universe collapses; one in which the ratio of actual density to the critical density is less than 1.

clusters - (see star clusters or galactic clusters)

color - A property of quarks that expresses their behavior in the presence of the strong nuclear force; analogous to electrical charge, but there are three color charges-red, green, and blue.

compressor - The compressor is part of a propulsion system used to squeeze the air, or to increase the pressure of the air flow.

condensation temperature - The temperature at which a gaseous substance becomes a liquid.

conduction
- The transfer of heat (or electricity) through a substance, resulting from a difference in temperature (or electrical potential) between parts.

contingency - An event that may possibly occur, but its occurrence is uncertain.

continuum spectrum - An electromagnetic spectrum containing all wavelengths of the visible region.

convection
- Heat transfer in a gas or liquid by the circulation of currents from one region to another.

convection zone or layer - A layer of the sun's structure that exists external to the radiative zone and below the photosphere. Convection currents circulate the sun's energy within the convection zone.

corona -
A faintly colored luminous ring that appears to surround celestial bodies.

coronal streamer -
Large scale magnetic structure observed in the sun's corona.

cosmic background radiation - Microwave radio emission coming from all directions and corresponding to a blackbody curve.

cosmic rays - A stream of ionizing radiation of extraterrestrial origin.

cosmic structures - Large structures such as stars, planets, galaxies, and clusters of galaxies formed late in the present epoch.

cosmogonists - Scientists concerned with the origin of the universe.

cosmogony - The study of the origin of the universe.

cosmological constant - A term used to express a force of "cosmic repulsion," the force that counteracts gravity.

cosmologists - Scientists who study the structure and changes in the present universe in order to predict the future of the universe.

cosmology -
An astrophysical study of the structure and dynamics of the universe.

crater - A depression on the surface of a planet (or a moon) usually caused by the impact of a meteoroid.

critical density - The cosmic density of matter required to "close" the universe and eventually halt cosmic expansion.

curation
- Careful storage of samples in unadulterated form for later scientific analysis.

 D

Dalton - An atomic mass unit. A unit of mass that equals 1/12 the mass of the most abundant isotope of carbon (12C), which is assigned a mass of 12.

dark line -
The absence of a given wavelength or set of wavelengths in a continuum spectrum.

dark matter - Material that is believed to make up more than 90% of the mass of the universe, but is not readily visible because it neither emits nor reflects electromagnetic radiation, such as light or radio signals. Its composition is unknown.

deceleration parameter - The quantity that defines the rate at which the expansion of the universe is slowing down; the braking effect of the galactical gravitational force; a function of cosmic matter density.

density -
The ratio of an object's mass to its volume.

derived data
- Values calculated from collected data.

deuterium - The isotope of hydrogen whose nucleus is one neutron and one proton.

deuteron - The nucleus of a deuterium atom, consisting of a proton and a neutron. It is regarded as a subatomic particle with a unit positive charge.

diversity - Differences or unlikeness.

Doppler effect - An apparent change in the frequency of waves (such as sound or light) that occurs when the source and observer are in motion relative to each other. The frequency increases when the source and observer approach each other, while the frequency decreases when they move apart.

Doppler shifts - Usually regards spectral lines. (see Doppler effect)

down quarks - One of six flavors of quarks that has a charge of -1/3 in units of elementary electric charge.

dwarf galaxies - Galaxies having only 1 million to 10 million stars. They have low mass and low escape velocities.

dwarf stars - Stars with masses less than 5 times the mass of the sun. (The sun is a dwarf star.)

dynamic - Pertaining to change or process over time resulting from physical forces of energy.

