A near-equatorial orbit is an orbit that lies close to the equatorial plane of the object orbited. Such an orbit has an inclination near 0°. On Earth, such orbits lie on the celestial equator, the great circle of the imaginary celestial sphere on the same plane as the equator of Earth.[dubious ] A geostationary orbit is a particular type of equatorial orbit, one which is geosynchronous. 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 the Guiana Space Centre in Kourou, French Guiana, or Alcantara Launch Centre in Brazil, can be good locations for spaceports as they provide some additional orbital speed to the launch vehicle by imparting the rotational speed of the Earth, 460 m/s, to the spacecraft at launch. The added velocity reduces the fuel needed to launch spacecraft to orbit. Since Earth rotates 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, in contrast to the varying ground 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 the Moon's orbit from the 28° N latitude of Cape Canaveral was originally estimated to reduce the payload capacity of the Apollo Program's Saturn V rocket by as much as 80%.
However, a non-inclined orbit need not be referenced only to an equatorial reference plane. If the plane of reference is the ecliptic plane, they are called an ecliptic orbit.
As non-inclined orbits lack nodes, the ascending node is undefined, as well as its related classical orbital elements, the longitude of the ascending node and the argument of periapsis. In these cases, alternative orbital elements or different definitions must be used to ensure an orbit is fully described.
- William 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 from the original on 2018-09-15. Retrieved 8 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.