COMET GLOSSARY

 

Apparent Motion A change in orientation between two objects which is superficially explainable in more than one way. For example, the apparent motion of the Sun rising and setting in the sky may appear to be the result of the Sun moving around the Earth, when in fact it is the result of the Earth's rotation around its axis.
   
Aerogel A nearly transparent, very lightweight material made primarily from silica. Dubbed a "Super Material", aerogel is the world's lightest solid, weighing as little as three times that of air, and exhibiting superb insulating properties.
   
Albedo A measurement of the amount of light an object reflects, expressed in fraction or percentage of the total light striking the object.
   
Asteroid A solid chunk of rock and iron, containing little or no ice, in orbit around the sun. An asteroid is often potato-shaped, and may be as small as a building, or almost as large as a small planet. Sometimes called a minor planet. Some asteroids may be former comets that have lost all of their volatile ices.
   
Astronomical Unit The average distance between the Earth and the Sun, about 93 million miles. The AU is used by astronomers as a standard unit of measurement to describe distances between objects in space.
   
Atmosphere An envelope of gases that surrounds a planet. The earth’s atmosphere provides air to breath, changing weather patterns, and protection from harmful solar radiation. Many other planets in the solar system have atmospheres as well, but only earth’s is capable of supporting human life.
Axis An imaginary line around which a planet, moon, comet, or asteroid spins, or rotates. The axis of rotation passes through the north and south poles of the object, and helps scientists visualize its orientation in space.
   
Chlorophyll A green pigment found in plants which traps light energy, allowing it to be converted into chemical energy.
   
CHON A nickname for the group of elements consisting of Carbon, Hydrogen, Oxygen, and Nitrogen, believed to be the most fundamental ingredients for organic life. Comets may have delivered the CHON elements to Earth very early in its history.
   
Chromatography The science of analyzing chemical compounds to determine their individual components.
   
Circuit  A closed path through which electricity may flow.
   
Coma The sparse, yet highly reflective gas cloud which forms around a comet nucleus as the solar wind sublimates its volatile ices, fully enveloping the solid nucleus.
   
Comet A solid clump of ice, rock, and dust, which follows a highly eccentric, elliptical orbit around the sun. The comet's solid core, its nucleus, is often potato-shaped, and may be as small as a building, or as large as a moon. A comet may develop a coma (gaseous cloud) and tail (extension of gas and dust) if it passes through the inner solar system, closer to the Sun's heat. The comet's size is further extended by an enormous hydrogen cloud that surrounds the coma.
   
Consumer A member of the food chain that relies on producers as their primary food source.
   
Controlled Environment  A system which takes advantage of technology to create safe living conditions artificially.
   
Day The approximate time it takes an object (such as the Earth) to spin once on its axis.
   
Declination

 

 One element of the astronomical coordinate system used to locate objects in the sky. Declination is essentially Earth's latitudes projected onto the sky; i.e., it is the "up and down" piece of information necessary to locate an object in the sky. Declination is usually expressed in degrees, minutes, and seconds of arc north (+) and south (-) of the celestial equator. Identified as the "Y" axis. See right ascension.
   
Density A measurement of  an object's mass per unit of volume.
   
Earth Year The time it takes Earth to complete one orbit around the Sun, or about 365¼ days.
   
Eccentricity The degree to which the shape of an ellipse deviates from that of a perfectly round circle. A circle is a special case of an ellipse, with an eccentricity of  0 (zero). A straight line is a special case of an ellipse, with an eccentricity of 1.0 (one).
   
Ecliptic Plane The plane, or imaginary disk, in space which represents Earth's orbit around the Sun.

The 23.5° difference between the ecliptic plane and the plane of Earth's equator projected onto the sky (the celestial equator) is the primary cause of seasonal climate changes on Earth.

 
   
Electric Current The flow of electrons through a conductor. Electrons, the most common charge carriers, are negatively charged. They flow from relatively negative points to relatively positive points.
   
Electron Electrons are a basic constituent of the atom; they are distributed around the nucleus in shells, and are responsible for the chemical properties of the atom. Electrons flowing in a conductor constitute an electric current.
   
Ellipse A closed, symmetrically curved shape drawn using two "center points", or foci, instead of the one foci used to produce a perfect circle. The greater the separation between foci, the greater the eccentricity of the ellipse. One of four conic sections that includes the circle, the ellipse, the parabola, and the hyperbola. Elliptical: having the shape of an ellipse.
   