 E

eccentricity - The ratio of the distance of a focus from the center of an ellipse to its semi-major axis. The flatness of an ellipse.

eclipsing binary star - A pair of stars in which the dimmer one regularly passes in front of and then behind the brighter one, decreasing and increasing the visible light from the pair in a sequential pattern.

ecliptic - The apparent annual path of the sun on the celestial sphere.

ecliptic plane - The plane of the Earth’s orbit around the sun.

ejecta - Material expelled from the impact site during a crater formation.

electromagnetic radiation - Carries energy through a medium, including empty space, where it travels at the speed of light in the form of a wave of tiny packets of energy (see photons).

electromagnetic spectrum - The entire range of electromagnetic radiation.

electron - A subatomic (elementary) particle carrying a unit negative charge.

electronegativity - A measure of the tendency to attract electrons in order to form a chemical bond.

element - Matter composed of atoms with identical numbers of protons in their nuclei.

ellipse - A closed curve drawn so that the sum of the distances from a point on the curve to two fixed points is always constant. A symmetrical oval.

elliptical galaxies - One of the three major classifications of galaxies. They are spherical or cigar-shaped and contain no disks or spiral arms.

emission lines - Bright spectral lines produced in a spectrum by a luminous source.

emission nebula - A cloud of interstellar gas lit by nearby stars.

emission spectrum - The spectrum of bright lines, bands, or continuous radiation that is provided by a specific emitting substance as it loses energy and returns to its ground state.

engineering development unit (EDU) - A model that serves as a mechanical prototype for proof of concept; matches form, fit, and function of the final product.

entropy - A measure of the amount of disorder in a system.

equatorial diameter - The diameter of a planet measured at its equator, which is greater than the pole to pole diameter if the planets is not spherical.

escape velocity - The speed needed by an object to escape from the gravitational field of a body.

Euclidean geometry - Geometry is the mathematics of lines drawn through space. In Euclidean geometry, space is postulated to be "flat," similar to a three-dimensional analog of a plane.

exit nozzle - Part of the propulsion system where exhaust gases are expelled.

exotic elements - Elements not existing in nature; manmade elements.

extrapolation - Inferring or estimating a value by projecting or extending known values.

extraterrestrial - Anything that originates, is located, or occurs outside Earth or its atmosphere.

 F

fairings - structure whose primary function is to produce a smooth outline and to reduce drag. For Genesis the fairing contains the spacecraft.

filament - A prominence that appears dark against the bright solar disk.

finite - Having definable limits; circumscribed or restricted, measurable.

first ionization potential (fip)
- A measure of the energy required to remove the outermost electron from an atom.

First Law of Thermodynamics - Heat is a form of energy. When this is taken into account, energy for a closed system (one that doesn't ineract in any way with anything else) is always conserved.

fission - The splitting of a large nucleus into smaller pieces.

flat universe - A universe that will continue to expand very slowly, just fast enough to avoid recollapse; one in which the ratio of actual density to the critical density is 1.

flavor - Designation of one of six quark types-up, down, strange, charm, top and bottom-that determines how they are influenced by the weak nuclear force.

fluence - The total number of particles delivered to an area; time-integrated flux; similar to a dosage.

foci - Two points on an ellipse in which the sum of the distances to the foci from any point on the ellipse is a constant.

focus - Singular for foci.

frequency - The number of wave crests that pass a stationary observer in one second.

fundamental forces - Agents that cause a change in a system. There are four known fundamental forces-strong nuclear force, weak nuclear force, gravity, and electromagnetic.

fundamental particles - Particles that cannot be divided into smaller particles. (see particles)

fusion - (see nuclear fusion)

fusion reaction - (see nuclear fusion)

 G

galactic clusters - Cosmic structures in which many galaxies are found in close proximity to each other.

galactic disk - The plate-shaped component of a spiral galaxy that constitutes the plane of the galaxy.

galactic dynamics - The orbital motion or rotation of stars and galaxies under the influence of gravitational force.

galactic halo - A spherical aggregation of stars, globular star clusters and thin gas clouds centered on the nucleus of a galaxy and extending beyond the known extremities of the galactic disk.

galaxy - A large aggregation of stars, bound together by gravity. There are three major classifications of galaxies-spiral, elliptical, and irregular.

gantry - A movable structure with platforms at different levels used for erecting and servicing rockets before launching.

gaseous planets - (see Jovian planets)

gauss - Unit of intensity of a magnetic field equal to a field of one line of force per cm2.

general relativity - (see relativity)

Geosynchronous Earth Orbit (GEO) - An orbit whose period is the same as the rotational rate of the Earth, so that the position of a satellite seems to stay in the same spot over the Earth.

giant planets - (see Jovian planets)

giant stars - Stars with masses from 5 to 100 solar masses.

globular cluster -
A roughly spherical group of stars, smaller than a galaxy. A cluster of 105 stars, mostly very old, red, low mass stars.

