Hill sphereThe Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. It is sometimes termed the Roche sphere. It was defined by the American astronomer George William Hill, based on the work of the French astronomer Édouard Roche. To be retained by a more gravitationally attracting astrophysical object—a planet by a more massive sun, a moon by a more massive planet—the less massive body must have an orbit that lies within the gravitational potential represented by the more massive body's Hill sphere.
Gravitational energyGravitational energy or gravitational potential energy is the potential energy a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (converted into kinetic energy) when the objects fall towards each other. Gravitational potential energy increases when two objects are brought further apart. For two pairwise interacting point particles, the gravitational potential energy is given by where and are the masses of the two particles, is the distance between them, and is the gravitational constant.
Deimos (moon)Deimos ˈdaɪməs (systematic designation: Mars II) is the smaller and outermost of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of and takes 30.3 hours to orbit Mars. Deimos is from Mars, much farther than Mars' other moon, Phobos. It is named after Deimos, the Ancient Greek god and personification of dread and terror. Moons of Mars#Discovery Deimos was discovered by Asaph Hall III at the United States Naval Observatory in Washington, D.C., on 12 August 1877, at about 07:48 UTC.
Gravitational potentialIn classical mechanics, the gravitational potential at a point in space is equal to the work (energy transferred) per unit mass that would be needed to move an object to that point from a fixed reference point. It is analogous to the electric potential with mass playing the role of charge. The reference point, where the potential is zero, is by convention infinitely far away from any mass, resulting in a negative potential at any finite distance.
Characteristic energyIn astrodynamics, the characteristic energy () is a measure of the excess specific energy over that required to just barely escape from a massive body. The units are length2 time−2, i.e. velocity squared, or energy per mass. Every object in a 2-body ballistic trajectory has a constant specific orbital energy equal to the sum of its specific kinetic and specific potential energy: where is the standard gravitational parameter of the massive body with mass , and is the radial distance from its center.
Atmospheric escapeAtmospheric escape is the loss of planetary atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) escape, and impact erosion. The relative importance of each loss process depends on the planet's escape velocity, its atmosphere composition, and its distance from its star. Escape occurs when molecular kinetic energy overcomes gravitational energy; in other words, a molecule can escape when it is moving faster than the escape velocity of its planet.
Bi-elliptic transferIn astronautics and aerospace engineering, the bi-elliptic transfer is an orbital maneuver that moves a spacecraft from one orbit to another and may, in certain situations, require less delta-v than a Hohmann transfer maneuver. The bi-elliptic transfer consists of two half-elliptic orbits. From the initial orbit, a first burn expends delta-v to boost the spacecraft into the first transfer orbit with an apoapsis at some point away from the central body.
Vis-viva equationIn astrodynamics, the vis-viva equation, also referred to as orbital-energy-invariance law or Burgas formula, is one of the equations that model the motion of orbiting bodies. It is the direct result of the principle of conservation of mechanical energy which applies when the only force acting on an object is its own weight which is the gravitational force determined by the product of the mass of the object and the strength of the surrounding gravitational field.
Amalthea (moon)Amalthea æməlˈθiːə is a moon of Jupiter. It has the third closest orbit around Jupiter among known moons and was the fifth moon of Jupiter to be discovered, so it is also known as Jupiter V. It is also the fifth largest moon of Jupiter, after the four Galilean Moons. Edward Emerson Barnard discovered the moon on 9 September 1892 and named it after Amalthea of Greek mythology. It was the last natural satellite to be discovered by direct visual observation; all later moons were discovered by photographic or digital imaging.
Primary bodyA primary (also called a gravitational primary, primary body, or central body) is the main physical body of a gravitationally bound, multi-object system. This object constitutes most of that system's mass and will generally be located near the system's barycenter. In the Solar System, the Sun is the primary for all objects that orbit the star. In the same way, the primary of all satellites (be they natural satellites (moons) or artificial ones) is the planet they orbit.
