RORSAT (Radar Ocean Reconnaissance Satellite)
Although prototype RORSATSs in the mid-1960s flew with only chemical batteries, their operational counterparts, beginning in the 1970s, carried reactors. This introduced a serious risk of contamination because all the RORSATs reentered after a few weeks or months in orbit. To counter the problem, each RORSAT consisted of three major components: the payload and propulsion section, the reactor, and a disposal stage used to lift the reactor into a higher orbit, with an altitude of 900–1,000 km, at the end of the mission. Thus while the main spacecraft decayed, the reactor continued to circle the Earth. Unfortunately, the lifetime of an object at this altitude is about 600 years, whereas uranium-235 and -238 have a half-life of more than one billion years. This means there is presently, in 1,000 km × 65° orbits, about 940 kg of highly-enriched uranium and a further 15 tons of other radioactive material including tens of thousands of droplets, 0.6–2 cm in diameter, which are the remains of the liquid sodium-potassium used to cool the RORSAT reactors.
There have also been accidents – the worst of them on Jan. 24, 1978, when a RORSAT malfunctioned and crashed within Canada's Northwest Territory, showering radioactive debris onto the Great Slave Lake and surrounding region.
Related category SATELLITES AND SPACE PROBES
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