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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.

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    $\begingroup$ The Chelyabinsk meteor itself seems to satisfy all of your criteria; if it somehow doesn't, we need clarification, else you've already answered your own question! $\endgroup$ – Kromey Mar 16 '16 at 0:05
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The Chelyabinsk meteor (2013) was about 20m in diameter, and fits most of your criteria:

  • It exploded high above the earth, and did not impact the ground.
  • We had no idea it was coming. This is because of its size and the fact that it was coming from the direction of the sun, making it more difficult to detect.
  • The meteor did not rain down large chunks. It exploded into a multitude of tiny pieces. It did cause a good amount of damage, but it was mostly broken glass resulting from the explosion's shockwave. If you search "Chelyabinsk meteor" on youtube, you will no doubt find videos of the damage happening (what a wonderful age we live in, with all the cameras!)

The Tunguska event (1908) was much bigger. As @CortAmmon notes, we don't really know what happened (there were far fewer cameras then). It is estimated that the meteor was in the neighborhood of 100m in diameter, though it is hard to nail down that number. It flattened a sizable chunk of Siberia, and a meteor this size would probably do the same to your city, which sounds like a little too much damage. Also, these days it is far more likely we would see a meteor this big coming. So you probably want a meteor with diameter in the neighborhood of 40m. There is still a risk we would see it coming, but less likely if it, like the Chelyabinsk meteor, came from the direction of the sun.

It is difficult to say whether a larger meteor would break into large chunks, or small bits like the Chelyabinsk meteor. This depends, I expect, mostly on the speed and make up of the meteor. I am not familiar enough with the science to say what speed the meteor should have, but I can suggest that a more iron-rich meteor would likely break into larger chunks than a rocky meteor because the iron is less brittle.

This simulator is fun to play around with, though it doesn't allow you to test the smallish meteors you are interested in. In particular, this simulator shows that lots of factors play into what happens to the meteor when it enters the atmosphere. Besides the meteor's size, its speed, angle of entry, and composition are very important.

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It is believed that this has already happened. The Tunguska event occurred in 1908. It was a large explosion that flattened 2000 square kilometers of forest. We still do not know exactly what happened, but the leading hypothesis is that a meteor broke apart mid air. The result is estimated to be roughly 1000 times more explosive force than the bomb dropped on Hiroshima.

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