Although it's true that plutonium is, in fact, one of the most poisonous (and carcinogenic) substances known to man, it's not necessarily the case that it would spread throughout the upper atmosphere from a spacecraft explosion. This is true because the most common use of plutonium in spaceflight is as a nuclear battery, whereby the heat given off from the plutonium serves to generate electricity for spacecraft systems. When used as such, the Pu is typically encased in a solid ingot of shielding metal; it is not simply a piece of Pu metal on its own. These ingots are extremely durable. Tests done on the one used in the Cassini probe showed that had it been dropped in the upper atmosphere, it was by far the most likely to simply plummet to earth intact and bury itself where it landed (or sank as the case might be). These ingots are designed to transmit warmth but not gamma radiation.