## ASTRONAUTICS

**absorption (radiant energy)**- Absorption is the process by which radiant energy is converted to thermal energy.

**absorptivity**- The absorptivity of a material is equal to the ratio of the radiant energy absorbed by the material to the radiant energy incident upon the surface of the material.

**aerobraking**– A spacecraft maneuver that uses atmospheric drag to reduce orbital speed and decrease the eccentricity of an initial highly elongated orbit.

**altitude (orbital)**- The term orbital altitude refers to the difference between magnitude of the orbiting object's position vector and radius of the central mass being orbited. For objects orbiting Earth, the orbital altitude refers to the difference between the magnitude of the orbiting object's position vector and Earth's mean radius (assumed to be 6,371 km).

**apogee**- For objects in orbit, apogee refers to the point along the orbit where the orbiting object is furthest from the central mass being orbited. For suborbital flight, apogee refers to the highest point (measured from the surface) achieved during flight.

**argument of latitude**- Argument of latitude is one of the Alternate Orbital Elements and is the angle measured from the ascending node to the spacecraft's position vector (used for circular orbits, where perigee is not defined).

**argument of perigee**- Argument of perigee is one of the Classical Orbital Elements and is measure of where the orbit is oriented within the orbital plane relative to the perigee. It is an angle measured between the ascending node and the perigee, in the direction of the orbiting objects motion.

**ascending node vector**- The ascending node vector is defined as the cross product of the unit vector normal to the fundamental plane (origin at the center of the Earth pointing to the North Pole) and the specific angular momentum vector.

**astronautics**- Astronautics is the science/engineering of systems capable of flight beyond a planetary atmosphere; including launch vehicles, spacecraft, and other space related technology.

**astronomical unit (AU)**- An AU is the average distance between the Earth and the Sun, 149,597,870,700 km (approximately 93 million mi).

**atmosphere**- The region of gases surrounding a celestial object.

**atmospheric drag**- Because the density of the atmosphere for lower orbits may not be considered negligible, atmospheric drag will produce a perturbation to the orbit of a spacecraft. Atmospheric drag is non-conservative (converts mechanical energy to thermal energy) and reduces the semi-major axis and eccentricity of an orbit.

**atmospheric reentry**– The term used to describe path of an object as it travels from space through the atmosphere to the surface of a planet. Atmospheric drags slows down an object as it enters a planets atmosphere (can produce extreme heating and disintegration of smaller objects).

**attitude (spacecraft)**- A spacecraft's attitude is its angular orientation in space relative to an external frame reference.

**aurora**- A phenomena that is produced when cosmic rays ionize the gases in the upper atmosphere, producing brilliant bands of light near the poles of the Earth. The aurora borealis (or northern lights) is the name given to the phenomena near the north pole and the aurora australis (or southern lights) is the name given to the phenomena near the south pole. Green and red light is given off by the ionization of oxygen while blue and purple light is given off by the ionization of nitrogen. Other planets in the solar system also experience the aurora phenomena.

**azimuth**- The angle, as measured from a tracking station, between true north and and the position of the spacecraft being tracked.

**blackbody**- A blackbody is an idealization used as the standard by which actual thermal radiation is compared. A blackbody is a perfect absorber and emitter of thermal radiation. While the radiant energy emitted by a blackbody depends on temperature and wavelength, it does not depend on direction, i.e. the energy emitted is diffuse. The intensity of the radiant energy emitted by a blackbody is only a function of temperature.

**blue-shift**- The wavelength of the light emitted/reflected by an object decreases (the color shifts toward the blue end of the visible spectrum) due to the fact that the object is moving toward from the person observing the object.

**bus (spacecraft)**- The term bus refers to the infrastructure (structure, electric power, attitude determination & control, communications, propulsion, etc.) that allows a spacecraft to perform a specified mission.

**classical orbital elements (COE)**- A set of 6 parameters that can be used to visualize an orbit (semi-major axis, eccentricity, inclination, right ascension of the ascending node, argument of perigee, and true anomaly).

**cold welding**- The process where two materials are fused in a vacuum environment.

**communication satellite**- A satellite placed in orbit with the express purpose of relaying radio, telephone, and/or television signals.

**constellation (satellite)**- A constellation of satellites is a group of satellites that operate in a coordinated manner.

**continuum assumption**- The continuum assumption assumes that a fluid may be treated as a continuous distribution of matter and is infinitely divisible without changing its physical nature, i.e. properties may be defined at a point. Under the continuum assumptions, macroscopic behavior can be modeled without employing a detailed model of molecular behavior.

**corpuscular radiation**- Corpuscular radiation refers to the energy associated with the charged particles (protons and free electrons) emitted by the Sun.

