25-year rule—a nonbinding guideline that stipulates that the post-operational orbital life of spacecraft orbiting below 2,000 km be restricted to 25 years.
ablation—the process whereby material is heated, vaporized, and lost from the surface of a projectile that is penetrating Earth’s atmosphere.
absolute magnitude—a measure of the luminosity a meteor would have as observed from a distance of 100 km.
AFI 91-217—Air Force Instruction 91-217 that along with Air Force Instruction 91-202 (U.S. Air Force Mishap Prevention Program) provides guidance for the development of a comprehensive mishap prevention program for current and future space systems, including the minimum risk probability parameters required for safe space operations and testing.
aperture—for instruments that collect electromagnetic radiation, the size of an instrument’s collecting area.
apogee—point in an orbit that is farthest from Earth.
astrodynamics—the study of the motion of human-made objects in space, subject to both natural and artificially induced forces
astronomical unit (AU)—mean distance of Earth from the Sun. Approximately 1.5 × 108 km.
atmospheric drag—resistance in the atmosphere caused when an object interacts with atmospheric particles.
NOTE: The definitions given in this appendix are drawn from such sources as Space Vehicle Design by Griffin and French (American Institute of Aeronautics and Astronautics, 1991); IUPAC Compendium of Chemical Terminology (1991); Traces of catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteororite Impact Structures by Bevan French (Lunar and Planetary Institute, 1998); NASA Handbook 8719.14: Handbook for Limiting Orbital Debris; and Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisions Making by Drouin et al; and from material available at sources such as www.nasa.gov, adsabs.harvard.edu, ntrs.nasa.gov, and www.celestrak.com.
ballistic limit—either the combination of geometry and material properties of a target (such as a debris shield) that is necessary to prevent a given impacting particle from perforating it at a specified velocity, or the minimum size of a particle that perforates a given target at a specified velocity, or the velocity beyond which a given particle will perforate a given target.
ballistic limit equation (BLE)—equation defining a curve (often referred to as a ballistic limit curve, or BLC) that is typically plotted in projectile diameter-impact velocity space and is a line of demarcation between those diameter-velocity combinations that result in target perforation and those combinations that do not.
breakup—destructive fragmentation of a space object. Breakups may be either accidental or intentional.
BUMPER—a NASA semi-empirical computer code that calculates the probability of system failure (e.g., spacecraft wall penetration) in light of specific design features.
cataloging—process of detecting, identifying, and determining the discrete orbit of a space object.
characteristic length—the arithmetic mean of the three major mutually perpendicular dimensions of an object.
cis-martian region—the region of space between the orbit of Earth and Mars.
confidence interval—the probability that the true number lies in a set of values.
conjunction—the point of closest approach of two objects.
conjunction (assessment risk) analysis—the process performed for mitigating the risk of an operational satellite colliding with a cataloged object. Known as Conjunction Assessment Risk Analysis (CARA) at NASA/GSFC for robotic spacecraft and conjunction analysis (CA) at JSC for the International Space Station and future U.S. human spacecraft operations.
cross-tagged—indicating a situation in which the observations for two (or more) closely separated objects are associated with the tracks of the other object(s).
damage predictor equation (DPE)—an equation that predicts damage (e.g., hole diameter, crater depth) to system components in terms of an impacting particle’s density, velocity, and angle of impact and the geometric and material properties of the target.
debris—see “orbital debris.”
Debris Assessment Software (DAS)—suite of tools (ORDEM, orbit propagators, and ballistic limit equations) that produces a first-order assessment of the risk of human casualty associated with uncontrolled space vehicle reentries.
decay—natural loss of altitude of a space object, culminating in reentry into Earth’s atmosphere.
deorbit—deliberate, forced reentry of a space object into Earth’s atmosphere by applying a retarding force, usually via a propulsion system.
destructive scattering—for electromagnetic waves that interact, the interference that produces a net decrease in the amplitude of the resultant waves.
electromagnetic pulse (EMP)—a short but intense burst of electromagnetic energy.
electromagnetic scattering—the process of readmitting energy that was removed from a beam of electromagnetic radiation.
electrostatic discharge (ESD)—a transfer of an electrostatic charge; also known as a shock.
emissivity—the ratio of the amount of radiation emitted by an object to the amount emitted by a blackbody at the same temperature.
ephemerides—plural form of ephemeris; a tabulation of computed positions and velocities (and/or various derived quantities such as right ascension and declination) of an orbiting body at specific times.