Graphite Epoxy Strap-On Motors (GEM) - solid rocket motors used on the Delta rocket's first stage.

gravitational constant - A constant, G, in the mathematical formula of Newton's definition of gravitational force, F=GmØm2/rœ. When the mass of objects is expressed in kilograms and r is in meters, then G = 6.6 x 10-ØØ N mœ/kgœ and the force unit, F, is Newtons.

gravitational energy - The capacity to do work by objects moving under the influence of the attraction between their masses.

gravitational lensing - A technique that takes advantage of the fact that matter distorts the space surrounding it, warping the light from background stars and galaxies, used to detect the presence of dark matter.

gravitationally bound - Pertaining to galaxies being held together by gravity and not drawn apart by the expansion of the universe.

gravitons - The quanta thought to convey gravitational force, analogous to photons, gluons, and bosons.

gravity - In Newtonian physics, the universal, mutual attraction of all massive objects for one another. Its force is directly proportional to the mass of each object, and decreases by the square of the distance separating the objects. In Einstein's general relativity, gravity is viewed as a consequence of the curvature of space induced by the presence of a massive object.

greenhouse effect
- The warming effect caused by the Earth's atmosphere allowing easy entry of visible light, but difficult escape of infrared radiation (heat).

groups - The levels of cosmic structures having densities and orbital velocities between galaxies and clusters. Most galaxies belong to small groups.

 H

hadrons - Elementary particles with large mass that are influenced by the strong nuclear force. There are two types of hadrons-mesons, which have zero spin, and baryons, which have half-integral spins.

halides - Binary compounds that contain halogen atoms—fluorine, chlorine, bromine, and iodine.

halo - (see galactic halo)

halogens
- A group of five nonmetallic elements, including fluorine, chlorine, bromine, iodine, and astatine.

heat
- Energy that can move or transfer from one object to another because of a difference in temperature, resulting in a gain or loss of internal energy of particle motion
.

helioseismology - The study of oscillations of the sun's photosphere that result from seismic events within the interior.

heliosphere - a bubble in space produced by the solar wind.

helium nucleus - Two bare bound protons, with one or two neutrons.

hertz - A unit of frequency equal to one cycle (or wave) per second.

heterogeneous - Consisting of dissimilar constituents (mixed).

histogram - A "bar graph."

homogeneous - The quality of consisting of similar parts or elements.

Hubble constant - The rate at which the universe expands, equal to 20 kilometers per second per 106 light years of distance.

Hubble law - The law that correlates the velocities at which distant galaxies are receding from one another to the distances between them.

Hubble's bubbles - (see voids)

hydrogen nucleus - A bare proton.

 I

ices - The solid state of substances (such as ammonia, carbon dioxide, and methane) that are found in the gaseous state at standard atmospheric conditions.

implode - Violent compression; opposite of explode.

inertia - The tendency of matter to remain at rest if at rest or to remain in motion in a straight line if in motion. (see Newton's First Law)

infinite - Being without limits of any kind; without end or indefinitely large; unmeasurable.

infrared (IR) - Invisible radiation, sensed as heat, which has wavelengths longer than red visible light and shorter than microwaves.

infrared radiation - Electromagnetic radiation having a wavelength longer than the longest visible wavelength but shorter than those in the microwave and radio ranges.

inhomogeneous - Not homogenous. (see homogenous)

inlet or air intake - Helps determine the amount of air flow into an engine.

insulator
- A material that is a poor conductor of heat, therefore delaying the transfer of heat (also can be used to describe electrical conduction).

interpolation - Determining or estimating a value between known values.

interstellar - Between the stars.

interstellar dust grains - Clumps of atoms and molecules in the interstellar medium.

interstellar gases - Molecules of gas in the interstellar medium.

