A

David

Darling

orbit

the shape of an orbit is one of the conic sections

The shape of an orbit is always one of the conic sections: circle, ellipse, parabola, or hyperbola. The orbit type depends on the velocity of the object with respect to the escape velcoity of the gravitatin body, as demonstrated by this diagram based on Newton's cannon.


An orbit is a curved path followed by an object under the gravitational influence of another body. It is one of the conic section family of curves, which includes the circle, the ellipse, the parabola and the hyperbola. A closed orbit, such as that followed by a satellite going around Earth, has the shape of a circle or an ellipse. An open orbit is one in which a spacecraft or other object does not follow a closed circuit around a gravitating body but simply has its path bent into the shape of a parabola or hyperbola.

 

As long as the orbit of an object keeps it in the vacuum of space, the object will continue to orbit without propulsive power because there is no frictional force to slow it down. If part or all of the orbit passes through Earth's atmosphere, however, the body is slowed by aerodynamic friction with the air. This causes the orbit to decay gradually to lower and lower altitudes until the object fully reenters the atmosphere and burns up.

 


circular orbit

A circular orbit is any orbit that has an eccentricity of zero – an ideal condition that is seldom if ever actually achieved. A satellite in a circular orbit travels at a constant speed. The velocity v needed to maintain such an orbit, known as the circular velocity, is given by v = √{G(M + m)/R}, where m and M are the masses of the orbiting object and the central body, respectively, R is the radius of the orbit, and G is the gravitational constant. The higher the altitude, the lower the speed, relative to the surface of Earth, needed to maintain a circular orbit.

 

circular orbit

 


elliptical orbit

In an elliptical orbit, the speed varies and is greatest at perigee (minimum altitude) and least at apogee (maximum altitude). Elliptical orbits can lie in any plane that passes through Earth's center.

 


equatorial orbit

An equatorial orbit is one that lies in a plane passing through the equator. The angle between the orbital plane and the equator plane is called the inclination of the orbit.

 


polar orbit

A polar orbit lies in a plane that passes through the north and south poles; in other words, it passes through Earth's axis of rotation. In other words, such an orbit has an inclination of 90 degrees. A spacecraft following such an orbit has access to virtually every point on Earth's (or some other planet's) surface, since the planet effectively rotates beneath it. This capability is especially useful for mapping or surveillance missions. An orbit at another inclination covers a smaller portion of the Earth, omitting areas around the poles. A polar orbit covers the entire globe every 14 days.

 

polar orbit
A polar orbit.

 

Placing a satellite into terrestrial polar orbit demands more energy, and therefore more propellant, than does achieving a direct orbit of low inclination. In the latter case, the launch normally takes place near the equator, where the rotational speed of the surface contributes a significant part of the final speed needed for orbit. Since a polar orbit is not able to take advantage of the free ride provided by Earth's rotation, the launch vehicle must provide all of the energy for attaining orbital speed.

 


sun-synchronous orbit

A sun-synchronous orbit is a special case of a near-polar orbit in which a satellite, in going around Earth, passes over the same points on Earth's surface at the same local times each day and a different swathe of territory on each orbit. This kind of orbit involves passing close to both poles and crossing the meridians at a carefully-chosen angle. Sun-synchronous orbits are typically low Earth orbits with altitudes of 550 to 850 kilometers.

 

sun-synchronous orbit
A sun-synchronous orbit.

 

A dawn-dusk orbit is a special case of a sun-synchronous orbit in which a satellite perpetually trails the shadow of Earth cast by the Sun. Because the satellite is close to the shadow, the part of Earth's surface directly below the satellite is always at sunset or sunrise, hence the name of this type of orbit. An advantage of it is that the satellite always has its solar panels bathed in sunlight so that it can produce power by this means continuously. RADARSAT-1, for example, travels in a dawn-dusk orbit with an altitude of 798 kilometers, period of 100.7 minutes, and inclination of 98.6°.

 

dawn-dusk orbit of RADARSAT-1
Dawn-dusk orbit of RADARSAT-1.

 


geosynchronous orbit

A geosynchronous orbit is a direct, circular, low-inclination orbit around Earth having a period of 23 hours 56 minutes 4 seconds and a corresponding altitude of 35,784 kilometers (22,240 miles, or 6.6 Earth radii). In such an orbit, a satellite maintains a position above Earth that has the same longitude. However, if the orbit's inclination is not exactly zero, the satellite's ground-track describes a figure eight. In most cases, the orbit is chosen to have a zero inclination, and station-keeping procedures are carried out so that the spacecraft hangs motionless with respect to a point on the planet below. In this case, the orbit is said to be geostationary.

