a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z

a

abiotic - Abiotic describes conditions and factors that are the nonliving elements of an ecosystem. Examples of abiotic factors include climate, air currents, temperature, moisture, light, and soil type. top

abort motor - The abort motor generates 1,800,000 Newtons (400,000 pounds) of thrust in a fraction of a second to rapidly move the crew to safety during a launch pad or in-flight emergency. top

absorb - Absorb means to take in. Light energy that is absorbed is not given off, it is taken in by the object that absorbs the light. As a result, the object may become warmer. top

absorption - Absorption means to soak in. Specifically, when referring to light, absorption means that light stops at the object and does not reflect or refract. Objects that absorb light appear dark or opaque. top

absorption spectrum - An absorption spectrum is a spectrum, broken by a specific pattern of dark lines or bands, observed when light passes through a gas. The absorption pattern is unique and can be used to identify the gas. top

abundant - Abundant means existing or occurring in large amounts. top

acceleration - Acceleration is any change in speed or velocity (when an object speeds up, slows down, or changes direction). Acceleration can be described as positive or negative (e.g., speeding up is positive acceleration, slowing down is negative acceleration). top

acid - An acid is a material that in water produces positively charged hydrogen ions (protons) in the solution. Examples of acids include vinegar, grapefruit juice, and hydrochloric acid. top

adaptation - An adaptation is a physical structure or behavior that helps an organism better survive in its environment. top

aerodynamic heating - Aerodynamic heating is the heating of a solid body produced as air or other fluid passes over the body. top

aerodynamics - Aerodynamics is the study of how efficiently air flows around an object. top

aerospace - Aerospace is Earth's atmosphere and the space beyond considered as a whole. Aerospace is also the branch of technology and industry focused on both aviation and space flight. top

air packs - An air pack is a breathing apparatus used by firefighters to provide breathable air. top

airfoil - An airfoil is a surface, such as a wing or propeller, designed to aid in lifting and controlling an aircraft by means of air currents. top

albedo - Albedo is the fraction of solar radiation that is reflected off the surface of an object. top

amplitude - Amplitude is a measure of the height of a sound wave which determines the sound's volume. top

analog - An analog is a situation on Earth that produces effects on the body similar to those experienced in space, both physical and mental/emotional. top

anisotropic - Anisotropic materials have a crystalline structure where the arrangement of atoms along one axis is different than that of another axis. Optically anisotropic materials rotate polarized light as it passes through them. top

annual - An annual is a plant that performs its entire life cycle from seed to flower to seed within a single growing season. All roots, stems and leaves of the plant die annually. top

anthrosphere - The anthrosphere is the part of the environment that is created or modified by humans for use in human activities and human habitats. top

Apollo Missions - The Apollo Missions were designed to land humans on the Moon and bring them safely back to Earth. Six of the missions (Apollos 11, 12, 14, 15, 16, and 17) achieved this goal. Apollos 7 and 9 were Earth orbiting missions to test the Command and Lunar Modules, and did not return lunar data. Apollos 8 and 10 tested various components while orbiting the Moon, and returned photography of the lunar surface. Apollo 13 did not land on the Moon due to a malfunction, but also returned photographs. The six missions that landed on the Moon returned a wealth of scientific data and almost 400 kilograms of lunar samples. Experiments included soil mechanics, meteoroids, seismic, heat flow, lunar ranging, magnetic fields, and solar wind experiments. top

argon - Argon is a colorless, odorless gas that is totally inert (does not react with other substances). Argon makes up 0.94% of the Earth's atmosphere and is the third most abundant atmospheric gas. top

arteries - Arteries are blood vessels that carry blood away from the heart. top

artificial satellite - An artificial satellite is a manufactured object that continuously orbits Earth or some other body in space. top

ash - Ash is the solid remains left after something is burned top

asteroid - An asteroid is a small, rocky object larger than a few hundred meters in diameter (and not classified as a planet or moon) that orbits the Sun; most asteroids are located in a band between the orbits of Mars and Jupiter. top

asteroid belt - The asteroid belt is a 1.5 astronomical-unit-wide area between the orbits of Mars and Jupiter where most asteroids are found. top

astrobiology - Astrobiology is the study of life in the universe. top

astronaut - An astronaut is a person who is trained to travel into space. top

astronomers - An astronomer is a person who studies objects in space. top

astronomy - Astronomy is the scientific study of the universe. top

at rest - At rest is a state of motionlessness or inactivity. top

Atacama Desert - The Atacama Desert is a cool, dry region in northern Chile that is approximately 600 to 700 miles long. It runs from the south bend of the Loa River to the mountains separating the Salado-Copiap? drainage basins. It is one of the driest places on Earth. top

atmosphere - The atmosphere is the envelope of gases that surrounding Earth or other planets. Earth's atmosphere is composed of nitrogen (78%), oxygen (21%), other gases (1%), water vapor (the amount varies greatly), and tiny particles of dust, sand, and/or salt). top

atom - Atoms are the smallest part of an element that maintains the chemical properties of that element. top

attitude - Attitude describes the position of a spacecraft relative to the direction of motion. top

attitude control motor - The attitude control motor helps to stabilize and reorient the Orion crew module before the crew module is released from the abort system to begin its controlled descent. top

attract - To attract is to pull together. top

attraction - A force between particles of matter that draws them together and resists their separation. top

axis - An axis is an imaginary straight line around which an object spins. The Earth's axis runs from the North Pole to the South Pole. top

b

balanced - Balanced comes from the verb "balance" meaning to "be equal with." top

balanced forces - Balanced forces are forces that act together on an object without changing its motion. top

ballast - Material inside a vehicle that give it additional mass for stability or other purposes. This material gives the vehicle more weight, a force pulling down toward the center of the Earth. Ballast is sometimes designed to be disposable. top

base - Bases are corrosive materials that produce negatively charged hydroxide ions when placed into water. Examples of bases include bleach and ammonia. top

bedrock - Bedrock is the hard, solid rock beneath surface materials such as soil and gravel. Bedrock also underlies sand and other sediments on the ocean floor. top

Bennu - Bennu is an asteroid largely made of carbon that was discovered September 11, 1999. Bennu became the destination for the OSIRIX-REx mission because it is relatively close, rotates slower, and is made up of primitive carbon compounds. top

Bernoulli's Principle - Bernoulli's principle states that as the velocity of a fluid (such as air) increases, the pressure exerted by that fluid decreases. top

biome - A biome is a major biotic community characterized by distinct climate and dominant forms of flora and fauna. top

biosphere - The biosphere is the region of the Earth's surface and atmosphere where living organisms exist. top

Biosphere 2 - Biosphere 2 is a model of Earth's biosphere located north of Tucson, AZ. top

biotic - Biotic factors are the living parts of an ecosystem. top

birefringence - Birefringence is a process where light of different polarizations travels at different speeds in different directions through a transparent medium. Birefringence is also called double refraction. top

black hole - A black hole is a region of space with gravitational force so strong that nothing can escape from it. top

blood vessels - Blood vessels are tubes that carry blood. They include arteries, veins, and the capillaries that connect them. top

breathing apparatus - A breathing apparatus is a device worn by firefighters and rescue workers to provide breathable air. top

bulb - A bulb is any plant that stores its complete life cycle in an underground storage structure. top

bunker gear - Bunker gear refers to the personal protective equipment used by firefighters. The term originated because the gear was stored next to a person's bunk. top

buoyancy - Buoyancy is an upward force on an object in a fluid, e.g., when you float in a pool or the ocean or a balloon floats in air. top

c

camber - Camber is the difference between the top and bottom curves of an airfoil. Airplane wings tend to have a longer curve on the upper than the lower wing surfaces (although the opposite is true of supersonic jets). top

capillaries - Capillaries are the thinnest blood vessels. Nutrients and gases can pass through capillary walls. Capillaries connect veins and arteries. top

carbon - Carbon is a chemical element that is necessary for life. Carbon is found in Earth's atmosphere, soils, and oceans. Carbon is also found in abundance in the sun, stars, comets and atmospheres of most other planets. Nearly every biological compound that makes up living things is composed of carbon. top

carbon cycle - The carbon cycle is the flow of carbon between carbon reservoirs such as rocks, oceans, plants, soil, fossil fuels, and the atmosphere. The carbon cycle is composed of fast and slow carbon components. The fast carbon cycle is largely the movement of carbon through life forms on Earth or the biosphere. The slow carbon cycle is the movement of carbon between rocks, soil, ocean, and atmosphere. top

carbon dioxide - Carbon dioxide is a colorless, odorless, non-poisonous gas that is a normal part of Earth's atmosphere. Carbon dioxide is a product of fossil fuel combustion. top

carbon monoxide - Carbon monoxide is a colorless, odorless gas that is a deadly poison. Carbon monoxide forms when a substance containing carbon burns without enough oxygen. Automobile exhaust and cigarette smoke contain carbon monoxide. Carbon monoxide has one atom of carbon joined to one atom of oxygen. top

carbon source pools - Carbon source pools are places where carbon is stored. Carbon source pools include rocks, plants, fossil fuels, oceans, soil, and the atmosphere. top

carbon-rich - Carbon-rich objects contain a great deal of the element carbon. top