**cosmic rays**- Charged particles (protons and free electrons) emanating from the Sun and from outside the solar system (galactic cosmic rays). Galactic cosmic rays are more energetic than cosmic rays from the Sun.

**delta-V**- Delta-V is the term that refers to a change in a spacecraft's velocity.

**delta-V boost**- The change in velocity that must occur to move a spacecraft from an Earth parking orbit to a hyperbolic-departure orbit (Patched Conic Approximation).

**delta-V retro**- The change in velocity that must occur to move a spacecraft from a hyperbolic-arrival orbit to the target planet parking orbit (Patched Conic Approximation).

**Doppler effect**- The Doppler effect refers to the change in the frequency of a wave due to the motion of a body reflecting the wave. Named after the Austrian physicist Christian Doppler (proposed the effect in 1842).

**drag**- Drag is one of the four principal aerodynamic forces acting on an aerospace system. Drag is the aerodynamic force (a vector quantity) exerted on an spacecraft in the direction rearward and parallel to the relative wind, the effect of the drag force is to decelerate the spacecraft.

**eccentric anomaly**- One of 3 angles used to define the position of a spacecraft in an elliptical Kepler orbit (the other angles being the true anomaly and the mean anomaly). Eccentric anomaly relates an elliptic Kepler orbit to a circular orbit.

**eccentricity (orbital)**- Eccentricity is one of the Classical Orbital Elements and is measure of how much an elliptical orbit deviates from that of a circular orbit. Eccentricity is equal to the ratio of the distance between the two foci of the orbit to the length of the major axis of the orbit. A circle has an eccentricity of zero, an ellipse has an eccentricity that is greater than zero and less than one, a parabola has an eccentricity of one, and a hyperbola has an eccentricity greater than one.

**eccentricity vector**- The eccentricity vector (also known as the perigee vector) a vector originating at the center of the Earth and extending through the perigee of an orbit.

**electromagnetic (EM) radiation**- EM radiation is a type of energy that is produced when an atomic particle is accelerated by an electric field. EM radiation may be modeled as either a wave or as a particle, depending upon the application. Visible light, infrared radiation, ultraviolet radiation, are some of the type of EM radiation that make up the EM spectrum. EM radiation emitted by the Sun can pose a risk to and influence the path/attitude of a spacecraft.

**ecliptic plane**- The ecliptic plane is the plane that contains the plane formed by the rotation of the Earth as it revolves around the Sun.

**elevation**- The angle, as measured from a tracking station, between the local horizon and the position of the spacecraft being tracked.

**emission (radiant energy)**- Emission is the process by which thermal energy is converted to radiant energy

**emissivity**- The emissivity of a surface is equal to the ratio of the radiant energy emitted by the surface to the radiant energy emitted by a blackbody at the same temperature.

**epoch**- a time or date used as a temporal reference point.

**exosphere**- The exosphere is a region of the atmosphere that, on average, is 600 km or more above the surface of the Earth. The exosphere is where the atmosphere merges with outer space and consists mainly of hydrogen and helium.

**flight path angle**- the angle formed by the instantaneous local horizontal direction and the instantaneous velocity vector of an orbiting object. The local horizontal is a vector perpendicular to the orbiting object's instantaneous position vector.

**focus (orbit)**- The term referring to the points in space that are used to define conic sections (circles, ellipses, parabolas, and hyperbolas). For orbits involving two objects, there is a primary (or occupied) focus and a vacant (or unoccupied) focus. For objects orbiting the Earth, the Earth is the primary focus. The plural of focus is foci.

**field of view (FOV)**- The FOV of a sensor is the solid angle (not to be confused with the ordinary angle used in 2D space) through which the sensor can detect electromagnetic energy. Knowledge of the sensor's FOV and orbital altitude can be used to determine the sensor's swath width (see definition of swath width).

**frequency**- The term frequency refers to the number of cycles of a wave per unit time.

**fundamental plane**- a plane of reference for a coordinate system that contains the origin of the coordinate system. For a geocentric-equatorial coordinate system, the fundamental plane is the plane formed by dividing the Earth into 2 hemispheres along the equator.

**geocentric-equatorial coordinate system**- an Earth-Centered Inertial (ECI) coordinate system that approximates an inertial reference frame. The geocentric-equatorial coordinate system has an origin origin at the center of the Earth, a fundamental plane formed by the Earth's equator (with the first coordinate axis perpendicular to this plane, starting at the origin and pointing toward the North Pole), the second coordinate axis (the principal direction) is formed by a line from the Earth's center to the center of the Sun at the Vernal Equinox, and the final coordinate axis formed by the use of the "right hand rule" involving the first two coordinate axes.

