Here we explore propulsion strategies for Icy Moon Worldships.
# Start far out (cheap escape) A Planet Nine like orbit is advantageous. Needing only **~0.6 km s⁻¹** to escape the Sun lets you focus nearly all energy on cruise speed rather than on the escape penalty.
# Push slowly for a very long time Continuous low thrust over millennia works on something the size of a world. - If mass drivers eject **10⁶ kg s⁻¹** of regolith at **5 km s⁻¹** (industrial but conceivable on a small world), thrust is **5 × 10⁹ N**. - Acceleration for a **7 × 10¹⁹ kg** world is **7 × 10⁻¹¹ m s⁻²**. - Velocity gain per year is **~2.2 m s⁻¹** (per millennium **~2.2 km s⁻¹**). - Over **5,000 years** this yields order **10 km s⁻¹** without catastrophic mass loss (consume about **7 × 10¹⁵ kg** per millennium at that ejection rate, which is a tiny skin of an icy mantle). This is the core virtue of a Worldship (time replaces peak power).
# Use beamed energy to avoid onboard power With Beamed Power or a directed energy launcher near the Sun, you accelerate the world without hauling the energy source. Two options: - **Beamed propulsion to attached sails or magnetic structures** (laser or microwave pressure on a reflective shroud or a photonic “ring sail”). The force is small but persistent and does not consume propellant. - **Beamed thermal steam** (focus energy onto external boilers that vent water vapour as a high temperature exhaust). This trades away onboard reactors during the departure phase. Both benefit from staging (use beaming for the first few AU where geometry is favourable, then switch to onboard systems).
# Leverage the Oberth manoeuvre with care In principle an Oberth Manoeuvre at perihelion is the most energy efficient way to add hyperbolic excess. In practice moving a fragile ice rock world to a very low solar perihelion is dangerous (heating and tides). A moderated version is possible (drop only to a few AU and then fire fusion drive clusters) but the benefits are much smaller than a daring near Sun swoop.
# Do not “rocket”, “tug” Mount external **fusion tugs** that carry high specific impulse engines and propellant while the world remains a passive payload. This keeps reaction mass off the habitat and allows swapping tugs as technology improves. Think of a train of power stages that dock, push for a century, then peel away.
# Accept cruise speeds that match civilisation timescales At **30 km s⁻¹** a traverse to **Proxima Centauri** (4.24 ly) takes **~42,000 years**. At **100 km s⁻¹** it is **~13,000 years**. The energy scales as *v²*. The step from 30 to 100 km s⁻¹ multiplies energy by **~11**. The design choice is social as much as technical (how long can a culture remain coherent).