ESA Master—a European Space Agency space debris and meteoroid environment model that describes the human-made and natural particulate environment of Earth and its incident flux on user-defined target orbits down to an impactor diameter of 1 micron.
Fast Air Target Encounter Penetration (FATEPEN)—a DOD computer program used in aircraft, vehicle, and ship vulnerability assessment.
flux—the number of objects impacting or passing through a unit area per unit time.
fragmentation—process by which an orbiting space object disassociates and produces debris.
Gaussian distribution—a continuous probability distribution that is often used to approximate a set of real, random values clustered around a single mean value.
GEOPROP—GEO propagator. A general-purpose orbital propagator model that includes in its calculations perturbations due to solar and lunar gravity, radiation pressure, and other major geopotentials.
geosynchronous Earth orbit (GEO)—see “orbital regions.”
graveyard orbit—an orbit into which spacecraft whose operational life have ended are intentionally placed to reduce the probability of collisions, and which currently includes orbits in two regions: higher than 2,000 km but outside GEO, and higher than GEO.
ground injury effects—potential injurious effects caused by the kinetic energy of a surviving debris object as it strikes the ground. Testing and analysis in support of range safety tests are used to estimate the likely injury on the ground if a person is struck by a piece of reentering debris.
Grün et al. interplanetary flux model (IFM)—an empirical compilation of many data sources that is used as the standard reference for meteoroid flux in near-Earth space.
Haystack—a high-power X-band radar that is a part of the Lincoln Laboratory’s Space Surveillance Complex and a contributing sensor to the U.S. Space Surveillance Network.
“height ceiling” effect—altitude cutoff above which a specular radar cannot detect the trails of objects reentering Earth’s atmosphere.
hypervelocity—relative velocity of two objects that, in general, exceeds the speed of sound in solid materials (about 5 km/s) and results in an impact response that is not dominated by material strength effects.
hypoxia—a reduction in the amount of oxygen reaching tissues in the body.
impact plasma—plasma generated from a meteoroid impact event.
inclination—angle between the orbital plane of a space object and the plane of Earth’s equator.
ionization efficiency—ratio of the number of ions formed to the number of electrons or photons used in an ionization process.
kinetic energy—the energy of motion.
launch collision avoidance (COLA)—the process of actively screening for potential collisions between a launch vehicle and known, tracked, on-orbit objects from liftoff through the end of the launch phase, and subsequently taking action to avoid any unacceptable conjunctions.
legally binding custom—a complex mix of precedent, practice, intent, and opinio juris (general conduct recognized within judicial decisions) within the geopolitical context of a given agreement.
LEO-to-GEO Environment Debris Model (LEGEND)—a NASA statistical, three-dimensional, debris evolutionary model for the study of the long-term debris environment for LEO, HEO, and GEO that provides debris characteristics as functions of time, altitude, longitude, and latitude.
light gas gun—a two-stage projectile launching device that uses a highly compressed light gas (such as hydrogen) to accelerate projectiles to typical speeds of 5-10 km/s under well-controlled conditions.
low Earth orbit (LEO)—see “orbital regions.”
luminous efficiency—fraction of total initial energy converted into visible radiation. It is a measure of the source’s ability to produce visible light.
Manned Spacecraft and Crew Survivability Code (MSCSurv)—a NASA computer code that calculates the probability of a spacecraft loss given a penetration of the crewed habitation modules.
meteor—phenomenon (involving heat, light, ionization) associated with a meteoroid impacting a planetary atmosphere.
meteoroid—small, solid particles orbiting the Sun, formally defined by the International Astronomical Union as being larger than an atom or molecule and smaller than an asteroid.
