AchondriteA class of stony meteorites that crystallized from magmas.
AnorthositeAn igneous rock consisting primarily of plagioclase; a major constituent of the lunar highlands.
AsteroidA moving object in the solar system of stellar appearance, without any trace of cometary activity.
Astronomical unit (AU)The distance from the earth to the sun, about 150 million km.
AtaxiteAn iron meteorite with high nickel content.
Ballistic limitMinimum thickness of a target (such as a debris shield) necessary to prevent penetration of an impacting particle.
BasaltA common igneous volcanic rock consisting predominantly of pyroxene and plagioclase.
BrecciaA rock composed of broken rock fragments cemented together by finer-grained material; a common product of a high energy impact process.
BreakupDestructive fragmentation of a space object. Break-ups may be either accidental or intentional. Since the early 1960s, debris created by in-orbit break-ups has represented the largest single constituent of the total space object population. They may be from an explosion of a single object or from the collision of more than one object.
Carbonaceous chondriteA primitive class of stony meteorite, the chemical composition of which has elemental abundances comparable to that of the Sun.
CatalogingProcess of detecting, identifying and determining the discrete orbit of a space object. In cataloging, data from sensor networks are used to create a set of orbital elements that describe an object’s discrete orbit. These orbital elements can be used to predict the future position of an object, they must be updated periodically to account for orbital perturbations. Space objects catalogs have been compiled and are maintained by different national governments and agencies.
ChondriteAn abundant type of stony meteorites characterized by the presence of chondrules.
ChondruleMillimetre-sized spherule of rapidly cooled silicate melt, found in abundance in chondritic meteorites.
CometA diffuse, asteroid like body of ices and solid particles whch orbits the Sun, often in a highly elliptical or parabolic orbit. In absorbing solar radiation, the ices evaporate and release dust, thus leading to a short lifetime on astronomical timescale within the inner solar system.
Cosmic dustsmall particles of cometary, asteroidal or interstellar origin. See also micrometeroid.
CrystallizationThe process of producing minerals with ordered atomic structures from a melt or by vapour condensation.
Damage EquationA formula or data which predicts the resultant damage to a target from a specific impacting particle of specified size, velocity density etc or vice versa.
DecayNatural loss of altitude of a space object leading to re-entry into the Earth’s atmosphere. At low altitudes the rate of decay is determined largely by atmospheric density and the object’s area-to-mass ratio but, for space objects in highly elliptical orbits, solar-lunar gravitational forces drive the rate of decay.
EjectaAny matter or fragment, generally small in size, ejected upon impact of a hypervelocity particle.
EccentricityThe measure of an elliptical orbit which deviates from circularity.
Electrostatic acceleratorA laboratory facility using a Van de Graaff high voltage generator to accelerate micron sized electrically charged particles up to velocities typical of particles and planets space..
Escape velocityThe velocity which a object must achieve to escape the gravitational field of its parent body.
FluxThe number of particles passing through a given area in a given time corresponding to a limiting size (cumulative) or within a given size range (differential). The space debris or meteoroid debris flux experienced by a spacecraft translates to the probability or damage.
FragmentationProcess by which an orbiting space object dissociates and produces debris. Fragmentation includes such processes as breakup and physical deterioration due to exposure and aging.
GlassSolid material without any crystal structure, for instance produced by rapid cooling of a silicate mix.
HexahedriteAn iron meteorite with low nickel content, consisting almost exclusively of kamacite.
HydrocodeNumerical computer capability to simulate hypervelocity impacts and the structural deformation, change of state, fragmentation, etc., which results from such impacts.
HypervelocityRelative velocity of two objects which, in broad terms, exceeds the speed of sound in solid material (about 5 km/s) and results in an impact response that is not dominated by static material strength effects. E.g. a fluid flow process.
Iron meteoriteMeteorite composed primarily of iron-nickel metal.
