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The worst ever radiation spill from a derelict space vehicle came in January 1978 when the nuclear-powered Cosmos 954 satellite crashed over northwestern Canada. The Soviets claimed the craft completely burned up on re-entry, but a massive recovery effort by Canadian authorities recovered a dozen fragments, most of which were radioactive.

The Phobos-Ground also contains a tiny quantity of the radioactive metal Cobalt-57 in one of its instruments, but Roscosmos said it poses no threat of radioactive contamination.

The spacecraft also carries a small cylinder with a collection of microbes as part of an experiment by the Pasadena, California-based Planetary Society that designed to explore whether they can survive interplanetary travel. The cylinder is attached to a capsule that was supposed to deliver Phobos ground samples back to Earth.

Igor Marinin, the editor of Russia’s Novosti Kosmonavtiki magazine, said on Russia’s NTV television that it will likely be destroyed in the fiery re-entry.