Cassini Mission - The purpose of the Cassini Mission, launched October 1997 and ended in September 2017, was to learn more about Saturn and its icy moons. top

cell - A cell is the smallest unit of living matter. top

chemical - A chemical is any substance with a definite composition made up of one or more elements. top

chemical change - A chemical change occurs when one or more substances change into entirely new substances with different properties. top

chemical equation - A chemical equation is a statement showing two equal amounts. For example, a chemical equation shows that two molecules of hydrogen and one molecule of oxygen makes two molecules of water. top

chlorophyll - Chlorophyll is a green chemical in plant cells that allows plants to use light energy to make food. top

chondrule - A chondrule is a spheroidal mineral grain present in large numbers in stony meteorites. The meteorites formed from asteroid material that is thought to have been present when the solar system formed. top

chromosphere - The relatively thin layer of the solar atmosphere located above the sun's surface. top

chronology - Chronology is the arrangement of events in the order of their occurrence. top

cirrus cloud - A cirrus cloud is a feathery cloud that is composed of ice crystals and that has the highest altitude of any cloud in the sky. top

citizen science - Citizen science is the collection and analysis of data, typically relating to the natural world, by members of the general public as part of a project with professional scientists. top

classification - Classification is the grouping of organisms or things based on characteristics they have in common. top

clay - Clay is soil with very small particles. top

climate - Climate is the long-term weather pattern of an area, including temperature, precipitation, and wind. top

climate change - Climate change is the variation in Earth's global climate over time, either cooling or warming. This term describes changes in the average state of the atmosphere over a long time scale, as opposed to brief changes in weather. top

cloud - A cloud is a large collection of very tiny droplets of water or ice crystals in the atmosphere, which forms when the air is cooled and condensation occurs. top

Clouds and the Earth's Radiant Energy System, or CERES - CERES is a sensor flown on satellites to collect data about Earth's systems. top

coal - Coal is a combustible black (or brownish-black) sedimentary rock, that consists largely of carbon (with varying amounts of hydrogen, sulfur, oxygen, and nitrogen). Coal is formed when dead plant matter decays into peat and is placed under intense heat and pressure for millions of years. Since it forms from the remains of ancient organisms, coal is known as a fossil fuel. The burning of coal is a primary source of energy for the world. top

collection - A collection is a group of objects or material accumulated in one location, usually for a specific purpose. top

coma - A coma is the cloud that forms around a comet's nucleus. This cloud is made when a comet travels near the sun. top

comet - A comet is a small object found in the solar system. It is made mostly of ice, cosmic dust, and rock. It follows an elliptical orbit around the sun and gives off gas and dust in the form of a tail as it passes close to the sun. top

communications - Communications is the use of messages to generate meaning across cultures, contexts, and media. top

composite materials - Composite materials are two or more different materials that are combined together. The combined materials do not lose their individual properties. The properties of the product are a combination of the properties of each material. top

composition - Composition is the nature of something's ingredients or constituents. It is the way in which a whole or mixture is made up. top

compound - A compound is a substance that is formed by the chemical combination of two or more elements. top

compound machine - A compound machine is a machine that consists of more than one simple machine. A wheelbarrow is an example of a compound machine that consists of a lever and a wheel and axle. Scissors, another example of a compound machine, consists of two levers and two wedges. top

compression - Compression is the process of molecules being pressed closer together. top

compression wave - A compression wave is a wave that is propagated by the compression of molecules in a substance. top

computer model - A computer model is a computer program that uses data, math and computer instructions to predict or simulate events that could occur or have already occurred. top

computer simulation - A computer simulation is the usage of a computer to imitate a real-world process or system. Computer simulation is a discipline that can assist in the design, creation, and evaluation of complex systems. Designers, program managers, analysts, and engineers use computer simulations to model a real or proposed system using computer software, which is useful when changes to the actual system are difficult to implement, expensive, or are impractical. Some examples of computer simulation familiar to most of us include weather forecasting and flight simulators used for training pilots. top

condensation - Condensation is the process of a gas changing to a liquid. top

conduction - Conduction is the transfer of heat between two solid objects that are touching. top

conductors - Conductors are materials that easily transfer heat or electricity. top

constellation - The definition of constellation you may be familiar with is a group of stars that can be connected together to form a pattern such as the "Big Dipper." NASA's mission which includes building a new vehicle, the Orion, capable of going to the Moon for lunar exploration and research, is called the Constellation Program. top

constraint - Any limit or restriction given for the design process is called a constraint. top

continuous - Continuous means going on without stopping or being interrupted. top

contracts - When the heart contracts, or gets smaller, the heart muscle is squeezing a larger space within the heart into a smaller space. top

controlled descent - A controlled descent is a landing where the speed and direction of the fall is modified. top

convection - Convection is the transfer of heat between flowing gases or liquids. top

coordinate system - A coordinate system is a grid placed on a map to help quickly locate specific locations. top

corona - The outer atmosphere of the sun. top

coronagraph - A telescope for observation of the sun's corona. top

coronal mass ejections (CME) - Huge bursts of solar wind rising above the sun's corona. top

cosmologist - A cosmologist is a scientist or astronomer who studies large scale structures and dynamics of the universe, including the origins of the universe. top

covalent bond - A covalent bond is a chemical bond formed between two atoms by sharing electrons between the atoms. top

crater - A crater is a bowl-shaped depression that forms on the surface of an object when a falling body strikes the object's surface or when an explosion occurs. top

crescent Moon - A crescent Moon is a lunar phase during which the Moon appears less than half full top

Crew Exploration Vehicle (CEV) - The Crew Exploration Vehicle (CEV) is America's new spacecraft for human space travel. The CEV will carry four crew members to the Moon. It can carry six crew members on missions to the International Space Station or to low-Earth orbit to transfer to a Mars-bound spacecraft. top

criteria - Criteria are rules guiding the design process, such as size, type of material, or dollar limit to build the model. top

cross-polarizers - Cross-polarizers are created when two pieces of polarizing film are placed at 90 angles to each other. top

cryolava - Cryolava is the liquid or gaseous forms of water, ammonia, and methane that erupt from a cryovolcano instead of molten rock. top

cryosphere - The cryosphere is the part of Earth's system that includes water in its frozen state. Earth's cryosphere includes snow, sea ice, lake ice, glaciers, permafrost, ice caps, and ice sheets. top

cryovolcano - A cryovolcano, or ice volcano, is a type of volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Cryovolcanoes are found on extremely cold moons of planets in the outer solar system. top

crystal lattice - A crystal lattice is the structure of an ionic solid in which orderly. top

CubeSat - A CubeSat is a type of miniaturized satellite for space research. top

cumulus cloud - A cumulus cloud is a low-level, billowy cloud that commonly has a top that resembles cotton balls. top

d

dark matter - Dark matter is the name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now has escaped all detection. There are many theories about dark matter, but the subject is still a mystery. top

data - Data is factual information, such as measurements or statistics, that are used as a basis for reasoning, discussion, or calculation top

Dawn Mission - The Dawn Mission, launched in 2007 and active until 2018, purpose was to learn more about the building blocks and early formation of the solar system. Dawn visited Vesta and Ceres, two of the largest bodies in the asteroid belt. top

decay - To decay is to rot or cause to rot as a result of bacterial, fungal, or chemical action. To decay is to decompose. top

decibel - The decibel (dB) is a unit of sound intensity. top

decompose - To decompose is to break down. The process of decomposing is the breaking down of dead plants and animals into tiny pieces. When these pieces mix with dirt they form soil. top

decomposer - A decomposer is an organism, such as bacteria and fungi, that break down wastes and the remains of other organisms into simpler substances. top

decrease - to get smaller top

definite - Definite means something that is known for certain or is clear. top

density - The density (d) of a material or object is a measure of how tightly the matter within it is packed together, and is given by the ratio of its mass (m) to its volume (V), or d = m/V. It is typically expressed in kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³) or grams per milliliter (g/mL). top

deposit - To deposit is to drop off. Wind and water often carry sand and mud particles from one place and later deposit them somewhere else. What they drop off is called a deposit (so deposit is both a verb and a noun). top

design - A design is a plan or drawing made to show the look and function of a spacecraft, building, garment, or other object before it is built or made. top

design process - The design process is a series of steps in designing and refining/improving something. Steps of the Design Process 1. Identify the problem 2. Identify criteria and constraints 3. Brainstorm possible solutions 4. Generate ideas 5. Explore possibilities 6. Select a design 7. Build a model or prototype 8. Refine the design, repeating steps 1-8 top

detection - Detection is the process of identifying the presence of something such as a fire. top

diameter - Diameter is a straight line passing from side-to-side through the center of a body or figure like a circle or sphere. top

diffraction grating - A diffraction grating is a surface with many closely spaced parallel grooves or splits in it which splits and diffracts light to produce the light's spectrum. top

direction - Direction is the line or course on which something is moving. top

dirt - Dirt is made of small particles formed from the breakdown of rocks. top

dispatching system - A dispatching system receives 911 calls and notifies the emergency service (for example, fire department) of the location and nature of the emergency. top

dormant - Dormant is when an organism's physical activity, growth, and development are slowed or temporarily stopped to conserve energy in unfavorable conditions. top

double refraction - Double refraction, also called birefringence, is a property of light in which a single ray of unpolarized light entering a certain medium is split into two rays, each traveling in a different direction. One ray is bent, or refracted, at an angle as it travels through the medium; the other ray passes through the medium unchanged. top

drag - Drag is the resistance on an object to movement through a fluid. It is a force that slows an object down. For example, swimmers and submarines experience drag as they move through water and birds and aircraft experience drag as they move through air. top

drop test - An experiment that measures speed, velocity or acceleration of a falling object, or the results of a fall or the impact of the fall, is called a drop test. top

dwarf planet - A dwarf planet is a celestial body that orbits the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it is nearly round shape in shape, has not cleared the neighborhood around its orbit, and is not a satellite. top

e

Earth Orbiting System - The Earth Orbiting System, or EOS, is a series of satellites that orbit Earth and collect various types of data. top

