In orbit there is no "down" to rest a structure on — everything is in continuous freefall. Assembly instead relies on a robotic arm to translate and dock modules, while residual drag and gravity-gradient forces (micro-g) slowly perturb an unsecured module's position, requiring active station-keeping.
Swing time t = θ / ω_arm (arm rotation to reach dock point)
Drift accel a ≈ μg · g₀ (residual microgravity acceleration)
Δv_correct = a · t_drift (station-keeping burn to null drift)
- Assembly speed — how fast the robotic arm swings each module from staging position to dock.
- Module mass — heavier modules have more inertia, so the same arm torque produces slower, more sluggish motion and more overshoot at dock.
- Residual accel. — tiny drag/gravity-gradient acceleration that drifts an undocked module if station-keeping is off.
- Station-keeping — toggles active correction thrusters that null drift on already-docked modules.
Real orbital assembly (ISS truss segments, future large solar-power stations) is done almost exactly this way: robotic arms like Canadarm2 grapple modules launched separately and berth them one at a time, since gravity can't be used to simply "stack" parts as on Earth.