Molniya orbit

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Ground trace from a Molnija satellite

A Molnija orbit is a highly elliptical orbit with an inclination of 63.4 ° and a period of exactly half a sidereal day . Molnija orbits are named after the series of Soviet Molnija communications satellites (from Russian Молния : "lightning") that have been using this type of orbit since the mid-1960s.

A satellite placed in a Molnija orbit remains largely stationary over a certain area of ​​the earth in apogee for around eight hours.

Development and properties

Molniya orbit with hour markers

Most of the territory of the former USSR, and especially Russia, is located in quite high northern latitudes. These areas can  only be covered insufficiently by geostationary satellites , i.e. satellites above the equator (inclination = 0 °), due to the unfavorably flat angle of incidence.

The other extreme case, a satellite with a polar orbit (inclination  = 90 °) facilitates contact with the satellite in such areas because it moves vertically over it, but is unsuitable for communication purposes, as it appears to the satellites only for a short time over the area of ​​use leaves. The use of a large number of such satellites is therefore necessary for continuous coverage.

The solution to the problem of providing the smallest possible number of satellites as high as possible above the usage area lies in a highly elliptical orbit and an inclination between  = 0 ° and  = 90 °. Due to the flattening of the earth, satellites usually cause a rotation of the apse , which shifts the argument of the periapsis and thus the position of the apogee and has to be compensated for by position corrections. In the Molnija orbit, this is  avoided by choosing an inclination of = 63.4 ° (“critical angle of inclination”), at which the orbital disturbances cancel each other out.

Since the orbital speed of a satellite according to Kepler's laws is inversely proportional to its distance to the center of gravity (in this case the center of the earth), it traverses the near-earth part of its orbit quickly and the distant part slowly. Due to an east-facing orbit, the angular speed is close to the rotational speed of the earth and so the apparent position of the satellite around the highest point of its orbit, the apogee , changes very little over a longer period of time. Due to the rotation of the earth, the apogee is not always above the same point. In order to avoid unusable apogee, the period of the orbit is chosen so that it is an integral divisor or a multiple of a day and the apogee regularly comes to lie over the same area on earth. A typical Molnija orbit has a period of approx. 12 hours, which means that the satellite appears over the usage area for approx. 8 hours in every second orbit, i.e. once a day. In this way, 24-hour coverage can be guaranteed for a certain area with only 3 satellites.

Track elements

A typical Molnija orbit shows the following characteristic orbital elements:

  • Inclination:  = 63.4 °
  • Orbit height in the perigee :  = 600 km above the earth's surface (this height is often chosen because no significant influences of the earth's atmosphere are to be expected here)
  • Orbit height in apogee:  = 39,750 km above the earth's surface
  • Length of the major semi-axis:  = 26,553 km
  • Eccentricity :  = 0.737
  •  Orbital period : U = 717.74  min

Such an orbit can be reached with relatively little effort from a spaceport in the far north , such as the Plesetsk Cosmodrome (62.8 ° north latitude). A start in an easterly direction already leads to a parking orbit with an inclination that corresponds to the northern latitude of the starting station. In order to achieve the desired inclination of  = 63.4 °, only minor course corrections are necessary. The necessary eccentricity of the orbit or length of the major semi-axis can be achieved by a simple acceleration thrust in the perigee.

Orbits similar to this concept with a period of 24 hours are the tundra orbit and the supertundra orbit .

use

A 1966 Soviet postage stamp shows a Molnija satellite and a schematic sketch of the orbit

The main application of the Molnija orbit was in the series of Soviet communications satellites of the same name. After two unsuccessful launches in 1964, Molnija 1-01 was the first satellite to be brought into this orbit on April 23, 1965. A satellite-based communication link between Moscow and Vladivostok was established for the first time on the following day . The Molnija-1 satellites were used for both military and civil long-range communications, including building the USSR-wide Orbita television transmission system. However, they only had a short lifespan and had to be constantly replaced. From the beginning of the 1970s, the system was replaced by Molnija-2 and until 1977 by Molnija-3 satellites.

