A
polar orbit is an
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
in which a
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
passes above or nearly above both
poleA 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 (usually a planet such as the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, but possibly another body such as the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
) being orbited on each revolution. It therefore has an
inclinationInclination 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
degreeA 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
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
. Except in the special case of a polar
geosynchronous orbitA geosynchronous orbit is an orbit around the Earth with an orbital period matching the Earth's sidereal rotation period. This synchronization means that for an observer at a fixed location on Earth, a satellite in a geosynchronous orbit returns to exactly the same place in the sky at exactly the...
, a satellite in a polar orbit will pass over the equator at a different
longitudeLongitude , identified by the Greek letter lambda , is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement...
on each of its orbits.
Polar orbits are often used for earth-mapping,
earth observationEarth observation satellites are satellites specifically designed to observe Earth from orbit, similar toreconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. Geostationary satellites hover over the same spot, providing...
, and reconnaissance satellites, as well as for some
weather satellitesA 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...
.
A
polar orbit is an
orbitIn physics, an orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star....
in which a
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
passes above or nearly above both
poleA 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 (usually a planet such as the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, but possibly another body such as the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
) being orbited on each revolution. It therefore has an
inclinationInclination 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
degreeA 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
equatorThe equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth...
. Except in the special case of a polar
geosynchronous orbitA geosynchronous orbit is an orbit around the Earth with an orbital period matching the Earth's sidereal rotation period. This synchronization means that for an observer at a fixed location on Earth, a satellite in a geosynchronous orbit returns to exactly the same place in the sky at exactly the...
, a satellite in a polar orbit will pass over the equator at a different
longitudeLongitude , identified by the Greek letter lambda , is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement...
on each of its orbits.
Polar orbits are often used for earth-mapping,
earth observationEarth observation satellites are satellites specifically designed to observe Earth from orbit, similar toreconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. Geostationary satellites hover over the same spot, providing...
, and reconnaissance satellites, as well as for some
weather satellitesA 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...
. 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 polar orbiting satellites to choose a
sun-synchronous orbitA 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 earths 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 orbitHighly elliptical orbit is an elliptic orbit characterized by a relatively low-altitude perigee and an extremely high-altitude apogee. These extremely elongated orbits can have the advantage of long dwell times at a point in the sky during the approach to and descent from apogee...
with its apogee above that area. This is the principle behind a
Molniya orbitA Molniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees and an orbital period of about 12 hours. Molniya orbits are named after a series of Soviet/Russian Molniya communications satellites which have been using this type of orbit since the mid 1960s.A satellite...
.
See also
- List of orbits
- Iridium satellite constellation
The Iridium satellite constellation is a large group of satellites used to provide voice and data coverage to satellite phones, pagers and integrated transceivers over Earth's entire surface...
- Vandenberg AFB, a major United States launch location for polar orbits.
External links