Accretion Disk: A relatively flat, rapidly rotating disk of gas surrounding a black hole, a newborn star, or any massive object that attracts and swallows matter. Accretion disks around stars are expected to contain dust particles and may show evidence of active planet formation. Beta Pictoris is an example of a star known to have an accretion disk.

Aeronomy: The study of the atmosphere of a planet, with particular attention to the composition, properties and motion of atmosphere constituents.

Altitude: Height in space of an object or point relative to sea level or ground level.

Antimatter: The 'opposite' to ordinary matter. For every particle of ordinary matter there is an almost identical antiparticle of antimatter: protons and antiprotons; electrons and positrons. The particle's mass is exactly the same as its antiparticle's mass, but their electrical charges and other fundamental properties  are opposite. When a particle meets its antiparticle, they annihilate each other.

Angstrom (Å): A unit of length equal to 10^-10 m.

Aphelion: The point on a planet's elliptical orbit at which it is furthest from the Sun.

Apogee: The most distant point from Earth on a satellite's orbit.

Asteroid: A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 km).

Asteroid Belt: A region of space between Mars and Jupiter where the great majority of asteroids is found.

Astrometry: The branch of astronomy concerned with measuring the positions of celestial bodies, such as stars and galaxies, and their real and apparent motions.

Astrophysics: The part of astronomy dealing with the physics and chemistry of astronomical objects and events.

Astronaut: A person who travels in space.

Astronomy: The study of the universe and the celestial bodies and phenomena that reside in it, including their composition, history, location, motion, positions, dimensions, distribution, energy and evolution.

Astrophysics: The part of astronomy that deals principally with the physics and chemistry of stars, stellar systems and interstellar material.

Astronomer: Scientist who observes and studies planets, stars and galaxies.

Atom: The smallest particle of an element that exhibits the chemical properties of the element.

Atmosphere: The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.

Astronomical Unit (AU): The average distance between the Earth and Sun (about 150 million kms - 93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system.

Aurora: A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colourful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the 'Northern Lights' (Aurora Borealis) or 'Southern Lights' (Aurora Australis), depending on in which polar region they appear.

Big Bang: A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe started roughly 15 billion years ago from an extremely dense and incredibly hot initial state.

Binary Stars: Binary stars are two stars that orbit around a common centre of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.

Black Hole: A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp.

Bok Globules: Dark clouds of dust and gas that are typically condensing to form stars, named after astronomer Bart Bok who studied them.

Brown Dwarf: A kind of 'failed' star: a small and opaque object whose mass is not sufficient to start, in its core, the nuclear reaction to transform hydrogen into helium. A brown dwarf cannot therefore produce enough energy to shine as a star. A brown dwarf's mass is not more than 0.08 solar masses.

Cassegrain Telescope: Popular design for large, two-mirror reflecting telescopes in which the primary mirror has a concave parabolic shape and the secondary mirror has a convex hyperbolic shape. A hole in the primary allows the image plane to be located behind the large mirror.

Cassini Division: A gap in Saturn's rings that divides the outer set from the inner set of rings.

Cassini-Huygens Mission: The joint NASA/ESA spacecraft studying Planet Saturn and its moons and carried onboard ESA's Huygens Probe. Named after Jean Dominique Cassini (1625-1712), the Italian-French astronomer who discovered several Saturnian satellites and the magnificent rings of Saturn, and Christiaan Huygens (1629-1695), who discovered Titan in 1655.

Celestial: Of or relating to the sky or visible objects in the sky, like the Moon, Sun, planets, comets, asteroids, stars, and galaxies.

Celsius (Centigrade): A temperature scale on which the freezing point of water is 0° C and the boiling point is 100° C.

Charge-Coupled Device (CCD): An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see. CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh on a screen door.

Chromosphere: Layer of the Sun's atmosphere located above the photosphere and below the corona. It is about 10 000 km deep and consists mainly of ionised hydrogen, helium and calcium. Its temperature ranges from 6000 °C at the photosphere boundary to 100 000 °C at the boundary with the corona. Its low density means it is usually only visible during a total solar eclipse.

Coma: The cloud of gas and dust that forms around a comet's nucleus. This cloud is created when the solar wind strikes the surface of the nucleus.

Comet: A ball of rock and ice, often referred to as a 'dirty snowball'. Typically a few kilometres in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley's Comet). A comet’s 'signature' long, glowing tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapours into space.

