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The product of the amount by which a spinning object resists a change of its spin rate, or its direction of spin. This can be described mathematically by: $\bar{H} =I \bar{ \Omega }$.
An object’s rate of spin. Also referred to as the angular velocity vector, or, $\bar{\Omega}$.
The point in the orbit of a celestial body (or satellite) at which it is furthest from Earth (i.e., the point of the largest radial distance in the geocentric-equatorial coordinate system).
An angle, $u$, measured in the direction of a satellite’s motion, along the orbital path from the ascending node to the satellite’s position.
Symbolically represented as $\omega$, this is the angle along the orbital path between the ascending node and perigee. This angle ranges between $0^{\circ}$ and $360^{\circ}$, and is always measured in the direction of the satellite’s motion.
In a geocentric-equatorial orbit, this is the node where a satellite goes from below the equator to above the equator (i.e., the satellite is moving north).
The field of study of spaceflight.
The direction of an object (e.g., celestial body, satellite, spacecraft, etc.) from an observer, expressed as the angular distance from true north.
Classical Orbital Element
Figures formed when intersecting a right circular cone with a plane. Conic sections may form a circle, an ellipse, a parabola, or a hyperbola, depending on the angle of the plane with respect to the cone.
Part of Newton’s Third Law of Motion that states the total momentum in a system remains the same.
A vector field in which total mechanical energy is conserved.
As used on this site, a collection of cooperating spacecraft.
A frame of reference (i.e., a set of numeric data) used to uniquely determine the position of an object in Euclidean space.
In a geocentric-equatorial orbit, this is the node where a satellite goes from above the equator to below the equator while heading south.
Also referred to as a prograde orbit, this is an orbit in which a satellite moves with Earth’s rotation, and has an inclination between $0^{\circ}$ and $90^{\circ$}.
The apparent lengthening of electromagnetic waves as a source and its observer move apart.
The amount by which an orbit deviates from circularity.
A vector that points from Earth’s center to perigee, and has a magnitude equal to the eccentricity of the orbit. Mathematically, $\bar{e}= \frac{1}{ \mu } \big[ \big( v^{2}- \frac{ \mu }{R} \big) \bar{R}- \big( \bar{R} \cdot \bar{V} \big) \bar{V} \big] $.
The plane of Earth’s orbit around the Sun.
The angle between the local horizon and a spacecraft.
A short-handed way of describing motion, based on the rate of change of time with the velocity of a body. In mathematical terms, an equation of motion refers to one of Newton’s Laws of Motion $\bar{p}=m \bar{V},\ \bar{F}=m \bar{a},$ or $F_{g}= \frac{G m_{1} m_{2} }{ R^{2} }$.
An orbit that follows the equator, having an inclination of either $0^{\circ}$, or $180^{\circ$}.
The cone of visibility for a sensor.
The angular measurement, or “headstart” needed by the target spacecraft for a rendezvous; represented by $ \phi _{final}$.
Falling under the influence of gravity.
A plane created by two axes of a coordinate system. Applied to spherical coordinates, this plane divides the sphere, e.g., Earth’s equator.
Earth centered.
A coordinate system based on Earth. The origin is Earth’s center; the fundamental plane is Earth’s equator; the principal direction is defined by the vernal equinox direction; and a third axis is found by applying the right-hand rule.
Refers to a still Earth; i.e., Earth not moving.
A circular orbit with a period of approximately $24$ hours, and an inclination of $0^{\circ}$. This type of orbit is particularly useful for communication satellites.
An inclined orbit with a period of approximately $24$ hours.
Basically, the tendency of two or more objects to attract one another.
Using a celestial body’s gravitational field and orbital velocity to “sling shot” a spacecraft; ultimately resulting in changing the spacecraft’s velocity with respect to the Sun.
Any circle that slices through the center of a sphere.
The mean solar time at Greenwich, UK.
A coordinate system with the center of the Sun as its origin.
Used for interplanetary transfer, this coordinate system’s origin in the center of the Sun, its fundamental plane is the ecliptic plane (see ecliptic plane), and its principal direction is in the direction of the vernal equinox.
A Measurement that describes the tilt of an orbital plane with respect to its fundamental plane.
An angle expressed as $\alpha$, and defined at the ascending node between the equator and the ground trace of an orbit.
Also referred to as a retrograde orbit, this is an orbit in which a satellite moves opposite of Earth’s rotation, and has an inclination between $90^{\circ}$and $180^{\circ}$.
A property of matter by which it remains in a state of rest or uniform motion until an external force is applied.
The energy possessed by an object due to its motion.
The period of time when a spacecraft can be launched directly into a specific orbit from its launch site.
An angle, $\gamma$, measured at the intersection of an orbit’s ground trace and longitude line.
