diff --git a/src/sim/galaxy.js b/src/sim/galaxy.js
new file mode 100644
index 0000000..dacce84
--- /dev/null
+++ b/src/sim/galaxy.js
@@ -0,0 +1,158 @@
+import { makeRng, hash32 } from './rng.js';
+
+const SYSTEM_NAMES = [
+  'Arcadia', 'Boreas', 'Cygni', 'Drakon', 'Eridu', 'Fenrir', 'Gorgon', 'Helix',
+  'Icarus', 'Juno', 'Kepler', 'Lyra', 'Mira', 'Nyx', 'Orion', 'Pavo',
+  'Quasar', 'Rigel', 'Sirius', 'Talos', 'Umbra', 'Vega', 'Wraith', 'Xandar',
+  'Yara', 'Zenith', 'Altair', 'Bellatrix', 'Castor', 'Deneb',
+];
+const PLANET_SUFFIX = ['Prime', 'II', 'III'];
+
+const PROFILES = [
+  { kind: 'agri', produces: { food: 10, meds: 3 }, consumes: { fuel: 4, tech: 3 } },
+  { kind: 'mining', produces: { ore: 12, fuel: 6 }, consumes: { food: 6, meds: 2 } },
+  { kind: 'industrial', produces: { tech: 6 }, consumes: { ore: 8, fuel: 5, food: 5 } },
+  { kind: 'refinery', produces: { fuel: 10 }, consumes: { ore: 6, food: 4 } },
+  { kind: 'resort', produces: { luxury: 5 }, consumes: { food: 7, meds: 4, fuel: 3 } },
+];
+const ALL_GOODS = ['ore', 'food', 'fuel', 'tech', 'meds', 'luxury'];
+
+export function otherEnd(lane, systemId) {
+  return lane.a === systemId ? lane.b : lane.a;
+}
+
+export function generateGalaxy(seed, { systemCount = 25 } = {}) {
+  const rng = makeRng(hash32('galaxy:' + seed));
+
+  // 1. Place systems with minimum spacing (relaxes if placement gets tight).
+  const systems = [];
+  let minDist = 95, attempts = 0;
+  while (systems.length < systemCount) {
+    const x = 60 + rng() * 880, y = 50 + rng() * 600;
+    if (systems.every((s) => Math.hypot(s.x - x, s.y - y) > minDist)) {
+      systems.push({ id: systems.length, name: SYSTEM_NAMES[systems.length % SYSTEM_NAMES.length], x, y, planets: [], lanes: [] });
+    }
+    if (++attempts > 2000) { minDist *= 0.9; attempts = 0; }
+  }
+
+  // 2. Lanes: each system to its 2 nearest neighbors, deduplicated.
+  const laneKeys = new Set();
+  const key = (a, b) => (a < b ? `${a}-${b}` : `${b}-${a}`);
+  for (const s of systems) {
+    const near = systems
+      .filter((o) => o.id !== s.id)
+      .sort((p, q) => Math.hypot(p.x - s.x, p.y - s.y) - Math.hypot(q.x - s.x, q.y - s.y));
+    laneKeys.add(key(s.id, near[0].id));
+    laneKeys.add(key(s.id, near[1].id));
+  }
+
+  // 3. Connectivity repair: link closest pair across components until one component.
+  const parent = systems.map((_, i) => i);
+  const find = (i) => (parent[i] === i ? i : (parent[i] = find(parent[i])));
+  const union = (a, b) => { parent[find(a)] = find(b); };
+  for (const k of laneKeys) { const [a, b] = k.split('-').map(Number); union(a, b); }
+  for (;;) {
+    const roots = new Set(systems.map((s) => find(s.id)));
+    if (roots.size === 1) break;
+    let best = null;
+    for (const s of systems) for (const o of systems) {
+      if (find(s.id) === find(o.id)) continue;
+      const d = Math.hypot(s.x - o.x, s.y - o.y);
+      if (!best || d < best.d) best = { a: s.id, b: o.id, d };
+    }
+    laneKeys.add(key(best.a, best.b));
+    union(best.a, best.b);
+  }
+
+  // 4. Materialize lanes with travel time and hazard.
+  const lanes = [...laneKeys].sort().map((k, i) => {
+    const [a, b] = k.split('-').map(Number);
+    const dist = Math.hypot(systems[a].x - systems[b].x, systems[a].y - systems[b].y);
+    const days = Math.min(4, Math.max(1, Math.round(dist / 80)));
+    const hazard = rng() < 0.2 ? +(0.3 + rng() * 0.4).toFixed(3) : +(rng() * 0.15).toFixed(3);
+    return { id: i, a, b, days, hazard };
+  });
+  for (const lane of lanes) {
+    systems[lane.a].lanes.push(lane.id);
+    systems[lane.b].lanes.push(lane.id);
+  }
+
+  // 5. Planets with economy profiles and a render orbit.
+  const planets = [];
+  for (const s of systems) {
+    const n = 1 + Math.floor(rng() * 3);
+    for (let i = 0; i < n; i++) {
+      const profile = PROFILES[Math.floor(rng() * PROFILES.length)];
+      const stock = {};
+      for (const g of ALL_GOODS) {
+        if (profile.produces[g]) stock[g] = Math.floor(150 + rng() * 150);
+        else if (profile.consumes[g]) stock[g] = Math.floor(40 + rng() * 80);
+        else stock[g] = Math.floor(80 + rng() * 70);
+      }
+      const planet = {
+        id: planets.length, systemId: s.id,
+        name: `${s.name} ${PLANET_SUFFIX[i]}`, kind: profile.kind,
+        produces: { ...profile.produces }, consumes: { ...profile.consumes },
+        stock, stockTime: 0,
+        orbit: { r: 90 + i * 55, a: rng() * Math.PI * 2 },
+      };
+      planets.push(planet);
+      s.planets.push(planet.id);
+    }
+  }
+
+  return { systems, lanes, planets };
+}
+
+// All-pairs hop counts (BFS per system). hops[a][b] = lanes to cross.