Comet Nucleus: The core of a comet, made up of ice, dirt and rock.

Comet Shoemaker-Levy 9 (SL-9): A comet that became gravitationally bound to Jupiter, colliding with the planet in July 1994. Prior to entering the planet’s atmosphere, the comet broke into several distinct pieces, each with a separate coma and tail.

Constellation: A geometric pattern of bright stars that appears grouped in the sky. Ancient observers named many constellations after gods, heroes, animals, and mythological beings. Leo (the Lion) is one example of the 88 constellations.

Corona (plural: Coronae): The outermost layer of the atmosphere of a star, including the Sun. The corona is visible during a solar eclipse or when special adapters or filters are attached to a telescope to block the light from the star’s central region. The gaseous corona extends millions of kilometres from the star’s surface and has a temperature in the millions of degrees.

Coronal Mass Ejection (CME): An outward eruption of billions of tonnes of material thrown into space from the Sun's corona. Caused by magnetic storms on the Sun. The most energetic of all solar explosions, they send material at high speed (200 - 400 km/s) into space, and have a great influence on the magnetosphere. They are most frequent around the time of solar maximum.

Cosmic Rays: High-energy atomic particles that travel through space at speeds close to the speed of light; also known as cosmic-ray particles.

Cosmology: The investigation of the origin, structure, and development of the universe, including how energy, forces, and matter interact on a cosmic scale.

Crater: A bowl-shaped depression caused by a comet or meteorite colliding with the surface of a planet, moon, or asteroid. On geologically active moons and planets (like Earth), craters can result from volcanic activity.

Dark Energy: A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.

Dark Matter: Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

Density: The ratio of the mass of an object to its volume.  d=m/V

Deep Space Network (DSN): NASA network of radio telescopes used to communicate with spacecraft operating far from the Earth. It includes three 70 m antennas located at Goldstone (California), near Madrid (Spain) and near Canberra (Australia).

Diffuse: Widely spread or scattered; not concentrated.

Doppler Shift: The change in observed frequency due to relative motion between source and observer.

Earth: The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kms (93 million miles) away from the Sun. Earth has one satellite — the Moon.

Electric Propulsion: Advanced propulsion system, as used on certain satellites (SMART-1), employing solar electric power to ionise and expel a propellant (such as Xenon gas) at high speed, causing it to move forwards. Compared with conventional chemical propulsion systems, electric propulsion requires less propellant to perform similar operations.

El Niño: Warm-water conditions off the western tropical coasts of the Americas, occurring irregularly but usually around Christmastime, caused by weakening trade winds and causing depleted fisheries, heavier-than-normal rain in the central and eastern Pacific and drought in the western Pacific.

Electromagnetic Radiation: A broad band of energy that consists of radio waves, microwaves, infrared rays, visible light, ultraviolet rays, extreme ultraviolet rays, X-rays, and gamma rays.

Electromagnetic Spectrum: The range of all wavelengths of radiation.

Electron: A very lightweight,  negatively charged part of an atom.

Element: A substance that is made of atoms with the same chemical properties, and which cannot be decomposed chemically into simpler substances.

Elliptical Orbit: An orbit which describes an ellipse or oval shape.

Emission: The release of electromagnetic radiation (e.g., light, X-rays, gamma rays) from excited atoms or molecules.

Escape Velocity: The minimum speed needed to escape the gravitational attraction of a celestial body and enter space. The Earth's escape velocity is 11.2 km/s.

Extinction: Dimming of light (attenuation) from a celestial body produced by the Earth’s atmosphere or by interstellar material.

Extrasolar Planet: A planet orbiting a star other than the sun.

Equinox: The date when the sun is directly overhead at the equator (0° latitude). On this day, at all places on the globe, night and day are of equal length (12 hours each). The spring or vernal equinox (for the northern hemisphere) occurs on about March 21. The fall or autumnal equinox (for the northern heisphere) occurs on or about September 23.

European Space Agency (ESA): An intergovernmental organisation with a mission to provide and promote - for exclusively peaceful purposes - the exploitation of space science, research & technology, and space applications. The 15 ESA Member States are: Austria, Belgium, Denmark, Finland, France, Germany, Ireland, Italy, Norway, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom.

Event Horizon: The spherical outer boundary of a black hole. Once matter crosses this threshold, the speed required for it to escape the black hole’s gravitational grip is greater than the speed of light.