interstellar medium - Material occupying the space between stars.

ion - An atom or molecule that has a net electrical charge from gaining or losing electrons.

ionization potential - A measure of the energy required to remove an electron from an atom or ion.

ionize - Turn into an ion or charged particle.

ionizing radiation - Electromagnetic radiation that has sufficient energy to cause the loss of one or more electrons from atoms or molecules, thereby ionizing them.

ionopause - (see anemopause)

ionosphere - A layer of the Earth's atmosphere ranging from about 100 to 700 km above the surface in which oxygen and nitrogen are ionized by sunlight, producing free electrons.

isotopes - One of two or more atoms having the same atomic number but differing in atomic weight and mass number.

isotropy - The state of having the same properties in all directions.

 J

jet propulsion - Propulsion of an airplane by jet engines by the forwardly directed forces of the reaction resulting from the rearward discharge of a jet of fluid.

Jovian planets - The four larger planets (Jupiter, Saturn, Uranus and Neptune) which, if they have solid surfaces, are covered by a deep gaseous atmosphere.

 K

Kelvin - A unit of absolute temperature.

Kepler's First Law of Planetary Motion - Law of Ellipses- The orbit of each planet is an ellipse, with the sun at one focus.

Kepler's Second Law of Planetary Motion - Law of Equal Areas- A line drawn from a planet to the sun sweeps out equal areas in equal times.

Kepler's Third Law of Planetary Motion - Harmonic Law- The square of the orbital period of a planet is directly proportional to the cube of its average distance from the sun.

 L

L1 - Lagrangian point; the point of stability between two celestial bodies; for Genesis, the L1 point is on the sun side of the Earth.

launch period - How many days a rocket can be launched into the proper orbit.

launch window - The dates or times (or set of times during a given day) that a rocket can launch, meet mission objectives, stay within safety guidelines, and be safely sent into its trajectory.

leptons - Elementary particles that have very small or no measurable mass and are not influenced by the strong nuclear force. Electrons and neutrinos are leptons.

libration - An oscillation in the apparent aspect of a secondary body (such as a satellite) as seen from the primary object around which it revolves.

light year - The distance that light travels in one year, 5.8 x 1012 miles or 9.3 x 1015 meters.

line spectrum - A spectrum that appears as distinct lines of particular wavelengths that are characteristic for the various elements.

Low Earth Orbit (LEO) - An orbit from about144 to 960 kilometers above the Earth commonly used by the Space Shuttle.

luminosity - The radiant energy output from an object.

luminous matter - Matter that emits or reflects electromagnetic radiation. Also called bright matter, it is made primarily of baryonic material.

 M

Mach 1 - The speed of sound.

MACHOs - Dark matter jargon for Massive Compact Halo Objects referring to objects in galactic halos made of protons and neutrons; also called baryonic matter.

magnetic field - The region of space near a magnetized body, in which magnetic forces are exerted.

magnetopause - The outer boundary of a planet's magnetosphere where the magnetosphere encounters the solar wind.

magnetosheath - The turbulent region between the bow shock and the magnetosphere of a planet.

magnetosphere - The region surrounding a planet where the behavior of electrically charged particles is determined by the planet's magnetic field.

magnetotail - An elongated "wake" of plasma found on the side of a planet opposite the sun.

major axis - The long axis on an ellipse.

manipulated variable - In an experiment the factor that is intentionally changed by the investigator. Also known as the independent variable.

mass - A measure of an object's resistance to acceleration; different from but proportional to the object's weight.

mass number -
The total number of neutrons and protons in the nucleus of an atom.

mass spectrometry -
The experimental determination of the mass of an atom or molecule by observing its behavior as it is accelerated in a magnetic field.

mass spectrum -
A graph obtained from a mass spectrophotometer that shows the atomic or molecular mass of the sample vs. the number of atoms or molecules having that mass.

mean -
A number that typifies a set of numbers of which it is a function. The value obtained by dividing the sum of a set of quantities by the number of quantities in the set.

mega parsec - One million parsecs.

metals
- A group of solid lustrous elements, often ductile and malleable, that form compounds easily with nonmetals and form alloys with other metals.

metalloids - A group of elements that either have a mixture of metallic and nonmetallic characteristics (like arsenic) or are generally nonmetallic but can form alloys with metals (like carbon).

meteorite - That portion of a meteoroid that survives passage through the atmosphere.

meteoroid - A meteoritic body in space.

microlensing - (see gravitational lensing)

micron - A unit of length equal to 1/1000 of a millimeter.

microwaves - Electromagnetic radiation with wavelengths of about 10-4 to 1 m.

