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Status: accepted · 6 Sep 2026. Supersedes the preview gesture in ADR-0034; the camera descent and surface arm remain as specified there.

Context

A dragged figure needs to stay under the pointer. A figure held on the local vertical above a selected latitude moves away from the hand, and its straight fall gives little indication of depth. The interaction calls for a curved launch from the hand toward the body, with a flexible connection to the place where that launch reaches the ground.

The preview and release also need one coordinate owner. React can retain a hit/miss flag while the pointer crosses a valid region, but a coordinate kept in that same state stops updating with the visible marker.

Decision

Pin the held end to the pointer, integrate a launch to choose the surface endpoint, and draw a damped spring chain between them.

The browser unprojects the pointer onto an eye-facing plane in front of the drawn body. Its placement and orientation convert that point to body radii in body-fixed axes. This uses the body's compression, so projecting the held end back through the camera returns the original pointer pixel.

launchArc integrates an inward and upward launch under inverse-square gravity. Its adaptive step follows the local orbital timescale, keeping distant launches bounded in cost. The transverse speed limits angular momentum below a grazing orbit's, so the launch reaches the unit sphere. The observatory converts that contact to latitude and longitude and places the endpoint at the drawn surface plus eye height.

SpringRope resamples the launch by arc length into 33 particles. Neighboring particles exert spring forces; damped attraction to the launch curve supplies its bending shape. The solver takes fixed 1/120-second steps, bounded to eight per presented frame. Both endpoints are pinned exactly; only the interior has inertia. Reduced motion follows the equilibrium curve immediately.

The observatory owns the landing coordinate. Release reads that owner before clearing the gesture. Pointer cancellation, lost capture, and a target change cannot commit a drop, and another pointer cannot take over the gesture.

The held figure and destination have distinct marks. Lines have pixel widths; the ground ring has a dark keyline for clouds and snow. Orbit traces and labels recede during aiming and descent through a temporary presentation stance, then return. Descent feedback shows height, progress, and a return-to-orbit action.

Alternatives considered

Move the figure above a directly selected latitude. This supplies a radial fall but separates the held figure from the pointer.

Draw a static Bézier curve. This gives an arch without inertia or tension. A spring chain responds to pointer motion while its pinned endpoints retain precise meaning.

Let the rope's moving endpoint decide release. This makes landing depend on whether the springs have settled. The destination is pinned to the launch contact, so the camera arrives at the visible marker even during motion.

Consequences

The pointer, marker, and camera arrival agree, and the curve responds to the hand without changing canonical simulation state. Tests compare camera positions with preview endpoints on Earth and Phobos, check spring behavior at 30, 60, and 120 frames per second, and project the held end back to browser pixels.

The launch is an aiming model in normalized body units. It contacts the datum sphere before the observatory adjusts the endpoint to the drawn surface; it is not a simulation of terrain collisions along a physical tether. The camera flies from its own position to the same destination, rather than jumping to the pointer to follow the rope. Choosing a site therefore means inspecting the surface marker, rather than placing the pointer directly on that site.