Exobiology: The study of biological processes that have or could have evolved away from the Earth.

Extraterrestrial: An adjective that means “beyond the Earth.” The phrase “extraterrestrial life” refers to possible life on other planets.

Extravehicular: Outside the spacecraft; activity in space conducted by astronauts (EVA = extravehicular activity).

Fahrenheit: A temperature scale on which the freezing point of water is 32° F and the boiling point is 212° F.

Flyby Spacecraft: A spacecraft that travels past a celestial object. Frequently, such a spacecraft is unmanned and takes images of the object.

Galaxy: A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The Milky Way Galaxy contains our solar system. Galaxies are classified or grouped by their shape. Round or oval galaxies are elliptical galaxies and those showing a pinwheel structure are spiral galaxies. All others are called irregular because they do not resemble elliptical or spiral galaxies.

Gamma-ray Astronomy: Field of astronomy which studies very energetic processes. Gamma-ray astronomers observe the formation of new elements in space (nucleosynthesis), very dense objects such as neutron stars or black holes, active galaxies, supernovae and the most powerful explosions in the Universe. Because gamma rays are absorbed by the Earth's atmosphere, gamma-ray astronomy has to be done from space.

Gamma-Ray Burst (GRB): A brief, intense, and powerful burst of gamma rays, the highest-energy, shortest-wavelength radiation in the electromagnetic spectrum. These bursts emanate from distant sources outside our galaxy and last only a few seconds. They are the brightest and most energetic explosions known.

Gamma Rays: Light with the shortest wavelengths and the highest energies and frequencies in the electromagnetic spectrum; also called gamma radiation.

Ganymede: One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometres (3300 miles) wide and larger than the planet Mercury.

Gas Giant: A large planet with a small, rocky core and a deep atmosphere composed mostly of hydrogen and helium. Our solar system contains four gas giants: Jupiter, Saturn, Uranus, and Neptune. This group is also known as Jovian planets.

Gaseous Nebula: A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.

General Theory of Relativity: A theory Einstein developed to explain how gravity influences space and time. Its fundamental principle is the equivalence of gravitational and inertial forces. General relativity is a geometric theory which states that gravity causes space-time to curve. This curvature affects the motion of objects in space-time. General relativity explains the bending of light by massive objects or the nature of black holes.

Geocentric: An adjective meaning “centered on the Earth.” Most early civilizations had a geocentric view of the universe.

Geosynchronous Orbit: Also known as geostationary. An orbit in which an object circles the Earth once every 24 hours, moving at the same speed and direction as the planet’s rotation. The object remains nearly stationary above a particular point, as observed from Earth.

Giotto Mission: ESA's first deep space probe. Made close fly-bys of comets Halley (1986) and Grigg-Skjellerup (1992) and obtained the first close range pictures of a comet nucleus. Other experiments measured the magnetic field, charged particle distribution, dust and gas composition and concentration.

Globular Cluster: A collection of hundreds of thousands of old stars held together by gravity. Globular clusters are usually spherically shaped and are often found in the halos of galaxies. Each star belonging to a cluster revolves around the cluster’s common center of mass.

Gravitational Lens: A massive object that magnifies or distorts the light of objects lying behind it. For example, the powerful gravitational field of a massive cluster of galaxies can bend the light rays from more distant galaxies, just as a camera lens bends light to form a picture.

Gravity: A physical force that appears to exert a mutual attraction between all masses. It is proportional to the mass of the object. In Einstein's Theory of General Relativity, it is explained as a curvature of space-time.

Gravity Assist: An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a gravity assist from Earth to increase its velocity by about 36,000 km per hr (22,300 miles per hour).

Gravitational Wave: Ripple in the structure of space-time which may occur individually or as continuous radiation. According to Einstein's Theory of General Relativity, they are emitted when extremely massive objects experience sudden accelerations or changes of shape. They travel through space at the speed of light. Gravitational waves remain undetected.

Great Red Spot: A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of two to three Earths. Galileo first observed the spot in the 17th century.

Gyroscope: A disc with a heavy rim mounted in such a way that its axis of rotation can adopt any position. Once the disc is spinning, the rotation axis remains fixed with reference to fixed stars, which makes the gyroscope useful for determining movement away from a fixed course. Three gyroscopes rotating about perpendicular axes help to maintain the orientation of a spacecraft in space by detecting rotation about any of the axes and initiating a mechanism to correct it.