Earth Science - Earth Science is the study of Earth and the universe around it. top

Earth system - The Earth system is a unified system comprised of six spheres: the anthrosphere, atmosphere, biosphere, cryosphere, geosphere, and hydrosphere. top

eclipse - The partial or total apparent darkening of the sun when the moon comes between the sun and Earth (solar eclipse), or the darkening of the moon when the full moon is in Earth's shadow (lunar eclipse). top

ecosystem - An ecosystem is the living things (biotic) and their nonliving (abiotic) environment. top

EDITH drill - EDITH drill stands for Exit Drills in the Home. An EDITH drill is a plan that families can use to quickly and safely exit a home in the event of a fire. top

efficiency - Efficiency is the work a machine does compared to the energy needed to run the machine. top

effort - Effort is the force placed on simple machines to move the load. For example, a person using a crow bar exerts a force (effort) on the bar to move an object. top

electromagnetic radiation - Electromagnetic radiation is energy radiated in the form of waves. It consists of electric and magnetic fields traveling at the speed of light. top

electromagnetic spectrum - The electromagnetic spectrum is the entire range of visible and invisible energy waves organized according to wavelengths. Visible light is a small part of the electromagnetic spectrum, which includes radio waves, ultraviolet waves, infrared waves, and microwaves. The shorter wavelengths have the highest energy. top

element - An element is a substance that cannot be separated or broken down into simpler substances by chemical means. top

emission spectrum - An emission spectrum is a spectrum of bright lines or bands of light of specific wavelength which are emitted when a gaseous element is exposed to high energy. Each element has its own unique pattern of bands. top

emit - To emit is to give off. LEDs give off, or emit, light. Colored LEDs emit very specific wavelengths of light. top

energy - (see also thermal energy) Energy is the ability to do work, and there are several different forms of energy (e.g., kinetic, potential, thermal, sound, light, chemical, etc.). While energy may be transformed from one form to another, the total energy remains the same within a closed system. top

energy budget - Earth's energy budget refers to the tracking of how much energy is flowing into and away from Earth, where the energy is going, and if the energy coming in balances the energy going out. The energy balance determines the climate of the Earth. top

engineer - Engineers use math and science to design new tools and devices to solve practical problems. top

engineering design process (EDP) - The engineering design process is a series of steps that engineers and others follow to come up with a solution to a problem. Often, the solution involves designing a product or process. The process generally includes the following steps: asking questions, imagining solutions, planning designs, creating and testing models, and making improvements. Because the engineering design process includes improving the product of process design, it is an ongoing process. top

environment - The environment is the complex of physical, chemical, and biological factors in which a living organism or community exists. top

equation - An equation is a number sentence that uses the equal sign (=) to show that two expressions have the same value (Example: 8 + 4 = 12.) top

erosion - Erosion is the process of carrying away soil or pieces of rocks. top

Europa - Europa is one of the many moons of Jupiter. It is slightly smaller than Earth's Moon and has a surface that is largely covered in ice. Europa is considered among the most promising environments suitable for life. Europa is thought to have an iron core, a rocky mantle and an ocean of salty water (below a shell of ice). top

evaporation - Evaporation is the process of changing from a liquid to gas. top

evapotranspiration - Evapotranspiration is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants. top

exert - Exert is to put forth strength or effort. top

expands - When the heart expands, or gets larger, the heart muscles stretch and the spaces within the heart spread out. top

extinct - An extinct animal or plant is one that has died out. top

extract - Extract is to remove or take out. top

extremophile - An extremophile is an organism that can survive in extremely harsh environments where it was thought no organisms could live, such as environments with intense heat, high acidity, extreme pressure, or extreme cold. top

f

fast carbon cycle - The fast carbon cycle is part of the movement of carbon through life forms on Earth or the biosphere as a part of the overall carbon cycle. The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. top

fault - A fault is a break in Earth's crust or the surface of a planet or moon where sections of rock often slide past each other. Faults are found at the boundaries of tectonic plates. top

Fermi Gamma-ray Space Telescope - Fermi is a space telescope that consists of two parts: the Large Area Telescope, or LAT, and the Fermi Burst Monitor. The LAT has a wide field of view and can detect gamma rays. The Fermi Burst Monitor observes gamma ray bursts which are sudden, brief flashes of gamma radiation that occur about once a day. top

fertile - Fertile is a term used to describe soil that is rich in nutrients. top

filament - A filament is a small thin wire inside an incandescent light bulb that is heated until it glows. top

fire - Fire is the flame, heat, and light produced by burning. It is a chemical reaction. Fire can only start if oxygen, heat, and fuel are present. top

fire-resistant - A material that is fire-resistant prevents the passage of excessive heat, hot gasses or flames. The bunker gear worn by firefighters is fire-resistant because it is designed to withstand burning and resist heat. top

fire-retardant - Fire-retardant describes a property of materials that slows or stops the spread of fire. top

fireproof - Fireproof is a term that describes the ability of an object or material to withstand intense heat and fire. top

flight - Flight is the act of moving through the air or space outside Earth's atmosphere, such as the movement taken by aircraft, birds or insects. top

fluid - A fluid is a substance that will flow. When a substance flows, the particles in the fluid can move past one another. Both liquids and gases are fluids. top

fluid shift - While in space, fluids in the body move from the lower part of the body toward the head. This movement is fluid shift. top

fluorescent light - A fluorescent light uses an electric current to heat gas particles inside a specially coated glass tube. When the particles hit the sides of the tube, a glowing light is produced. top

foam fire suppressant - A foam fire suppressant is a foam used to suffocate and put out fires. Foam fire suppressants work by coating the fuel, preventing its contact with oxygen and, as a result, putting out the fire. top

force - A force is whatever can cause an object with mass to accelerate (change its direction or speed). Force may be expressed with both magnitude (speed or velocity) and direction making it a vector quantity. top

forced convection - Forced convection occurs when a pump or other mechanism moves a heated fluid. top

fossil fuel - Fossil fuel is a general term for crude oil, coal, natural gas, or heavy oils. These fuels are created by exposure to heat and pressure in the earth's crust over hundreds of millions of years. top

fraction - A fraction is a way to compare equal parts to a whole (example 2/8 is 2 equal parts out of 8 equal parts). top

freezing point - The freezing point of a liquid is the temperature at which the liquid changes state from a liquid to a solid. top

frequency (v) - Frequency is the number of waves that pass a fixed point in a given period of time. The frequency of electromagnetic radiation is measured in hertz (Hz) which is defined as the number of waves per second. top

fuel - Fuel is something that stores energy and can later supply heat. Fuels that are burned include oil, gas, coal, and wood. It is a necessary component for fire. top

fulcrum - A fulcrum is the base or pivot on which a lever turns when moving or lifting something. The fulcrum of a seesaw is the bar on which the board rests. top

full Moon - A full Moon is a phase of the Moon during which its full daylight hemisphere can be seen from Earth. top

g

g - The force of Earth's gravity. top

galaxy - A galaxy is a collection of gas, dust, and stars that are bound together by gravity. top

Galileo Mission - The Galileo Mission was launched by NASA to learn more about Jupiter and its moons. Through the mission, which lasted from October 1989 to September 2003, we learned more about the extensive volcanic activity on the moon Io. top

gantry - NASA's gantry is a large apparatus in Hampton, Virginia that was built to test Apollo space capsules. Now it helps researchers test the Orion space capsule (see Figure 3). top

gas - A gas is a state of matter that has no definite shape and no definite volume. The molecules in a gas move rapidly in all directions and spread out to fill the space they are in. top

gas giant - A gas giant is a planet that has a very deep atmosphere; the gas giants include Jupiter, Saturn, Uranus, and Neptune. top

GEDI - GEDI, or the Global Ecosystem Dynamics Investigation, produces high resolution laser ranging observations of the 3D structure of the Earth. GEDI's precise measurements of forest canopy height, canopy vertical structure, and surface elevation greatly advance our ability to characterize important carbon cycling processes, biodiversity, and habitat. top

geometry - Geometry is a branch of mathematics that deals with the measurement, properties, and relationships of points, lines, angles, surfaces, and figures. top

geosphere - The geosphere is the solid portion of Earth and the processes that shape Earth's surface. top

geosynchronous orbit - A geosynchronous orbit is a satellite orbit at approximately 35,800 kilometers above the Equator in which objects travel at the same speed as Earth. Objects in this orbit remain stationary in reference to Earth. top

germinate - Germinate is when something starts to grow, particularly when a seed sprouts into a new plant. top

geyser - A geyser is a hot spring that periodically sends up fountain-like jets of water and steam into the air. top

gibbous Moon - A gibbous Moon is a phase of the Moon in which more than half, but not all, of the Moon's daylight hemisphere is visible from Earth. top

glacial advance - Glacial advance is an increase in the thickness and area of a glacier. This term also describes the time period it takes for the increase in glacial thickness to occur. top

glacial retreat - Glacial retreat occurs when backward melting at the front of a glacier takes place at a rate exceeding forward motion. top

global climate change - Global climate change is the long-term fluctuations in temperature, precipitation, wind, and all other aspects of Earth's climate. top