Milky Way galaxy - The spiral galaxy of which our sun is a part.

million instructions per second (MIPS) - A measure of computer information processing speed.

minor axis - The short axis on an ellipse.

molecular cloud - A collection of chemical molecules in the interstellar medium mutually attracted by gravity.

momentum - The quantity of motion in a body. For linear momentum, it is the mass of the body multiplied by its velocity.

 N

nanometer - A unit of distance measurement that equals 10-9 meters.

neutrino -
Mysterious and poorly understood sub-atomic particles that have no charge and little, if any, mass.

neutron
- An uncharged, subatomic particle of approximately the same mass as the proton.

Newton's First Law Of Motion - Objects at rest tend to stay at rest unless acted on by an unbalanced force. Objects in motion tend to stay in motion in a straight line unless acted on by an unbalanced force.

Newton's Second Law Of Motion - As force is increased, acceleration increases. As mass is increased, acceleration decreases. Therefore, force equals mass times acceleration. (force = mass x acceleration)

Newton's Third Law Of Motion - For every action or force there is an equal, opposite and simultaneous reaction or force.

noble gases - A group of elements that, except for helium, combine to form compounds only under extreme conditions. Helium does not form compounds, even under these conditions.

nonmetals - A group of elements that gain or share electrons to form compounds with metals or other nonmetals. They are poor conductors of heat and electricity, even in the solid state.

nova - Literally a "new" star; an exploding star which suddenly becomes visible from Earth.

nuclear fusion - The process of two atomic nuclei fusing to produce a new nucleus, the mass of which is less than the sum of the masses of the two fusing atoms.

nuclear reaction - A chemical reaction which, unlike most, occurs between the nuclei of atoms rather than between their exterior electrons.

nuclei - Plural of nucleus.

nucleon - Protons and neutrons, those particles that make up the nucleus of an atom.

nucleus - The protons and neutrons, found at the center of an atom, that contribute most of the atom's mass.

nuclide - An atomic nucleus specified by its atomic number and atomic mass.

 O

omega - An index of matter density of the universe, defined as the ratio of actual density to the critical density.

open universe - A universe that will continue to expand forever; one in which the ratio of actual density to the critical density is greater than 1.

operational definitions - In an experiment, a description of how the investigator has decided to measure a variable.

 P

parsec - A unit of length in astronomy; 3.258 light years.

particles - Fundamental units of matter and energy.

perihelion
- The closest point to the sun in an orbit.

periodic - Having a repeating pattern.

photons - Tiny, discrete packets (or units) of electromagnetic energy

photosphere - The visible surface of the sun; the zone that occurs at the top of the convection zone, where atoms no longer block radiative flow. As the hot atoms cool, they release their excess energy as photons of radiant energy, some of which fall into the visible light portion of the spectrum.

pitch - That property of sound which is determined by the frequency of vibration of sound waves which strike the ear.

planet - A cosmic object that is more massive than an asteroid but less massive than a star and shines by reflected light.

planetesimal - Accretions of solar nebula material having a mass of 10$+18$ g, which combine to form more massive bodies.

plasma - An electrically neutral yet highly ionized gas comprised of ions, electrons, and neutral particles; often considered to be the fourth state of matter.

plasma universe - A theory of the universe that asserts that parts of the universe expand while others contract.

positron - A positively charged electron.

potential energy - The energy of a particle or system of particles derived from position, or condition, rather than from motion.

preconception - A concept formed in advance of further instruction; if inaccurate, may make learning difficult.

