A book about a medieval type moon colony that is slowly dying because comets are no longer crashing into the moon to release more air and water. It features a girl who he finds in an egg shaped spaceship that takes them to a space station orbiting Earth. There she meets a boy called Ondray.
Reunion, by John Gribbin and Marcus Chown.
From the Amazon description:
A sequel to "Double Planet". The moon has been colonized for more than 1000 years, but now the comets have stopped coming and the atmosphere is slipping away. Only one young girl holds the secret that can return the comets - but first she needs help, and Earth is the only place to find it.
Additionally, looking at the chapter list, book 2 within this is called "Ondray"