These forces of inertia and gravity have to be perfectly balanced for an orbit to happen. If the forward movement inertia of one object is too strong, the object will speed past the other one and not enter orbit. If inertia or momentum is much weaker than the pull of gravity, the object will be pulled into the other one completely and crash. Is there gravity in space? What is an orbit? What is a satellite? What is gravity? The orientation of the orbit plane in space is described by additional parameters.
Figure 2 shows the orbit oriented in space with respect to a fundamental plane, in this case the plane of the Earth's equator, called the celestial equator. The angle between the orbit plane and the celestial equator is the orbit's inclination, i. See also: Celestial sphere. The first point in Aries denotes the intersection of the celestial equator with the ecliptic plane and is used frequently in celestial mechanics as a universal reference point.
See also: Celestial mechanics ; Ecliptic. The argument of perigee determines the orientation of the orbit's line of symmetry in space. For orbital motion involving only two bodies and no other perturbing forces, the first five orbital elements are constant that is, the shape of the orbit and its orientation in space do not change and the sixth element, the true anomaly, increases with time and locates the body on the orbit.
Parabolic and hyperbolic orbits, which have eccentricities greater than or equal to unity, are unbounded and extend to infinity. These orbits occur infrequently in nature but are occasionally observed for comets with cosmic origins that pass through the solar system. These comets follow hyperbolic orbits with respect to the Sun. Geocentric hyperbolic orbits are necessary for artificial satellites and space probes to interplanetary destinations. Spacecraft on these orbits escape the Earth's gravitational field en route to Mars, Jupiter, or other planets and in general will not return.
See also: Comet ; Escape velocity ; Hyperbola ; Parabola. Orbital motion becomes more complicated with the introduction of other external forces, including the gravity of additional bodies, the effects of the Earth's nonspherical shape, atmospheric drag and radiation pressure. These forces perturb the orbit from a conic section into a form that may not repeat and may be chaotic that is, hard or impossible to predict over long time scales.
For example, the equatorial bulge of the Earth causes both the line of nodes and the argument of perigee to drift at constant rates called the precession of the nodes or of perigee. Atmospheric drag, which extracts energy from an orbit through friction, causes a spacecraft or passing asteroid to spiral down to and eventually impact the surface of a planet.
See also: Chaos ; Perturbation astronomy ; Radiation pressure. Modern investigations of orbital motion have discovered new classes of orbits that involve three or more bodies under the influence of gravity.
The interaction of multiple gravitational fields and the judicious selection of initial conditions permit orbits in the shape of a horseshoe, triangle, square, peanut, or figure eight, among others.
Many spacecraft have exploited these unique orbits for mission-related purposes, but they are also found in nature. They apply to any object that orbits another: planets orbiting the Sun, moons orbiting a planet, spacecraft orbiting Earth. The orbit of a planet around the Sun or of a satellite around a planet is not a perfect circle. The Sun or the center of the planet occupies one focus of the ellipse. A focus is one of the two internal points that help determine the shape of an ellipse. The distance from one focus to any point on the ellipse and then back to the second focus is always the same.
Eccentricity is the measure of the "roundness" of an orbit.
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