Good points all, Kaptain K.

One can always argue that the neutronium in question is not stable, but a slowly-decaying remnant of a larger chunk. Or, some process or environmental condition is keeping it stable artifically (certainly plausible if it's a weapon rather than a natural object).

But our OP is quite incorrect in thinking an Earth-mass neutronium sphere would not be a planet killer. It would be far more so than the asteroid/comet in the Armageddon/Deep Impact movies. Those might wipe out the Earth's surface, but a collision with the neutronium would totally destroy the planet, probably vaporize it (or at least completely melt it), and the planet that recondensed would be quite different from the Earth we know. For example, it would have twice the mass, thus much higher surface gravity, and its core would be neutronium (with iron outside that).

Even a near miss would probably be a planet-killing event. The tides around an 80-meter object with the mass of the Earth would be, well, astronomical. It could rip the Earth to pieces without even coming into contact with it! The Moon would be the least of our worries.

Think about it -- you're talking about shooting a planetary-scale mass into the vicinity of the Earth. That's not trivial, friends!

Now, if the amount of neutronium is the teaspoonful originally suggested, you're talking about something less massive than the "Armageddon" object -- more like Mount Everest. That could be nasty, but might not end civilization as we know it. Still, I wouldn't want to be anywhere near the impact site.