After eleven days of travel they arrive at the asteroid and see only a radio mast and the entrance to a tunnel. When they fly their ship into the tunnel a door closes behind them, lights come on, and the tunnel fills with air and artificial gravity. Finding the air breathable, the five impromptu astronauts explore the asteroid. They discover that it is a vast, empty fortress with unknown weapons still in their racks and a control room where the fluting signal originates.