Meteoroid Environment Model (MEM)—a NASA semi-empirical computer code for estimating the distribution of meteoroid velocity and direction for near-Earth and interplanetary (Mercury to asteroid belt) natural particulates down to 1 µg, to provide flux on spacecraft surfaces.
Meteoroid Environment Office—NASA’s technical lead for defining the meteoroid environment using radar and optical measurements, for performing data analysis, and for developing models that can be used together with tests results from the Hypervelocity Impact Technology Facility at JSC.
microcrater—crater produced on spacecraft surfaces or from rocks on airless solar system bodies on the order of 1 mm in size or smaller.
micrometeoroid—colloquialism for a “small” meteoroid.
mission-related debris—objects dispensed, separated, or released as part of a planned mission.
mission-related object—an object intentionally released from a spacecraft or rocket body during the course of a mission.
Monte Carlo Method—any method that searches for a solution to a problem by generating random numbers using a set of parameters and then determining what percentage of those random numbers possess certain properties.
NASA Technical Standard 8719.14—establishes requirements for (1) limiting the generation of orbital debris, (2) assessing the risk of collision with existing space debris, (3) assessing the potential of space structures to impact the surface of Earth, and (4) assessing and limiting the risk associated with the end of mission of a space object.
“new construction” satellite—a satellite that has features to limit or prevent the creation of orbital debris.
“no explosion” guideline—removing from a space structure at the end of mission stored energy that could result in an explosion or deflagration of the space structure, to preclude generation of new orbital debris after end of mission.
Object Reentry Survival Analysis Tool (ORSAT)—a NASA semi-empirical model for determining survivability of reentering hardware, using a suite of tools that perform trajectory, atmospheric, aerodynamic, thermodynamic, and thermal/ablation physics calculations.
“old construction” satellite—a satellite that does not include features or ways to minimize the creation of orbital debris.
Olympus Satellite—a European Space Agency (ESA) experimental communications satellite that experienced multiple anomalies on August 11, 1993, near the peak of the Perseid meteor shower that year.
orbit—the path taken by an object that revolves around another object. Special orbits include the following:
• highly elliptical orbit—an orbit with an eccentricity of greater than 0.5, including geostationary transfer orbit and the Molniya orbits.
• unbound orbit—an orbit whose energy with respect to the Sun is greater than zero; all such orbits trace out hyperbolas (or in the limiting case of an orbit with e = 0, a parabola) with the Sun at one foci.
orbit conjunction message—a message format that allows communication between government and commercial agencies about spacecraft orbits.
orbital debris—space objects in Earth orbit that are not functional or operational spacecraft, including spent rocket bodies, mission-related objects, fragments from breakups and deterioration, sodium potassium radiator coolant from Russian nuclear-powered spacecraft, and aluminum oxide particles from solid rocket exhaust; such debris can be artificial or natural.
orbital lifetime reduction—acceleration of the natural decay of spacecraft and other space objects’ orbits to reduce the time that they remain in orbit.
orbital regions—space objects travel in a wide variety of orbits at various altitudes. The following are some of the more frequently used orbits:
• circular semisynchronous orbit—circular orbit (such as that used by the Global Positioning System) with a period of about 12 hours. The mean altitude of such an orbit is approximately 20,200 km.
• geostationary Earth orbit—nearly circular orbit with a period of approximately 1,436 minutes and an inclination close to zero degrees. In such an orbit, the satellite maintains a relatively stable position directly above the equator, at a mean altitude of approximately 35,785 km. In practice, “geostationary” satellites exhibit small orbital eccentricities and slight inclinations, resulting in an apparent wobble about a fixed location.
• geostationary transfer orbit (GTO)—elliptical orbit with an apogee around GEO and a perigee in LEO. This orbit is used to transfer spacecraft from LEO to GEO. The rocket bodies used to accomplish this transfer often remain in this orbit after the spacecraft separates and circularizes its orbit using an apogee kick motor.
• geosynchronous Earth orbit (GEO)—roughly circular orbit with any inclination and a period of approximately 1,436 minutes.
