It may be an asteroid or it may be a comet,  but they are all Rocks from Space and some day one of those rocks WILL be too close to Earth!


– a –

  • ABLATION: Removal of material by heating and vaporization as a meteorite passes through Earth’s atmosphere.


  • ACHONDRITE: A class of stony meteorite formed by an igneous process; the lack of chondrules.

  • ALBEDO: the fraction of incident radiation, light, that is reflected by a surface. 1.0 = white, 0.0 = black

  • ALBEDO FEATURE: A dark or light marking on the surface of an object that may not be a geological or topographical feature.

  • ALLOCTHONOUS: Material that is formed or introduced from somewhere other than the place it is presently found. In impact cratering this may refer to the fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets it’s outer flanks after the impact event.

  • AMOR ASTEROIDS: A ‘Near Earth Asteroid’ which has a perihelion distance just beyond Earth orbit. 1.017 AU and 1.3 AU. Designation List

  • ANTIPODAL POINT: the point that is directly on the opposite side of the planet.

  • APHELION: the point in its orbit where a planet is farthest from the Sun.

  • APOLLO ASTEROIDS: A ‘Near Earth Asteroid’ which has a semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU. Designation List

  • ASTEROID BELT: Between the orbits of Mars and Jupiter where most asteroids are located.

  • ASTEROID FAMILIES: Apollo, Trojen, Aten, Amor, Hirayama

  • ASTEROID NUMBER: asteroids are assigned a serial number when they are discovered.

  • ASTEROIDS are small, mostly rocky bodies orbiting the Sun. Asteroids range in size from 1000 kilometers in orbit the Sun between Mars and Jupiter and are the source of most meteorites. They are classified into a number of types according to their spectra (and hence their chemical composition) and albedo (There are actually several classification schemes in use today.) There are also a dozen or so other rare types.

      • C-type, includes more than 75% of known  asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical   composition as the Sun minus hydrogen, helium, and  other volatiles; 

      • S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron and magnesium silicates; 

      • M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron. 


  • ASTROBIOLOGY: Study of the origin, distribution, and destiny of life in the universe.

  • ASTROBLEME: literally means “star wound” and refers to an ancient, eroded, meteoritic crater.

  • ASTRONOMICAL UNIT: (AU)  149,597,870.691 km; the average distance from the Earth to the Sun. 

  • ATEN :  asteroids that are always closer to the Sun than the Earth is; they have a period shorter than 1 year (the semi-major axis is smaller than Earth’s). Designation List

– b –

  • BARRINGER CRATER: also known as Meteor Crater, Arizona, USA.

  • BASALT: common volcanic igneous rock

  • BOLIDE: a fireball that produces a sonic boom

  • BRECCIA: Rock consisting of angular, coarse fragments embedded in a fine-grained matrix. 

– c –

  • CALDERA: a crater formed by an explosion or by a collapsed volcanic vent

  • CARBON: An element with atomic number 6; symbol: C. Carbon is one of the four elements essential for life. (The others are hydrogen, oxygen, and nitrogen.)

  • CARBONATE: A compound containing carbon and oxygen.

  • CATACLASTIC: A texture found in metamorphic rocks in which brittle minerals have been broken, crushed, and flattened during shearing.

  • CATENA:   A chain of craters. 

  • CAVUS:   Hollow, irregular depression. 

  • CENTAURS : A diverse group of rocks in the outer solar system that displays episodic cometary behavior and eccentric orbits. Centaurs are objects that probably came from the Kuiper belt.(also a class of rocket). The plot of the outer solar system.

  • CENTRAL PEAK: The exposed core of uplifted rocks in complex meteorite impact craters. The central peak material typically shows evidence of intense fracturing, faulting, and shock metamorphism.

  • CERES: The largest and first discovered (1801) asteroid in the Asteroid Belt.

  • CHAOS: distinctive area of broken terrain. 