primordial nebula - (see solar nebula)

prism - An object, usually a triangular-based polyhedron of glass, that causes a mixture of wavelengths of light to be separated or dispersed. Isaac Newton produced a spectrum with a prism in 1672.

probability - Branch of mathematics that deals with the predicted outcomes of random chance events.

probability cloud - Description of the most likely region where an electron may be found.

prominence -
A structure in the corona consisting of cool plasma supported by magnetic fields.

propel - To drive forward or onward by or as if by means of a force that imparts motion.

protium
- The isotope of hydrogen whose nucleus is one proton (no neutrons).

proton
- A subatomic particle having a single positive charge and constituting the nucleus of the ordinary hydrogen atom.

proton-proton fusion - A nuclear fusion process in which both reacting atoms are ionized hydrogen atoms that lack electrons.

protostars - Young stars.

 Q

quanta - Fundamental units of energy.

quarks - Fundamental particles of which protons and neutrons are made; held together by strong nuclear force. Quarks have color, flavor, and fractional electric charges.

quasar - A quasi-stellar object; objects far from Earth, moving extremely swiftly away from Earth, and emitting visible light and radio frequency radiation.

 R

- The emission or transmission of energy in the form of waves through space or another medium
.

radiative zone or layer - About 70% of the sun; the section where energy travels outward by radiation.

rare earth metals - Elements with atomic numbers from 57 through 71.

ratio - A comparison of two quantities, expressed as a fraction.

rays - A system of bright, elongated streaks, sometimes associated with a crater.

reciprocal centimeter $\left(cm-1\right)$ - The quotient of unity divided by a measurement in centimeters, or 1/cm.

red giant stars - Later evolutionary stages of low-mass stars, which collapse then expand.

red shift - An apparent increase in wavelength (becoming more red) of the radiation from an object moving away from the observer.

redundancy - Planning in an identical copy of a functional component to serve as backup in case of failure of the primary component.

relative abundance (of isotopes) - The proportion in a sample of the masses of all naturally occurring isotopes of an element to the mass of the most abundant isotope of that element.

relativity - Einstein's general theory of relativity was his theory of gravitational force. His special theory of relativity was his theory of the electrodynamics of moving systems.

responding variable - In an experiment, the factors that are affected by the manipulated variable. Also known as the dependent variable.

Rocketdyne - A part of the Boeing company created to build rocket engines in support of national defense and U.S. involvement in space.

rotational period - The time necessary for a body to make one turn about its axis.

Rydberg equation - An equation that predicts the positions of the lines in an emission spectrum of the hydrogen atom.

 S

salt - A chemical compound formed by combining the anion of an acid with the cation of a base.

Second Law of Thermodynamics - Some forms of transformations of one kind of energy to another are not observed to occur in natural processes. The observed transformations in a closed system are always characterized by a nondecreasing entropy. In open systems where the entropy may be kept constant, the allowed transformations are always characterized by a decrease in the amount of (free) energy available to do useful work.

semi-major axis - Half of the major axis.

shock wave - A region of abrupt change in pressure and density caused by supersonic flow around a body.

silicon (SI) - A nonmetallic chemical element, number 14, which is a major component of many types of rocks and minerals on Earth; used to make glass, concrete, and semiconductors.

solar core - The central region of the sun's structure that consists of protons, neutrons, nuclei, and free electrons and is the site of proton-proton fusion reactions.

solar nebula - A diffuse mass of interstellar dust and/or gas that is thought to be the remnants of the material out of which the sun formed.

solar wind - The components of the sun that are spewed into the void of outer space.

space-time - The four-dimensional description of events as depicted in the theory of relativity.

specific impulse - The thrust produced per unit rate of consumption of the propellant that is usually expressed in pounds of thrust per pound of propellant used per second and that is a measure of the efficiency of a rocket engine.

spectra - Plural of spectrum.

spectrograph - A device used to spread-out radiation into its component wavelengths.

spectroscope - A tool which produces, observes, and records the spectra of elements.

spectroscopy - The study of spectra.

spectrum - The distribution of energy from a radiant source, arranged in order of wavelengths.

spin - Angular momentum.

spiral galaxy - One of the three major classifications of galaxies; their characteristics include a plate-shaped disk containing spiral arms.

stagnation point - The point at which the solar wind plasma comes closest to the center of a planet.

star - A cosmic structure that generates energy by means of nuclear fusion at its core.