• high Earth orbit (HEO)—any Earth orbit with a mean altitude greater than 2,000 km.
• low Earth orbit (LEO)—orbit with a mean altitude of less than 2,000 km.
• Molniya orbit—highly elliptical orbit with an inclination of 63.4 degrees (or 116.6 degrees, which has never been used), a period of about 12 hours, apogee above the Northern Hemisphere near geosynchronous altitude, and perigee in the Southern Hemisphere. Molniya orbits have historically been used to provide communications and early-warning services; they are suited to this task because spacecraft in Molniya orbits spend most of their time above the middle latitudes of the Northern Hemisphere.
• Sun-synchronous orbit (SSO)—retrograde LEO orbit in which the orbit plane processes at the same rate Earth revolves around the Sun. A spacecraft in SSO experiences the same ground lighting conditions each day, which can be useful for Earth observation missions.
Orbital Debris Environment Model (ORDEM)—a NASA semi-empirical computer code for environment characterization for current and short-term future (~30 years) debris impact flux down to 10 µm in Earth orbit (LEO and GEO) based on returned samples, remote observations, modeling, and historical changes and trends.
payload—the cargo carried by a vehicle.
perigee—point in an orbit that is closest to Earth.
Poisson consensus model—model that consolidates theory, measurements, and assumptions into an average event rate where Poisson statistics apply. This process requires the integration of various statistical distributions (such as velocity, and angle of impact).
polarization—action whereby waves of electromagnetic radiation are limited to vibrations in specific directions.
power-law distribution—also known as the Pareto distribution, which states that smaller objects or events are more common and larger objects or events are rare.
probabilistic risk assessment (PRA)—a process used to determine the overall risk associated with a particular program or a mission stage by factoring in all known risks, and their corresponding uncertainties, if known.
projectile—an object that is propelled forward by an external force.
PROP3D—Propeller Three Dimensional Analysis. A NASA general-purpose orbital propagator model.
• active protection—steps taken, once a spacecraft is in orbit, to reduce the level of risk to which it is exposed; includes, for example, the elimination of debris in the path of an orbiting spacecraft, the avoidance of collisions and the removal of large objects to eliminate future potential debris-generating events.
• passive protection—steps applied before a spacecraft is launched, including, for example, spacecraft shielding, redundant system design, and orbit selection to lower exposure to particulates.
• operation protection—steps designed to protect a spacecraft against damage from particles that are too large to actively protect against or too small to be seen and avoided; includes intelligent spacecraft attitude profiles, and smart working and living arrangements (e.g., placing astronauts in areas not directly exposed to the particulate flux).
radar cross section (RCS)—a measure of how much radar energy is reflected from an object, usually a function of the absolute size of the target, the target’s material, and the geometry of the encounter.
radar head echo data—data on radar reflection from plasma that forms in the vicinity of meteoroids.
reentry casualty—as defined by NASA (within ORSAT and DAS models), a person who is injured by reentering space hardware; as defined by the European Space Agency, a person who is killed by a reentering object.
remediation—restoration of a contaminated environment by the removal of the contamination (e.g., orbital debris).
remote sensing—the measurement or acquisition of information on some property of an object or phenomenon, by a recording device that is not in physical or intimate contact with the object or phenomenon under study.
reorbit—intentional action to change a space object’s orbit at the end of its operational life. Typically, this process involves putting the space object into an orbit where it is expected to pose a reduced hazard (including both collision and reentry hazards).
risk—the combination of the probability of an event and the consequence of that event.
rocket body—any stage of a launch vehicle (including apogee kick motors) left in Earth orbit at the end of a spacecraft delivery (launch and orbital insertion) sequence.
satellite—a natural or artificial body in orbit around another body, such as a planet.
satellite anomoly—mission-degrading or mission-terminating events affecting on-orbit operational spacecraft.
satellite breakup assessment model (SBRAM)—a NASA computer code used to determine the short-term hazard (hours to days) from a single breakup event and separate from “background risk.”
satellite orbital debris characterization impact test (SOCIT)—a series of hypervelocity tests conducted between 1991 and 1992 by the DOD to simulate the hypervelocity breakup of a payload.
self-propagation—chain reaction resulting in an increase in the amount of orbital debris.
signal-to-noise ratio (SNR)—a measurement, in decibels of the strength of a signal compared with the background noise.
size estimation model (SEM)—an algorithm for estimating orbital debris size that relates radar cross section to size.
slag—the remains of the smelting of metallic ore; also produced by solid rocket motors that have aluminum as a fuel source.