  • CHICXULUB CRATER: The submerged crater at the tip of the Yucatán Peninsula is an impact crater that dates from 65 million years ago. It is 120 miles wide and 1 mile deep. It is probably the site of the K-T meteorite or comet impact that caused the extinction of the dinosaurs and other groups of organisms.

  • CHONDRITE: A common type of meteorite that contains chondrules (see definition below). Chondrites come from asteroids that did not melt when formed and are designated as H, L, LL, E, or C depending on chemical compositions. The H, L, and LL types are called ordinary chondrites. The L chondrites are composed of silicate minerals (mostly olivine and pyroxene, but feldspar as well), metallic nickel-iron, and iron sulfide (called troilite). Most L chondrites are severely shocked-damaged, probably by a large impact on the asteroid in which they formed. The C (or carbonaceous) chondrites are the most primitive meteorites. They contain water-bearing minerals and carbon Compounds including a variety of organic molecules such as amino acids.

  • CHONDRULE : Roughly spherical objects found in a type of meteorite called chondrites. Most chondrules are 0.5 to 2 millimeters in size and are composed of olivine and pyroxene, with smaller amounts of glass and iron-nickel metal. The shapes of the mineral grains in them indicate that chondrules were once molten droplets floating freely in space.

  • COMA: The dust and gas surrounding an active comet’s nucleus

  • COMET: A medium-sized icy object orbiting the Sun; smaller than a planet.  It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, CO2, and other gasses) and a tail (dust and ionized gasses). Because of the force of the solar wind its long tail of gas and dust always points away from the sun,  The tail can be up to 250 million km long. Comets are only visible when they’re near the sun in their highly eccentric orbits. 

  • COMMENSURATE ORBITS: Asteroid orbits whose periods are simple multiples or fractions of Jupiter’s orbital period.

  • COMPLEX IMPACT CRATER:   A large crater with a single or many peaks in the middle of the crater. Examples are found on the Moon and on Earth. 

  • COSMIC DUST: Microscopic silicate or iron particles. IPD’s (Interplanetary dust particles) can collect in buried mud, the atmosphere, or in space. Smaller than the dust-like particles we see burning up during a meteor shower. Cosmic Dust originates withing comets and asteroids, and perhaps in the solar nebula. The dust may be carbon-rich, and may have provided an source of organic material for early Earth.

  • COSMIC RAY EXPOSURE AGE: An age determined by the presence of specific isotopes produced by cosmic ray bombardment.

  • COSMIC VELOCITY: The velocity of an orbiting body in space.

  • CRATER: Bowl shaped depression caused by impact or a depression around a volcanic vent.

  • CRATER RAYS:   Lines of ejecta radiating from a crater.

  • CRATON : The stable portions of continents composed of shield areas and platform sediments. Typically cratons are bounded by tectonically active regions characterized by uplift, faulting, and volcanic activity.

  • CRETACEOUS PERIOD: A geological term denoting the interval of Earth history beginning around 144 million years ago and ending 66 million years ago with the Chixalub impact and the demise of the age of dinosaurs.

  • CRYSTALLINE : – Rock types made up of crystals or crystal fragments, such as metamorphic rocks that recrystallized in high-temperature or pressure environments, or igneous rocks that formed from cooling of a melt. Crystallization; the formation of minerals with an ordered atomic crystalline structure.

  • CRYSTAL LATTICE: An orderly arrangement of atoms in a mineral.

– d –

  • DENSITY: measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3. 

  • DIAPLECTIC GLASS: A natural glass formed by shock pressure from any of several minerals without melting. It is found only in association with meteorite impact craters.

  • DIFFERENTIATION: Chemical zonation caused by differences in the densities of minerals; heavy materials sink, less dense materials float. 

  • DINOSAURS: large reptiles that lived until 65 million years ago. Most probably wiped out by an impact. 

  • DISASTER: From the greek “dis” and “aster”, the opposite of a sprinkle of light (stars).

  • DISTRIBUTION ELLIPSE: An area over several kms in which meteorites tend to fall with the more massive fragments at one end.