Habitable Zone: A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone would have a surface temperature slightly higher than the freezing point of water.

Hale-Bopp Comet: The brightest comet to appear in the night sky for many decades. Discovered by Alan Hale and Thomas Bopp on 22 July 1995, it reached perihelion on 1 April 1997 and was a naked eye object for many months. Its nucleus appears to be very large, about 40 km across.

Halley's Comet: The most famous periodic comet. Its aphelion is beyond the orbit of Neptune, but it returns to the inner Solar System every 76 years. Named after the 17th century British scientist, Edmond Halley, who first recognised its regular pattern of reappearances. Studied by a fleet of spacecraft during its 1986 apparition, including ESA's Giotto.

Heat-Sink: A region or structure that can absorb and hold large quantities of heat.

Heat Shield: A thick layer that protects from heat.

Heliocentric: With the Sun in the centre. Our Solar System is heliocentric.

Helioseismology: The study of the Sun's interior by measuring oscillations (ripples) as they appear at the surface. The oscillations are caused by sound waves that originate at different depths inside the Sun. By measuring their travel times and the distance they travel, scientists can study conditions in the Sun's interior.

Heliosphere: The volume of space around the Sun which contains the charged particles and magnetic field carried in the solar wind. Its outer boundary (which has never been crossed) is called the heliopause and marks the beginning of interstellar space. This is thought to be 50 - 100 AU from the Sun.

Humidity: The presence of water vapour in air.

Impact Crater: A large depression on a moon or a planet. An impact crater is created when an asteroid, a comet, or a meteorite strikes the moon or the planet with great force.

Impact Event: A collision between two solar system bodies that releases exceptionally large amounts of energy. Some examples are the 1908 Siberian Tunguska impact by a comet or an asteroid and the asteroid that struck Earth 65 million years ago, which may have led to the extinction of the dinosaurs and other species of the Cretaceous-Tertiary era.

Inflation: The theory that the universe expanded very rapidly shortly after the Big Bang.

Interplanetary Space: The region of space surrounding our Sun. Asteroids, comets, Earth, and the solar wind are examples of things occupying interplanetary space.

Interstellar: Located between or among the stars.

Interstellar Dust: Microscopic solid grains, composed largely of common elements such as graphite, silicates.

Interstellar Gas: Sparse gas in interstellar space.

Interstellar Medium (ISM): The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. For centuries, scientists believed that the space between the stars was empty. It wasn't until the eighteenth century, when William Herschel observed nebulous patches of sky through his telescope, that serious consideration was given to the notion that interstellar space was something to study. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.

Interstellar Reddening: The interstellar dust scatters blue light more effectively than red light - which means that most of the light that reaches us through the interstellar dust is reddish.

Io: The innermost of Jupiter’s four large moons. Due to Jupiter's gravitational might, Io is geologically active; its surface is peppered with volcanoes that send sulphurous eruptions into its thin atmosphere. Io appears to have the most active volcanoes in the solar system.

Io Plasma Torus: A bagel-shaped region of trapped sulphur ions around Jupiter that originates from the surface of Io, one of Jupiter’s moons. Gravitational tidal forces between Jupiter, other Galilean moons, and Io cause tidal friction in Io’s interior, producing geysers that spew sulphur at tremendous speeds. Some of the sulphur ions leave Io’s surface and become trapped around Jupiter.

Ion: An atom that has become electrically charged by the addition or loss of one or more electrons.

Ionization: The process by which ions are produced, typically by collisions with other atoms or electrons, or by absorption of electromagnetic radiation.

Ionized gas: A gas that has been ionized so that it contains free electrons and charged particles, a plasma if it is electrically neutral overall.

Ionosphere: A region of the Earth’s upper atmosphere where solar radiation ionizes the air molecules. This region affects the transmission of radio waves and extends from 50 to 400 km (30 to 250 miles) above the Earth's surface.

Jovian Atmosphere: The atmosphere surrounding the giant, massive planet Jupiter. The Jovian atmosphere is composed primarily of hydrogen (90 percent) and helium (10 percent). Other minor ingredients include water, hydrogen sulphide, methane, and ammonia.

Jovian Planets: The planets Jupiter, Saturn, Uranus, and Neptune. They are called Jovian planets because of similarities in their composition and location. This group is also known as the “giant planets,” the “gas planets” and, when grouped with the planet Pluto, the “outer planets.”

