The Comet Rudder

A comet on a hyperbolic trajectory is already heading out of the solar system. It carries volatile propellant — ice that sublimates into jets. If you could ride a comet and steer its jets, you’d have a deep-space vehicle with its own fuel supply and a free departure trajectory. The engineering question is stability: can jet-actuated corrections keep a comet-rider on course through a multi-field disturbance environment?

Andrée derives the stability framework. Tracking errors arise from the interplay between disturbances (gravitational perturbations, outgassing variability, debris encounters) and corrective jet firings. Four stability categories emerge: disturbance-energy stability (bounded input energy produces bounded tracking error), outer-loop contraction (errors shrink over time despite persistent disturbances), actuator-memory stability (past corrections don’t accumulate into oscillation), and rotation-mediated Floquet stability (the comet’s spin creates periodic dynamics that must remain bounded).

The paper also provides diagnostic signatures: if a comet’s trajectory shows improving short-term error correction, reduced error persistence, regular residuals, and bounded recovery after disturbances, it is being actively steered rather than passively tumbling. These diagnostics apply equally to planetary defense — distinguishing a naturally perturbed object from one under deliberate control.

The structural point: the comet is simultaneously the vehicle, the fuel tank, and the disturbance source. Its outgassing both propels and perturbs. Stability requires using the same physical process for correction that generates the errors needing correction — a control problem where the actuator is the noise source.