In Jan's request, Sullivan sees the chance to do something that will put him in the history books, and, truthfully, he would do it anyway. He agrees.
Jan writes Maia a letter.
He tells her his plan is to hide in the whale and hitch a ride to the Overlords' world.
He theorizes the trip will take a few months for him, thanks to the speeds of the Overlords' ships. In Earth time, however, the journey will be at least forty years.
This makes the journey an eighty-year round trip, meaning everyone Jan knows will be dead upon his return—assuming a return is even in the cards.
Sullivan recounts the necessary provisions Jan will need for a safe trip, and Jan tells him he plans to solve the supply problem with narcosamine, which basically means he'll hibernate for most of the trip.
Sullivan is glad Jan thought so far ahead. After all, he doesn't want to think he's sending the boy on some highly complicated suicide mission.
We return to Jan's letter: He tells Maia goodbye and says he's looking forward to meeting her grandchildren.