Jovian Winds: The hurricane-force, high-velocity motion of gas molecules in Jupiter’s atmosphere. The wind speed increases as one travels deeper into Jupiter’s atmosphere. The various patterns of atmospheric winds are easily identified in Jupiter’s upper cloud layer.

Jupiter: The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined. Jupiter is a gaseous planet with a very faint ring system. Four large moons and numerous smaller moons orbit the planet. Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years.

Kelvin Scale: The temperature scale most commonly used in science, on which absolute zero is the lowest possible value. On this scale, water freezes at 273 K and boils at 373 K.

Kepler’s Laws: Three laws, derived by 17th century German astronomer Johannes Kepler, that describe planetary motion.

Kepler’s First Law: The orbits of planets are ellipses, with the Sun at one focus. Therefore, each planet moves in an elliptical orbit around the Sun.

Kepler’s Second Law: An imaginary line connecting any planet to the Sun sweeps over equal areas in equal intervals of time.

Kepler’s Third Law: The square of any planet’s orbital period is proportional to the cube of its mean distance from the Sun.

Kourou: French-operated launch site in French Guiana. Location at 5° N latitude provides fuel-efficient, near equatorial launches.

Lagrange Points: Points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others' respective gravities balance. There are five points and are labelled L1 through L5. L1, L2 and L3 lie along the centreline between the centres of mass between the two masses; L1 is on the inward side of the secondary, L2 is on the outward side of the secondary; and L3 is on the outward side of the primary. L4 and L5, the so-called Trojan points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary.
L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/25.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.

Light Year: The distance that light can travel one year in a vacuum, which is about 5.8 trillion miles or 9.5 trillion kms.

Local Bubble: A hot, low density region containing the Sun and a few other nearby stars.

Local Group: A small cluster of more than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and the Milky Way galaxy.

Long-Period Comet: A comet having an orbital period greater than 200 years and usually moving in a highly elliptical, eccentric orbit. Comets have orbits that take them great distances from the Sun. Most long-period comets pass through the inner solar system only once. Hale-Bopp is an example of a long-period comet.

Luminosity: The amount of energy radiated into space every second by a celestial object, such as a star. It is closely related to the absolute brightness of a celestial object.

Lunar Eclipse: A darkening of the Moon, as viewed from Earth, caused when our planet passes between the Sun and the Moon.

Magellanic Clouds: The Magellanic Clouds are two dwarf irregular galaxies. Known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the galaxies are in the Local Group. The closer LMC is 168,000 light-years from Earth. Both galaxies can be observed with the naked eye in the southern night sky.

Magnetic Field: A region of space in which magnetic forces may be detected or may affect the motion of an electrically charged particle. As with gravity, magnetism has a long-range effect and magnetic fields are associated with many astronomical objects.

Main Sequence: The stable phase of a star's lifetime, when outward pressure from internal fusion process using hydrogen for fuel is balanced by the inward force of self-gravitation. This phase is usually the longest phase of a star's lifetime. Our Sun is a main sequence star.

Mars: The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and Phobos.

Marshall Space Flight Center (MSFC): NASA center overseeing the research, development, and implementation of three primary areas essential to space flight: reusable space transportation systems, generation and communication of new scientific knowledge, and management of all space lab activities. Located in Huntsville, Alabama, the center aided in the design, development, and construction of the Hubble Space Telescope.

Megaparsec (MPC): Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.

Mercury: The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water.

Microgravity: Microgravity is a term commonly applied to a condition of free-fall within a gravitational field in which the weight of an object is reduced compared to its weight at rest on Earth.

Meteor: A meteoroid which is in the process of entering Earth's atmosphere.  It is called a meteorite after landing.

Meteorite: The remains of a meteoroid that plunges to the Earth’s surface. A meteorite is a stony or metallic mass of matter that did not completely vaporize when it entered the Earth’s atmosphere.

Meteoroid: A small, solid object moving through space. A meteoroid produces a meteor when it enters the Earth’s atmosphere or another planet.

Micrometeoroid: A very small meteoroid with a diameter of less than a millimeter. Micrometeoroids form the bulk of the interplanetary solid matter scattered throughout the solar system.

Milky Way Galaxy: The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.

Molecule: A combination of two or more atoms bound together electrically; the smallest part of a compound that has the properties of that substance.

Molecular Cloud: A relatively dense, cold region of interstellar matter where hydrogen gas is primarily in molecular form. Stars generally form in molecular clouds. Molecular clouds appear as dark blotches in the sky because they block all the light behind them.

