RORSAT

Radar Ocean Reconnaissance SATellite or RORSAT is the western name given to the Soviet "Upravlyaemyj Sputnik Aktivnyj (Управляемый Спутник Активный)" (US-A) satellites. These satellites were launched between 1967 and 1988 to monitor NATO 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 Canada's Northwest Territories.

* 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 a majority of the nuclear cores were successfully ejected into high orbits, these orbits still decay. If left untouched, the highly radioactive cores will return to the Earth's atmosphere after several hundred years.

RORSATs have also been identified as a major contributor to the space debris environment in the low Earth orbit region. During 16 reactor core ejections, approximately 128 kg of the NaK-78 (a fusible alloy eutectic of 22 and 78 % w/w sodium and potassium respectively) from the primary coolant system of the BUK reactor have been released. While the smaller debris particles resulting from these events have already decayed, larger particles (up to 5.5 cm in diameter) are still in orbit. Since the metal coolant was irradiated with neutrons from the nuclear reactor it will contain quantities of the radioactive argon-39,Fact|date=June 2008 which has a half-life of 269 years. However, this is a minor concern, as the droplets will burn up completely in the upper atmosphere once they reenter. The major concern is the risk of an impact into an operational satellite. At a typical impact velocity of 10 km/s, a 1cm³ NaK droplet causes significant da

Mass of 1 cubic centimetre NaK droplet = 0.866g [cite web | title=BASF Corporation – Sodium-Potassium Alloy (NaK) | url=http://www.basf.com/inorganics/pdfs/tech_datasheet/NaK.pdf | accessmonthday=September 13 | accessyear=2006 ]

K_e = 0.5 × M × V²

K_e = 0.5 × 0.000866kg × 10,000²m/s = 43,300 joules.

American radar satellites

The United States National Reconnaissance Office 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. The others will follow until the end of 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.Fact|date=February 2007

References

External links

* [http://www.astronautix.com/craft/usa.htm Encyclopedia Astronautica article on the US-A RORSAT program.]
* [http://www.svengrahn.pp.se/trackind/RORSAT/RORSAT.html The US-A program and radio observations thereof]
* [http://www.space.com/news/mystery_monday_040329.html Space.com on the RORSATs' leaky radiator problem]

Journal Articles

* 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.