– e –

  • EARTH GRAZER: A meteoroid (or other space debris) that enters the Earth’s atmosphere and disintegrates, traveling nearly parallel to Earth’s surface

  • ECCENTRICITY: The astronomical measurement describing an orbit’s deviation from the circular.

  • EJECTA: Material thrown out from and deposited around an impact crater.

  • ELLIPSE: oval. the orbits of the planets are ellipses, not circles.

  • ENERGY: The capacity for doing work. Energy can change from one form (heat, chemical, nuclear, potential energy) into another but is always conserved.

  • EON: Two or more geological Eras form an Eon, which is the largest division of geological time, lasting hundreds of millions of years.

  • EPOCH: A division of a geologic period; it is the smallest division of geologic time, lasting several million years.

  • EXPONENTIAL NOTATION: “1.23e4” means “1.23 times 10 to the fourth power” or 12,300; “5.67e-8” means “5.67 divided by 10 to the eighth power” or 0.0000000567.

– f –

  • FACULA: Bright spot. For example, the Memphis Facula around bright region of Ganymede, a moon of Jupiter.

  • FALLS: Rare, a rock found as the result of an ‘observed’ meteorite impact.

  • FINDS : The majority of meteorites are recorded as finds, those specimens which were not observed to fall.

  • FIREBALL: a meteor brighter than magnitude -3 caused by millimeter-sized (or bigger) meteoroids disintegrating in the atmosphere.

  • FISSURE: a narrow opening or crack of considerable length and depth.

  • FRACTIONATION: Fractional crystallization: a process in which minerals crystallize out of a magma at specific temperatures, thereby changing the composition of the magma.



– g –

  • GEOCENTRIC DISTANCE: The distance from Earth.

  • GEOLOGIST: Scientist who studies Earth, its materials, the physical and chemical changes that occur on the surface and in the interior, and the history of the planet and its life forms. Planetary geologists extend their studies to the Moon, planets, and other solid bodies in the Solar System.

  • GEGENSCHEIN: Faint, diffuse, glowing region on the ecliptic opposite of the Sun produced by IDP’s (interplanetary dust particles)

  • GIANT IMPACT THEORY: An explanation for the origin of the Moon from Earth debris which collected in space after a projectile the size of planet Mars smashed into a growing Earth.

– h –

  • HEXAHEDRITE: An iron meteorite containing less than 5 percent nickel.

  • HIGH-PRESSURE MINERAL PHASES: Mineral forms that are stable only at the extremely high pressures typical of Earth’s deep interior but not its surface. Such pressures are generated instantaneously during meteorite impact. For example, stishovite is the high-pressure polymorph of quartz, a common crustal mineral.

  • HIRAYAMA ASTEROIDS: A asteroids which travel in a cluster along the same orbit.

– i –

  • ICE: In planetary science, terms refers to water, methane, and ammonia which usually occur as solids in the outer solar system. 

  • IGNEOUS ROCK: When molten rock cools, igneous rock is formed.

  • IMPACT BASIN: An impact crater that has a rim diameter greater than 185 miles (300 km). There are over 40 impact basins on the Moon. These catastrophic impacts produce faulting and other crust deformations.

  • IMPACT CRATER: What remains of collisions between an asteroid or meteorite and a planet or moon.

  • IMPACTITES:  A collective term for all rocks being affected by impact as the result of a collision with another planetary body. 

  • IMPACT MELT: Rocks melted during impact.  They are extremely uniform in their composition but variable in their texture. They are composed predominantly of the target rocks but may contain a small amount of the impactor.

  • IMPACT SHOCK: Rocks shocked during impact. Usually seen in quartz minerals.

  • INCLINATION: the angle between the plane of its orbit and the ecliptic.

  • IRON METEORITE: An iron meteorite is a meteor made of the metal iron that has fallen to Earth.

– k –

  • KILOGRAM: (kg) = 1000 grams = 2.2 pounds, the mass of a liter of water. 

  • KILOMETER: kilometer (km) = 1000 meters = 0.62 miles.