Moon: A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.

NASA (National Aeronautics And Space Administration: A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates space exploration efforts as well as traditional aeronautical research functions.

Naval Research Laboratory (NRL): The US Navy's corporate laboratory, which conducts a broadly-based multidisciplinary program of scientific research and advanced technological development directed toward maritime applications of new and improved materials, techniques, equipment, system, and ocean, atmospheric, and space sciences and related technologies.

Nebula: A cloud of gas and dust located between stars and/or surrounding stars. Nebulae are often places where stars form.

Neptune: The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.

Neutrino: A neutral, weakly interacting elementary particle having a very tiny mass. Stars like the Sun produce more than 200 trillion trillion trillion neutrinos every second. Neutrinos from the Sun interact so weakly with other matter that they pass straight through the Earth as if it weren’t there.

Neutron Star: An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 km or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.

Newtonian Reflector: A type of reflecting telescope whose eyepiece is located along the side of the telescope. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light at a right angle through the side of the telescope to the eyepiece.

NOAA: National Oceanic and Atmospheric Administration.

Orion Nebula: A hot cloud of ionized gas that is a nearby region of recent star formation, located in the sword of the constellation of Orion.

Oort Cloud: Swarm of billions of comets thought to surround the Solar System between 2000 and 20 000 AU from the Sun. First proposed by E. Öpik in 1932 and later developed by J. Oort in the 1950s. Its existence is based on studies of long-period comet orbits, which seem to have their aphelia in this zone.

Orbit: The act of travelling around a celestial body; or the path followed by an object moving around a celestial body. For example, the planets travel around, or orbit, the Sun because the Sun’s gravity keeps them in their paths, or orbits.

Orbital Decay: Reduction in the size (and period) of a satellite's orbit due to gravitational effects or aerodynamic drag, eventually causing it to re-enter the atmosphere.

Orbit Maintenance: Keeping a spacecraft's orbit within a given set of orbital parameters, the reference orbit, over time. See orbit maintenance maneuvers, orbital decay.

Orbit Maintenance Manoeuvres: Changes in a spacecraft's velocity, usually through thruster firings, designed for orbital maintenance.

Orbital Plane: The plane, passing through a planet's centre of gravity, in which a satellite orbits.

Parsec: A unit of distance equal to 3.26 light years; 3.09 x 10¹³ km or 1.92 x 10¹³ miles.  A parsec is the distance at which the radius of the Earth’s orbit subtends an angle of one second of arc.

Periodic Comet: A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.

Phases: Regularly occurring changes in the appearance of the Moon or a planet. Phases of the Moon include new, full, crescent, first quarter, gibbous, and third quarter.

Planetesimal: A small body of rock and/or ice, under 10 kms (6 miles) across, formed during the early stages of the solar system. Planetesimals are the building blocks of planets, but many never combined to form large bodies. Asteroids are one example of planetesimals.

Plasma: A substance composed of charged particles, like ions and electrons, and possibly some neutral particles. Our Sun is made of plasma. Overall, the charge of a plasma is electrically neutral. Plasma is regarded as an additional state of matter because its properties are different from those of solids, liquids, and normal gases.

Plasma: A gas consisting of equal numbers of ionized atoms and electrons. A fourth state of matter - not a solid, liquid, or gas.  In a plasma, the electrons are pulled free from the atoms and can move independently. The individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality.

Pluto: A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune’s orbit where Pluto resides. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. Pluto’s orbit is more elliptical than those of the eight solar system planets.

Protoplanet: A small body that attracts gas and dust as it orbits a young star. Eventually, it may form a planetary body.

Protostar: A collection of interstellar gas and dust whose gravitational pull is causing it to collapse on itself and form a star.

Quasar: Abbreviation of "quasi-stellar object". Quasars are unusually energetic objects which emit up to 1,000 times as much energy as an entire galaxy, but from a volume about the size of our solar system.

Radar: A system using beamed and reflected radio-frequency radiation, usually microwave, to detect objects and measure ranges.

Spectrometer: An instrument for examining recorded and measured electromagnetic spectra.

Scattering: The change in the paths of photons without absorption or change in wavelength.

Solar Cycle: The periodic changing of the Sun’s magnetic field, which determines the number of sunspots and the amount of particles emitted in the solar wind. The period of the cycle is about 11 years.