 


geostationary orbit

A geostationary orbit (GSO) is a direct, circular geosynchronous orbit at an altitude of 35,786 kilometers (22,223 miles) that lies in the plane of Earth's equator. A satellite in this orbit always appears at the same position in the sky and its ground-track is a point. Such an arrangement is ideal for some communication satellites and weather satellites since it allows one satellite to provide continuous coverage of a given area of Earth's surface.

 

geostationary orbit
1. Geostationary satellites 'parked' over equator travel at same direction and speed as Earth revolves. Each "footprint" covers 40% of globe. Directional antennae are aimed and fixed in position with no need for tracking

 

2. Satellites at lower orbits must travel faster than Earth revolves to avoid being pulled out of orbit by gravity, so they need tracking. Many do not follow an equatorial path


 

The first satellite was placed into geostationary orbit was Syncom 3 in 1964. It orbited above the Pacific Ocean and beamed pictures from the Tokyo Olympics to the US later that year – the first trans-Pacific TV transmission.

 

The possibility of spacecraft in geostationary orbits was first discussed by Herman Potocnik (who wrote under the pseudonym Herman Noordung) and Konstantin Tsiolkovsky. In 1945, Arthur C. Clarke discussed how a set of such satellites could form a global communications network.

 


geosynchronous / geostationary transfer orbit (GTO)

A geosynchronous / geostationary transfer orbit (GTO) is an elliptical orbit, with an apogee of 35,784 kilometers, a perigee of a few hundred kilometers, and an inclination roughly equal to the latitude of the launch site, into which a spacecraft is initially placed before being transferred to a geosynchronous or geostationary orbit.

 

geostationary transfer orbit

 

After attaining GTO, the spacecraft's apogee kick motor is fired to circularize the orbit and thereby achieve the desired final orbit. Typically, this burn will also reduce the orbital inclination to 0° so that the final orbit is not only geosynchronous but also geostationary. Because the greater the initial inclination, the greater the velocity change (delta v) needed to remove this inclination, it is important that launches of GSO satellites take place as close to the equator as possible. For example, in a Delta or Atlas launch from Cape Canaveral the transfer orbit is inclined at 28.5° and the required delta v increment at apogee is 1,831 meters per second; for an Ariane launch from Guiana Space Centre the inclination is 7° and the delta v is 1,502 meters per second; while for a Zenit flight from the Sea Launch platform on the equator the delta v is 1,478 meters per second. By the rocket equation, assuming a (typical) specific impulse of 300 seconds, the fraction of the separated mass consumed by the propellant for the apogee maneuver is 46% from Cape Canaveral, 40% from Kourou, and 39% from the equator.

 

As a rough guide, the mass of a geostationary satellite at the start of its operational life (in GSO) is about half its initial on-orbit mass when separated from the launch vehicle (in GTO). Before carrying out the apogee maneuver, the spacecraft must be reoriented in the transfer orbit to face in the proper direction for the thrust. This reorientation is sometimes done by the launch vehicle at spacecraft separation; otherwise, it must be carried out in a separate maneuver by the spacecraft itself. In a launch from Cape Canaveral, the angle through which the satellite must be reoriented is about 132°.

 


Low, medium, and high Earth orbits

 

low Earth orbit

Definitions of low Earth orbit (LEO) vary. According to some, LEO includes orbits having apogees and perigees between about 100 kilometers and 1,500 kilometers. Others extend that range up to 2,000 or 3,000 kilometers. In some cases, the distinction between LEO and MEO (medium Earth orbit) is dropped and LEO is considered to be any orbit below geosynchronous altitude. The majority of all satellites, as well as the Space Shuttle and International Space Station, operate from LEO.

 


medium Earth orbit

Medium Earth orbit (MEO) is an orbit that is intermediate in altitude between that of low Earth orbit and geostationary orbit at 35,900 kilometers.

 

medium Earth orbit

 


high Earth orbit

HEO may stand for 'Highly elliptical orbit'. Definitions of this vary, one being an orbit with a perigee (low point) below 3,000 kilometers and an apogee (high point) above 30,000 kilometers.

 

HEO may also stand for 'High Earth orbit' – an orbit whose apogee lies above that of a geostationary orbit at 35,800 kilometers.