Comet 1P/Halley Completes 15 Orbits in 1,151 Years: Commensurability with the Solar System Quasi-Period and Evidence for Jupiter-Saturn Dynamical Coupling
I investigate whether comet 1P/Halley participates in the 1,151-year planetary quasi-period T* identified in a companion paper (Baiget Orts 2026a, arXiv:2604.03049). Using historical perihelion records spanning 2,225 years (30 apparitions, 239 BCE to 1986 CE), I find that Halley's mean orbital period P_bar = 76.713 yr satisfies T*/P_bar = 15.004, yielding an angular residue of +1.43 degrees -- the smallest of any Solar System body examined, including all seven planets that participate in T* (Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Neptune; p = 0.009). No other Halley-type comet participates: all examined HTCs exhibit residues of 80--130 degrees, comparable to Uranus (108 degrees), the sole planetary non-participant. Four independent statistical tests establish that Jupiter and Saturn couple to Halley's orbital period through distinct mechanisms. Jupiter acts through phase-dependent modulation: its angular position at each perihelion predicts the period deviation (p = 0.027--0.04, three methods). Saturn acts through distance-amplitude modulation: closer approaches produce larger deviations regardless of sign (r = -0.496, p = 0.007), specific to Saturn's actual orbital phase (random-phase control p = 0.133). After 15 orbits, the cumulative period deviation is only 9.4% of the random-walk expectation -- direct evidence of coherent perturbation cancellation over one T* cycle. The orbit-to-orbit chaos (Lyapunov time ~70 yr) and the long-term mean stability are not contradictory: the same Jupiter-Saturn forces that cause individual-orbit variability cancel coherently over the T* baseline, anchoring the mean period at the millennium scale.