I have always been found the relativistic impactor charming in its simplicity. Mine was only 10% of the speed of light, and it would do a pretty good job.
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It seems apparent that all these efforts are unnecessary. We are doing a great job of eliminating ourselves quite efficiently. They merely need to watch & wait; even most of the infrastructure will likely survive.
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Yes. See the “Trek’s End” "Development Notes” for details. My impactor was a 5 km asteroid (or, more likely, primordial body from Kuiper belt or Oort cloud) with density around that of a stony meteorite, or about 3 g/cm³, moving at 10% of the speed of light, 3×107 m/sec. This gives a kinetic energy of 1036 ergs, which is around ten times that required to boil all the oceans and sterilise the biosphere down to around 1 km.
My assumption was that the aliens would choose a locally-sourced massive object at lower velocity because they’d know the Terries, with their primitive spacefaring capabilities, wouldn’t be able to do anything about it even if they saw it coming. The energy requirement would be almost negligible compared to accelerating an ultrarelativistic impactor and delivering it over interstellar distances.
Don’t try this at home.
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Not at all surprised that you did the calculations behind the story. I am surprised, but suppose I shouldn’t be, that you not only wrote up a webpage documenting the calcs, but also (naturally) attached the programs for the calcs. Kinda cool you did it in perl.
…squeezed my fingers to launch myself…
…3 kg with an index finger alone. I weigh about 75 kg, so that works out to an acceleration of 0.067 m/s².
Not sure if I follow the math here… I might be wrong, but you might want to re-check? (e.g. my thinking is 3kgf = 3kgf*9.8N/kgf = 29.4N. Acceleration = 29.4N / 75kg = 0.39m/s2)
However this doesn’t change your conclusion - of course fingertip-strength should suffice for zero-g maneuvering. Watching astronauts in ISS seems consistent with this.
…better L/D than a watermelon…
“L/D” is pilot-speak for “lift to drag ratio”, the distance an unpowered aircraft travels forward for each unit of distance it descends. Gliders have very high L/D, while watermelons do not, and are thus prone to sloppy landings.
Excellent visual image of watermelons making sloppy landings. But is a watermelon’s landing problem fundamentally an L/D problem? Or should we model a watermelon as a stressed-skin aircraft, and see it’s just a case of the watermelon rind tensile strength doing poorly in the face of an unfavourable cube/square scaling w.r.t. the mass of its contents? 
Besides, an F-104 might suggest L/D is optional for a landing you can walk away from.
…no more than a second to cross the two Earth diameters…
I clearly remember that as the most memorable part from my first reading of the story several years ago. It really impressed me along the lines of “ohmygod hugely fast, and yet observable and comprehensible”. Kinda cool to think that the motion of a 0.1c object is insanely fast yet not instantaneous when seen from GEO distances. Thank you very much for that.
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