In orbit there is no "down" to rest a structure on — everything is in continuous freefall. Assembly relies on a two-link 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 = theta / w_arm (arm rotation to reach dock point)
Drift accel a ~ microg * g0 (residual microgravity acceleration)
dv_correct = a * t_drift (station-keeping burn to null drift)
- Assembly speed — how fast the arm swings each module from staging position to dock.
- Module mass — heavier modules have more inertia, so the same arm torque produces slower 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.
This 2D view is a top-down projection of the same arm-kinematics and drift model as the 3D original, solved with a two-bone inverse-kinematics chain instead of a full 3D skeleton.