<b>51. YUCT Gravitational Catastrophism</b>

(51) This paper presents a unified model of periodic mass extinctions, terrestrial catastrophes, and the origin of life within the Yakushev Unified Coordination Theory (YUCT). We show that a massive transient object — the remnant of a second Earth satellite — after a chaotic three‑body interaction with the Earth–Moon system, was ejected into a highly elliptical orbit with a period of 26.1 Myr. Its repeated passages through the inner Oort cloud trigger comet showers, while the same Galactic plane crossing that destabilises the Oort cloud also reduces the coordination efficiency K_eff of the Earth’s mantle. This reduction lowers mantle viscosity and friction along fault zones, producing a synchronous cascade: impact spikes, flood basalt volcanism, geomagnetic excursions, accelerated rifting/collision, ocean anoxia, and mass extinctions — all sharing the same 26–30 Myr cycle. We derive the period directly from the universal YUCT exponent β = 2/3 and quantify each cascade step with testable formulas. The model explains the lunar dichotomy, the mass of Earth’s water, and the 27.5‑Myr geological pulse independently documented by Rampino et al. We further argue that the collision sterilised the Earth–Moon system, rejecting panspermia and implying a local Hadean abiogenesis. Three falsifiable predictions are made. The YUCT cascade thus unifies astrophysics, geophysics, palaeontology, and astrobiology under a single quantitative framework.