diff --git a/nav.py b/nav.py
new file mode 100644
index 0000000..ca03c84
--- /dev/null
+++ b/nav.py
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+#!/usr/bin/env python3
+"""Navigation computer of the SS Probably Fine.
+
+Usage: python3 nav.py DESTINATION
+"""
+
+import sys
+
+# Distances between neighboring waypoints, in million km.
+DISTANCES = {
+    ("earth", "moon"): 0.4,
+    ("earth", "gateway"): 0.4,
+    ("gateway", "mars"): 225.0,
+}
+
+# Every route starts at Earth.
+ROUTES = {
+    "moon": ["earth", "moon"],
+    "mars": ["earth", "gateway", "mars"],
+}
+
+NAMES = {
+    "earth": "Earth",
+    "moon": "Moon",
+    "gateway": "Lunar Gateway",
+    "mars": "Mars",
+}
+
+CRUISE_SPEED = 0.87  # million km per day
+FUEL_PER_MKM = 0.142  # tonnes of fuel per million km
+LAUNCH_FUEL = 380.0  # tonnes of fuel to escape Earth's gravity
+
+
+def distance(a, b):
+    if (a, b) in DISTANCES:
+        return DISTANCES[(a, b)]
+    return DISTANCES[(b, a)]
+
+
+def legs(route):
+    return [(route[i], route[i + 1]) for i in range(len(route) - 1)]
+
+
+def plan(destination):
+    route = ROUTES[destination]
+    total = 0.0
+    fuel = 0.0
+    for start, end in legs(route):
+        d = distance(start, end)
+        total += d
+        fuel += d * FUEL_PER_MKM
+        if start == "earth":
+            fuel += LAUNCH_FUEL
+    return route, total, total / CRUISE_SPEED, fuel
+
+
+def main():
+    destination = sys.argv[1]
+    route, total, days, fuel = plan(destination)
+    print("Route: " + " -> ".join(NAMES[w] for w in route))
+    print(f"Distance: {total:.1f} million km")
+    print(f"Travel time: {round(days)} days")
+    print(f"Fuel required: {round(fuel)} t")
+
+
+if __name__ == "__main__":
+    main()