**geometric altitude**- The geometric altitude is the altitude measured relative to sea level.

**geopotential altitude**- The geopotential altitude is a measure of specific potential energy at a specified height relative to the Earth’s surface (converted to a geometric height (altitude) by assuming that the acceleration due to gravity is constant at the value that exists at the Earth’s surface.

**GPS (Global Positioning System)**- The GPS system is a constellation of satellites (31 operational satellites as of 10.17.17) used to provide time and geolocation data to users of the system. For the system to operate properly, a user needs simultaneous access to 4 GPS satellites.

**gravity**- Gravity is a force of attraction between any two objects. In a 2-body system, it is proportional to the product of the mass of the 2 bodies and is inversely proportional to the square of the distance between the 2 bodies.

**gravity assist**- An orbital maneuver involving a planet's gravitational field and orbital velocity to change a spacecraft's orbital velocity (magnitude and/or direction) with respect to the Sun. If a spacecraft passes "behind" a planet during a gravity assist maneuver, its energy will increase with respect to the Sun and is a spacecraft passes "in front of" a planet during a gravity assist maneuver, its energy will decrease with respect to the Sun.

**ground tracks**- Ground tracks are imaginary lines on the surface of the Earth that are formed by the projection of an object's orbit onto the Earth surface.

**gyroscopic motion**- The tendency of a rotating object to maintain its axis of rotation, so as to conserve angular momentum.

**heat transfer**- The movement of thermal energy due to a temperature difference. There are 3 types of hear transfer: conduction (heat transfer by intermolecular interaction), convection (heat transfer by the sum of conduction and advection, advection being the transfer of energy by bulk fluid motion), and thermal radiation heat transfer (heat transfer by the net transmission of thermal radiation between two or more objects).

**heliocentric**- A model where the origin of the frame of reference is at the center of the sun. A heliocentric model of the solar system has the planets revolving around the sun.

**heliocentric-ecliptic coordinate system**- A Sun-Centered Inertial (SCI) coordinate system that approximates an inertial reference frame. The heliocentric-ecliptic coordinate system has an origin origin at the center of the Sun, a fundamental plane that corresponds to the ecliptic plane (with the first coordinate axis perpendicular to this plane and passing through the center of the Sun, i.e. through the origin), the second coordinate axis (the principal direction) is formed by a line from the Earth's center to the center of the Sun at the Vernal Equinox, and the final coordinate axis formed by the use of the "right hand rule" involving the first two coordinate axes.

**Hohmann transfer**- A Hohmann transfer is an orbital maneuver that allows a spacecraft to move between two co-planar orbits via an elliptical transfer orbit in a manner that minimizes energy usage. A Hohmann transfer is a 2-step maneuver, an initial delta-V (tangent to the initial orbit) to move into an elliptical transfer orbit and a second delta-V (tangent to the final orbit) to move into the final orbit.

**impulsive burn**- The term impulsive burn refer to the use of a spacecraft's propulsion systems (to "burn" propellant) and thereby change the spacecraft's momentum, i.e. produce a delta-V.

**inclination**- The inclination of an orbit is one of the Classical Orbital Elements and is a measure of the tilt of the orbital plane. It is an angle formed by the vector normal to the orbital plane (the specific angular momentum) and the vector normal to the fundamental plane (For the geocentric-equatorial coordinate systems, the vector normal to the fundamental plane is a vector running from the center of the Earth through the North Pole.)

**inertia**- Inertia refers to the tendency of an object to maintain its state of motion.

**J2 effect**- J2 is a mathematical terms that refers to the "strength" of the flattening associated with an oblate spheroid. The J2 effect refers to the orbital perturbation this flattening, i.e. the "bulge" at at the planet's (Earth's) equator, has on orbital behavior. Since the J2 effect is a gravitational effect, the J2 effect is conservative, i.e. it does not change the mechanical energy of an orbit. The J2 effect does result in a torque being applied to a spacecraft's orbit, changing the ascension of the ascending node and the argument of perigee of its orbit.

**Karman Line**- The Karman line is an imaginary line above which aeronautical flight is not possible in the Earth's atmosphere. It is located at 100 km above the surface of the Earth and is the internationally accepted boundary separating of space.

**Kepler's First Law of Planetary Motion**- The orbits of the planets are ellipses with the Sun at one focus.

**Kepler's Second Law of Planetary Motion**- A line joining a planet to the Sun sweeps out equal areas in equal times.

**Kepler's Third Law of Planetary Motion**- The square of the orbital period - the time it takes to complete a single orbit - is directly proportional to the cube of the mean (average) distance between the Sun and the planet.

