>> if you hit it at its apogee, a lot of it will burn up in the atmophere.
Having several pounds of plutonium burn up in the atmosphere just means it will be spread over a very wide area which is much worse than a chunk of it falling somewhere.
Pu-239 is not particularly dangerous, however. It can cause cancer if inhaled in sufficient quantities — which doesn't need much, but is unlikely if it's scattered in the atmosphere.
