Anear-equatorial orbit is anorbit that lies close to theequatorial plane of theprimary body orbited. Such an orbit has aninclination near 0°. On Earth, such orbits lie near thecelestial equator, thegreat circle of the imaginarycelestial sphere that iscoplanar with the equator of Earth.[dubious –discuss] Ageostationary orbit is a particular type of equatorial orbit, one which isgeosynchronous. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to observers on the surface of the Earth.
Equatorial orbits can be advantageous for several reasons. For launches of human technology to space, sites near the Equator, such as theGuiana Space Centre inKourou,French Guiana, orAlcantara Launch Centre in Brazil, can be good locations forspaceports as they provide some additionalorbital speed to thelaunch vehicle by imparting therotational speed of the Earth, 460 m/s, to the spacecraft at launch.[1] The addedvelocity reduces the fuel needed to launch spacecraft to orbit. Since Earthrotates eastward, only launches eastward take advantage of this boost of speed. Westward launches, in fact, are especially difficult from the Equator because of the need to counteract the extra rotational speed.
Equatorial orbits offer other advantages, such as to communication: a spaceship in an equatorial orbit passes directly over an equatorial spaceport on every rotation,[1] in contrast to the varyingground track of an inclined orbit.
Furthermore, launches directly into equatorial orbit eliminate the need for costly adjustments to a spacecraft's launch trajectory. The maneuver to reach the 5° inclination of theMoon's orbit from the 28° N latitude ofCape Canaveral was originally estimated to reduce the payload capacity of theApollo Program'sSaturn V rocket by as much as 80%.[1]
Non-inclined orbit
editAnon-inclined orbit is anorbitcoplanar with aplane of reference. Theorbital inclination is 0° for prograde orbits, andπ (180°) forretrograde ones.[citation needed]
If the plane of reference is a massivespheroid body'sequatorial plane, these orbits are calledequatorial, and the non-inclined orbit is merely a special case of the near-equatorial orbit.
However, a non-inclined orbit need not be referenced only to an equatorial reference plane. If the plane of reference is theecliptic plane, they are called anecliptic orbit.
As non-inclined orbits lacknodes, theascending node is undefined, as well as its related classicalorbital elements, thelongitude of the ascending node and theargument of periapsis. In these cases, alternative orbital elements or different definitions must be used to ensure an orbit is fully described.[2]
Ageostationary orbit is ageosynchronous example of an equatorial orbit, non-inclined orbit that is coplanar with the equator ofEarth.
See also
editReferences
edit- ^abcWilliam Barnaby Faherty; Charles D. Benson (1978)."Moonport: A History of Apollo Launch Facilities and Operations". NASA Special Publication-4204 in the NASA History Series. p. Chapter 1.2: A Saturn Launch Site. Archived fromthe original on 2018-09-15. Retrieved8 May 2019.
Equatorial launch sites offered certain advantages over facilities within the continental United States. A launching due east from a site on the Equator could take advantage of the earth's maximum rotational velocity (460 meters per second) to achieve orbital speed. The more frequent overhead passage of the orbiting vehicle above an equatorial base would facilitate tracking and communications. Most important, an equatorial launch site would avoid the costly dogleg technique, a prerequisite for placing rockets into equatorial orbit from sites such as Cape Canaveral, Florida (28 degrees north latitude). The necessary correction in the space vehicle's trajectory could be very expensive - engineers estimated that doglegging a Saturn vehicle into a low-altitude equatorial orbit from Cape Canaveral used enough extra propellant to reduce the payload by as much as 80%. In higher orbits, the penalty was less severe but still involved at least a 20% loss of payload.
- ^Prussing, John E.; Conway, Bruce A. (1993).Orbital Mechanics (1st ed.). New York, New York: Oxford University Press. p. 49.ISBN 0-19-507834-9.