What size of near-earth asteroid could break up in the atmosphere to do sufficient damage to a city without destroying it?

I need a meteor to deliver an extraterrestrial algae-like cellular life form to an urban environment/city.

1. The meteor needs to be far enough that it will not impact the Earth and will not cause sufficient alarm.
2. The meteor needs to be inconspicuous enough that a city would not be evacuated upon approach (in particular, it would be better if it was undetected until it entered the atmosphere, but I don't know if that's feasible), but it still would cause extensive damage (such as with the Chelyabinsk meteor).
3. The meteor needs to deliver several large chunks capable of destroying/damaging buildings and many smaller chunks over the city.

I don't know anything about astrophysics, but a hard-science answer would be appreciated.

posted over 8 years ago

socrates‭

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