+export function hopMatrix(galaxy) {
+  const n = galaxy.systems.length;
+  const m = Array.from({ length: n }, () => Array(n).fill(Infinity));
+  for (let src = 0; src < n; src++) {
+    m[src][src] = 0;
+    const queue = [src];
+    while (queue.length) {
+      const cur = queue.shift();
+      for (const laneId of galaxy.systems[cur].lanes) {
+        const next = otherEnd(galaxy.lanes[laneId], cur);
+        if (m[src][next] === Infinity) {
+          m[src][next] = m[src][cur] + 1;
+          queue.push(next);
+        }
+      }
+    }
+  }
+  return m;
+}
+
+// Dijkstra by travel days. Returns { lanes: [laneId...], days } or null.
+export function findRoute(galaxy, fromSys, toSys) {
+  if (fromSys === toSys) return { lanes: [], days: 0 };
+  const dist = new Map([[fromSys, 0]]);
+  const prevLane = new Map();
+  const visited = new Set();
+  for (;;) {
+    let cur = null, best = Infinity;
+    for (const [s, d] of dist) if (!visited.has(s) && d < best) { best = d; cur = s; }
+    if (cur === null) return null;
+    if (cur === toSys) break;
+    visited.add(cur);
+    for (const laneId of galaxy.systems[cur].lanes) {
+      const lane = galaxy.lanes[laneId];
+      const next = otherEnd(lane, cur);
+      const nd = best + lane.days;
+      if (nd < (dist.get(next) ?? Infinity)) {
+        dist.set(next, nd);
+        prevLane.set(next, laneId);
+      }
+    }
+  }
+  const laneIds = [];
+  let s = toSys;
+  while (s !== fromSys) {
+    const laneId = prevLane.get(s);
+    laneIds.unshift(laneId);
+    s = otherEnd(galaxy.lanes[laneId], s);
+  }
+  return { lanes: laneIds, days: dist.get(toSys) };
+}
diff --git a/tests/galaxy.test.js b/tests/galaxy.test.js
new file mode 100644
index 0000000..15b629c
--- /dev/null
+++ b/tests/galaxy.test.js
@@ -0,0 +1,67 @@
+import { describe, it, expect } from 'vitest';
+import { generateGalaxy, hopMatrix, findRoute, otherEnd } from '../src/sim/galaxy.js';
+import { GOODS } from '../src/sim/constants.js';
+
+describe('galaxy', () => {
+  const g = generateGalaxy('test-seed', { systemCount: 25 });
+
+  it('is deterministic for the same seed', () => {
+    const g2 = generateGalaxy('test-seed', { systemCount: 25 });
+    expect(JSON.stringify(g2)).toBe(JSON.stringify(g));
+    const g3 = generateGalaxy('other-seed', { systemCount: 25 });
+    expect(JSON.stringify(g3)).not.toBe(JSON.stringify(g));
+  });
+
+  it('has the requested systems, each with 1-3 planets and at least one lane', () => {
+    expect(g.systems).toHaveLength(25);
+    for (const s of g.systems) {
+      expect(s.planets.length).toBeGreaterThanOrEqual(1);
+      expect(s.planets.length).toBeLessThanOrEqual(3);
+      expect(s.lanes.length).toBeGreaterThanOrEqual(1);
+    }
+  });
+
+  it('is fully connected (every system reachable from system 0)', () => {
+    const hops = hopMatrix(g);
+    for (let i = 0; i < g.systems.length; i++) {
+      expect(hops[0][i]).toBeLessThan(Infinity);
+    }
+  });
+
+  it('lanes have sane travel days and hazard, and back-reference systems', () => {
+    for (const lane of g.lanes) {
+      expect(lane.days).toBeGreaterThanOrEqual(1);
+      expect(lane.days).toBeLessThanOrEqual(4);
+      expect(lane.hazard).toBeGreaterThanOrEqual(0);
+      expect(lane.hazard).toBeLessThanOrEqual(0.7);
+      expect(g.systems[lane.a].lanes).toContain(lane.id);
+      expect(g.systems[lane.b].lanes).toContain(lane.id);
+    }
+  });
+
+  it('planets have profiles, stocks for every good, and an orbit for rendering', () => {
+    for (const p of g.planets) {
+      expect(Object.keys(p.produces).length).toBeGreaterThan(0);
+      expect(Object.keys(p.consumes).length).toBeGreaterThan(0);
+      for (const good of GOODS) expect(p.stock[good]).toBeGreaterThanOrEqual(0);
+      expect(p.stockTime).toBe(0);
+      expect(p.orbit.r).toBeGreaterThan(0);
+    }
+  });
+
+  it('findRoute returns a connected lane path with summed days', () => {
+    const from = 0, to = g.systems.length - 1;
+    const route = findRoute(g, from, to);
+    expect(route).not.toBeNull();
+    let sys = from, days = 0;
+    for (const laneId of route.lanes) {
+      const lane = g.lanes[laneId];
+      expect([lane.a, lane.b]).toContain(sys);
+      sys = otherEnd(lane, sys);
+      days += lane.days;
+    }
+    expect(sys).toBe(to);
+    expect(route.days).toBeCloseTo(days, 5);
+    expect(findRoute(g, 3, 3)).toEqual({ lanes: [], days: 0 });
+  });
+});