An angle expressed by the symbol $\delta$, measured along the equator, between the node closest to the launch opportunity being considered and the longitude where the orbit crosses the launch site latitude.
Measured from the direction of the vernal equinox to the point where the launch site passes through the orbital plane.
The angular distance, $\alpha_{lead}$, that the target spacecraft travels during the intercepting spacecraft’s time of flight.
The two points that mark the intersection of the orbital plane and fundamental plane in a coordinate system.
The product of an object’s mass and its velocity. Mathematically speaking, this can be defined by the equation: $\bar{p}=m \bar{V}$.
The time since the vernal equinox passed over a particular (local) longitude line.
Represented by $\Pi$, the angle measured from the principal direction to perigee, in the direction of a satellite’s motion.
A measurement of how much matter an object contains.
Expressed as $M$, this is an angle that must be expressed mathematically (the little meanie has no physical meaning--it’s an angle in an imaginary orbit that corresponds to a celestial body’s eccentric anomaly). Mathematically, it is a product of a spacecraft’s mean motion and the amount of time that has passed since the spacecraft’s last perigee passage, or: $M = nT$.
Expressed as $n$, the angular speed of a spacecraft.
The average time between the Sun’s successive passages over a given longitude.
A semi-synchronous, eccentric orbit used for specific communications satellites (particularly those providing coverage in northern latitudes, i.e., Russia, Canada, etc.).
The moment arm, or $\bar{R}$, is the measurement of the perpendicular length between an joint axis and the line of force acting upon that joint.
An object’s resistance to spin, represented mathematically as $I$.
The amount of resistance possessed by an object in motion.
A measurement (represented as $\Delta N$) of how much an orbit’s ground track moves to the west from one orbit to the next.
A fixed path on which a spacecraft (or anything, really) travels around a planet or other celestial body.
A technique used to change the direction of a spacecraft’s velocity.
A technique used to change the magnitude of a spacecraft’s velocity.
A physically identifiable starting point for a coordinate system.
A temporary orbit used by a spacecraft until it transfers to its final mission orbit.
A simplifying assumption that breaks the interplanetary trajectory into three separate regions, and only takes the gravitational attraction between the spacecraft and one celestial body in each region into consideration. (A major assumption of this approximation is that all orbital planes lie in the ecliptic plane).
The point in the orbit of a celestial body (or satellite) at which it is closest to Earth (i.e., the point of the smallest radial distance in the geocentric-equatorial coordinate system).
The wobble in an orbit.
An orbit that follows a path between the North and South Poles, having an inclination of $90^{\circ}$.
The energy possessed by an object due to its relative position to other objects.
Defined by pointing a unit vector toward a visible, distant object, e.g., the north star.
(See Direct Orbit)
The largest radial distance of an ellipse. In a geocentric-equatorial coordinate system, this is called apogee.
The smallest radial distance of an ellipse. In a geocentric-equatorial coordinate system, this is referred to as perigee.
A satellite’s (or spacecraft’s) distance from its tracking site.
A coordinate system with the spacecraft’s center of mass (at the start of re-entry) as its origin, the spacecraft’s orbital plane as its fundamental plane, and “down,” or the Earth’s center as its principal direction.
The three dimensional path a spacecraft must follow when entering Earth’s atmosphere, to avoid skipping out or burning up.
(See Doppler-Shift)
The arrival of two or more spacecraft at the same point in an orbit at the same time.
(See Indirect Orbit)
Represented by $\Omega$, an angle describing orbital orientation with respect to the principal direction.
An orbit with an orbital period of $12$ hours.
One half the distance across the long axis of an ellipse.
The time between passages of the vernal equinox over a given longitude.
The volume of space within which a celestial body’s gravitational force dominates.
A retrograde (indirect), low Earth orbit, with an inclination ranging between $95^{\circ}$ and $105^{\circ}$. This orbit is often used for remote sensing satellites.
The linear width, or diameter, of specific total area that is visible (typically, as seen by a sensor) on Earth's surface at one time.
An Earth-fixed reference frame with its origin at the launch site and the horizontal as its fundamental plane.
A measurement resulting from the sum of an object’s position, or potential energy, and its motion, or kinetic energy. Mathematically, this can be defined as $E = PE + KE$.
The path an object follows through space.
An intermediate orbit that transfers a spacecraft from its parking orbit to its final mission orbit.
The angle, $\nu$,along the orbital path from perigee to the satellite’s position vector. This angle ranges between $0^{\circ}$ and $360^{\circ}$, and is always measured in the direction of the satellite’s motion.
An angle, $l$, measured from the principal direction to the satellite’s position vector, in the satellite’s direction of motion.
A data format, consisting of two 69-character lines, used to encode a list of orbital elements of an Earth-orbiting satellite.
A measurement of an object’s mass plus the influence of gravity.