Molniya orbit

A Molniya orbit is a highly elliptical satellite path used for communication and remote sensing in high-latitude regions. Its name, meaning "lightning" in Russian, comes from the Soviet and Russian Molniya satellite series, which began using this orbit in the mid-1960s for both civilian and military applications. The orbit has an inclination of 63.4 degrees, an argument of perigee of 270 degrees, and a period of roughly half a sidereal day. A satellite in this orbit lingers over the northern hemisphere for most of its journey, then zips quickly across the southern hemisphere. This makes it ideal for covering areas like Russia or Canada, where geostationary satellites—fixed above the equator—offer only a low, often problematic viewing angle. For high latitudes, a Molniya orbit serves a similar purpose to a geostationary orbit for the equator, though multiple satellites are needed for uninterrupted coverage.

The orbit was devised by Soviet scientists in the 1960s as a less energy-intensive alternative to geostationary orbits, which require significant fuel to reach a high perigee and adjust inclination, especially when launched from Russian latitudes. OKB-1 sought a cheaper option and settled on a highly elliptical orbit with an apogee over Russia. The name "Molniya" refers to the lightning-fast pass through perigee. The first successful satellite to use this orbit, Molniya 1-1, launched on 23 April 1965, following two launch failures and one satellite failure in 1964. These early Molniya-1 satellites handled civilian television, telecommunications, and long-range military communications, and also carried cameras for weather monitoring and possibly for assessing clear areas for Zenit spy satellites. Their lifespan was about 1.5 years due to orbital perturbations, requiring constant replacements.

The Molniya-2 series followed, providing both military and civilian broadcasting and forming the Orbita television network across the Soviet Union. It was later replaced by the Molniya-3 design. A planned satellite called Mayak, intended to supplement and replace the Molniya satellites in 1997, was cancelled, and the Molniya-3 was eventually succeeded by the Meridian satellites, first launched in 2006. The Soviet US-K early warning satellites, part of the Oko system, have used Molniya orbits since 1967 to watch for American rocket launches. From 1971, American Jumpseat and Trumpet military satellites were also placed in Molniya orbits, possibly to intercept Soviet communications from the Molniya satellites; details remain classified. The American SDS constellation, active since 1976, uses a mix of Molniya and geostationary orbits to relay signals from lower-flying satellites to ground stations in the United States. A Russian constellation called Tyulpan, designed in 1994 for high-latitude communications, never moved past planning. In 2015 and 2017, Russia launched two Tundra satellites into a Molniya orbit for its EKS early warning system, despite the name "Tundra."

Much of Russia and the former Soviet Union lies at high northern latitudes. Broadcasting to these areas from a geostationary orbit requires significant power due to low elevation angles, extra distance, and atmospheric attenuation. Sites above 81 degrees latitude cannot see geostationary satellites at all, and elevation angles below 10 degrees often cause problems. A Molniya orbit satellite, with an apogee altitude as high as that of geostationary satellites and an apogee sub-satellite point at 63.4 degrees north, provides excellent visibility over Russia, northern Europe, Greenland, and Canada for a large portion of its orbit. It also requires less launch energy than a geostationary orbit.