GLOBE - GLOBE, or the Global Learning and Observations to Benefit the Environment Program, is an international science and education program that provides students and the public worldwide with the opportunity to participate in data collection and the scientific process, and contribute meaningfully to our understanding of the Earth system and global environment. top

gram - A gram is a unit used to measure the mass of small objects. There are 1,000 grams (g) in a kilogram. top

graph - A graph is a picture that represent mathematical information in an organized manner with lines, shapes, and colors. top

gravitational force - Gravitational force is the force of attraction between all masses in the universe. top

gravitational pull - Gravitational pull is the attraction that all masses in the universe exert on one another. The gravitational pull of the Moon on Earth's oceans causes tides. top

gravity - Gravity is a force between objects based on their masses and the distance between the objects. The force of gravity on the moon is less than the force of gravity on Earth because the moon has only 1/6 the mass of Earth. Earth's gravity is described as 1g. top

Great Salt Lake - The Great Salt Lake, located in several wide basins in Utah, is the largest natural lake west of the Mississippi River. The Great Salt Lake is approximately 75 miles long and about 35 miles wide. Salt is brought in by tributaries. As water in the Lake evaporates, salt is left behind. The salinity of the lake is 270 parts per thousand as compared to the average salinity of the world ocean, which is 35 parts per thousand. top

greenhouse gases - Greenhouse gases are gases that contribute to the warming of the Earth's atmosphere by reflecting solar radiation from Earth's surface. Carbon dioxide, ozone, and water vapor are examples of greenhouse gases. top

ground truth - Ground truthing is a validation process where a person on the ground (or sometimes in an airplane) makes a measurement of the same phenomenon a satellite is measuring, at the same time the satellite is measuring it. The two answers are compared to evaluate how well the satellite instrument is performing. The actual measurements taken on Earth are called "ground truth." top

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habitat - A habitat is the place where an organism is naturally found in an ecosystem. top

heat - Heat is the amount of thermal energy absorbed, released, or transferred by a material. This is typically expressed as q, and is measured in joules (J). top

HIAD - (Hypersonic Inflatable Aerodynamic Decelerators) HIADs are inflatable heat shield structures made up of incredibly strong, yet flexible fabric designed to maintain shape and withstand head during atmospheric reentry. top

highlands - Highlands are mountainous regions of land. top

hill - A hill is raised above the surrounding land, but is smaller than a mountain. top

horizon - The horizon is a layer of soil that is different from both the layers above and below it. top

hot spring - A hot spring is a spring where the water temperature is 6-9?C (10-15?F) warmer than the mean annual air temperature of the area around it. top

Hubble Space Telescope, HST - The Hubble Space Telescope is a large telescope that orbits Earth. It is named after astronomer Edwin P. Hubble (1889-1953). top

humus - Humus is part of the soil that is made up of decayed organic materials. top

hydrosphere - The hydrosphere is the part of Earth that is composed of water. top

hypotenuse - A hypotenuse is the side opposite the right angle in a a right triangle. top

i

ice - Ice is frozen water. top

ice age - An ice age is a cycle cold period marked by periods of glacial advance with episodes of glacial retreat. top

ice core - Ice cores are cylinders of ice obtained by drilling into a glacier. top

ice sheet - An ice sheet is the layer of ice covering a large land mass, notably Antarctica and Greenland. Ice sheets form from the compression of snow as new snow builds on top of it. top

ICESat-2 - ICESat-2 (short for Ice, Cloud and land Elevation Satellite), launched Sept. 15, 2018, uses lasers and a very precise detection instrument to measure the elevation of Earth's surface. By timing how long it takes laser beams to travel from the satellite to Earth and back, scientists can calculate the height of forests, glaciers, sea ice, lakes and more. top

incandescent light - An incandescent light produces light when a metal wire inside a glass bulb is heated and gets hot enough to glow. The glass bulb contains an inert gas that does not react with the filament. top

inclination - Inclination is the angle between a reference plane and another plane or axis of direction. For an artificial satellite, the reference plane is the Equator. The inclination of a satellite's orbit is the angle that the orbit crosses the Equator. If a satellite has a 0° inclination then it would be orbiting over the Equator. If a satellite has a 90° inclination, then its orbit is perpendicular to the Equator and it would pass over the poles. top

inclined plane - An inclined plane is a sloping plank or ramp that makes it easier to raise heavy loads. For example, pushing a heavy wagon up an inclined plane takes less force than lifting it. top

incoming energy - Incoming energy is energy coming to Earth from the Sun. About 29% of the solar energy that arrives at the top of the atmosphere is reflected back to space by clouds, atmospheric particles, or bright ground surfaces like sea ice and snow. This energy plays no role in Earth's climate system. About 23% of incoming solar energy is absorbed in the atmosphere by water vapor, dust, and ozone, and 48% passes through the atmosphere and is absorbed by the surface. Thus, about 71% of the total incoming solar energy is absorbed by the Earth system. top

increase - to get larger top

inertia - Inertia is the tendency of an object to continue doing what it is doing, either moving or resting, unless acted on by an outside force. The inertia of an object is related to its mass (the greater the mass, the greater the inertia). top

infrared - Infrared wavelengths are longer than visible light and give off heat. top

infrared thermometer - An infrared thermometer is a tool that measures the heat being given off by an object. top

insulator - An insulator is a material or substance that does not conduct heat, sound, or electricity easily. top

intensity - Sound intensity is the amount of energy transferred by a sound wave per unit time. top

Io - Io, one of Jupiter's moons, is the most volcanically active world in the Solar System, with hundreds of volcanoes, some erupting lava fountains dozens of miles (or kilometers) high. top

ionic - Ionic compounds are compounds in which the atoms are held together by ionic bonds. An ionic bond is a chemical bond in which one atom loses one or more electrons to form a positive ion and another atom gains one or more electrons to form a negative ion. The force of attraction between the positive and negative ions forms the bond. top

irregular polygon - An irregular polygon is a polygon that does not have all angles or sides equal. top

ISON - Comet ISON was a sungrazing comet discovered in September of 2012. The comet was expected to put on a spectacular show in Earth's sky in late 2013, but instead fell apart shortly after rounding the Sun's far side on Nov. 28, 2013. top

isotropic - Isotropic materials have a crystalline structure where atoms are arranged in the same way along each axis. top

ISS - The ISS, or International Space Station, is a large spacecraft and science laboratory in orbit around Earth that serves as a home for crews of astronauts and cosmonauts from different countries who are learning about living and working in space. The ISS orbits Earth at an average altitude of approximately 250 miles. It travels at 17,500 mph, which means it orbits Earth every 90 minutes. top

j

joint - A joint is a place where two or more pieces come together. top

joule (J) - The joule is a unit of energy. One joule is the energy expended when 1 Newton of force is applied to move an object a distance of 1 meter. top

Jupiter - Jupiter is the 5th planet from the Sun and is a gas giant that lacks an Earth-like surface. It is twice as massive as all the other planets combined. Jupiter has an atmosphere largely made of Hydrogen and Helium. It has more than 75 moons. Nine spacecraft have visited Jupiter. Seven flew by and two have orbited the gas giant. Juno, the most recent, arrived at Jupiter in 2016. Jupiter's Great Red Spot is a gigantic storm that's about twice the size of Earth and has raged for over a century. top

k

Katherine G. Johnson - Katherine Goble Johnson is an African American mathematician who made critical calculations for manned spaceflights in her 35-year career with NASA Langley in Hampton, VA. Katherine Johnson's calculations were essential to the success of Project Mercury spaceflights of astronauts Alan Shepard and John Glenn. Katherine Johnson's life has been portrayed in the movie and book Hidden Figures. Mrs. Johnson received the Presidential Medal of Freedom in 2015 from then-president Barack Obama. top

kinetic energy - Kinetic energy is the energy of a moving object. top

kingdom - A kingdom is the largest of six groups into which an organism can be classified. top

Kuiper Belt - The Kuiper Belt is a donut-shaped ring of space around the Sun and in the region beyond Pluto, that contains many frozen comet bodies that contain remnants from our Solar System's formation. top

l

LADEE (Lunar Atmosphere and Dust Environment Explorer) - The Lunar Atmosphere and Dust Environment Explorer (LADEE), launched in 2013, was designed to study the Moon's thin exosphere and the lunar dust environment. During an extended mission, LADEE gathered detailed information about the structure and composition of the thin lunar atmosphere. LADEE also succeeded in key engineering tests. The mission proved the effectiveness of the new modular common spacecraft bus, or body, an innovation which could drastically reduce the cost of spacecraft development. It also hosted NASA's first dedicated system for two-way communication using laser instead of radio waves. top

latent heat - Latent heat is heat energy that is released or absorbed by a substance when it changes from one phase to another. top

latitude - Latitude is the number of degrees north or south of the Equator. The Equator is 0°N or S, and the North and South Poles are 90° N and 90° S respectively. To visualize this think of Earth as a circle divided into 360°. top

launch abort system, or LAS - The launch abort system offers a safe, reliable method of moving the entire crew out of danger in the event of an emergency on the launch pad or during the climb to Earth orbit. top

lava - Lava is the hot, melted rock that comes out of an erupting volcano or split in the surface of Earth or solid-surface planets or moons. top

lava flow - A lava flow is a moving stream of melted (liquified) rock. top

Law of Conservation of Matter - The Law of Conservation of Matter is a principle of matter conservation that states that the mass of an object or collection of objects never changes over time, no matter how the constituent parts rearrange themselves. top

