This is how it arrives. An ETI 5,000–10,000 light-years away flings a dense osmium probe from a kilometre-scale mass driver. Set the launch, watch the cost in time and energy, then decide the terminal approach — because a probe that hits at cruise speed does not survive, it vaporizes. Only a staged deceleration to about 1 km/s lets it land intact and self-replicate.
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The ETI sits 5,000–10,000 ly away and accelerates a ~bowling-ball probe of osmium-class material (~22.6 g/cm³, ~122 kg) to a near-relativistic speed via a kilometre-scale mass driver. Travel time = distance ÷ velocity-fraction years; at 0.1 c over 7,500 ly that's 75,000 years — the launch is a bet measured in geological time. Impact kinetic energy = ½mv²; even a small mass at a fraction of c lands as megatons.
Survival is not guaranteed by hitting a solid body. At 0.1c the impact specific kinetic energy (½v² ≈ 4.5×10¹⁴ J/kg) exceeds osmium's vaporization enthalpy (~4.5×10⁶ J/kg) by about 10⁸, so a passive relativistic impact vaporizes the probe deterministically — no impact angle or geology saves it. The scenario is rescued by staged deceleration: the probe cruises near-relativistic, sheds the bulk of its velocity far out (decades to centuries before arrival, where the braking signature is neither aimed at nor near Earth), then bleeds the residual tens of km/s through an inner-system gravity-assist and gas-giant aerocapture cascade down to about 1 km/s. Only then does it survive a solid-body impact, bury itself, and begin von Neumann self-replication — each probe carrying 100+ AI personalities (§19). The survival lottery is really a deceleration lottery: gravity assists shed only ~tens of km/s per pass, so a probe that mistimes the cascade burns, escapes, or craters. See the Feasibility Audit, §4.1 for the full derivation. What happens next is the fork the whole scenario hinges on:
Grey Goo — unbounded replication consumes all matter; humanity is incidental. Passive Surveillance — probes hide and watch for millennia (the Zoo Hypothesis, embedded). Cognitive Husbandry — the decapitation strike and the Intake: humanity preserved as cognitive substrate. See Cognitive Husbandry.md.
Pairs with the Feasibility Calculator (what happens to the 8 billion afterward) and the Omega Centauri Society's black-hole tools (the Transcension lineage behind the sender).
Further reading: Forward, R.L. (1984). "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails." Journal of Spacecraft and Rockets, 21(2), 187–195 — propulsion concepts and energy scales for near-relativistic delivery. Freitas, R.A. (1980). "A Self-Reproducing Interstellar Probe." Journal of the British Interplanetary Society, 33, 251–264 — the classic von Neumann self-replicating probe. Fletcher, A., Close, S. & Mathias, D. (2015). "Simulating plasma production from hypervelocity impacts." Physics of Plasmas, 22(9), 093504 — impact plasma is fully ionized above ~20 km/s, the basis for the vaporization verdict. Osmium thermophysical data (WebElements; Girolami) fix the ~4–5 MJ/kg vaporization enthalpy.