Artificial satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

in low

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

orbits are rarely placed in

retrograde orbit Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession or ...

. This is partly due to the extra velocity (and propellant) required to launch into orbit against the direction of the Earth's rotation. Most commercial Earth-observing satellites use retrograde sun-synchronous orbits to ensure that observations are performed at the same local time each pass of any given location, while almost all communication satellites use prograde orbits.http://www.sac.gov.in/Satcom_Overview.doc "Orbits of almost all communication satellites are prograde orbits, as it takes less propellant to achieve the final velocity of the satellite in prograde orbit by taking advantage of the earth's rotational"

Examples

Israel has successfully launched seven

Ofeq Ofeq, also spelled Offek or Ofek ( he, אופק, ''lit.'' Horizon) is the designation of a series of Israeli reconnaissance satellites first launched in 1988. Most Ofeq satellites have been carried on top of Shavit 2 launch vehicles from Palmac ...

satellites in retrograde orbit aboard a

Shavit Shavit ( he, שביט, link=no, meaning ''comet'') may refer to: People Surname * Ari Shavit (born 1957), Israeli reporter and writer * Bradley Shavit Artson (born 1959), American rabbi * Dorit Shavit (born 1949), Israeli ambassador * Edna Shav ...

launcher. These reconnaissance satellites complete one Earth orbit every 90 minutes and initially make about six daylight passes per day over Israel and the surrounding countries, though this optimal Sun-synchronized orbit degrades after several months. They were launched in retrograde orbit so that launch debris would land in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

, and not on populated neighboring countries on an eastward flight path. The United States launched two

Future Imagery Architecture Future Imagery Architecture (FIA) was a program awarded to Boeing to design a new generation of optical and radar imaging US reconnaissance satellites for the National Reconnaissance Office (NRO). In 2005 NRO director Donald Kerr recommended th ...

(FIA) radar satellites into 122° inclined retrograde orbits in 2010 and 2012. The use of a retrograde orbit suggest that these satellites use

synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...

. Earth-observing satellites may also be launched into a

sun-synchronous orbit A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it ...

, which is slightly retrograde. This is typically done in order to keep a constant surface

illumination angle The angle of incidence, in geometric optics, is the angle between a ray incident on a surface and the line perpendicular (at 90 degree angle) to the surface at the point of incidence, called the normal. The ray can be formed by any waves, such as ...

, which is useful for observations in the visible or infrared spectrum's.

SEASAT Seasat was the first Earth-orbiting satellite designed for remote sensing of the Earth's oceans and had on board one of the first spaceborne synthetic-aperture radar (SAR). The mission was designed to demonstrate the feasibility of global satelli ...

and

ERS-1 European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of 2 satellites, ERS-1 and ERS-2. ERS-1 ERS-1 launched 17 July 1991 from Guiana Space Centre ...

are examples of satellites launched into sun-synchronous orbits for this reason.

Space warfare and accidents

Arthur C. Clarke Sir Arthur Charles Clarke (16 December 191719 March 2008) was an English science-fiction writer, science writer, futurist, inventor, undersea explorer, and television series host. He co-wrote the screenplay for the 1968 film '' 2001: A Spac ...

wrote an article called "War and Peace in the Space Age", in which he suggested that an artificial satellite in retrograde orbit could use "a bucket of nails" to destroy an SDI (anti-warhead) satellite. This premise was questioned on account of the vastness of space and the low probability of an encounter. Nevertheless, a satellite in retrograde orbit could pose a major hazard to other satellites, especially if it were placed in the

Clarke belt A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitude ...

, where

geostationary satellite A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitude ...

s orbit. This risk highlights the fragility of

communication satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. ...

s and the importance of international cooperation in preventing space collisions due to negligence or malice.

References

Sources and external links

Notes on Arthur C. Clarke
{{DEFAULTSORT:Artificial Satellites In Retrograde Orbit Orbits

Examples

Space warfare and accidents

See also

References

Sources and external links