Upravlyaemy Sputnik Aktivnyy ( for Controlled Active Satellite), or US-A, also known in the Western world as Radar Ocean Reconnaissance Satellite or RORSAT (GRAU index 17F16K), was a series of 33 Soviet reconnaissance satellites. Launched between 1967 and 1988 to monitor NATO and merchant vessels using radar, the satellites were powered by nuclear reactors. US-A satellites were part of the Legenda satellite system.
Because a return signal from an ordinary target illuminated by a radar transmitter diminishes as the inverse of the fourth power of the distance, for the surveillance radar to work effectively, US-A satellites 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 the satellites 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.
The US-A programme was responsible for orbiting a total of 33 nuclear reactors, 31 of them BES-5 types with a capacity of providing about two kilowatts of power for the radar unit. In addition, in 1987 the Soviets launched two larger TOPAZ nuclear reactors (six kilowatts) in Kosmos satellites (Kosmos 1818 and Kosmos 1867) which were each capable of operating for six months. The higher-orbiting TOPAZ-containing satellites were the major source of orbital contamination for satellites that sensed gamma-rays for astronomical and security purposes, as radioisotope thermoelectric generators (RTGs) do not generate significant gamma radiation as compared with unshielded satellite fission reactors, and all of the BES-5-containing spacecraft orbited too low to cause positron pollution in the magnetosphere.positron pollution from TOPAZ
The last US-A satellite was launched 14 March 1988.
One of the last RORSAT follow-ups, called Kosmos 1867, was on 19 July 2025, carried out from the IAAM Foundation tracking stations, coordinated by the Mallorcan astronomer Amado Carbonell Santos, on the island of Mallorca, Spain.
Incidents Launch failure, 25 April 1973. Launch failed and the reactor fell into the Pacific Ocean north of Japan. Radiation was detected by US air sampling airplanes. Kosmos 367 (04564 / 1970-079A), 3 October 1970, failed 110 hours after launch, moved to higher orbit. Kosmos 954. The satellite failed to boost into a nuclear-safe storage orbit as planned. Nuclear materials re-entered the Earth's atmosphere on 24 January 1978 and left a trail of radioactive pollution over an estimated 124,000 square kilometres of Canada's Northwest Territories. Kosmos 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 7 February 1983. Kosmos 1900. The primary system failed to eject the reactor core into storage orbit, but the backup managed to push it into an orbit below its intended altitude.
Other concerns Although most nuclear cores were successfully ejected into higher orbits, their orbits will still eventually decay.
US-A satellites were a major source of space debris in low Earth orbit. The debris is created two ways:
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 BES-5 reactors. The smaller droplets have already decayed/reentered, but larger droplets (up to 5.5 cm in diameter) were still in orbit as of 2012. Since the metal coolant was exposed to neutron radiation, it contains some radioactive argon-39, with a half-life of 269 years. There is no risk of surface contamination, as the droplets will burn up completely in the upper atmosphere on re-entry and the argon, a chemically inert gas, will dissipate. The major risk of the now solid objects is impact with operational satellites.
An additional mechanism for radioactive material release is through the impact of space debris hitting intact contained coolant loops. A number of these old satellites are punctured by orbiting space debris—calculated to be 8 percent over any 50-year period—and release their remaining NaK coolant into space. The coolant self-forms into frozen droplets of solid sodium-potassium of up to approximately several centimeters in size,C. Wiedemann et al, "Size distribution of NaK droplets for MASTER-2009", Proceedings of the 5th European Conference on Space Debris, 30 March-2 April 2009, (ESA SP-672, July 2009). and these solid objects then become a significant source of space debris themselves.A. Rossi et al, "Effects of the RORSAT NaK Drops on the Long Term Evolution of the Space Debris Population", University of Pisa, 1997.
List of US-A satellites There were 38 Rorsat satellite launches from Baikonur, all with reported mass of 3,800 kg. +Rorsat satellite launchesLaunch dateSatellite nameLaunch vehiclePerigee (km)Apogee (km)Inclination (deg)Period (min)1968 March 22Cosmos 209Tsyklon87692765.30103.001969 January 25US-A mass modelTsyklon -100 - -1970 October 3Cosmos 367Tsyklon 29151,02265.30104.501971 April 1Cosmos 402Tsyklon 29651,01165.00104.901971 December 25Cosmos 469Tsyklon 29481,00664.50104.601972 August 21Cosmos 516Tsyklon 29061,03864.80104.501973 April 25n/a - failureTsyklon 2 - - - -1973 December 27Cosmos 626Tsyklon 290798265.40103.901974 May 15Cosmos 651Tsyklon 289094665.00103.401974 May 17Cosmos 654Tsyklon 29241,00664.90104.401975 April 2Cosmos 723Tsyklon 289996164.70103.601975 April 7Cosmos 724Tsyklon 285294365.60102.901975 December 12Cosmos 785Tsyklon 29071,00465.10104.201976 October 17Cosmos 860Tsyklon 292399564.70104.301976 October 21Cosmos 861Tsyklon 292898764.90104.201977 September 16Cosmos 952Tsyklon 291199064.90104.101977 September 18Cosmos 954Tsyklon 225126565.0089.701980 April 29Cosmos 1176Tsyklon 287396264.80103.401981 April 21Cosmos 1266Tsyklon 291194164.80103.601981 August 24Cosmos 1299Tsyklon 292696265.10103.901981 March 5Cosmos 1249Tsyklon 290497665.00103.901982 August 30Cosmos 1402Tsyklon 225026665.0089.601982 June 1Cosmos 1372Tsyklon 291996664.90103.901982 May 14Cosmos 1365Tsyklon 288197965.10103.601982 October 2Cosmos 1412Tsyklon 288699864.80103.901984 June 29Cosmos 1579Tsyklon 291497065.10103.901984 October 31Cosmos 1607Tsyklon 290899465.00104.101985 August 1Cosmos 1670Tsyklon 28931,00764.90104.101985 August 23Cosmos 1677Tsyklon 28801,00164.70103.901986 August 20Cosmos 1771Tsyklon 29091,00065.00104.201986 March 21Cosmos 1736Tsyklon 293699565.00104.401987 February 1Cosmos 1818Tsyklon 277579965.01100.611987 June 18Cosmos 1860Tsyklon 290099265.00104.001987 July 10Cosmos 1867Tsyklon 279781365.01100.81987 December 12Cosmos 1900Tsyklon 269673566.1099.101988 March 14Cosmos 1932Tsyklon 29201,00865.10104.40
See also SNAP-10A, an experimental nuclear reactor launched into orbit by the United States Space-based radar List of Kosmos satellites
References
External links Encyclopedia Astronautica article on the US-A RORSAT programme. The US-A program and radio observations thereof
Science & Global Security, 1989, Vol. 1. Background on space nuclear power. scienceandglobalsecurity.org