SOLVED. about characters in a book that are alive, book is on a space shuttle [s]

[Jaye]

It was set mostly on shuttle and in a book inside the story. All of the characters in it were alive, and their book was on a space ship, headed for maybe Jupiter. There were 3 people on board the ship and they were going to put it in a time capsule when they got to the planet. The book was meant to be one of Einstein's but a little girl swapped their dust jackets. I think the people on the ship went through a de-ageing experience, the youngest died of it.

Characters I remember:
Milo; possibly the youngest crewmember
Sylvia; might have been the books main character
Laurel; in the book young then turned up old? Might have been the author of Sylvia's book
There was a queen with a lapdog

Any help would be great, thanks, it's been at least six years since I read it.

[Daphne]

The Great Good Thing by Roderick Townley, or maybe I think it has a sequel? Main character's name is Sylvie.

edited to say: This must be the third book in the series, The Constellation of Sylvie.

[Jaye]

Yes, that's it! Thank you so much.