LCROSS (Lunar CRater Observing and Sensing Satellite) - LCROSS, or Lunar CRater Observing and Sensing Satellite, mission objective was to confirm the presence or absence of water ice in a permanently shadowed crater near a lunar polar region. LCROSS launched with the Lunar Reconnaissance Orbiter (LRO) aboard an Atlas V rocket from Cape Canaveral, Fla., on June 18, 2009. The missions found evidence that the lunar soil within shadowy craters is rich in useful materials, and the moon is chemically active and has a water cycle. Scientists also confirmed the water was in the form of mostly pure ice crystals in some places. top

lever - A lever is a bar for lifting loads. The lever rests on a fulcrum. To raise a heavy weight at one end, you press down at the other end. top

LIDAR - LIDAR, which stands for Light Detection and Ranging, is a remote sensing method that uses light, in the form of a pulsed laser, to measure variable distances to the Earth. These light pulses--combined with other data-- provide precise, three-dimensional information about the shape of the Earth and its surface characteristics. top

life - Life is a condition that distinguishes animals and plants from nonliving matter. Living things have the ability to grow, metabolize, reproduce, respond to stimuli, and change before dying. top

life cycle - Life cycle is a series of stages through which something passes during its lifetime. Example: The life cycle of a star is the series of stages that a star goes through from its formation from a nebula to its demise. The life cycle of a frog would be the stages (egg, tadpole, froglet and adult) that a frog goes through. top

life processes - Life processes are things organisms must carry out in order to survive. Life processes that all living things must conduct include moving, getting energy from food, responding to changes in the environment, growing, reproducing, getting rid of wastes, taking in and using nutrients. top

lift - Lift is an upward force resulting from pressure differences (e.g. different pressures on the top and bottom of a bird's or an aircraft's wing moving through a fluid) due to the air above the wing traveling faster than the air below the wing, because the upper surface is longer than the lower surface. top

light - Light is a form of energy that consists of electromagnetic waves of particular frequencies and wavelengths that radiate outward. top

light bank - A light bank is a group of lights that are connected. A Solid State Lighting Module, or SSLM, is a light bank made up of rows of light-emitting diodes. top

liquid - A liquid is a state of matter having a definite volume but not a definite shape; a fluid. top

living - Living is a condition that distinguishes organisms from inorganic objects and dead organisms. top

load - The load is a measure of the force or weight applied to a machine or surface. top

loam - Loam is soil made up of a mixture of sand, silt, and clay. top

location - A position or site occupied or available for occupancy or marked by some distinguishing feature. top

longitude - Longitude is the number of degrees east or west of the Prime Meridian, the 0° E or W line going through Greenwich, England and the North and South Poles. The Prime Meridian divides the globe into eastern and western hemispheres. It runs between the Poles through the Pacific Ocean on the side of the globe opposite England. This line is called the International Date Line. Degrees longitude are 0° E or W at the Prime Meridian and 180° E or W at the International Date Line. top

longitudinal wave - A longitudinal wave is a wave whose particles vibrate parallel to the direction the wave is traveling. top

low-Earth orbit - Low-Earth orbit (LEO) is the path in which a spacecraft or satellite moves around the Earth. This path may be between 320 and 800 kilometers (200-500 miles) above the Earth's surface. top

lowlands - The lowlands are sections of ground lower than the surrounding area. A valley is an example of lowlands. top

LRO (Lunar Reconnaissance Orbiter) - RO launched on an Atlas V rocket on June 18, 2009, beginning a four-day trip to the moon. LRO spent its first three years in a low polar orbit collecting detailed information about the moon and its environment. After this initial orbit, LRO transitioned to a stable elliptical orbit, passing low over the lunar south pole. With a suite of seven powerful instruments, LRO has collected a treasure trove of data, making an invaluable contribution to our knowledge about the moon. top

Luna - Luna is the official name of Earth's moon. top

lunar crane - The lunar crane, part of the Lunar Surface Manipulation System, was developed because of the need for a versatile system capable of performing multiple functions on the lunar surface, such as unloading components from a lander, transporting components to an operational site and installing them, and supporting service and replacement during component life. The lunar lander is a complex machine that consists of several simple machines including a lever, pulley and wheel and axle. top

lunar eclipse - A lunar eclipse is when the moon passes through Earth's shadow at full moon. top

luster - Luster is a way a mineral reflects light from its surface. Words like shiny and dull describe the luster of a mineral. top

m

machine - A machine is a device that uses energy and does work. A machine can be as simple as a lever or screw or as complex as an airplane or computer. top

magma - Magma is molten (melted) rock beneath the Earth's (or other planet or moon's) surface that is melted by intense heat. top

magnetic field - The magnetic field is the region around a magnet where its force attracts or repels materials. top

magnitude - Magnitude is the size (or amount or quantity) of a measurement or object. top

mare - Mare are the large, dark areas of the moon that are made of basalt. top

Mars - Mars is the fourth planet from the Sun. Mars is about one-sixth the size of Earth. It is sometimes called the "Red Planet" because of its reddish color, which it gets from the iron in its soil. Mars is extremely cold (average temperature is -80?F). Mars is rocky and has canyons, volcanoes and craters all over its surface. top

Mars 2020 Rover - The Mars 2020 Rover is part of the Mars 2020 mission looking at potential for life on Mars. The Mars rover is car-sized (about 10 feet long, 9 feet wide, and 7 feet tall) and weighs less than a compact car (2,314 pounds). The Mars 2020 Rover will look for signs of habitable conditions in the past, signs of past microbial life, and use its drill to collect core samples of Mars' surface. top

Mars helicopter - The Mars Helicopter is a planned robotic helicopter that will travel to Mars with the Mars 2020 rover. It will test the technology necessary for a controlled flight in Mars' thin atmosphere, scouting out possible interesting targets for future studies on the Red Planet and the best driving routes for future Mars rovers. top

Mars Reconnaissance Orbiter - The Mars Reconnaissance Orbiter is a multipurpose spacecraft that was launched on Aug. 12, 2005 to advance our understanding of Mars. The purpose of the Mars Reconnaissance Orbiter was to make detailed observations about the planet and to examine potential landing sites for future surface missions. top

mass - The amount of matter in an object is the object's mass. Objects are made up of atoms containing varying numbers of protons, neutrons, and electrons which determine their mass. top

matter - Matter is often defined as anything that has mass and takes up space (has volume), and it is the generic term for the substance of which all physical objects are composed. Matter can be in several different states, including solids, liquids or gases. top

measurement - Measurement is a means to quantify the size, length, or amount of something (as established by measuring). top

medium - A medium is a physical environment through which a disturbance, such as a light our sound wave, can travel. Example: Glass is a medium through which light can pass. top

melting point - The melting point of a solid is the temperature at which the solid changes state from a solid to a liquid. top

meteor - A meteor is a bright streak of light in Earth's atmosphere that occurs when a meteoroid burns. top

meteorite - A meteorite is a meteor that hits Earth's surface. top

meteorology - Meteorology is the scientific study of Earth's atmosphere, especially in relation to weather and climate. top

meter - Meter are the base unit of measurement in the metric system. A meter is approximately 39.37 inches. top

meteroid - A meteoroid is a small, rocky body that travels through space. top

microgravity - The condition of microgravity (a small amount of gravity) exists when objects are in free fall, like the space shuttle and other objects orbiting the Earth. The objects would actually fall to the Earth if they weren't moving very quickly in a different direction. top

microorganism - An organism that can only be seen with a microscope. Microorganisms, such as protozoans, bacteria, and some algae, typically consist of a single cell. top

mixture - A mixture is a physical combination of two or more substances. Each substance in a mixture retains its own physical and chemical properties. top

model - A model represents something else. A model might be a drawing or a 3-D object. Models are smaller than the original object or made out of less expensive materials than the actual object. Many iterations (repetitions or versions) of a model are often needed before the actual object (for instance, the Orion spacecraft) can be built. The first version is often a drawing scaled down to fit on a piece of paper. The next may be a series of structures made out of paper, cardboard, plastic, or other readily available materials. top

modeling - Modeling is the production of a physical, conceptual, or mathematical representation of a real phenomenon that is difficult to observe directly. top

molecule - A molecule is a group of two or more atoms held together by a covalent bond. top

molten - Molten is a when a solid substance is made into a liquid by being heated. The heat inside Earth, for example, changes solid rock into molten rock called magma. top

momentum - Momentum is a quantity defined as the product of the mass and velocity of an object. For example: momentum (p) = mass (m) x velocity (v). top

monomer - A monomer is a small molecule that is linked with large numbers of other small molecules to form a chain or a network (polymer). top

moon - A moon is a natural satellite that revolves around another body that is greater in mass. top

Moon phases - Moon phases are the different appearances of the Moon over the course of approximately one month. As the lit portion of the Moon appears to change shape, each distinct stage of the change is called a phase. top

motion - Motion is a change in position over time. top

mountain - A mountain is any place on Earth that rises sharply and is well above its surroundings. top

n

National Transonic Facility - The National Transonic Facility (NTF) is a high-pressure, cryogenic, closed-circuit wind tunnel that uses super cold nitrogen gas at high pressure to duplicate true flight aerodynamics. The NTF can accommodate models as small as one-fiftieth the size of an actual aircraft. Notable vehicles tested in the NTF have included the Boeing 777 and 767, the Space Shuttle and Booster, the B-2 Bomber, and the F-18 Hornet. top

navigating - Navigating is to plot and manage the course or position of a ship, aircraft, etc. top

near-Earth object - Near-Earth objects (NEOs) are comets and asteroids that have been nudged by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood. top

Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) - The Near-Earth Object Wide-field Infrared Survey Explorer, or NEOWISE, is a NASA infrared-wavelength space telescope active from December 2009 to February 2011. The NEOWISE project helped collect physical data on a number of minor planets and detected over158,000 asteriods and more than 155 comets. top

net force - The net force is the total of all of the forces acting on an object. These forces are vectors, which means they have direction as well as magnitude. top

neutron star - A neutron star is the type of star formed when a massive star explodes as a supernova, leaving behind an ultra dense core. top

new Moon - A new Moon is the phase of the Moon in which no part of the Moon is visible from Earth. top

Newton's Law of Gravitation - The force of attraction between two masses where its strength depends on their masses and the distance between them. top

Newton's Second Law of Motion - Newton's Second Law of Motion states that the acceleration of an object depends on the mass of the object and the amount of force applied. The first part of the law explains why the acceleration of a 3 kg bowling ball would be greater than that of a 7 kg bowling ball. The second part of the law explains why the acceleration of the 7 kg bowling ball will be greater if a larger force is applied to it. top

Newtonian fluid - A Newtonian fluid is a fluid that reacts the same way no matter how much stress, or force, is applied to it. top

nitrogen - Nitrogen is a naturally occurring colorless, odorless, tasteless gas element. Nitrogen makes up about 78% of the Earth's atmosphere. It makes up proteins and is essential for life. top

non-Newtonian fluid - A non-Newtonian fluid is a fluid that changes behavior depending on the amount of stress, or force, applied to it. top

nondestructive evaluation - Nondestructive evaluation (or nondestructive testing) is a variety of analysis techniques used in the science and technology industry to evaluate the properties of a material, component or system without causing damage. top

nonliving - Nonliving is not living, inanimate, or no longer alive or characterized by life. top

nucleus - The nucleus of the comet is the solid, rocky part of the comet. top

nutrients - Nutrients are substances that an organism (plant or animal) needs in order to survive and grow. top

o

ocean - An ocean is any of the large bodies of salt water into which the great ocean is divided. Oceans cover nearly three fourths of the surface of Earth. top

Olympus Mons - Olympus Mons is a shield volcano found on Mars. Olympus Mons is the largest volcano in the solar system measuring 624 km (374 mi) in diameter (approximately the same size as the state of Arizona) and 25 km (16 mi) high. It is roughly 100 times the size of Earth's largest volcano, Mauna Loa. top

Oort Cloud - The Oort Cloud is a spherical region surrounding our solar system that contains billions of comets; it extends from the Kuiper Belt to almost halfway to the nearest star. top

opaque - Opaque describes an object through which light is unable to pass. top

orbit - An orbit is the path of a celestial body or an artificial satellite as it revolves around another body. top

orbital decay - Orbital decay is the reduction in altitude of a satellite's orbit caused by gravity and drag from the atmosphere. top

orbital period - Orbital period is the time it takes a satellite to complete one orbit. top

organic molecules - Organic molecules are molecules containing carbon. The ability of carbon atoms to form chains means that there are millions of organic molecules. Organic molecules are responsible for life on Earth. top

organism - An organism is any living thing that can carry out its life on its own. top

Orion - NASA's Orion crew exploration vehicle will replace the space shuttle after it is retired. Orion is the flagship of NASA's programs for space exploration beyond low Earth orbit and a key element of NASA's Constellation Program to explore the Moon, Mars and beyond. top

OSIRIS-REx - OSIRIS-REx, or Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer, is a spacecraft that traveled 1.2 billion-miles to the asteroid Bennu. OSIRIS-REx's mission is to collect samples from Bennu that will help scientists investigate how planets formed and how life began, as well as improve our understanding of asteroids that could impact Earth. top

outgoing energy - Outgoing energy is energy radiated from Earth's surface and atmosphere. The atmosphere radiates heat equivalent to 59% of incoming sunlight; the Earth's surface radiates only 12 percent. The total amount of outgoing energy (71%) equals the amount of incoming energy, creating an energy balance. top

oxygen - Oxygen is a gas found in air that cells need to live (represented by O2). top

ozone - Ozone is a gas molecule that is made up of three oxygen atoms. top

p

parent material - Parent material is the underlying geological material in which soil horizons form (or from which the soil developed). top

particle - A particle is a tiny amount or small piece of something. top

parts per million by volume (ppmv) - Parts per million by volume is the measure of the concentration of a gas within the atmosphere. In this case, it measures the concentration of carbon dioxide in the air. top

path of totality - The path of this shadow, the path of totality, is where observers will see the Moon completely cover the Sun for about two and a half minutes. top

pattern - A repeated design or recurring sequence. top

payload - Payload is the part of an aircraft or spacecraft vehicle's load that is necessary to the purpose of the flight. Payloads often include cargo, passengers, and instruments. top

permafrost - Permafrost is any soil or rock that is frozen throughout the year. top

pH - pH is a measure of the concentration of hydrogen ion (H+) in a substance which determines if a solution is acidic or basic. top

phase - In physical science, a phase describes the physical state of matter. The four common phases of matter are solid, liquid, gas, and plasma. In astronomy, a phase is the change in the illuminated area of one celestial body as seen from another celestial body; the Moon's phases are a result of the changing positions of Earth, Sun, and Moon. top

phase change - A phase change occurs when matter changes from one state to another. This is any combination of changing from a solid to a liquid to a gas. top

photon - A photon is a quantum, or discrete amount, of light energy. Photons have no mass and behave like both a particle and a wave. top

photosphere - The photosphere is the visible surface of the Sun. top

photosynthesis - Photosynthesis is the process in green plants and certain other organisms that uses energy from sunlight to make food (a sugar called glucose) from water and carbon dioxide. top

physical change - A physical change is a change of matter from one form to another without a change in chemical properties. top

physical feature - Physical features include bodies of water and landforms. Any body that is part of the topography (oceans, plateaus, mountains, lakes, rivers, volcanoes, canyons, etc.) is a physical feature. top

physical map - A physical map shows the location of major landforms such as mountains, plains, and deserts. It also shows country borders and major cities. top

physical model - A physical model is a copy of an object that represents that object. It can be the same size as the object, or larger or smaller than the object. Models are very useful for experiments, visualization, or education whether by professionals (scientists and engineers, for example) or others. top

plains - Plains are large, flat, level ground. Plains are often covered with low grasses and have very few or no trees. top

Planck's constant (h) - Planck's constant is a physical constant relating the energy of a photon to its frequency. The value of this constant is approximately 6.626 x 10-34 joule·second. top

planet - A planet is a celestial body that orbits the sun, is round because of its own gravity, and has cleared the neighborhood around its orbital path top

planetary volcanoe - Planetary volcanoes are volcanoes that are found on the planets (including Earth) and their moons. top

Plasma - A fourth state of matter (in addition to solid, liquid, and gas) that exists in space. In this state, atoms are positively charged and share space with free negatively charged electrons. top

polar orbit - A polar orbit is an orbit in which the satellite passes over the North and South Poles on each orbit, and eventually passes over all points on Earth. top

polariscope - A polariscope is an instrument used for measuring or exhibiting the polarization of light or for examining substances in polarized light, often to determine stress and strain in glass and other substances. top

polarized light - Polarized light waves are vibrating in one direction as they pass through or are reflected by certain media. top

polarizing lens - Polarizing lenses filter out some of the light waves at particular angles, but let waves at other angles through. The waves of light that pass through a polarizing filter tend to be aligned in one plane. top

pole - A pole is one of the two ends of a magnet where the magnetic force is the strongest. top

political map - A political map shows the location of cities and the borders of countries and states. top

polymer - A polymer is long or large molecule consisting of a chain or network formed by chemically bonding many repeating units, or monomers, together. top

position - The point or area occupied by a physical object. top

PPE - PPE stands for personal protective equipment. Personal protective equipment is worn to minimize exposure to hazards that cause serious injuries and illnesses. PPE for firefighters includes a fire helmet; fire-resistant pants, jackets and gloves; and steel-toed, waterproof boots. top

precipitation - Precipitation is water in the atmosphere that falls to Earth as rain, snow, hail, sleet, or freezing rain. top

precision - Precision is the condition of being exact and accurate. top

pressure - Pressure is the force per unit area. Even though you can't feel it, air has pressure. Air molecules move continuously. The more times they bump into each other or a surface, the greater the pressure. Air pressure is increased by: 1. increasing the number of molecules in the same amount of space (volume); 2. increasing the temperature (it makes the air molecules move faster); or 3. decreasing the volume. top

probability - Probability is the likelihood or chance that something will occur. top

process - A process is the continuous action or a series of changes taking place in a definite manner. top

projectile - A projectile is any object that is thrown or otherwise launched. It is affected by Earth's gravity. A projectile may start at a given height and move toward the ground in an arc. Regardless of its path, a projectile will follow these rules:

• Projectiles maintain a constant horizontal velocity (in the absence of air resistance).
• On Earth, projectiles experience a constant vertical acceleration of 9.8 m/s2 (32 ft/s 2) downward (in the absence of air resistance).