**Kessler syndrome**- The Kessler syndrome refers to the likelihood of a series of cascade MMOD collisions that each result in additional cascade collisions. Such a scenario, should it occur, would prove catastrophic for the use of Earth-orbiting satellites.

**kinetic energy**- energy associated with the motion of an object relative to a fixed reference.

**Knudsen number**- The Knudsen number is the ratio of the molecular mean free path to a characteristic length. It is used to determine if the continuum assumption may be used to model a flow field (Kn < 0.01 continuum flow regime | 0.01 < Kn <0.1 slip flow regime | 0.1<Kn<10 transition regime | Kn>10 free molecular flow regime)

**Lagrange points**- Lagrange points are locations in space where the combined gravity forces of 2 larger masses (for a 3-body problem) equal the centripetal force needed to maintain the location of a 3rd mass relative to the 2 larger masses, i.e. Lagrange points are stable equilibrium points where a 3rd mass located at one of these points will remain motionless relative to the 2 larger masses.

**lapse rate**- The ratio of the change in temperature to a unit change in increasing altitude. The slope of the temperature distribution in a given zone for a standard atmosphere.

**latitude**- Latitude is the angle between the equatorial plane and a vector drawn from the center of the Earth to the point of interest on the Earth, i.e. an indication of how far north or south a location on the Earth is relative to the equator. Isolatitude lines form east-west lines parallel to the equator.

**launch vehicle**- The term launch vehicle refers to the system that is used to deliver a payload to space, i.e. the launch vehicle provides the necessary change in velocity to achieve orbit for an orbital mission.

**lead angle**- The angle formed between the target spacecraft's position vector when a coplanar rendezvous is initiated and the target spacecraft's position vector when the rendezvous maneuver is completed.

**light year**- The distance that light travels in one year's time (9.4607 x 10¹² km).

**line of nodes**- The intersection of the orbital plane and the fundamental plane forms a line. The 2 points where this line crosses an object's orbit are known as the nodes.

**local horizontal**- The term local horizontal refers to a line drawn normal, i.e. at a right angle, to the instantaneous position vector of an object in orbit.

**longitude**- Longitude is the angle between the plane containing a "great circle" that runs through Greenwich, England and a vector drawn from the center of the Earth to the point of interest on the Earth. Longitude is measured westerly from the Greenwich plane. A great circle is a circle formed on the surface of the Earth that lies on an imaginary plane that passes through the Earth's center.

**longitude of perigee**- Longitude of perigee is one of the Alternate Orbital Elements and is the angle measured from the principal direction to the perigee (used for equatorial orbits, where there are no nodes and thus the right ascension of the ascending node and the argument of perigee are not defined).

**mass**- Mass is the amount of matter associated with an object.

**mean anomaly**- One of 3 angles used to define the position of a spacecraft in an elliptical Kepler orbit (the other angles being the true anomaly and the eccentric anomaly). While it has no physical meaning, mean anomaly is equal to the mean motion times the time since the last perigee passage.

**mean motion**- Mean motion is the average angular speed needed for an orbiting object to complete a single orbit.

**mesosphere**- The mesosphere is a region of the atmosphere that, on average, ranges from approximately 51 km to approximately 86 km above the surface of the Earth. Meteors burn up in the mesosphere. The stratosphere and mesosphere make up the middle atmosphere.

**micrometeorites and orbital debris (MMOD)**- Natural satellites, non-functioning artificial satellites, and other debris orbiting the Earth. The US Joint Space Operations Center tracks, using radar/optical telescopes and space-based sensors, tracks MMOD larger than 10 cm in diameter (see Kessler syndrome).

**mission operations systems**- Mission operations systems refers to all the infrastructure needed to successfully complete a space mission.

**mission statement**- A space mission statement states the objective of the mission, the users/customers who will benefit from the mission, and the operational concept used to ensure the smooth operation of the various mission elements.

**moment of inertia**- The moment of inertia of an object is a quantity used to describe its rotational inertia, moment of inertia is to angular momentum as mass is to linear momentum.

**momentum**- Momentum (a vector quantity) refers to the resistance of an object in motion to change its speed or direction. Linear momentum is the product of the mass of an object times its velocity while angular momentum is the product of the moment of inertia of an object times it angular velocity.

**momentum (specific angular)**- The specific angular momentum is the angular momentum vector divided by the mass of the object under consideration and is equal to the cross product of an orbiting object's position vector with its velocity vector.

