Dragonfly establish communications; the AI brings the Flouwen and the crew into contact and the two species begin to exchange cultural and scientific knowledge. The crew witnesses the birth of a Flouwen youngling, while one crew member takes a ride on a Flouwen who is fascinated by human technology. The Flouwen realize the humans are travelling to the pole and warn them that they are approaching a period where the configuration of the planetary system will cause the ocean on Eau to partially flow to Roche. They try to stop the humans from continuing into this violent event by pinning the spacecraft to the ocean floor with ice as ballast (water ice sinks in the ammonia-water solution of the ocean). The humans then realize that the interplanetary waterfall poses a threat to the lander crew remaining on Roche. Fortunately, the tidal stresses cause nearby dormant volcanoes to become active again. This melts an underwater glacier and floods the area with warm water, upon which the ice floats off the plane. The crew manages to get airborne and return to Roche. They rendezvous with the lander just as water is reaching it, and the entire landing party makes it back to the Prometheus. An epilogue is set many years later, when a follow-on mission arrives from Earth and is met by the sole surviving crew member, the others having died in accidents or from old age. This episode is expanded in the last sequel novel, Rescued from Paradise. Forward's light sail propulsion system The light sail system consists of three functional parts: a powerful laser, a large focusing lens, and a giant space-sail. The idea behind the solar sail is that the laser provides a small force on the sail when the sail reflects the light. This small force provides the acceleration of the spaceship. With the ship's primary source of energy coming from the outside, it is not limited to traveling distances that it had enough fuel for. The light used in the system is from an array of a thousand laser generators, which are focused through lenses and aimed at the sail. The lasers provided up to 1,500 terawatts of power. Two different lenses were used to magnify the laser beams. The acceleration lens is 100 km in diameter and is able to accelerate the ship at 0.01g; the deceleration lens is 300 km in diameter and is able