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Radar Ocean Reconnaissance SATellite or RORSAT is the western name given to the Sovietmarker Upravlyaemyj Sputnik Aktivnyj (Управляемый Спутник Активный) (US-A) satellites. These satellites were launched between 1967 and 1988 to monitor NATOmarker and merchant vessels using active radar. RORSATs were launched under cover name of Cosmos satellites.

Because a return signal from a target illuminated by a radar transmitter diminishes as the inverse of the fourth power of the distance, for the surveillance radar to work effectively, RORSATs had to be placed in low earth orbit. Had they used large solar panels for power, the orbit would have rapidly decayed due to drag through the upper atmosphere. Further, the satellite would have been useless in the shadow of earth. Hence the majority of RORSATs carried type BES-5 nuclear reactors fuelled by uranium-235. Normally the nuclear reactor cores were ejected into high orbit (a so-called "disposal orbit") at the end of the mission, but there were several failure incidents, some of which resulted in radioactive material re-entering the Earth's atmosphere.

Notable incidents

  • RORSAT launch failure, April 25, 1973. Launch failed and the reactor fell into the Pacific Ocean north of Japan. Radiation was detected by U.S. air sampling airplanes.

  • Cosmos 954. The satellite failed to boost into a nuclear-safe storage orbit as planned. Nuclear materials re-entered the Earth's atmosphere on January 24, 1978 and left a trail of radioactive pollution over an estimated 124,000 km² of Canadamarker's Northwest Territoriesmarker.

  • Cosmos 1402. Failed to boost into storage orbit in late 1982. The reactor core was separated from the remainder of the spacecraft and was the last piece of the satellite to return to Earth, landing in the South Atlantic Ocean on February 7, 1983.

  • Cosmos 1900. The primary system failed to eject the reactor core into storage orbit, but the backup managed to push it into an orbit 80km (50 miles) below its intended altitude.

Other concerns

Although most nuclear cores were successfully ejected into high orbits, they will still decay after several hundred years.

RORSATs were a major source of space debris in low Earth orbit. During 16 reactor core ejections, approximately 128 kg of NaK-78 (a fusible alloy eutectic of 22 and 78 % w/w sodium and potassium respectively) escaped from the primary coolant systems of the BUK reactors. The smaller droplets have already decayed, but larger droplets (up to 5.5 cm in diameter) are still in orbit. Since the metal coolant was exposed to neutron radiation it contains some radioactive argon-39, with a half-life of 269 years. This is a minor concern as the droplets will burn up completely in the upper atmosphere on reentry and the argon, a chemically inert gas, will dissipate. The major risk is impact with operational satellites. At a typical impact velocity of 10 km/s, a 1cm³ NaK droplet would cause significant damage; the kinetic energy is 43 kJ, about that of a bowling ball at 495 km/h (310 mph).

American radar satellites

The United States National Reconnaissance Officemarker operates a series of terrain-mapping radar satellites known as Lacrosse. These do not have a maritime capability, but the U.S. Air Force and Space Command are developing a satellite constellation known as Space-Based Radar (or SBR). SBR will fulfill the maritime function of RORSATs, as well as have the ability to track aircraft and potentially ground-based vehicles.

German radar satellites

Germany currently is developing the SAR Lupe radar satellite program, which consists of five identical satellites. Three satellites were launched between December 2006 and November 2007. Others followed in 2008.

Chinese radar satellites

Recently the Chinese have purchased Russian technology for this type of satellite. Some commenters assume their interest is in observing the Taiwan Strait.


  • Wiedemann, C., Oswald, M., Stabroth, S., Klinkrad, H., Vörsmann, P., Size distribution of NaK droplets released during RORSAT reactor core ejection, Advances in Space Research, Vol. 35, 2005, pp. 1290-1295.
  • Wiedemann, C., Oswald, M., Stabroth, S., Klinkrad, H., Vörsmann, P., Modeling of RORSAT NaK Droplets for the MASTER 2005 Upgrade, Acta Astronautica, Vol. 57, 2005, pp. 478-489.

External links

See also

  • SNAP-10A, a nuclear reactor launched by the United States

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