Polar orbit
Encyclopedia
A polar orbit is an orbit
in which a satellite
passes above or nearly above both pole
s of the body being orbited (usually a planet such as the Earth
, but possibly another body such as the Sun
) on each revolution. It therefore has an inclination
of (or very close to) 90 degree
s to the equator
. Except in the special case of a polar geosynchronous orbit
, a satellite in a polar orbit will pass over the equator at a different longitude
on each of its orbits.
Polar orbits are often used for earth-mapping, earth observation
, and reconnaissance satellites, as well as for some weather satellite
s. The Iridium satellite constellation
also uses a polar orbit to provide telecommunications services. The disadvantage to this orbit is that no one spot on the Earth's surface can be sensed continuously from a satellite in a polar orbit.
It is common for near-polar orbiting satellites to choose a sun-synchronous orbit
: meaning that each successive orbital pass occurs at the same local time of day. This can be particularly important for applications such as remote sensing of the atmospheric temperature, where the most important thing to see may well be changes over time, which you do not want to see aliased onto changes in local time. To keep the same local time on a given pass, it is desirable for the orbit to be as short as possible, which is to say as low as possible. However, very low orbits of a few hundred kilometers would rapidly decay due to drag from the atmosphere. A commonly used altitude is approximately 1000 km; this produces an orbital period of about 100 minutes. The half-orbit on the sun side then takes only 50 minutes, during which local time of day does not greatly vary.
To retain the sun-synchronous orbit as the Earth revolves around the sun during the year, the orbit of the satellite must precess at the same rate. Were the satellite to pass exactly over the pole, this would not happen. But because of the Earth's equatorial bulge, an orbit inclined at a slight angle is subject to a torque which causes precession; it turns out that an angle of about 8 degrees from the pole produces the desired precession in a 100 minute orbit.
A satellite can hover over one polar area a large part of the time, albeit at a large distance, using a polar highly elliptical orbit
with its apogee above that area. This is the principle behind a Molniya orbit
.
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
in which a satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
passes above or nearly above both pole
Geographical pole
A geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...
s of the body being orbited (usually a planet such as the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, but possibly another body such as the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
) on each revolution. It therefore has an inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
of (or very close to) 90 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s to the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
. Except in the special case of a polar geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
, a satellite in a polar orbit will pass over the equator at a different longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
on each of its orbits.
Polar orbits are often used for earth-mapping, earth observation
Earth observation satellite
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc....
, and reconnaissance satellites, as well as for some weather satellite
Weather satellite
The weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...
s. The Iridium satellite constellation
Iridium satellite constellation
The Iridium satellite constellation is a large group of satellites providing voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface. Iridium Communications Inc...
also uses a polar orbit to provide telecommunications services. The disadvantage to this orbit is that no one spot on the Earth's surface can be sensed continuously from a satellite in a polar orbit.
It is common for near-polar orbiting satellites to choose a sun-synchronous orbit
Sun-synchronous orbit
A Sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...
: meaning that each successive orbital pass occurs at the same local time of day. This can be particularly important for applications such as remote sensing of the atmospheric temperature, where the most important thing to see may well be changes over time, which you do not want to see aliased onto changes in local time. To keep the same local time on a given pass, it is desirable for the orbit to be as short as possible, which is to say as low as possible. However, very low orbits of a few hundred kilometers would rapidly decay due to drag from the atmosphere. A commonly used altitude is approximately 1000 km; this produces an orbital period of about 100 minutes. The half-orbit on the sun side then takes only 50 minutes, during which local time of day does not greatly vary.
To retain the sun-synchronous orbit as the Earth revolves around the sun during the year, the orbit of the satellite must precess at the same rate. Were the satellite to pass exactly over the pole, this would not happen. But because of the Earth's equatorial bulge, an orbit inclined at a slight angle is subject to a torque which causes precession; it turns out that an angle of about 8 degrees from the pole produces the desired precession in a 100 minute orbit.
A satellite can hover over one polar area a large part of the time, albeit at a large distance, using a polar highly elliptical orbit
Highly Elliptical Orbit
A highly elliptical orbit is an elliptic orbit with a low-altitude perigee and a high-altitude apogee. It is a type of high Earth orbit....
with its apogee above that area. This is the principle behind a Molniya orbit
Molniya orbit
Molniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of -90 degree and an orbital period of one half of a sidereal day...
.
See also
- List of orbits
- Vandenberg AFB, a major United States launch location for polar orbits.
- OrbitsOrbitIn physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
External links
- Orbital Mechanics (Rocket and Space Technology)