As reported in the press, a Navy Aegis missile cruiser in the Pacific Ocean will try an unprecedented shoot-down of the out-of-control, school-bus-size U.S. spy satellite loaded with a toxic fuel as it begins its plunge to Earth. Marine General James E. Cartwright, the vice chairman of the Joint Chiefs of Staff, said that the Navy missile will be fired as the satellite re-enters the atmosphere and "has a reasonably high opportunity for success."  The Navy has been developing -- and has successfully tested -- a ballistic missile intercept capability aboard several of its Aegis cruisers and destroyers. 

Aegis is an advanced radar/fire-control system that was originally developed to destroy incoming anti-ship cruise missiles. The Navy has 22 cruisers and some 60 destroyers with the Aegis system and between 90 and 122 vertical-launch cells for surface-to-air and Tomahawk land-attack missiles. The Navy is upgrading several of these ships for the ballistic missile defense role.

The three previous spy satellites that fell to earth with nuclear reactors on board were Soviet Radar Ocean Reconnaissance Satellites (RORSAT). The RORSAT was part of the world's first satellite system orbited for ocean surveillance to detect warships on the high seas. The Soviet Union began tests of the system in 1967 and the first operational RORSATs went into orbit in 1974. 

Two types of satellites were used in tandem: Electronic Intelligence (ELINT) satellites that were "passive" and sought to "lock on" to electronic signals emanating from ships, especially radar transmissions. The 18,400-pound EINT satellites became operational about 1970. Pairs of ELINT satellites were coordinated with a single RORSAT, guiding the latter to a suspected target ship. The 10,000-pound RORSAT could then use active radar to precisely locate and target Western warships. Later RORSAT satellites could send targeting data directly to missile-armed ships, surface ships, and submarines as well as to ground stations (as did the early RORSATs).

The power requirements for the RORSAT's radar was provided by a small nuclear reactor that carried some 110 pounds of enriched uranium (U235) to produce up to ten kilowatts of power for some 90 to 120 days in space.  When the service life of these Kosmos-series RORSATs was complete the reactor section carrying the radioactive fuel and weighing about one ton was designed to be boosted into higher orbits -- more than 550 miles -- where they would circle the earth for more than 500 years and then cause no danger when they did come down and burn in the atmosphere. (They normally orbited at an altitude of about 130 miles.)

But three reactor sections malfunctioned and plunged into the atmosphere: Kosmos 954 in January 1978, with portions of the craft landing in Canada; Kosmos 1402 in February 1983, which fell into the Indian Ocean; and Kosmos 1990 in February 1989. Apparently no significant pieces of the last survived reentering the atmosphere.

No attempt was made to intercept those SPYSAT reactor sections when they plunged to earth.