What goes up must eventually come down, including satellites that are currently orbiting the earth. After their work is done, they will be deliberately tossed back into the atmosphere where they will burn up in the high altitudes. Little, if any, will actually hit the ground. But sometimes a satellite fails and falls back to earth unpredictably, posing hazard to human population, wildlife and the environment. And what if the satellite contained an active nuclear reactor?
During the Cold War period, both the United States of America and the Soviet Union launched dozens of nuclear-powered satellites into space. The first was Transit-4A, launched by the US in 1961. It was one of numerous satellites launched between 1959-88 to provide satellite navigation service to the US Navy. It was the first satellite navigation system before GPS. The Transit system had a total of 41 orbiting satellites, of which a handful of them were nuclear-powered. Many of them, while no longer functional, are still in orbit with their reactor core intact.
Between 1964 and 1978, the US launched a further seven nuclear-powered satellites for the Nimbus program. The Nimbus satellites helped meteorologists study the earth’s weather, its changing climate, the ozone layer, sea ice and so on. The technology and lessons learned from the Nimbus missions are the heritage of most of the Earth-observing satellites NASA and NOAA have launched over the past decades. Out of the seven Nimbus satellites, two are still in orbit.
The Soviet Union began launching nuclear satellites into space in 1965. For the next 22 years they launched over thirty satellites, each carrying a small nuclear reactor on board to power the various instruments. These satellites were mostly of reconnaissance type meant to spy upon the US Navy’s vessels and submarines. The satellites used radar to detect marine traffic, and because the radar signal rapidly loses power with distance, the satellites had to be placed in low earth orbit where there was significant drag from air molecules. The air resistance prohibited the use of large solar panels, leaving nuclear power as an attractive and probably the only alternative.
The majority of these satellites carried type BES-5 nuclear reactors fueled by uranium-235. Because of space and weight constraints, the fuel was highly enriched to weapons-grade level so that the reactors were fast, efficient and small, not to mention, extremely powerful. A typical BES-5 nuclear reactor weighed less than 400 kg and generated 100 kW of thermal power, of which about 3 kW was converted to usable electric power. The reactor was mounted inside a separate unit that could be jettisoned further up into space into a higher orbit once the satellite reached the end of its operational life. This way the dead satellite could safely re-enter earth’s atmosphere without the risk of radioactive contamination. But as far as space missions are concerned, things don’t always work out as planned.
Cosmos 954 (also spelled “Kosmos 954”) was launched on 18 September 1977 from the Baikonur Cosmodrome. It orbited the earth between 259 and 277 kilometers every 89.5 minutes. On board was a liquid sodium–potassium thermionic converter driven by a nuclear reactor containing around 50 kilograms of uranium-235. Within weeks of launch, the satellite’s orbit became erratic and it was understood that Cosmos 954 would have a very short life. As Soviet operators struggled to control their failing spacecraft, they realized that Cosmos 954 would fall back to earth very soon. To make matter worse, the system which was intended to dispose the spent reactor core into a safe orbit failed.
In a rare gesture of responsibility and accountability, the Soviet Union preemptively notified the United States, as well as all nations that lay directly under the flightpath of the ailing satellite, about a possible radioactive fallout. In a series of secret meetings with the US, officials of the Soviet Union provided details about Cosmos 954’s nuclear reactor. The US in turn warned its NATO partners that Cosmos 954 was expected to fall, and offered to help clean
up any radioactive contamination that might result.
Cosmos-954 BES-5 type reactor scheme
On 24 January 1978, a few minutes before sunrise, Cosmos 954 entered the Earth’s atmosphere and broke up over Canada. Debris from the satellite fell along a 600-kilometer path from Great Slave Lake to Baker Lake, including portions of the Northwest Territories, Alberta, and Saskatchewan. The subsequent search and clean-up operation cost Canada nearly CA$14 million, while the US spent some USD 2.5 million. Canada later billed the Soviet Union CA$6 million, of which only half the amount was paid.
The original satellite was believed to have weighed between 4 and 5 tons. Of this, only about 65 kilograms of material was recovered. Except for one large fragment, all were radioactive.
An immediate concern after the fall of Cosmos 954 was that a sufficiently large part of the core might have survived re-entry and crashed through the ice to become submerged in water, where it might have become critical. This is because water acts as a moderator, slowing down neutrons and allowing the chain reaction to start. Experts calculated that as little as 22 kg of highly enriched uranium could become critical under these conditions. Despite extensive search, the core was never found. It was concluded that the core had disintegrated almost completely.
In a paper published in August 1984 in Health Physics, the authors noted that at least a quarter of the reactor (about 7 to 8 kg) had fallen in the form of fine particles less than 1mm in diameter. These micro particles fell like an invisible slow fog on the Northwest Territories and on the barren Arctic and sub-Arctic land. The remaining three quarters evaporated into a fine mist and remained suspended in the atmosphere for years, before slowly descending to the earth’s surface. By this time, radioactive decay would have removed most of the shorter-lived radionuclides posing little health risks. As for the millimeter-sized particles, the authors noted that if an individual accidentally swallowed one, it would pass through the digestive tract and out of the body within 48 hours, giving the person a radiation dose no more than a conventional X-ray.
Looking for radioactive debris. Photo: Nevada National Security
Incidentally, Cosmos 954 was not the first nuclear-powered satellite to fail. In 1973, a launch failure of a similar satellite caused the reactor to drop into the Pacific Ocean north of Japan. Another Soviet spy satellite, Cosmos 1402, malfunctioned and fell into the Indian Ocean in 1983. The ejection system failed to jettison the reactor to a higher orbit causing the reactor to fall separately a few days later over the South Atlantic Ocean, completely disintegrating as it did so.
There is a possibility that such an event could occur again in future. There are many nuclear-powered satellites launched during the Cold War period that are still orbiting the earth. Although their orbits are high and currently stable, collision with space debris and meteorites could knock them out of their designated orbits and towards earth.
Nuclear power is still used in space exploration, such as in rovers and scientific instruments on another planetary body, but no longer to power earth orbiting satellites. The hazards from a possible failure far outweighs any potential gain from such a system.
A piece of the satellite lying in snow. Photo: Library and Archives Canada
A Cosmos 954 debris at the Canada Science and Technology Museum.
# Atomic Energy Control Board, Canada, https://inis.iaea.org/collection/NCLCollectionStore/_Public/12/595/12595268.pdf
# CIA, https://www.cia.gov/library/readingroom/docs/CIA-RDP85B01152R000200260006-4.pdf
# Health Physics, https://www.ncbi.nlm.nih.gov/pubmed/6480350
# Alexander F. Cohen, https://digitalcommons.law.yale.edu/cgi/viewcontent.cgi?article=1316&context=yjil