A specialised, highly elliptical orbital path pioneered by the Soviet Union for military and communications satellites. It is named for the satellites that use it ("Molniya" is Russian for "Lightning"). It continues to be used by Russian, as well as U.S. surveillance satellites.
Back at the beginning of the Cold War, as countries were just beginning to greedily stockpile nuclear weapons, the two main factions were concerned about how they would know when the other decided to annihilate the world by launching their missiles. Each side came up with their own methods for doing this, but they all boiled down to keeping a sharp eye on the enemy's nuke farms.
The blue corner created something called the Defense Support Program, which used a fleet of satellites in geostationary orbits, monitoring the earth's surface for ballistic missile exhaust trails using infrared sensors. This system is still in use today.
The red corner used a variation on this theme. They still employed satellites, but placed them in a highly elliptical orbits which would allow them to observe the missile fields of the blue corner for extended periods of time. It was felt that geostationary orbits were too limited since it was not possible to observe latitudinal extremes. Satellites in Molniya orbits can "hover" over a particular area for hours at a time.
The first Molniya satellite was launched in 1964, since then more than 150 have followed. They are designed primarily to support the communication and surveillance networks of the Russian military, but also have bandwidth for the Russian satellite tv network.
The Molniya orbit is an egg-shaped orbit which brings the satellite 450-650km above the southern hemisphere at the closest approach, and almost 40,000km above the northern hemisphere at the most distant. Below is an exaggerated representation:
* - satellite's path
High pass over northern hemisphere
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Low pass over southern hemisphere
A body in an uneven orbit will slow down as it approaches apogee (comet orbits are not unlike this, albeit vastly elongated), and in this case the distance of the apogee from earth means satellites in this orbit are able to observe the northern hemisphere uninterrupted for almost eight hours per day. I should point out here that Molniya orbits are an orbital path, not an orbital plane. A Molniya orbit could just as easily be used to observe say, Asia, as it could be to observe the Americas.
Soviet missile launch-detection satellites would fly this orbit, positioned such that the continental U.S. was right on the edge of the earth's sphere, as seen by the satellite's cameras. Once missiles had launched and risen about ten kilometres they would stand out against the background of space so could easily be detected, more easily than detecting exhaust heat against the background of the earth's surface (which geostationary satellites have to do). The fact that light from other sources - reflections from the ground or clouds - had to pass through more atmosphere before it reached the satellite also made false alarms less likely.
Only one false alarm is known to have been generated by Molniya satellites: on September 26, 1983 a rare alignment of a Molniya satellite with the sun and the area it was observing caused it to mistake sunlight reflected off high-altitude clouds for several nuclear missile launches from the U.S. At the time the system had only been in place a short time and only indicated five missiles had been launched - inconsistent with U.S. strategic nuclear doctrine of a single massive strike - so no alert passed beyond the commanding officer of the installation monitoring the satellites.
In order to maintain a constant watch on American missile fields, a constellation of carefully-spaced satellites on the same orbit were required so the area being watched was always in view of at least one of them. These satellites were in several groups of four, each in slightly different orbits to observe slightly different spots. Groups of these satellites are still in use today for government and military communications, though due to their relatively high turnover and Russia's decreased budget, they are less numerous now than they were at the height of the Cold War.
Sources:
- U.S. Centennial of Flight Commission; "Molniya Orbit"; <http://www.centennialofflight.gov/essay/Dictionary/MOLNIYA/DI166.htm>
- Forden, Geoffrey; "False Alarms on the Nuclear Front"; <http://www.pbs.org/wgbh/nova/missileers/falsealarms.html>
- Pike, John; "Defense Support program"; <http://www.fas.org/spp/military/program/warning/dsp.htm>
- Lindborg, Christina; "Molniya"; <http://www.fas.org/spp/guide/russia/comm/elliptical/molniya.htm>
- Parrington, Lt. Col. Alan J. (USAF); "Towards a Rational Space"; <http://www.airpower.maxwell.af.mil/airchronicles/apj/alan.html>
- Some ascii art site whose circle I nabbed