Energy The capacity of a body or system to do work.
   
Escape Velocity The speed at which an object must travel in order to break free from the gravitational clutches of a star, planet, moon, etc.
   
Evaporate To change from a liquid state to a gas state.
   
Freeze To change from a liquid state to a solid state.
   
Gas A state of matter in which molecules are so loosely bound that its volume will increase or decrease depending on the size and shape of its container.
   
Geostationary Orbit An orbital state in which a satellite maintains a fixed position over a specific geographical reference point. The speed of the satellite's progress around the Earth is adjusted until it completes one full rotation around the Earth's axis in exactly the same time the Earth itself takes to rotate around its axis. Also called a synchronous orbit.
   
Gravity An attracting force that all matter possesses. Gravity causes all objects to attract all other objects. The larger and more solid an object is, the greater the power of its gravity to attract other objects toward itself. Gravity compresses the planets and stars into spheres, and controls the motions of all objects in the solar system and throughout the universe.
   
Hydrogen Cloud A sparse cloud of hydrogen atoms that surrounds the coma of a comet, often extending millions of miles into space.
   
Indicator A chemical agent used to ascertain the presence, or degree of presence, of a particular chemical property. Example: Ph indicator reveals whether a solution is basic or acidic by means of a range of color change.
   
Ion An atom or group of atoms that has either lost one or more electrons making it positively charged (a cation), or gained one or more electrons making it negatively charged (an anion).
   
Kuiper Belt A band of comet nuclei in the area of Pluto’s orbit. Many astronomers believe that Pluto itself is actually a very large comet nucleus.
   
Liquid A state of matter in between gas and solid. Although a liquid's molecules can assume the shape of a container, they are too tightly bound to change in volume. They are not bound tightly enough to retain a three-dimensional shape, however.
   
Magnitude A measurement of the amount of light received from a star or other luminous object. The brightest star in our sky, the Sun, has a magnitude of -26.7; Polaris, the North Star, has a magnitude of +2.0.
   
Mass A measure of the total amount of matter, or "stuff", in an object. An object's mass is the same on the moon as it is on earth, for example, although its weight on the moon is only a sixth of its earth weight.
   
Matter A physical substance, having mass and occupying space. At its most basic level matter consists of fundamental particles such as electrons and quarks. Quarks are the fundamental building blocks of protons and neutrons which are the components of atomic nuclei.
   
Melt To change from a solid state to a liquid state.
   
Meteoroid A small fragment of rock and/or iron, from a comet or asteroid, in orbit around the sun. A meteoroid could be as small as a grain of sand or as large as a building.
   
Meteor A small fragment from a comet or asteroid that passes through the earth’s atmosphere, creating a streak of bright light (sometimes called a “shooting star”) as it burns up due to friction with the air.
   
Meteorite A small fragment from a comet or asteroid that has survived its entry into earth’s atmosphere as a meteor and landed safely on the earth's surface.
   
Microgravity A term used to describe the apparent weightlessness experienced in earth orbit. The condition results from the spacecraft's state of freefall around the planet, and not the absence of gravitational attraction. The prefix "micro" reminds scientists that small accelerations are occurring all the time, and any implication that gravity can be escaped at common orbital altitudes of two to four-hundred miles is absurd and misleading. See "Gravity: It's the Law!" for more information.
   
Minor Planet Another name for asteroid.
   
Month The approximate time it takes the Moon to complete one orbit around the Earth.
   
Moon The natural satellite of a planet.
   
Nucleus The solid core of a comet, consisting of a mixture of various ices and dust. Inside of Jupiter's orbit, the ice within the nucleus may begin to sublimate, forming a coma, tails, and a hydrogen cloud.
   
Observable Can be detected by the senses, with or without the aid of instruments such as microscopes, telescopes, Geiger counters, ohmmeters, spectrometers, etc.
   
Oort Cloud An envelope of cometary nuclei that surrounds the entire solar system. Comets may not have originated in the Oort cloud, but rather may have been gravitationally “flung” there following close encounters with the sun, Jupiter, or even nearby stars.
   
Orbit The path in space described by a satellite rotating around a larger body, where the motion of the orbiting body is dominated by their mutual gravitational attraction.
   
Organic Compound Class of complex carbon-based compounds found in living organisms: carbohydrates, lipids, proteins, and nucleic acids.
   
