We imagine a Worldship built from a remote Binary Planet system — two icy bodies orbiting one another far beyond Neptune, perhaps at hundreds of astronomical units from the Sun.
Page type: science
This arrangement offers structural stability, tidal heating and redundancy, but it also presents the fundamental challenge of **propulsion**. Moving such massive worlds out of their distant orbit and onto an interstellar trajectory requires a blend of engineering patience and abundant energy.
# Starting Conditions At a distance of 300–1000 AU, orbital velocity around the Sun is roughly 1 km s⁻¹ and escape speed is about 1.4 km s⁻¹. To depart the Solar System from there, the binary must gain only a few hundred metres per second of delta v. The hard part is not escaping but reaching a useful cruise velocity — ideally between 5 and 20 km s⁻¹ — enough to cross the inner light years within a few hundred thousand years. This is modest by interstellar standards yet achievable with millennial engineering.
# Propulsion Requirements Assume each body in the binary has a radius of 250–400 km and density near 1.8 g cm⁻³. Each weighs about 10²⁰ kg. To add 1 km s⁻¹ to that mass requires roughly 5 × 10²⁵ joules. To reach 10 km s⁻¹ needs about 5 × 10²⁷ joules — comparable to the energy our civilisation produces over a million years. The only feasible approach is **continuous low thrust** sustained for millennia.
# Mass Driver Propulsion The simplest and most robust design is the Mass Driver. Linear accelerators embedded around the equator eject regolith or ice pellets at several kilometres per second.
Each reaction launches a small amount of propellant outward while imparting equal momentum inward. If operated in phased synchrony, thousands of driver tubes can produce continuous thrust while keeping the system balanced - wikipedia 
For example, if the drivers expel 10⁶ kg s⁻¹ at 5 km s⁻¹, total thrust is 5 × 10⁹ newtons. For a 10²⁰ kg world, acceleration is about 5 × 10⁻¹¹ m s⁻², which adds roughly 1.5 km s⁻¹ per millennium. Over several thousand years, this gradually lifts the binary onto an interstellar trajectory without catastrophic heating or stress.
# Fusion Tug Propulsion Beyond low thrust, more advanced propulsion can come from external Fusion Tugs. Each tug carries high specific impulse engines (exhaust velocities of 50–100 km s⁻¹) and propellant tanks.
Dozens of these craft dock to the world and apply thrust in long arcs, exchanging energy with the worldship but leaving the inhabited caverns undisturbed. When a tug expends its fuel, it can detach and be replaced by another. This method modularises propulsion and allows gradual upgrades over centuries.
# Binary Push-Pull Propulsion In a binary configuration, both worlds can act as reaction masses for each other. By firing synchronised mass drivers that exchange material across the gap (hundreds of kilometres apart), each planet effectively pushes against its twin. This "gravitational catapult" keeps the total momentum of the pair balanced while raising their shared velocity. Material ejected in mid-space can be magnetically guided or recovered, recycling propellant for closed-loop thrust cycles.
# Beamed Power Assist During the early acceleration stage, we can exploit Beamed Power from the inner Solar System. Gigawatt-scale laser or microwave arrays direct energy at absorber sails or rectenna arrays on the binary’s surface. That power drives boilers or plasma engines without requiring local fuel. Once the pair reaches thousands of AU, beamed energy becomes less efficient, and the worldship transitions to onboard fusion or mass driver propulsion.
# Fusion Steam Rockets The binary’s ice can serve directly as working fluid. Reactors heat water to plasma and vent it through magnetic nozzles, producing thrust. Steam rockets offer relatively high thrust at modest efficiency — useful for initial orbit changes or for braking manoeuvres at a destination system. The waste steam also reshapes surface cavities into shielding layers as it refreezes.
# Energy Sources Primary power will come from Fusion Reactors using deuterium mined from the ice. Each kilogram yields up to 3 × 10¹⁴ joules in theory. Secondary systems include fission startup reactors and geothermal power from residual radiogenic heat. Tidal flexing between the two planets adds a continual low-grade energy source, maintaining warm oceans and preventing total freeze-out during the long acceleration phase.
# Maintaining Binary Stability The mutual orbit of the two bodies provides both rotational reference and tidal heating. However, thrust must be applied carefully to avoid disturbing the orbit. Phased thrust cycles alternately push along and perpendicular to the orbital plane to keep the pair bound while changing their barycentric velocity. Magnetic tethers or low-density gas bridges between them can balance torques and share energy between hemispheres.
# Radiative and Structural Considerations Each thrust phase produces waste heat, which must be radiated through outer galleries and louvres. The bulk ice acts as a thermal buffer, while radiators near the poles dissipate gigawatts of energy to space. Internal habitats, buried beneath tens of kilometres of ice, remain stable. The slow acceleration (less than 10⁻⁹ g) ensures structural loads remain negligible compared to tidal forces or spin stresses.
# Cruise and Navigation Once the binary pair reaches a few km s⁻¹, it continues accelerating for millennia until the desired cruise speed is reached. After shutdown, momentum carries it into interstellar space. Orientation is maintained using gyroscopic stabilisers, mass driver torque control and star tracker navigation. The worldship remains essentially a moving ecosystem — a small planetary system on a very slow arc between the stars.
# Arrival and Braking At the target system, braking can be achieved with **Magnetic Sails** that interact with the interstellar medium, or by ejecting propellant in the opposite direction over centuries.
Another option is gravitational capture using the new system’s gas giants, if the trajectory is planned far in advance. Once captured, the binary can be repurposed as twin colonies orbiting the local star.
# Summary A binary planet worldship trades speed for survivability. Its propulsion systems — mass drivers, fusion tugs, and beamed power — operate gently but persistently, turning time itself into a propulsion source. The pair’s mutual gravity provides both structure and warmth, while slow, deliberate engineering replaces fragile spacecraft with enduring, inhabited worlds that glide between the stars.
# See
- Worldship Energy
- Fusion propulsion - wikipedia
- Mass driver - wikipedia
- Beamed power propulsion - wikipedia