**navigation**- Navigation refers process of monitoring and controlling the motion of a spacecraft as it moves through space. Critical elements for space navigation include a system for measuring the position & velocity of the spacecraft, an inertial reference frame, a model of the solar system (for planetary missions), and a model capable of predicting the dynamics of the spacecraft.

**Newton's First Law of Motion**- An object at rest will remain at rest and an object in motion will remain in motion with the same velocity unless acted upon by an external force.

**Newton's Second Law of Motion**- An external force acting on a fixed mass system produces a change in the momentum of the system. The net external force acting on the fixed mass system is equal to the time rate of change of the momentum of the system, in an inertial reference frame.

**Newton's Third Law of Motion**- For every external force acting on a system, there is an equal reaction force acting in the opposite direction, i.e. there are no isolated forces (forces occur in pairs).

**Newton's Law of Universal Gravitation**- The force of gravity between two objects is directly proportional to the produce of their two masses and inversely proportional to the square of the distance between them.

**nodal regression rate**- The rate of change of the right ascension of the ascending node due to the oblateness of the Earth.

**node (ascending)**- The term ascending node refers to the situation where, during the course of the orbit, the orbiting object passing from the Southern Hemisphere to the Northern Hemisphere through one of the nodes, i.e. from below to above the node.

**node (descending)**- The term descending node refers to the situation where, during the course of the orbit, the orbiting object passing from the Northern Hemisphere to the Southern Hemisphere through one of the nodes, i.e. from above to below the node.

**north (magnetic)**- A point on the surface of the Earth (northern hemisphere) where the magnetic field lines converge. A compass points toward magnetic north. Note that magnetic north changes location over time.

**north (true)**- A point on the surface of the Earth (northern hemisphere) where the lines of longitude converge, i.e. the "North Pole." The Earth rotates on the axis containing the true north and south poles. Note that true north does not change location with time.

**oblate ellipsoid**- An oblate ellipsoid is a spheroid shape generated by rotating an ellipse about is minor axis, i.e. a spheroid planet with an equatorial diameter that is greater than the distance between the poles. The Earth may be considered an oblate ellipsoid.

**orbit**- The term orbit refers to the curved path, or trajectory, taken by one object as it revolves around a second object. An orbit results from the interaction of the gravitational forces produced by the objects.

**orbit (co-apsidal)**- Co-apsidal orbits are orbits where the major axes (the line of apsides) of two or more orbits are aligned. All circular orbits are co-apsidal orbits.

**orbit (coplanar)**- Coplanar orbits are orbits where objects orbiting the same central mass have the same orbital inclination.

**orbit (geostationary)**- A geostationary orbit is a prograde circular equatorial orbit with an orbital period equal to a sidereal day for Earth orbiting objects.

**orbit (geosynchronous)**- A geosynchronous orbit is an orbit with an orbital period equal to a sidereal day for Earth orbiting objects.

**orbit (Molniya)**- A Molniya orbit is a highly elliptical orbit with a period equal to one-half that of a sidereal day for Earth orbiting objects.

**orbit (parking)**- A parking orbit is a temporary orbit used by a spacecraft until it is transferred to the orbit needed to successfully complete the specified mission.

**orbit (phasing)**- Used for co-orbital rendezvous maneuvers, a phasing orbit (also known as a waiting orbit) is the orbit that will return the interceptor spacecraft to the rendezvous location after a single orbital period.

**orbit (semi-synchronous)**- A semi-synchronous orbit is a prograde orbit with an orbital period equal to one-half a sidereal day for Earth orbiting objects.

**orbit (Sun-synchronous)**- A Sun-synchronous orbit (or heliosynchronous orbit) is a retrograde low-Earth orbit (LEO) that passes over a point on the central mass's surface at the same local solar time each orbit.

**orbit (transfer)**- A transfer orbit is a temporary orbit used to move a spacecraft from one orbit to another.

**orbit cranking**- The use of a gravity assist maneuver to change the direction of a spacecraft's velocity.

**orbit pumping**- The use of a gravity assist maneuver to change the magnitude of a spacecraft's velocity.

**orbital energy**- The orbital energy (also known as mechanical energy) of a satellite is the sum of the kinetic and potential energies of the satellite.

**orbital mechanics**- The branch of astronautics that mathematically describes and predicts the orbital behavior of satellites (also called astrodynamics, orbital analysis, and orbitology).

**orbital precession**- Orbital precession is the change in the orientation of a rotating object's axis or rotation due to the oblateness of the central mass. This produces a "wobble" about the axis of rotation.

**orbital plane**- The plane defined by the position and velocity vectors associated with an orbiting object. The specific angular momentum vector is normal (perpendicular) to the orbital plane.