prominence - A prominence is a huge column of relatively cool gas arcing out above the photosphere (the visible surface of the sun) and can be seen from Earth. top

property - A property is a characteristic that can be observed or measured. top

prototype - A prototype is an original or model on which something is based. top

pulley - A pulley is a wheel with a rope, chain, or belt passed over it. A load is attached to one end of the rope and the other end is pulled. A single pulley lets you change the direction of the pull. A set of pulleys makes it possible to lift heavy loads with less effort. top

pulsar - A pulsar is a rotating neutron star which generates regular pulses of radiation. top

pulse rate - A pulse rate is how many times a heart beats in one minute. top

q

qualitative data - Qualitative data are sets of information that describe attributes. top

quantitative data - Quantitative data are values that can be counted or measured. top

r

R-value - The R-value measures a material's insulating properties. top

radiation - Radiation is the transfer of heat through space. Radiation is a form of energy that is emitted in the form of rays, electromagnetic waves, and/or particles. In some cases, radiation can be seen (visible light) or felt (infrared radiation), while other forms--like x-rays and gamma rays--are not visible and can only be observed with special equipment. top

range - Range is a set of different things of the same general type. Example: Stars are found in a range of colors. top

rarefaction - Rarefaction is the process of molecules becoming more spread out (as opposed to compression). top

ratio - A ratio is a comparison of two quantities. top

reaction - A reaction is a chemical process in which substances combine and change, such as heating iron and sulfur together to produce iron sulfide. top

recycled - Used materials that are made into new products are called recycled materials. Recycling reduces the waste of useful materials, and it reduces the need for new materials. Recycling sometimes reduces energy, costs, and pollution. top

redesign - To redesign is to revise something in appearance, function, or content. Redesigning to make improvements is an important part of the engineering design process. top

reduced gravity - Reduced gravity is less gravity than normally experienced on Earth, or less than 1g. top

reflect - Reflected light is light that hits a surface and bounces off. top

reflection - Reflection is the bouncing of light waves (or sound -- echo) off a surface. top

refract - When light is refracted it is bent. Light refracts when it passes from one material to another. top

refraction - Refraction is the bending of light as it passes from one material to another. top

regolith - A fine dust called regolith covers the moon. Regolith is created when micrometeoroids bombard the moon's surface, breaking up moon rocks. (The vowels are pronounced like those in "LEGO" and "miss.") top

rehydrate - To rehydrate is to put water back into the body that is removed during spaceflight or after exercise on Earth. top

relief map - A relief map shows the variation in land heights. The different heights are shown as lines or different colors. top

repel - Repel means to push away. top

reusable - Objects or materials that can be used again are called reusable. The objects may be used for the same purpose or different purposes. top

revolution - Revolution is the motion of a body that travels around another body in space; one complete trip along an orbit top

revolve - To circle around something or move in an orbit. top

rock - A rock is a naturally formed solid that is usually made up of one or more types of minerals. top

root - A root is the part of a plant that takes in water and minerals, and anchors the plant into the soil. top

Rosetta - Rosetta is a spacecraft developed by the European Space Agency (ESA) that was sent on a ten-year mission to catch the comet "67P/Churyumov-Gerasimenko" (C-G) and gather data that could answer some of our questions about comets. Rosetta terminated its mission after landing on C-G on September 30, 2016. top

rotate - To spin or turn around in a circle. top

rotation - Rotation is the spin of a body on its axis. top

s

S'COOL - S'COOL, or Students' Cloud Observations On-Line, is a project which involves school children in real science. They provide ground truth measurements to assist in the validation of the CERES instrument. They can also compare the surface- and space-based observations to learn more about clouds and climate. top

Saffire - Saffire is a series of three fire safety experiments designed to better understand how fire behaves in microgravity. The goal of Saffire is to learn how fire works in space to ensure a safe environment for astronauts. top

salinity - Salinity is a measure of the amount of dissolved salts in a given liquid. top

sample - A sample is a specimen, or small part of something that is intended to show the quality, style, or nature of the whole. top

sample return capsule - A sample return capsule, or SRC, is a spacecraft whose goal is to collect and return samples from an extraterrestrial location to Earth for analysis. ISIRIS-REx is tasked with carrying samples of the asteroid Bennu back to Earth and protecting it during a high-speed entry into Earth's atmosphere. top

sand - Sand is rock material that has been eroded into small grains. top

satellite - A satellite is an artificial or natural body that revolves around another celestial body that is greater in mass. top

satellite data - Satellite data is data collected by a human-produced machine that is launched into space. Satellites contain cameras and sensors that gather information on clouds, oceans, land, ice; measure the amount of ozone and carbon dioxide in the atmosphere and the amount of energy absorbed and emitted by Earth; and monitor wildfires and volcanoes. top

saturated solution - A saturated solution contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specified temperature. top

scale - Scale is the ratio of the length in a drawing, or model, to the length of the real object. If you scale an object, you size or measure it proportionately. top

scale factor - Scale factor is the ratio of any two corresponding lengths in two similar geometric figures. top

science - Science is the pursuit and application of knowledge and understanding of the natural and physical world using a systematic approach. Scientific knowledge is based on evidence arrived at through observation and repeated experimentation. top

scientific protocol - Scientific protocol is the exact set of procedures to be followed in scientific research to make sure that collected data can be compared. top

scientists - Scientists use systematic methods to study the world around them. They use an organized approach to observe and study the world. They ask questions, look for patterns, and try to find general rules for the natural world. top

screw - A screw is a kind of simple machine. It is actually an inclined plane that twists around a central axis. top

seasons - A year on Earth is usually divided into four quarters (spring, summer, autumn or fall, and winter) caused by the tilt of Earth and the amount of direct sunlight received. top

sediments - Sediments are particles that have been deposited by some natural process, such as blowing wind or moving water. top

seismometer - A seismometer is an instrument that responds to and records ground motions, such as those that might be caused by earthquakes, volcanic eruptions, and explosions. top

self-healing material - Self-healing materials are able to repair damage by closing the gap around a penetrating object. top

silt - Silt is made of particles smaller than sand. top

similar - Geometric shapes are similar if their corresponding sides are proportional and corresponding angles are equal. top

simple machine - A simple machine is any of several devices that have few or no moving parts that are used to modify motion and force in order to perform work. Simple machines include inclined planes, levers, wedges, wheel and axles, pulleys, and screws. top

simulation - A simulation is something that substitutes for the real thing. For instance, flight simulators are mockups for pilots to practice so they do not crash real aircraft under different weather and equipment emergencies. A simulation can have many qualities of the authentic experience without all the expenses and dangers. A model simulates - or is a simulation of - a real object or event. top

slope - Slope is a measure of the difference in position between two points on a line. Slope is a measure of a line's rise (the amount it changes along the y axis) divided by its run (the amount it changes along the x axis). Slope can be used to refer to the steepness of a volcano and can be calculated by dividing the height of a volcano by its width. top

slow carbon cycle - The slow carbon cycle is the movement between rocks, soil, ocean, and atmosphere as a part of the overall carbon cycle. Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. top

smoke detector - A smoke detector is a device that is used in homes, businesses, and schools to sense smoke, which can be an indicator of fire. top

Soda Lake - Soda Lake, in southern California, is in the Mojave Desert. The water in Soda Lake quickly dries, leaving basic (alkaline) deposits behind (including sodium carbonate and sodium bicarbonate). top

soil - Soil is a mixture of minerals, weathered rocks, and decayed plant and animal material. top

solar - Having to do with the sun. top

solar eclipse - A solar eclipse is when the Moon passes between the sun and Earth casting its shadow on the Earth. top

solar flares - Solar flares are violent eruptions of gas on the sun's surface. top

solar radiation - Solar radiation refers to energy that travels in rays or waves and originates from the sun. top

solar system - The solar system includes the sun, the planets, and other bodies that revolve around the sun including comets. top

solid - A solid is a state of matter having a definite shape and a definite volume. top

solid rocket boosters - Solid rocket boosters use a propellant/fuel in solid form. Two solid rocket boosters enable the shuttle to reach Earth orbit. top

solute - The solute is the dissolved component of a solution. The solute is usually, but not always, present in a smaller amount than the solvent. top

solution - A solution is a mixture in which the components are evenly mixed so that every part of the mixture is the same as any other. top

solvent - The solvent is the component of a solution that dissolves one or more solutes. top

sound - Sound is a form of energy produced and transmitted by vibrating objects. top

sound wave - A sound wave is a series of compressions and rarefactions traveling through a substance. top

space - Space is everything in the universe from where the Earth's atmosphere ends (about 100 km above the Earth) and beyond. It includes the Moon, planets, other stars, the Milky Way, black holes, and the near-vacuum medium between celestial bodies. top

spacecraft - A spacecraft is any vehicle designed for travel into space, such as the space shuttle. top

specific heat - Different materials require different amounts of heat to produce similar changes in their temperatures. In other words, materials have different specific heat capacities, often called, specific heat. The specific heat capacity of a material is the amount of energy it takes to raise the temperature of 1 gram of the material 1 degree Celsius. Specific heat capacity can be measured in joules per gram per degree Celsius (J/g ?C). top

spectrometer - A spectrometer is a tool that helps scientists study comets. Spectrometers can be found on satellites, rockets, airplanes, and telescopes. A spectrometer works like our eyes, but it breaks light into colors like a rainbow. top

spectroscope - A spectroscope is an instrument used to produce and observe spectra. top

spectroscopy - Spectroscopy refers to the science and practice of using spectrometers and spectroscopes and of analyzing spectra. top

speed - Speed is the time it takes an object to travel a certain distance. Speed equals distance divided by time or s = d / t. top

speed of light (c) - The speed of light is the speed of electromagnetic radiation in a perfect vacuum. The speed of light is the same for all frequencies of electromagnetic radiation, 3.0x10 8 m/s. top

sphere - A sphere is a solid figure whose surface consists of an infinite number of consecutive points that are all positioned at the same distance from its center. top

star - A star is a huge ball of very hot, glowing gas that gives off both heat and light. top

states of matter - States of matter are the distinct forms of matter. Three common states of matter are solid, liquid, and gas. top

statistics - Statistics is a branch of mathematics concerned with the collection, analysis, interpretation, and presentation of large amounts of numerical data. top