Light gas gunTwo-stage gun device that use a highly compressed light gas (hydrogen or helium) to accelerate projectiles to typical speeds of 5-8 km/s under well controlled conditions.
MetamorphismRe-crystallization, in the solid state, of a rock in response to high temperature or pressure.
MeteorA streak of light in the sky produced by transit of a meteoroid through the Earth’s atmosphere.
MeteoriteExtraterrestrial material that survives passage through the atmosphere and reaches the Earth’s surface as a recoverable object.
MeteoroidA small object which causes a meteor phenomenon.
MicrometeoroidSmall meteoroid typically in the micron size range.
ModelsEquations or computer data which characterises the orbital debris or meteoroid population. It may be used to fill gaps in the existing measurements data, to interpret new data and to project the characteristics of the future debris environment. Application of the model to an object in space, with damage equations, can predict spacecraft lifetimes.
OrbitThe path, usually elliptical followed by one object revolving around another.
Orbital debrisSpace objects in Earth orbit that are not functional spacecraft. Spent rocket bodies, mission related objects, fragments from breakups and deterioration, nonfunctional spacecraft, and aluminum particles from solid rocket exhaust, ejecta are all considered debris.
Orbital lifetime reductionAccelerating the natural decay of spacecraft and other space objects to reduce their time in orbit.
Orbital regionsSpace objects travel in a wide variety of orbits at various altitudes; The following are some of the more frequently used orbits:
Low earth orbit (LEO)Orbit with a mean altitude of less than 2000 km. Most commercial satellites operate in LEO.
Sun-synchronous orbit (SSO)Retrograde LEO orbit in which the orbit plane precesses at the same rate the earth revolves around the sun. A spacecraft in SSO experiences the same ground lighting conditions each day this can be useful for Earth observation missions.
High earth orbit (HEO)Any earth orbit with a mean altitude greater than 2000 km.
Circular semi-synchronous orbitCircular orbit (such as used by the GPS satellites) with a period of about 12 hours (mean altitude 20000 km).
Highly elliptical obitOrbits with an eccentricity of grater than 0.5, including GTO and Molnya orbits (inclination 65° and period 12 hours).
Geostationary transfer orbit (GTO)Elliptical orbit with apogee around GEO and a perigee in LO. This orbit is used to transfer spacecraft from LEO to GEO. Rocket bodies are often left in this type of orbit.
Geostationary earth orbit (GEO)Near-circular orbit with a period of 1436 minutes and an inclination close to 0°. The satellite maintains a relatively stable position directly above the equator at a mean altitude of about 35785 km.
Parent bodyObjects of asteroidal size or larger from which meteorites or cometary dust is derived. Also large man-made objects from which orbital debris are derived.
PerturbationAny disturbance or minor sinuosity imposed on a otherwise elliptical orbit; this may be caused by gravitational attraction of a third body, solar radiation pressure or atmospheric drag.
Semi-major axisThe long axis of an ellipse; one characteristic of orbits.
Silicate inclusionsPieces of silicate rock contained in iron meteorites.
Solar cycle activityPeriodic fluctuations in the energy output of the sun. In general these fluctuations exhibit a sinusoidal variation with a period of 11 years. During periods of years of high solar activity, the Earth’s atmosphere is heated, causing it to expand. This expansion increases the atmospheric density encountered by space objects particularly those in orbits lower than 1000 km, causing them to decay more rapidly; This may lead to a decrease in the overall population of objects in Earth orbit during solar maximum periods.
SpallationPhenomenon that occurs when a high-velocity impact causes a stress wave to interact with the free surface of a target. If the resulting tensile stress caused by this interaction exceeds the tensile yield stress of the material, matter can separate from the target (or “spall”) and be projected from the surface (see “ejecta”) at a high velocity (between the original impact velocity and 1/10 of that velocity).
Stony-iron meteoriteA class of meteorites consisting of comparable parts silicate minerals or rocks and iron-nickel metal.
Volatile elementAn easily volatilized element that condenses from a gas at low temperature.