**origin**- The origin is a central spatial point of reference for a coordinate system. For a geocentric-equatorial coordinate system, the origin is the center of the Earth.

**out-gassing**- The process by which gases and trapped volatile compounds dissolved or trapped within a material is release in the presence of a vacuum environment.

**parallax**- The difference in the apparent position of an object when viewed from two different positions, i.e. from two different perspectives.

**patched-conic approximation**- A technique used to define an interplanetary trajectory. The patched-conic approximation breaks the spacecraft's trajectory into 3 distinct regions where, in a given region, only a single gravitational force acts on the spacecraft.

**payload**- The part of the spacecraft for which the mission was performed, i.e. the part of the spacecraft that defines the reason for undertaking the mission. Payloads are the cargo, satellites, passengers, and/or other items carried by a launch vehicle to space.

**perigee**- For objects in orbit, perigee refers to the point along the orbit where the orbiting object is closest to the central mass being orbited.

**perigee rotation rate**- The rate of change of the argument of perigee due to the oblateness of the Earth.

**period (orbital)**- The orbital period is the time its takes to complete a single orbit, i.e. the time for the orbiting object to revolve once around the central mass.

**perturbation**- A perturbation is a change to the behavior of a system that results from the application of an outside influence or force.

**perturbations (orbital)**- Changes in an actual orbit, relative to that predicted by orbital mechanics, due to unaccounted for gravitation forces. Orbital perturbations can produce complex orbital behavior and thus changes to the Classical Orbital Elements (COEs).

**phase angle**- The angle formed between the interceptor spacecraft's position vector and the target spacecraft's position vector at the start of a coplanar rendezvous maneuver.

**photon**- A photon is a massless particle used to model a quantum of electromagnetic radiant energy and force. A photon moves at the speed of light in a vacuum.

**plane change (simple)**- A simple plane change is an orbital maneuver (performed at the ascending or descending node) that produces a change in the orbital inclination where the direction, but not the magnitude, of the spacecraft's velocity vector changes. For polar orbits, a simple plane change can be used to change the right ascension of the ascending node instead of the orbital inclination.

**plane change (combined)**- A combined plane change is an orbital maneuver that involves a change in the orbital inclination where both the direction and the magnitude of the spacecraft's velocity vector changes. Combined plane changes change both the size and orientation of the orbit.

**plasma**- Plasma is the fourth state of matter (the others being solid, liquid, & gas). In general, a plasma consist of an ionize gas comprised of positive ions and free electrons. Solar plasma consists of protons and free electrons.

**potential energy**- energy associated with the position of an object relative to a fixed reference.

**principal direction**- A unit vector within the fundamental plane that starts at the the origin and points to a fixed direction in space. For a geocentric-equatorial coordinate system, the principal direction is formed by a vector that begins at the center of the Earth and points toward the center of the Sun at the vernal equinox.

**prograde**- The term prograde (or direct) refers to an orbit where the orbiting body moves in the direction of the Earth's orbit, i.e. easterly.

**radiosity (radiant energy)**- Radiosity is the sum of the radiant energy emitted by the surface of a material and the energy reflected by the surface of a material.

**radius of apoapsis**- The radius of an orbit when the orbiting object is furthest from the central mass being orbited, i.e. when the orbiting object is furthest from the occupied focus of the orbit. For objects orbiting the Earth, this is known as the radius of apogee.

**radius of periapsis**- The radius of an orbit when the orbiting object is closest to the central mass being orbited, i.e. when the orbiting object is closest to the occupied focus of the orbit. For objects orbiting the Earth, this is known as the radius of perigee.

**range**- The distance from a tracking station to the position of the spacecraft being tracked.

**red-shift**- The wavelength of the light emitted/reflected by an object increases (the color shifts toward the red end of the visible spectrum) due to the fact that the object is moving away from the person observing the object.

**reflection (radiant energy)**- Reflection is the process of redirecting radiant energy incident upon the surface of a material.

**reflectivity**- The reflectivity of a material is equal to the ratio of the radiant energy reflected by a material to the radiant energy incident upon the surface of the material.

**relativity (Galilean)**- The motion of an object depends upon the frame of reference of the observer.

**relativity (Einsteinian)**- Time depends upon the frame of reference of the observer.

**remote sensing**- Remote sensing refers to the observation of a quantity and/or process from a distance. This often involves the use of aircraft or spacecraft (satellites) equipped with sensors capable of detecting electromagnetic energy.

**rendezvous (coplanar)**- A coplanar rendezvous maneuver resulting in the interceptor and target spacecraft, initially in orbits with the same orbital inclination but different semi-major axes, having the same orbit and arriving in the same spatial location at the same time. A coplanar rendezvous can be achieved by a Hohmann transfer.