Star 37 FM - The motor that propels the third stage of the Delta II rocket containing the Genesis spacecraft.

star clusters - Aggregations of stars that are gravitationally bound, but are smaller and less massive than galaxies.

static - Pertaining to bodies at rest, without motion, at equilibrium; not being acted upon by physical forces of energy.

steady state - A condition that exists when the value of some parameter remains constant over time because the rate of accumulation of this parameter equals the rate of its loss.

stellar wind - A flow of charged particles out of a star.

stochastic process - A process or event that involves the occurrence of random chance events.

strong nuclear force - Fundamental force of nature that binds quarks together; holds protons and neutrons together to form the nucleus of an atom. A gluon is a quanta of strong nuclear force.

structure - (see cosmic structure)

sunspot - A temporary, disturbed area of the solar photosphere that appears dark because it is cooler than the surrounding areas; concentration of strong magnetic flux on the sun's surface.

supercluster - A cluster of tens of thousands of galaxies, typically about one hundred million light-years in diameter.

supergiant stars - Stars with masses over 100 solar masses.

 T

telemetry - A system for taking measurements in the spacecraft or satellites and transmitting them to ground stations.

terrestrial - Belonging to the Earth.

terrestrial planets - Those planets - Mercury, Venus, Earth, Mars (and Pluto?) - having a hard, rocky surface.

thermal equilibrium - A condition that exists between two objects when they have reached the same temperature and heat energy no longer flows between them.

thermodynamics - The study of heat and its transformation into mechanical energy.

thermostat - A practical device used to open and close an electrical circuit through the bending of a bimetallic strip, due to a difference in the rate of contraction and expansion of the different types of metal on each side of the strip.

Third Law of Thermodynamics - There is an absolute zero to temperature (0 Kelvin) where all forms of matter become perfectly ordered (usually to perfect crystals).

thought experiment - An experiment that cannot be or is not carried out in practice; it is reasoned through by thought and intuition.

tidal force - A gravitational force that causes a changing effect on the shape of an object.

tired light - A theory of the universe that maintains that light, as it travels vast distances, loses energy, creating a redshift distance relationship that is not due to cosmic expansion.

trajectory - A path, usually curved, of an object moving through space.

tritium - The isotope of hydrogen whose nucleus is one proton and two neutrons.

 U

ultraviolet radiation - Radiation of wavelengths between that of violet visible light and x-rays.

up quarks - One of six flavors of quarks that has a charge of +2/3 in units of elementary electric charge.

 V

valence electron - An outer electron which can participate in forming a bond.

variables - Factors that is subject to change in an experiment.

vector - A quantity that is expresses direction and magnitude.

visible spectrum - Electromagnetic radiation with wavelengths from about 400 nm to around 750 nm.

voids - Spaces between superclusters that are relatively free of luminous matter. Also called bubbles.

volume - The space occupied by an object.

 W

waves - Propagation of energy by means of coherent vibration.

weak nuclear force - Fundamental force of nature that governs the process of nuclear radioactive decay.

weighted average - A mean or average that is adjusted to reflect value or proportion.

WIMPs - Dark matter jargon for Weakly Interacting Massive Particles; includes electrons, which have very little mass, and neutrinos, which may have zero mass. Also called nonbaryonic matter.

worm holes - Tunnels through space-time linking one black hole to another.

 X

x particles - Theoretical particles that may have decayed into quarks and antiquarks as the temperature significantly decreased after the Big Bang.

x-ray emissions - Radiation in the x-ray portion of the electromagnetic radiation emanating from an object.

x-rays - A relatively high-energy photon with wavelengths in the range of 0.01 to 10 nanometers.

 Y

 Z

 + Freedom of Information Act + The President's Management Agenda + FY 2002 Agency Performance and accountability report + NASA Privacy Statement, Disclaimer, and Accessiblity Certification + Freedom to Manage