The Molniya orbit is a highly elliptical orbit that possesses an inclination of 63.4 degrees, an argument of perigee of 270 degrees and an orbital period of approximately half a sidereal day.
The name of this orbit comes from the Molniya satellites program which was a series of civilian and military communications satellites developed by the Soviet Union and then by Russia. Indeed these satellites have used this particular orbit since the mid-1960s, therefore giving its name to it. The Molniya satellites program was developed at the same time as the Soyuz and Voskhod programs.
Satellites placed in this orbit are used for different purposes: television broadcasting, telecommunications, military communications, weather monitoring as well as some classified purposes. Generally it is designed to provide communications and remote sensing coverage but over high latitudes.
Russia, part of the Soviet Union at the time, is located at high northern latitudes. And in order to broadcast, satellites had to be placed in a geostationary orbit.
A geostationary orbit, which can also be called a geosynchronous equatorial orbit, is a circular orbit above Earth’s equator that will follow the direction of Earth’s rotation. The point is that with a satellite placed in the geostationary orbit, the satellite antennas located on Earth will not have to rotate to track it but can be pointed permanently at the position where the satellites are located, since they are synchronised. Satellites placed in this orbit are often communications, weather and navigation satellites. Because of the risk of interference, it is mandatory to leave a certain distance between the satellites. Therefore there are only 180 positions available, which makes it a scarce resource.
However for countries located at high northern latitudes, placing a satellite on a geostationary orbit requires a lot of power due to the low elevation angles. For this reason, Energia, a Russian manufacturer of ballistic missile, spacecraft and space station components, had initiated research to find other orbits which would have the same properties as the geostationary orbit but without the high energy demanding launch. In the 1960s Soviet scientists found the Molniya orbit that perfectly fulfils the role using a highly elliptical orbit with an apsis over the Soviet Union territory.
On October 10, 1960, the first Molniya satellite was launched, but it failed because of the malfunction of Block 1, a stage of the Vostok rocket. The program was ultimately successful with the launch of Molniya 1-1 in 1965.
The average service life of the first models of this satellite was not very long: no more than a year and a half due to the fact that their orbit was disrupted by perturbations, like the gravitational effects of the Sun or the Moon.
Then the succeeding series, called the Molniya-2, was design to provide both military and civilian broadcasting and was used to create, in 1967, the Orbita television network, which is considered as the first national network of satellite television.
The last series was the Molniya-3 design before it was replaced by the Meridian satellites, launched from 2006. Just as the Molniya satellites, Meridian was designed to serve both civil and military purposes. Built by the main Russian satellite manufacturer, ISS Reshetnev, Meridian satellites seem to be technologically more advanced and apparently have a lifespan of seven years.
Even though the Soviets were the first to use this type of orbit, Americans did not wait long before using it in their turn. As soon as 1971, military satellites Jumpseat and Trumpet were launched into Molniya orbit, thus their precise mission and capabilities are classified. It was soon to be followed by the American SDS constellation, that used a combination of Molniya and geostationary orbits. Their purpose was to relay signals from lower flying satellites back to ground stations.
The Molniya program introduced the concept of satellite constellation, like the Starlink constellation developed by Space X. A constellation requires at least three spacecraft in Molniya orbits, and then each one will be active for a period of eight hours per orbit.
Even if the Molniya satellites are no longer produced and used currently, the orbit is still very much used. In 2015 and 2017, Russia launched two Tundra satellites into a Molniya orbit as part of the EKS warning system which can identify ballistic missile launches from outer space and thus give advance notice of a nuclear attack.
Almost forty years after its first launch, a Molniya satellite was last launched on February 18, 2004.