point in an orbit that is furthest from the Earth.
BALLISTIC LIMIT —
minimum thickness of a target (such as a debris shield) necessary to prevent an impacting particle from perforating it.
destructive fragmentation of a space object. Breakups may be either accidental or intentional. Since the early 1960s, debris created by in-orbit breakups has represented the largest single constituent of the total space object population.
process 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 an object's future position, but must be updated periodically to account for orbital perturbations. Space object catalogs have been compiled and are maintained by different national governments and agencies.
see "Orbital Debris."
DEBRIS FLUX —
amount of debris passing through a given area in a given time. Area, as well as flux, can be defined in terms of either surface area or cross-sectional area. The debris flux experienced by a spacecraft is directly proportional to the probability of impact.
natural loss of altitude of a space object culminating in reentry 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 usually drive the rate of decay. Decay may be accelerated by lowering the perigee of an object's orbit.
deliberate, forced reentry of a space object into the Earth's atmosphere by applying a retarding force, usually via a propulsion system.
process by which an orbiting space object disassociates and produces debris. Fragmentation includes such processes as breakup and physical deterioration due to exposure and aging. The planned, controlled, and intentional release of objects (see ''Mission-related Object") is not considered fragmentation.
GEOSYNCHRONOUS EARTH ORBIT (GEO) —
see entry under "Orbital Regions."
numerical computer capability to simulate hypervelocity impacts and the structural deformation, changes of state, fragmentation, etc., that result from such impacts.
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.
angle between the orbital plane of a space object and the plane of the Earth's equator.
LIGHT GAS GUN —
two-stage gun 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 entry under "Orbital Regions."
MISSION-RELATED OBJECT —
object intentionally released from a spacecraft or rocket body during the course of a mission. These objects normally perform no useful service after release and are sometimes referred to as "operational debris." Examples of mission-related debris include spacecraft-launch vehicle separation and stabilization devices,
ORBITAL DEBRIS —
space 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 are all considered debris.
ORBITAL LIFETIME REDUCTION —
accelerating the natural decay of spacecraft and other space objects to reduce the time that they remain in orbit. Orbital lifetime reduction can be achieved through propulsive maneuvers, deployment of balloons or other drag-enhancing devices, and other methods.
ORBITAL REGIONS —
Space 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.
Sun-Synchronous Orbit —
retrograde LEO orbit in which the orbit plane processes 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 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.
Highly Elliptical Orbit —
orbit with an eccentricity of greater than 0.5, including GTO and the Molniya orbits.
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.
Molniya Orbit —
highly elliptical orbit with an inclination of 63-65 degrees, a period of about 12 hours, and an apogee above the Northern 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.
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.
Geosynchronous Earth Orbit (GEO) —
roughly circular orbit with any inclination and a period of approximately 1,436 minutes. The ground tracks of inclined geosynchronous satellites follow a figure eight-shaped pattern, completing a full circuit once a day, with the center of the figure eight fixed directly above the equator at an altitude of 35,785 km.
discharging all stored energy sources on a space object in order to reduce the chance of breakup. Typical passivation measures include venting excess propellant and discharging batteries.
point in an orbit that is closest to the Earth.
intentional changing of a space object's orbit at the end of its operational life. Typically, this involves putting the space object in an orbit where it is expected to be less of a hazard (including both collision and reentry hazards).
ROCKET BODY —
any stage of a launch vehicle (including apogee kick motors) left in Earth orbit at the end of a spacecraft delivery sequence. Typical space missions leave only one rocket body in Earth orbit, but some launches leave as many as three separate rocket bodies in different orbits. Some rocket bodies may carry special devices for experimental purposes and be given names associated with the experiment. Rocket bodies are normally as large or larger than the spacecraft they carry and often retain residual propellants that may later be a source of energy for breakup.
SOLAR CYCLE ACTIVITY —
periodic fluctuations in the energy output of the Sun. In general, these fluctuations exhibit an approximately sinusoidal variation with a period of 11 years. During periods 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 1,000 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.
orbiting object designed to perform a specific function or mission, (e.g., communications, navigation, or weather forecasting). A spacecraft that can no longer fulfill its intended mission is considered
nonfunctional. (Spacecraft in reserve or standby modes awaiting possible reactivation are considered functional.)
SPACE OBJECT —
any object in space. The term space object includes the natural meteoroid environment, as well as orbiting objects such as individual spacecraft, rocket bodies, fragmentation debris, and mission related objects. It should be noted that the space law community has yet to come to consensus on the classification of debris as space objects.
SPACE SURVEILLANCE NETWORK (SSN) —
collection of groundbased radar and electro-optical sensors used by the U.S. Space Command to track and correlate man-made space objects.
SPACE SURVEILLANCE SYSTEM (SSS) —
Russian counterpart of the U.S. SSN. The SSS is located throughout the former Soviet Union and is comprised principally of radar, optical, and electro-optical sensors.
phenomenon that occurs when a high-velocity impact causes a stress wave to interact with the free back surface of a thick target. If the resulting tensile stress caused by this interaction exceeds the tensile yield stress of the material, a thin sheet of material can separate from the target (or "spall") and be propelled from the surface at a velocity nearly equal to the original impact velocity of the particle producing the stress wave.