**rendezvous (co-orbital)**- A co-orbital rendezvous maneuver results in the interceptor and target spacecraft, initially in the same orbit except that each spacecraft possesses a different true anomaly, having the same orbit and arriving in the same spatial location at the same time. A co-orbital rendezvous is achieved through the use of a phasing (or waiting) orbit.

**retrograde**- The term retrograde (or indirect) refers to an orbit where the orbiting body moves in the direction opposite to that of the Earth's orbit, i.e. westerly.

**right ascension of the ascending node**- Right ascension of the ascending node is one of the Classical Orbital Elements and is measure of the swivel of the orbital plane with respect to the principal direction. It is the angle formed from the principal direction to the ascending node of the orbit.

**satellite**- A satellite is an object (artificial or natural) that is orbiting another celestial body. The term was first coined by Galileo Galilei.

**scintillation (starlight)**- Scintillation is the "twinkling" of starlight due to the fact that the light is being refracted as it passes through Earth's atmosphere. The temperature/composition of the Earth's atmosphere varies with location and time. This produces changes in the refraction index of the atmosphere and the resulting twinkling of starlight.

**semi-major axis**- The semi-major axis of an orbit is one-half the distance of a line segment that runs through the widest point of the perimeter of the orbit (passing through the center and both foci of the orbit). The semi-major axis is one of the Classical Orbital Elements and is a measure of the size of the orbit.

**sidereal time**- Sidereal time is a timescale used to measure the Earth's rotational period relative to "fixed" stars (not the Sun). The mean sidereal day is 23 hours, 56 minutes, 4.0916 seconds, i.e. the time it takes Earth to rotate once relative to the principal direction.

**solar flares**- A solar flare is a sudden burst of charged particles (protons and electronics) that erupt from the Sun due to the interaction between the particles and the Sun's magnetic field. Filament eruptions are elongated solar flares. Coronal mass ejections are large clouds of solar plasma that can escape the Sun's gravity during long solar flares or filament eruptions.

**solar wind**- The solar wind consists of charged particles (protons and electronics) that, because of the interaction between the particles and the Sun's magnetic field, continuously stream outward from the surface of the Sun.

**spacecraft**- The term spacecraft refers to the system of systems that make up a vehicle (or platform) designed to travel through space.

**specific mechanical energy**- The sum of the kinetic and potential energy, i.e. the mechanical (orbital) energy, divided by the spacecraft's mass. For circular and elliptical orbits, specific mechanical energy is negative, for parabolic orbits specific mechanical energy is zero, and for hyperbolic orbits specific mechanical energy is positive.

**spectroscopy**- Spectroscopy involves the measurement and examination of how matter interacts with and emits electromagnetic energy over a range of wavelengths.

**speed of light**- The speed of light is 299,792,458 m/s (in a vacuum).

**sphere of influence (SOI)**- The region of space around an object where the object's gravitational attraction is considered significant, i.e. non-negligible.

**stage**- To increase the overall change in velocity of a launch vehicle, stages (multiple rockets, each with their own rocket engine and propellant) are stacked and operate in series. For example, once the propellant of a stage has been exhausted, it is detached from the launch vehicle and the next stage is ignited.

**standard atmosphere**- A standard atmosphere is a model of the atmosphere that serves as a common reference for the design of flight vehicles that operate within the Earth's atmosphere. A standard atmosphere model will provide temperature, pressure, and density of a motionless, dry atmosphere as a function of altitude.

**stratosphere**- The stratosphere is a region of the atmosphere that, on average, ranges from approximately 11 km to approximately 51 km above the surface of the Earth. Increases in atmospheric temperature due to the absorption of ultraviolet radiation in the ozone layer occurs in this region. The troposphere and stratosphere contain 99% of the Earth's atmosphere.

**swath width**- The swath width of an orbiting sensor is the diameter of the planetary surface area over which a sensor is capable of detecting electromagnetic energy.

**synodic period**- The synodic period is the time between successive planetary conjunctions, i.e. the time between successive opportunities for a given interplanetary trajectory.

**thermal radiation**- Thermal radiation is the electromagnetic radiation that is emitted by a body due to the motion of the atomic particles for which the body consists. Any material with a temperature above absolute zero emits thermal radiation.

**thermodynamics**- A branch of science focusing on transfer of energy and the relationships between the different forms of energy, particularly the transfer of energy due to a temperature difference (heat transfer) and the transfer of energy due to macroscopic force interactions (work).

**thermodynamics (Zeroth Law)**- If two systems are in thermal equilibrium with a third system, then the two systems are in thermal equilibrium with each other.

