The Stellar Death Clock II: From the Closing Biotic Window Problem to an Engineering Plan

The brightening of the Sun imposes a deterministic deadline on Earth’s biosphere: a literature spanning three decadesbrackets the collapse of the plant biosphere between ∼0.5 and ∼1.9 Gyr (Franck et al., 2000; Graham et al., 2024; HaqqMisra and Wolf, 2026), with the oxygen clock of Ozaki and Reinhard (2021) (1.08±0.14 Gyr) potentially the earliestof the well-quantified clocks. This is the closing biotic window problem, and its correct treatment is worst-case planning:the deadline must be assumed at the short end of the bracket, because the penalty asymmetry between early and lateengineering is total. Comparing the three response families — interstellar exodus, Mars terraforming, and orbitalcomigration — on energy per unit of preserved biomass selects comigration as the only non-dominated strategyon this ledger. An engineering plan is then presented: the first installment of an asteroid-assisted maintenancemigration (1.0 →∼1.053 au), executed by a single recycled ∼145-km main-belt object cycling in a resonant corridoranchored at Q ≈ 11.2 au with a perihelion slightly interior to Earth’s orbit (nominal period 15 yr, stepped down byeach Earth encounter and restored by periodic Jovian refuelling), each oblique crossing encounter transferring aladder-defined orbital-energy increment to the Earth–Moon system, with a mean crossing-radius displacement of∼1.8 km reported as a phase-dependent diagnostic. The execution is partitioned into ∼32 campaigns of ∼2 Myrof encounters separated by 1-Myr fallow periods for secular stabilization and state re-acquisition, keeping everycampaign inside the ∼5 Myr Lyapunov time of the chaotic inner Solar System. The plan is specified in closed analyticform: N ≈ 4.2×106 encounters, a nominal ∼96-Myr execution schedule of the planning model, a ∼5-fold nominalmargin against the worst-case deadline (∼11-fold against the oxygen clock; the double-pessimistic delivery cornerstill completes inside both under the same model), and a civilization-paid program energy budget ten orders ofmagnitude below the exodus vector per unit of preserved biomass. The plan is stated independently of any particularimplementation; its numerical validation is deliberately left pending for future work of the series. The closing bioticwindow problem is thus not only qualitatively motivated but quantitatively specified — and it should be posed now— while its deadline, and the execution of its remedy, are both still measured in hundreds of millions of years.