  • KINETIC ENERGY: Kinetic energy is the energy that an object has because of its motion. An object’s kinetic energy is equal to 0.5 times its mass times its velocity squared. In the metric system, kinetic energy is measured in joules or kg-m2/s2.

  • KIRKWOOD GAPS: relatively empty regions between the main concentrations of asteroids in the Main Asteroid Belt

  • KUIPER BELT OBJECTS: a disk-shaped region past the orbit of Neptune containing at least 70,000 small icy bodies. It is now considered to be the source of the short-period comets. The Kuiper belt was named after the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951.

  • K-T BOUNDARY: The boundary of the Cretaceous-Tertiary eras which marks the end of the age of dinosaurs and the beginning of the age of mammals.

– l –

  • LIGHT-YEAR: = 9.46053e12 km (= 5,880,000,000,000 miles = 63,239 AU); the distance traveled by light in a year. 

  • LIMB: the outer edge of the apparent disk of a celestial body

  • LONG PERIOD COMET (LPC): are those comets with a period greater than 200 years. 

– m –

  • MACULA: dark spot. 

  • MAGMA: Molten rock containing dissolved minerals and gasses which crystallize out to form igneous rock.

  • MANTLE: A zone in a differentiated parent body between the core and the crust.

  • MAGNITUDE: The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude. 

  • MAIN BELT: located between Mars and Jupiter roughly 2 – 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroids in the group). 

  • MARIA: Large impact basins on the Moon filled with basalts.

  • METEOR: Also “shooting star.” The light from an object, as small as a grain of dust, as it burns through Earth’s atmosphere. Larger rocks, that give off more light, are known as bolides or fireballs.

  • METEORITE: A rock of extra-terrestrial origin found on Earth 

  • METEOROID: A small rocky object orbiting the Sun; smaller than an asteroid 

  • MINOR PLANETS: A term used for asteroids.

– n –

  • NEAs: (Near Earth Asteroids) are asteroids which have orbits that bring them within 121 million miles or 195 million kilometers (1.3 A.U.) of the Sun. There are over 250 near-Earth asteroids known and they are classified into three groups: Atens, Apollos, and Amors. NEA’s are the only asteroids that can crash into our planet.

  • NEAT: (Near-Earth Asteroid Tracking) is a NASA/JPL system that tracks near-earth asteroids using the 1.2-meter- diameter (48-inch) Palomar telescope to track asteroids that come close to the Earth.

  • NECs: (Near Earth Comets) Short-period comets (period less than 200 years) with orbits that bring them within 121 million miles or 195 million kilometers (1.3 A.U.) of the Sun.

  • NEOs:   (Near Earth Objects) are comets and asteroids that have ventured close to the Earth’s orbit.

– o –

  • OLD:   a planetary surface that has been modified little since its formation typically featuring large numbers of impact craters

  • OORT CLOUD: the spherical cloud of rock and debris, about six trillion icy objects, surrounding our planetary system which extends approximately 3 light years, approximately 30 trillion, kilometers from the Sun.

  • OVOID: resembling an egg in shape

– p –

  • PARSEC: = 206265 AU = 3.26 light year

  • PEAK RING: Central uplift characterized by a ring of peaks rather than a single peak. Peak rings are typical of larger terrestrial craters above about 50 km in diameter.

  • PERIHELION: The point in the path of a celestial body that is nearest to the sun.

  • PERIOD: The time it takes a rock to orbit the sun. For example: long period comets >200 years and short period comets < 200 years.

  • PERTURB: To cause a planet or satellite to deviate from a theoretically regular orbital motion.

  • PHAs (Potentially Hazardous Asteroids) MOID=0.05 A.U. (Minimum Orbital Insertion Distance) These rocks may never impact Earth but their closeness is worth a sharp watch. There are over 200 known PHA’s. (link includes daily orbital elements and tables of known PHA’s and further info) See Torino Impact Scale.

  • PLANAR FEATURES: Microscopic features in grains of quartz or feldspar consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain’s crystal structure.

  • PLANET: Any large body that orbits a sun. From Greek meaning wanderer.

– r –

  • RESOLUTION: the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details.

