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A several-gram spacecraft travelling at relativistic speeds would pack quite a punch. Definitely the most dangerous piece of space junk ever created. It's interesting to imagine making our first contact with intelligent extra-terrestrials by causing a small Tunguska-like event.

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The spacecraft themselves would be essentially invisible: A gram-size interstellar probe striking the Earth’s upper atmosphere at 20 percent the speed of light would release roughly a kiloton of energy, indistinguishable from airbursts produced by meter-scale space rocks that regularly pepper our planet at a rate of about once per year.

So they also want to fund an array that would start looking for a robot impact from other civilizations and recognize it as such.

Tunguska is beginning to make sense now.

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