Solar Eclipse: A phenomenon in which the Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse occurs when the Moon completely obscures the Sun’s disk, leaving only the solar corona visible. A solar eclipse can only occur during a new phase of the Moon.

Solar Maximum: The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is highest.

Solar Minimum: The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.

Solar System: The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational influence.

Solar Wind: Streams of charged particles flowing from the Sun at millions of kms an hour. The composition of this high-speed solar wind may vary, but it always streams away from the Sun. The solar wind is responsible for the Northern and Southern Lights on Earth and causes the tails of comets to point away from the Sun.

Solstice: The date when the sun reaches its highest (summer solstice) or lowest (winter solstice) noon-time point in the sky. On the summer solstice in the northern hemispheres (on or about June 22), the subsolar point is 23.5°N (Tropic of Cancer). On the summer solstice in the southern hemisphere (on or about December 22) the subsolar point is 23.5°S (Tropic of Capricorn).

Space Shuttle: A reusable U.S. spacecraft operated by astronauts and used to transport cargo, such as satellites, into space. The spacecraft uses rockets to launch into space, but it lands like an airplane.

Specific Impulse: A measurement of a rocket's relative performance. Expressed in seconds, the number of which a rocket can produce one pound of thrust from one pound of fuel. The higher the specific impulse, the less fuel required to produce a given amount of thrust.

Star Tracker: A device that continuously and automatically records the position and heading of selected stars for spacecraft attitude control.

Sunspot: A region on the Sun’s photosphere that is cooler and darker than the surrounding material. Sunspots often appear in pairs or groups with specific magnetic polarities that indicate electromagnetic origins.

Supernova: The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months. A supernova remnant is the glowing, expanding gaseous remains of a supernova explosion.

Supernova Remnant: The remains of a gigantic explosion marking the end of the life of either a massive star or a white dwarf accumulating mass from a neighbour.

Telescope: An instrument used to observe distant objects by collecting and focusing their electromagnetic radiation. Telescopes are usually designed to collect light in a specific wavelength range. Examples include optical telescopes that observe visible light and radio telescopes that detect radio waves.

Temperature: A measure of the average of random speeds of the microscopic particles in matter.

Terrestrial Planets: The four planets of the inner solar system (Mercury, Venus, Earth, and Mars) are called terrestrial planets because they are made up mostly of rock.

Thruster: A control jet in a spacecraft, usually using a monopropellant like hydrogen peroxide or hydrazine.

Topography: The shape of a surface, including its relief and the relative position of features.

Triton: The largest of Neptune’s satellites. Triton has an atmosphere and is roughly the size of Earth's moon. It has an “ice cap” of frozen nitrogen and methane with “ice volcanoes” that erupt liquid nitrogen, dust, and methane compounds from beneath its frozen surface.

Ultraviolet: A type of electromagnetic radiation that is invisible and beyond the violet part of the visible spectrum of light. It has shorter wavelengths (100 – 4000 Å) and carries higher energy than visible light.

Universe: The totality of space and time, along with all the matter and energy in it. Current theories assert that the universe is expanding and that all its matter and energy was created during the Big Bang.

Uranus: The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth’s distance from the Sun and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant outer planet has a visible ring system and over 20 moons, the largest of which is Titania. Uranus is tipped on its side, with a rotation axis in nearly the same plane as its orbit.

Van Allen Belt: A region containing charged particles trapped in the Earth’s magnetic force field (magnetosphere). The belt’s lower boundary begins at about 800 kms (496 miles) above the Earth’s surface and extends thousands of kms into space.

Variable Star: A star whose luminosity (brightness) changes with time.

Vandenberg: USAF-operated launch site on the southern California coast. Location at 121° W longitude provides easy polar and high-latitude launches over open ocean.

Venus: An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produce a “greenhouse effect” on the planet. Venus’s surface temperature is roughly 480° C (900° F), making it the hottest planet in the solar system.

X-Rays: Electromagnetic radiation with very short wavelengths and very high energies and frequencies. X-rays fall between gamma rays and ultraviolet radiation; also called X-radiation or Roentgen ray.

X-ray Telescope: A special telescope used to detect X-rays – high-energy electromagnetic radiation. The high energy of X-rays means they will go through rather than bounce off a “normal” telescope mirror. Instead, the mirrors are arranged so the X-rays skip across them much like a stone skips across the surface of a lake.
 

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