  • RESONANCE: A state in which one orbiting object is subject to periodic gravitational perturbations by another.

– s –

  • SEMIMAJOR AXIS: the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by rp = a(1-e) and ra = a(1+e).

  • SHATTER CONE: Striated conical fracture surfaces produced by meteorite impact into fine-grained brittle rocks such as limestone.

  • SHIELD: Any of several extensive regions where ancient Precambrian crystalline rocks are exposed at the Earth’s surface, for example, the Canadian Precambrian Shield

  • SHOCKED QUARTZ: Quartz that has undergone deformation due to extreme pressure and heat. It has been found in the layer that marks the K-T boundary.

  • SHOCK METAMORPHISM: The production of irreversible chemical or physical changes in rocks by a shock wave generated by impact, or detonation of high-explosive or nuclear devices.

  • SHOEMAKER, Gene: Pioneered the field of impact cratering with his landmark studies of Barringer Crater (a.k.a. Meteor Crater) in Arizona, and as a chief geologist he taught the Apollo astronauts about rocks on the moon. Also a pioneer of theories of impact-induced mass extinctions in the geologic past, and the search for Earth-crossing asteroids and comets. He and his wife Carolyn and David Levy were the team that discovered the famed comet that smacked into Jupiter in 1994.

  • SHOEMAKER-LEVY 9: SL-9 was a short-period comet that was discovered by Eugene and Carolyn Shoemaker and David H. Levy. As the comet passed close by Jupiter, Jupiter’s gravitational forces broke the comet apart. Fragments of the comet collided with Jupiter for six days during July 1994, causing huge fireballs in Jupiter’s atmosphere that were visible from Earth.

  • SHORT PERIOD COMETS (SPC): are those comets with a period less than 200 years

  • SIDEROPHILE ELEMENTS: An element with a weak affinity for oxygen and sulfur such as iridium, osmium, platinum, and palladium, that, in chemically segregated asteroids and planets, are found in the metal-rich interiors. Consequently, these siderophile elements are extremely rare on Earth’s surface.

  • SI UNITES 1564-1616 SI is an abbreviation for systeme internationale, which has become the worldwide adopted standard for units of measurement.

  • SILICATE: a compound containing silicon and oxygen (e.g. olivine)

  • STISHOVITE: A dense, high-pressure phase of quartz that has so far been identified only in shock-metamorphosed quartz-bearing rocks from meteorite impact craters.

  • SUBLIME: (or sublimate) to change directly from a solid to a gas without becoming liquid 

– t –

  • TARGET ROCKS: The surface rocks that an asteroid or comet impactor smashes into in an impact event.

  • TORINO IMPACT SCALE: Developed in 1999, a scale which describes the risks of a threat of any NEO.

  • TROJANS: located near Jupiter’s Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more. This name derives from a generalization of the names of two of the largest asteroids in Jupiter’s Lagrange points: 624 Hektor and 911 Agamemnon.  Saturn’s satellites are also sometimes called Trojans.

  • TECTONIC: deformation forces acting on a planet’s crust.

  • TEKTITES: Natural, silica-rich, homogeneous glasses produced by complete melting and dispersed as droplets during terrestrial impact events. They range in color from black or dark brown to gray or green and most are spherical in shape. Tektites have been found in four regional deposits or “strewn fields” on the Earth’s surface: North America, Czechoslovakia (the moldavite tektites), Ivory Coast, and Australasia.

  • TIDAL HEATING: frictional heating of a satellite’s interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites.

– v –

  • VELOCITY: Both the speed and the direction that a body is moving. It has more information than speed alone. Velocity is a vector

  • VOLATILE: As a noun, this refers to substances that are gasses at ordinary temperatures. In astronomy, it includes hydrogen, helium, water, ammonia, carbon dioxide and methane. 

– y –

  • YOUNG: In describing a planetary surface, “young” means that the visible features are of relatively recent geologic origin. Older features have been eroded or destroyed by lava flows. Young surfaces exhibit few impact craters and are typically varied and complex.