Communications Satellites, Part 4: Red Lightning—Soviet Molniya Spacecraft

The Soviet Union was slow to develop and deploy communications satellites for domestic and international communications in part because, situated in a large landmass with most of its satellite nations adjacent in Eastern Europe, wired or microwave relay communication links were less expensive, more reliable, and better performing. When they became interested in the potential of satellite communications, their geographical situation and technological constraints weighed upon the design. After initial experiments with low Earth orbit satellites, the U.S. had moved toward geostationary platforms (see Part 3 in this series), launching its first, Syncom 1, in 1963, followed by others in the series. A geosynchronous or geostationary orbit is difficult to reach, requiring a launcher to deliver 3.8 km/sec of delta-v in addition to the 9.5 km/sec or so just to get to low Earth orbit. Given the launch vehicles available at the time, the U.S. Syncom satellites had an on-orbit mass of just 39 kg, with 25 kg of that payload, and transmitter power of only two watts. This required a huge antenna on Earth to communicate with the satellite in its distant orbit.

Further, while a satellite in geostationary orbit is easy to track, not requiring a steerable antenna on Earth, it is necessarily in an equatorial orbit (otherwise it would bob up and down in latitude over the day), and is difficult to communicate with from high latitudes (such as much of the Soviet Union’s territory) where the satellite is low on the horizon and its signal subject to atmospheric absorption.

While the Soviet Union had much larger rocket boosters than the U.S. in the early 1960s, its electronics technology lagged far behind and was unable to produce a functional geostationary satellite as small and light as the U.S. Syncom design. So, designers opted for a design called Molniya (Молния—“Lightning”), a massive 1600 kg platform, stabilised in three axes, equipped with six large solar panels and steerable antennas, with a powerful transmitter requiring a modest-sized antenna on Earth. These satellites were launched into what has come to be called the Molniya orbit, with eccentricity 0.74, inclination 63.4°, and a semi-major axis of 26,000 km. A satellite in this orbit has a period of half an Earth day and will “hang” near apogee for hours, moving only slowly, at a high latitude easily visible from all of Soviet territory, including the Arctic. Because the perigee remains close to Earth, the delta-v required to reach this orbit is much less than for a geosynchronous satellite, and within the capability of existing Soviet boosters. The rapid passage through perigee near the Earth gave the satellites their name, “Lightning”.

One disadvantage of the Molniya orbit is that it takes the satellite through the Van Allen radiation belts four times a day, resulting in rapid degradation of solar panels not designed for such an environment. This caused early Soviet Molniya satellites to have limited lifetimes, with some later satellites designed with backup solar panels to be deployed when the original panels began to fail. Eventually, radiation-hardened solar panels reduced the damage and increased satellite lifetime.

Molniya satellites and their successor design, Meridian, have been in use continuously by the Soviet Union and then Russia since 1965. Today, they are mostly used for civil and military communications to Arctic regions of Russia. The U.S. also operates satellites in Molniya orbits, particularly for reconnaissance of the Soviet/Russian landmass.