**thermodynamics (First Law)**- The energy of a isolated system does not change. The term isolated implies that the systems has no communication, i.e. no transfer of mass, momentum, energy, etc., with the surroundings. If such communication exist, then this exchange must be included in the analysis.

**thermodynamics (Second Law)**- The entropy of a natural completely isolated system will only increase over time. The term isolated implies that the systems has no communication, i.e. no transfer of mass, momentum, energy, etc., with the surroundings. If such communication exist, then this exchange must be included in the analysis. The term natural refers to a system that physically exists and not an idealized system that may be used for the analysis.

**thermodynamics (Third Law)**- The entropy of a crystalline substance approaches zero as the temperature approaches absolute zero.

**thermosphere**- The thermosphere is a region of the atmosphere that, on average, ranges from approximately 85 km to approximately 600 km above the surface of the Earth. This is a region of very "thin" air heated by the sun. The thermosphere region of the atmosphere is where the space shuttle orbited and where auroras occur. Significant chemical reactions occur in this region. The thermosphere is known as the upper atmosphere.

**thruster**- A thruster is a rocket propulsion system used to affect changes in the spacecraft's orbit or attitude (see definition of attitude).

**transmission (radiant energy)**- Transmission is the process of radiant energy passing through a material without interacting with the material.

**transmissivity**- The transmissivity of a material is equal to the ratio of the radiant energy transmitted through a material to the radiant energy incident upon the surface of the material.

**tropopause**- The tropopause is the region of the atmosphere separating the troposphere and the stratosphere. The jet stream is located in the tropopause. The troposphere and tropopause make up the lower atmosphere.

**troposphere**- The troposphere is a region of the atmosphere that, on average, ranges from 0 km to approximately 11 km above the surface of the Earth. It is the lowest region of the atmosphere and the troposphere is where weather occurs. The troposphere is approximately 3 times thicker at the equator (18 km) than at poles (6 km).

**trajectory**- Trajectory refers to the path followed by an object as it moves through space or the behavior of a complex system as a function of time.

**true anomaly**- The polar angle measured from the perigee of the orbit to the orbiting objects position along the path of the orbit, in the direction of the orbiting objects motion. True anomaly at epoch refers to the true anomaly at a specified time of reference.

**true longitude**- True longitude is one of the Alternate Orbital Elements and is the angle measured from the principal direction to the orbiting objects position vector (used for circular equatorial orbits, where there is no perigee and no nodes, thus true anomaly, the right ascension of the ascending node and the argument of perigee are not defined).

**ullage maneuvers**– A spacecraft maneuver that provides the acceleration needed to move liquids to a desired location in a storage tank. Without such maneuvers, fluid management in space is problematic.

**V-infinity Earth**- Also known as the hyperbolic excess velocity or Earth-departure velocity, V-infinity Earth is the velocity a spacecraft needs (velocity relative to the Earth) to leave the Earth's sphere of influence.

**V-infinity target**- A spacecraft's velocity (relative to the target planet) as it enter's the target planet's sphere of influence.

**vacuum**- Technically, the term vacuum refers to a region of space that is completely devoid of matter. In a space environment, the term vacuum refers to a region of space that is nearly devoid of matter, i.e. will still have a finite density, and a spacecraft in the vacuum of space may still experience atmospheric drag.

**Van Allen Radiation Belt**- A region in space where charged particle (Cosmic Rays) are captured by the Earth as a result of the interaction between the particles and the Earth's magnetic field. These particles provide a serious risk to spacecraft operating in this region of space. Cosmic rays escaping the Van Allen Radiation Belt and interacting with the Earth's upper atmosphere produce the auroras.

**vapor pressure**– Vapor pressure is the pressure when the liquid phase (or solid phase) of a material is in equilibrium with the material’s gaseous phase. A material’s vapor pressure indicates how easy a material will evaporate and vapor pressure increases with temperature. High vapor pressure implies a substance evaporates easily, i.e. when it is volatile.

**vernal equinox**- The Vernal Equinox (also known as the March Equinox) is the time during the spring when the Earth's equatorial plane passes through the Sun's disk and the length of the day and night are approximately equal.

**wait time**- The time the interceptor spacecraft must wait until it is in the correct orbital position to initiate a coplanar rendezvous maneuver.

**wavelength**- The wavelength of a wave is the distance between 2 successive points on a wave characterized by the same phase of oscillation.

**weight**- Weight is one of the 4 principal forces acting on an aerospace system. Weight is the force (a vector quantity) acting on an object in the presence of a gravitational force field. The magnitude of the weight is equal to the product of the mass times the acceleration due to gravity, i.e. W=mg.