A bottom-up cost analysis comparing orbital and terrestrial AI datacenter economics. Using a parametric economic model, we test three chip architectures and three launch cost scenarios against Bernstein Research, Stargate, and Nvidia terrestrial compute cost benchmarks.We estimate a 1 GW orbital compute constellation costs $46B/GW at 2028 deployment, already below Stargate's $50B benchmark, and falls below Bernstein's optimistic $35B/GW floor by 2032 as Wright's Law compresses satellite manufacturing costs. SpaceX's vertical integration, Terafab's chip economics, and accelerating terrestrial cost inflation are converging to make orbital compute economically defensible sooner than the market appreciates.