Paper Chromatography The withdrawal of a substance from a mixture onto a special filter paper.
   
Photosynthesis Chemical reaction used by producers, such as green plants, to produce food. Light energy is used to produce chemical energy, converting carbon dioxide and water into sugar and oxygen.
   
Photodissociation The decomposition of molecules into atomic or molecular fragments as a result of absorbing light.
   
Planet A large, usually spherical object in orbit around a star. Some planets are rocky, like the Earth. Others are gaseous, like Jupiter. Asteroids also orbit a star, but are too small to be classified as a planet. There is currently no widely accepted definition of what makes a planet a planet.
   
Plasma Matter containing more or less equal amounts of positively charged ions and negatively charged electrons. It is usually extremely hot and found in or near stars.
   
Probe An unmanned spacecraft which transports scientific instruments into space, returns its data via telemetry, and is not retrieved after its mission is complete.
   
Producers Any of various organisms (such as green plants) which produce their own organic compounds from simple precursors (such as carbon dioxide and inorganic nitrogen), many of which are food sources for other organisms.
   
Revolve To orbit. All objects in our solar system revolve around, or orbit, the Sun. A moon revolves around, or orbits, a planet. A planet and its moon, in turn, revolve, or orbit, around the Sun together.
   
Right Ascension

 

 One element of the astronomical coordinate system used to locate objects in the sky. It is essentially Earth's longitudes projected onto the sky; i.e. it is the "left and right" piece of information necessary to locate an object in the sky. RA is usually expressed in hours, minutes, and seconds. Identified as the "X" axis.
   
Rotate To spin. A planet rotates, or spins, on its axis.
   
Satellite Any object that orbits or revolves around another object. A planet's moons are natural satellites; artificial satellites are often placed in orbit to collect data, assist in communications, etc.
   
Sidereal Year The time required for an object to complete a single orbit around another object, as measured against the background stars, equal to 365.2564 mean solar days.
   
Small Body A category of objects in the solar system which includes asteroids, comets, and meteoroids.
   
Solar Relating to the Sun.
   
Solar System The Sun, the nine major planets and their satellites, all comets, asteroids, and meteroids, and any other objects held in orbit around the Sun by gravitational attraction.
   
Solar Wind Stream of plasma, mainly electrons and protons, which flows from the Sun's corona at up to 900 km/s. It is found throughout the Solar System as far away as the heliopause. See also Heliosphere.
   
Solid A state of matter in which a three-dimensional regularity of structure resulting from the close proximity of atoms or molecules exists.
   
Space  
   
Spacecraft A vehicle for transporting people or things into space.
   
Star A self-luminous sphere of gas. 
   
State of Matter One of three physical states in which matter can exist, i.e. solid, liquid, or gas. Plasma is sometimes considered a fourth state of matter.
   
Sublimate To change directly from a solid state to a gas state, or visa versa.
   
Sun The star at the center of our solar system. The Sun contains over 99% of all mass in our solar system.
   
Switch A device which can either break or complete an electrical circuit, thereby preventing or allowing the flow of electricity as needed.
   
Synodic Period The time between occurrences of a particular configuration of a planet or planets, as seen from the Earth.
   
System An organized group of related objects or components that form a whole; a group of two or more objects that work together. A change in one or more objects may effect the system itself.
   
Tail (comet) Streaming particles of gas and dust pushed away from the coma of a comet by the solar wind.
Terminator The edge of the shadow that moves slowly across the earth's or any other object's surface as it spins on its axis. The shadow is produced as light from the Sun is blocked on the side of the earth facing away the Sun.
   
Triangulation A method used to determine the location of an unknown point, in which lines are drawn to three different reference points. The point at which the three lines intersect marks the unknown location.
   
Volume The amount of space something takes up.
   
Volatile That quality of a substance which allows it to vaporize easily. The molecules which make up volatile substances do not exert strong attractive forces upon one another, allowing evaporation or sublimation to occur at relatively low temperatures.
   
Weight The degree of force with which the earth's gravity pulls an object towards its center. An object's weight is different from planet to planet, although its mass remains the same.
   
Weightlessness The absence of any observable effects of gravity. Scientists prefer the term microgravity, because no object is totally immune to gravitational influence.
   
Year The time it takes an object (such as the Earth) to complete one orbit around its Sun, with respect to the background stars. See Sidereal Period.
   
 

 

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