“soft law”—a set of rules or guidelines that are not necessarily legally binding but are still legally significant.
solar cycle activity—periodic fluctuations in the energy output of the Sun. High solar output can cause expansion of Earth’s atmosphere.
solar flux—a historical representation of past solar flux component levels (i.e., 10.7 cm wavelength) and predicted future levels of intensity.
solid rocket motor—the part of a spacecraft that produces thrust by burning a solid fuel.
spacecraft—orbiting object designed to perform a specific function or mission (e.g., communications, navigation, or weather forecasting).
spacecraft anomaly—an unexpected or unplanned event or incidence that occurs to a spacecraft.
space fence—a U.S. Air Force tracking system that will replace the Naval Space Surveillance Network and will become part of the Air Force Space Surveillance System; it will be able to detect objects in space down to 5 cm in size.
space object—any object in space, including the natural meteoroid environment, as well as orbiting objects such as individual spacecraft, rocket bodies, fragmentation debris, and mission-related objects.
Space Surveillance Network (SSN)—collection of ground-based radar and electro-optical sensors used by the U.S. Space Command to track and correlate human-made space objects.
Space Surveillance System (SSS)—Russian counterpart of the U.S. Space Surveillance Network.
spallation—phenomenon that occurs when a high-velocity impact causes a stress wave to interact with the free back surface of a thick target and possibly eject material from the rear surface of the target without perforating the target.
special perturbations data—data set of higher accuracy obtained using a force model with higher fidelity than CARA/CA.
specific heat—the amount of heat required to raise the temperature of 1 gram of a substance 1 degree Celsius.
spectral energy distribution—the power carried by electromagnetic radiation with a wavelength, as a function of wavelength.
specular-scattering radar—a radar capable of specular scattering, indicating that the radar beam lies perpendicular to the axis of the rail, resulting in Fresnel reflections.
specular trail—trail plasma measured when a meteoroid is traveling perpendicular to the line-of-sight of a radar.
sporadic meteoroid—meteoroid not found in a stream. Although such meteoroids do not have a clear common parentage, the sporadic background meteoroid population as detected at Earth shows strong directionality reflecting the general orbital properties of the parent body population.
SSP 30435—Space Station Program Natural Environment Definition for Design; includes the environmental models used to design the space station as they were defined in 1994.
state vector—provides the location, velocity, and future course that an object will take in space using a 3-dimen-sional frame of reference.
stream meteoroid—meteoroid with a common, recent (less than 100,000 years) parentage, resulting in all meteoroids associated with the stream following nearly identical heliocentric orbits.
Standard Breakup Model (SBM)—semi-empirical model that determines the number, mass, velocity, and ballistic coefficient distributions of fragments down to 1 mm produced from a breakup event.
terminal velocity—the constant, maximum speed that a falling object reaches when the downward force of gravity equals the upward force of drag.
trajectory—the path taken by a falling object.
two-line element set (TLE)—a data set made up of two 69-character lines that enables deducing the position and velocity of a satellite when used in collaboration with NORAD’s SGP4/SDP4 orbital model.
uncertainty—lack of knowledge and understanding of the structure of a risk and the connections between the stages of the evolution of a risk.
VAX computer—Virtual Address eXtension computer.
Whipple shield—a type of hypervelocity impact shield used to protect crewed and uncrewed spacecraft from collisions with small particles, typically consisting of two metallic or non-metallic panels placed a small distance apart from each other.
zodiacal light—a dim cone of light produced by scattering of sunlight off interplanetary dust. It can be seen in the night sky above the horizon just before or just after sunset.