Interstellar comets arrive with everything a deep-space platform needs except control. They carry volatiles convertible to propellant, rotate naturally (providing attitude variation), and travel on hyperbolic trajectories through and beyond the solar system. The question is what minimal modification converts a comet from a passenger into a vehicle.
The answer is four thrusters. One primary jet and three secondary jets at 120-degree intervals. The trick is relaxing the control requirement from full six-degree-of-freedom steering to forward-cone steering — sufficient for practical navigation but far less demanding. The secondary jets synthesize continuous in-plane steering through differential thrust. The primary jet handles out-of-plane control through a subtler mechanism: as the comet rotates, the torque direction from the primary jet sweeps predictably, and phase-scheduled firing — timing thrust pulses to the rotation cycle — produces net out-of-plane steering.
This is control through exploitation rather than opposition. Instead of fighting the comet's rotation, the system uses it. The rotation that makes attitude control seem impossible is exactly what enables out-of-plane maneuvers with a single additional thruster. The natural dynamics aren't an obstacle — they're a free actuator that just needs to be timed correctly.
The reachability analysis formalizes this under bounded-curvature constraints: how much can the trajectory bend given the available thrust and the body's moment of inertia? The steering envelope — the set of achievable trajectory changes — maps the gap between what the comet does on its own and what four thrusters can add. That gap is surprisingly navigable.