apply gravity towards Jupiter for objects orbiting Jupiter

src/actor.rs

@@ -44,6 +44,7 @@ impl Plugin for ActorPlugin {
         app.add_systems(
             Update,
             (
+                handle_gravity,
                 handle_input.run_if(in_control),
                 handle_collisions,
                 handle_damage,
@@ -51,9 +52,16 @@ impl Plugin for ActorPlugin {
         );
         app.add_systems(PostUpdate, (handle_vehicle_enter_exit,));
         app.add_event::<VehicleEnterExitEvent>();
+        app.insert_resource(GravityUpdateTimer(Timer::from_seconds(
+            1.0,
+            TimerMode::Repeating,
+        )));
     }
 }
 
+#[derive(Resource)]
+pub struct GravityUpdateTimer(Timer);
+
 #[derive(Copy, Clone)]
 pub enum DamageType {
     Unknown,
@@ -157,6 +165,8 @@ pub struct WantsMaxVelocity(pub f64);
 pub struct WantsToLookAt(pub String);
 #[derive(Component)]
 pub struct Identifier(pub String);
+#[derive(Component)]
+pub struct OrbitsJupiter;
 
 #[derive(Component)]
 pub struct LifeForm {
@@ -632,3 +642,27 @@ fn handle_gforce(
         }
     }
 }
+
+fn handle_gravity(
+    time: Res<Time>,
+    mut timer: ResMut<GravityUpdateTimer>,
+    mut q_pos: Query<(&Position, &mut LinearVelocity), With<OrbitsJupiter>>,
+    id2pos: Res<game::Id2Pos>,
+) {
+    if !timer.0.tick(time.delta()).just_finished() {
+        return;
+    }
+
+    let pos_jupiter: DVec3 = if let Some(pos_jupiter) = id2pos.0.get(cmd::ID_JUPITER) {
+        *pos_jupiter
+    } else {
+        return;
+    };
+
+    // this assumes prograde orbits for every object
+    for (pos, mut v) in &mut q_pos {
+        let relative_pos = pos.0 - pos_jupiter;
+        let accel = nature::gravitational_acceleration(relative_pos, nature::JUPITER_MASS);
+        v.0 += accel;
+    }
+}

src/cmd.rs

@@ -668,6 +668,7 @@ fn spawn_entities(
                     1.0
                 } * state.model_scale,
             );
+            let orbits_jupiter = state.id != ID_JUPITER;
 
             // Spawn the actor
             let actor_entity;
@@ -680,6 +681,9 @@ fn spawn_entities(
                     ..default()
                 });
                 actor.insert(SleepingDisabled);
+                if orbits_jupiter {
+                    actor.insert(actor::OrbitsJupiter);
+                }
                 actor.insert(world::DespawnOnPlayerDeath);
                 actor.insert(actor::HitPoints::default());
                 actor.insert(Position::from(absolute_pos));