STEM - STEM is an acronym for the disciplines of science, technology, engineering, and math. top

stratus cloud - A stratus cloud is a low-altitude gray cloud that has a flat, uniform base. top

stress - Stress, in materials science, is the applied force or system of forces that tends to deform or damage an object. top

Sun - The closest star to Earth and is in the center of our solar system. top

sun-synchronous polar orbit - A sun-synchronous polar orbit is a special kind of polar orbit. When traveling in this orbit, a satellite not only travels over the North and South Poles, but it passes over the same part of Earth at roughly the same time each day. Tracking and Data Relay Satellite or TDRS - TDRS is a system of nine geosynchronous communications satellites. They are used to communicate from Earth to orbiting satellites, the space shuttle, and the International Space Station. top

sunspots - A region on the surface (photosphere) of the sun that is temporarily cool and dark compared to surrounding areas. top

suppress - To suppress a fire means to stifle it or put it out and thus prevent its spread. top

Surveyor - The Surveyor probes were the first U.S. spacecraft to land safely on the Moon between 1966-1968. top

survival - Survival is when a living thing continues to live and thrive. top

survive - To survive is to remain alive or to continue to thrive and prosper. top

swarm - A swarm is a group of CubeSats that are launched in close proximity to one another and have the capability of talking to each other. These small satellites can share data between themselves and take turns in relaying science data to the ground. One satellite can send data from all of the other satellites in the swarm. top

symmetrical - When an object is balanced, or equal on both sides, it has symmetry and is called symmetrical. top

system - A system is a set of interconnected parts that make up a complex whole. top

t

tail - The comet's tail forms when the comet travels near the sun. The tail always points away from the sun. top

technician - A technician is a highly trained and skilled person who performs specific tasks, such as performing tests and collecting data, often using sophisticated equipment and techniques. Technicians generally work in partnership with engineers, scientists, and other professionals. top

technology - Technology is any alteration of the natural world made to meet a human need or desire. top

tectonic plates - Tectonic plates are the huge sections of the Earth's crust that are moving very slowly. Tectonic plates are where most earthquakes and volcanoes occur and where some mountains form. top

telescopes - Telescopes are tools that help you see objects that are far away. top

temperature - Temperature is a measure of the average heat or thermal energy of the particles in a substance. top

Terra - Terra is the official name of Earth. top

terrestrial planet - A terrestrial planet is one of the very dense planets that are located close to the Sun. top

test - A test is a procedure used to determine the quality or effectiveness of a procedure or product. Tests, or trials, are often used to see how well something works. top

texture - Texture is the way something feels. For instance, sand feels rough (has a grainy texture) while smaller mud particles have a smoother or slimy texture. top

thermal energy - Thermal energy is the energy of movement of the molecules within a substance. The higher the temperature, the faster the molecules move, thus temperature can be used as a measure of thermal energy. top

thermal protection system - A thermal protection system is a combination of materials used to insulate and reduce the amount of heat transferred to a spacecraft as it enters an atmosphere. top

thermometer - A tool that is used to measure temperature. top

thrust - Thrust is a force that propels an object. Thrust must be greater than drag for an object to move forward. top

tides - The regular rise and fall of water due to the gravitational pull of the Moon and, to a lesser degree, the Sun. top

tilt - Tilt is a sloping position or movement of an object. Earth's axis is tilted about 23.5 degrees from its orbital plane. Because of this tilt, the sun shines on different latitudes at different angles throughout the year causing seasons. top

topsoil - Topsoil is the dark, top layer of soil that is rich in humus and minerals. top

trace element - A trace element is a chemical element present in minute (very small) quantities. The atmosphere contains trace amounts of the elements neon, helium, methane, krypton and hydrogen. top

trajectory - Trajectory is the path that an object takes moving through space. top

translucent - Translucent describes an object through which only some light is able to pass. top

transmit - When referring to light, transmit or transmission is when light passes through an object without having an effect on the object top

transparent - Transparent describes an object through which light is able to pass. top

transpiration - Transpiration is evaporation of water from the leaves of a plant. Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. top

trend - A trend is a general direction of movement. On a graph, the trend is the overall direction (either increase or decrease) of the values graphed. top

u

ultraviolet - Ultraviolet wavelengths are shorter than visible light. Shorter waves have more energy. Ultraviolet radiation can burn and cause skin cancer. top

unbalanced forces - Unbalanced forces are forces that do not cancel each other out when acting together on an object. top

universe - The universe, or cosmos, is space and all the planets, stars, and other forms of energy and matter that make it up. top

v

vacuum - A vacuum is a space entirely devoid of matter. top

valley - A valley is a place that is naturally lower than the surrounding land. Valleys are often located between mountains or hills. top

variable - A letter that represents a group of numbers is called a variable. Variables are italicized. That's how you can tell them apart from units such as grams (g). An example of some variables used in this activity: s = speed, d = distance, t = time or s = d / t . Two variables you use all the time to plot points on a graph are the x and y coordinates [e.g., Plot point (x, y)]. top

vector - A vector is a variable (something that can change/vary) that is composed of both an amount and a direction. An example of a vector is velocity. What makes velocity different than speed is the direction of travel. Wind velocity is a way vectors are used in everyday life. When the weather report states the wind is 40 kilometers per hour (25 miles per hour) out of the west, the wind's velocity, not speed, is what is being reported. top

vector quantity - A vector quantity is any force that has both size and direction. For example, speed becomes a vector once you give it a direction. top

veins - Veins are blood vessels that carry blood back to the heart. top

velocity - The speed and the direction of travel of an object is the object's velocity. Velocity is similar to speed, but whereas an example of speed would be, "the wind was blowing at 40 miles per hour," velocity would be expressed as "40 miles per hour from the SE." Direction becomes important when dealing with navigation of boats, aircraft, wind and water currents, etc. top

Venus - Venus is the second planet from the Sun and is Earth's closest neighbor in the solar system. Venus is slightly smaller in size than Earth. It is typically the brightest object in the sky after the Sun and the Moon. Venus is surrounded with thick clouds of sulfuric acid. Its surface is covered with craters, volcanoes, mountains, and big lava plains. Temperatures on Venus are hot enough to melt lead. top

Vertical Spin Tunnel - The vertical spin tunnel, which works much like the recreational indoor skydiving locations, features a vertically-rising test airstream into which free-flying unpowered aircraft models are hand-launched to evaluate spinning and spin recovery behavior, tumbling resistance, and recovery from other out-of-control situations. top

vibrate - Vibrate is to move to and fro or from side to side top

viscosity - Viscosity is a measure of how strongly a liquid or gas resists flowing. A substance with a high viscosity, such as honey, flows very slowly. Water has a very low viscosity and flows easily. top

visible light - Visible light is the part of the electromagnetic wave that can be seen by the human eye. top

visualize - To visualize means to form a mental image or to make something visible to the eye. top

volcanism - Volcanism is the eruption of molten rock (magma) and gases onto the surface of the Earth or a solid-surface planet or moon. top

volcano - A volcano is a vent in the surface of Earth or solid-surface planet or moon where molten material and gases from the inside comes up and flows/explodes out onto the surface. top

volume - The volume of an object is how much space it occupies, and it is typically expressed in milliliters (mL), cubic centimeters (cm or cc), liters (L) or cubic meters (m3). top

w

waning - Waning is an adjective that means "decreasing," as in a "waning crescent Moon." top

water - Water is a substance made when two atoms of hydrogen bond to one atom of oxygen. In temperatures between 0?C-100?C, it is a clear liquid that has no color, taste, or smell. In its frozen state (below 0?C), water is called ice. In its gaseous state, (above 100?C), water is called water vapor. top

water cycle - The water cycle is one part of the Earth system. It involves the movement of water from Earth's surface to the atmosphere and back through the processes of evaporation, condensation, and precipitation. top

water vapor - Water vapor is the water present in the atmosphere in a gaseous form. top

wavelength - Wavelength is the distance between two crests or two troughs on a wave. Light is classified by its wavelength. Wavelength is usually measured in meters. top

waxing - Waxing is an adjective that means "increasing," as in a "waxing crescent Moon." top

weather - Weather is the current state of the atmosphere, measured in terms of temperature, pressure, humidity, wind speed and direction, cloudiness and precipitation. top

weathering - Weathering is a process through which rocks or other materials are broken down. Wind, moving water (rivers and waves) and glaciers all cause weathering. top

wedge - A wedge is a simple machine with 2 or more sloping surfaces. An ax is a wedge used to split wood. top

weight - Weight is the force of gravity on an object. Here on Earth, it means how hard the Earth pulls down on objects. Because the moon is smaller than the Earth, the moon wouldn't pull down on an object as hard and it would weigh a lot less there. However, since the object would still be made up of the same number and type of particles, its mass and density would be the same on the Earth and on the moon. top

wheel and axle - A wheel and axle is a simple machine in which a round frame (wheel) turns on a central shaft or rod (axis). top