With slight adjustments, the same orbits were used by Soviet spy satellites whose apogee was over the United States. Geostationary orbits are suitable for observing the USA, but due to the sensor technology used, high-contrast observation angles were necessary, which could only be reached from higher latitudes. An example of this is the US-KS early warning satellite for the detection of US rocket launches, with subsequent improvements allowing the use of geostationary orbits.

The USA partly used Molnija orbits for its part for spy satellites , whereby the satellites' long stay in the northern latitudes, which is so beneficial for Soviet communication, was used to eavesdrop on them. The electronic reconnaissance satellites Jumpseat and their successor Trumpet also used Molnija orbits. Another application is the Satellite Data System, SDS, for forwarding data from spy satellites operating over Russia to US ground stations. The SDS allowed the real-time data transmission from the low-flying KH-11 - reconnaissance satellites during their flyby on their near-pole webs below the SDS satellites.

Molnija orbits are unsuitable for manned space travel, as they repeatedly cross the high-energy Van Allen belt . The radiation exposure in the Van Allen Belt is also a problem for on-board electronics and is one of the main reasons for the relatively short lifespan of satellites in a Molnija orbit.

literature

  • Ernst Messerschmid, Stefanos Fasoulas: Space systems: An introduction with exercises and solutions , Springer, 2005, ISBN 3-540-21037-7

Individual evidence

  1. a b c Ch. D. Brown: Elements of Spacecraft Design. American Institute of Aeronautics and Astronautics, 2002, ISBN 1-56347-524-3 , pages 107-110, ( excerpt ).
  2. a b M. Capderou: Handbook of Satellite Orbits: From Kepler to GPS. Springer Verlag, 2014, ISBN 978-3-319-03415-7 , p. 393, ( reading sample ).
  3. M. Capderou: Satellites: Orbits and Missions. Springer Verlag, 2005, ISBN 2-287-21317-1 , pp. 228-229, ( reading sample ).
  4. ^ Y. Zhang, Y. Xu & H. Zhou: Theory and Design Methods of Special Space Orbits. National Defense Industry Press and Springer Nature Singapore Pte Ltd., 2017, ISBN 978-981-10-2947-9 , pp. 12-13, ( excerpt ).
  5. a b St. Q. Kidder & Th. H. Vonder Haar: On the Use of Satellites in Molniya Orbits for Meteorological Observation of Middle and High Latitudes. In: Journal of Atmospheric and Oceanic Technology , Volume 7, 1990, pp. 517-522, ( digitized ).
  6. M. Capderou: Satellites: Orbits and Missions. Springer Verlag, 2005, ISBN 2-287-21317-1 , p. 224, ( reading sample ).
  7. a b P. A. Gorin: Molniya. In: St. B. Johnson (ed.): Space Exploration and Humanity: A Historical Encyclopedia Volume 1, ABC-CLIO, 2010, ISBN 978-1-85109-514-8 , pp. 415-417, ( reading sample ).
  8. ^ B. Chertok: Rockets and People - Volume III: Hot Days of the Cold War. The NASA History Series, 2009, ISBN 978-0-16-081733-5 , pp. 453-490, ( reading sample )
  9. RW Sturdevant: OK . In: St. B. Johnson (ed.): Space Exploration and Humanity: A Historical Encyclopedia Volume 1, ABC-CLIO, 2010, ISBN 978-1-85109-514-8 , p. 794, ( reading sample ).
  10. P. Norris: Watching Earth from Space: How Surveillance Helps Us - and Harms Us. Springer Science & Business Media, 2010, ISBN 978-1-4419-6937-8 , pp. 228-229, ( reading sample ).
  11. UN Doniants, Yu. P. Ulybyshev & EF Zemskov: Elliptic Orbit Communication System "Molniya-Zond": Spacecraft, Launch and Orbit Stationkeeping. In: Conference Paper: 56th International Astronautical Congress, at Fukuoka, Japan , 2005, 8 p., ( Digitized version ).