A Worldship built from an icy body or a Planet Nine like object solves shielding and resources but raises a central problem: energy. Moving a small world out of solar orbit and onto an interstellar trajectory demands either very large bursts of power or very long periods of continuous thrust (and usually both).
Page type: science
# How much delta v do we need Two targets matter (escape and cruise) - **Escape from the Sun at large distance** Circular speed at distance *r* is `v_circ ≈ 29.8 km s⁻¹ / √(r in AU)`. Escape speed is `v_esc = √2 · v_circ`. Examples (order of magnitude) - At 30 AU (Neptune like): `v_circ ≈ 5.4 km s⁻¹`, `v_esc ≈ 7.7 km s⁻¹`. A body co moving with the local circular flow needs roughly **2.3 km s⁻¹** of delta v to escape. - At 500 AU (Planet Nine like): `v_circ ≈ 1.3 km s⁻¹`, `v_esc ≈ 1.9 km s⁻¹`. Needed delta v is **about 0.6 km s⁻¹**. - **Interstellar cruise** Practical human timescales push toward **tens to hundreds of km s⁻¹** (thousands to tens of thousands of years to nearby stars). Century class crossings require **percent of light speed**, which is far beyond worldship propulsion built from local volatiles.
# What does that mean in energy terms Kinetic energy `E = ½ M v²`. - Take a modest worldship core with radius **250 km**, bulk density **1.8 g cm⁻³** (ice rock mix). Mass `M ≈ 7 × 10¹⁹ kg`. - To add **1 km s⁻¹**: `E ≈ 3.5 × 10²⁵ J`. - To add **10 km s⁻¹**: `E ≈ 3.5 × 10²⁷ J`. For comparison (context only) - Total annual human energy use is order **10²⁰ J** (so **1 km s⁻¹** for this body equals **hundreds of thousands of current human years** of energy). - Ideal deuterium–tritium fusion yields about **3 × 10¹⁴ J per kg** of fuel (real systems deliver less due to efficiency). Supplying **10²⁷ J** even ideally needs **10¹³ kg** of fusion fuel (and significant reaction mass if you use a rocket like exhaust).
# Why rockets from a world are hard If you throw local material from Mass Drivers or steam rockets you face the rocket equation (Δv = *v*ₑ ln(*m₀*/*m₁*)). With exhaust `vₑ ~ 5 km s⁻¹` (aggressive but plausible for mass drivers) - Δv **1 km s⁻¹** needs mass ratio `≈ e^{0.2} ≈ 1.22` (expel **22 percent** of the world’s mass as propellant). - Δv **5 km s⁻¹** needs mass ratio `≈ e^{1} ≈ 2.72` (expel **63 percent** of the mass). Using high exhaust velocity sources like Fusion Propulsion helps (for example `vₑ ~ 50–100 km s⁻¹` for advanced fusion plumes) but then the engineering burden shifts to fuel production, power handling and nozzle structures on a planetary scale.
# What actually becomes the bottleneck - **Energy generation and handling** (reactors, beamed arrays, thermal management). - **Momentum exchange hardware** (mass driver throughput, exhaust nozzles that survive for millennia). - **Structural integrity** (keeping caverns safe under gentle but relentless thrust and spin). - **Time** (planning for multi millennial construction and acceleration programmes).
## A workable outline 1. **Choose a distant body** (R ~ 200–400 km, ρ ~ 1.5–2.0 g cm⁻³) to minimise escape cost and maximise resources. 2. **Industrialise the mantle** (build mass drivers, radiators, fusion power, closed ecologies). 3. **Spin up gently** (internal rings provide living gravity while the body itself turns very slowly). 4. **Begin beamed assist** (century scale push while building outbound fusion tugs). 5. **Transition to tug driven fusion cruise** (target 5–20 km s⁻¹ over a few millennia). 6. **Optional magsail brake** at destination using interstellar plasma (reduces propellant needed for arrival).
# Bottom line Energy is the main issue. But by choosing a far outer Solar System origin, pushing very gently for very long periods, and offloading power to beamed systems and external tugs, a worldship can in principle reach interstellar escape and modest cruise speeds without destroying itself. The price is measured in **industrial scale**, **patient centuries** and **civilisational continuity**, not in spectacular single burns.