Quire a few others have noted that in his "Thunder" series John Varley is continuously referring to Robert Heinlein's novels. But what I haven't seen so far is the notion, which struck me after the second book of the series and seems still more pertinent after the third, that what Varley is doing here is reimagining Heinlein's entire sequence of juveniles for another age.
From 1947 through 1962, Heinlein wrote a total of 14 juveniles. The first dozen were published by Scribner's; they rejected STARSHIP TROOPERS as too adult and this effectively ended the series, although a final junvenile, PODKAYNE OF MARS, was published tree years later.
The Heinlein novels. although all stand alone, in fact describe a sequence of future events. The first of them, ROCKET SHIP 'GALILEO', gives an account of the first trip to the Moon; later ones introduce Mars, Venus, Jupiter's moon Ganymede and the Asteroid belt; after that, the novels make the leap to the stars, initially in early explorations, later on in the series into a galaxy largely explored by mankind. In this respect, PODKAYNE OF MARS is a throwback to the earlier part of the sequence.
What strikes me is that Varley is more or less writing the same sequence of stories, beginning on Earth, going on to Mars and, in the newest book, envisioning future trips to the stars. I hope he continues there. These novels are very close in feel to the Heinlien stories, but firmly placed in the context of a future bleivable in the present. Heinlein was never able to let his characters actually have sex, as Scribner's editors wouldn't allow it; instead he made fun of their prejudices by making his main characters so naive that even early 1950s teenagers must have gotten the point (in TUNNEL THROUGH THE SKY, the hero lives for a month in a cave on an alien planet with a girl without realizing that she isn't a boy; when another boy happens along, he knows it within minutes). I note that someone else commenting on these books feels that their sexual openness should bar them from any child's reading list; this to me seems pure idiocy. Kids today grow up watching ads, TV shows and movies depicting sex openly and continuously; Varley's novels accept it as part of life. I find that commendable and if anything adding to their merit as superior juvenile science fiction.
The Heinlein project was in a sense unique. Almost no other major sf author made a similar effort to write at the top of his or her form for young readers, possibly with the exception of Andre Norton and Ted White. Varleys novels are in my view the best sf juveniles to be published over the last at least three decades; if there is anyt writing around today more prone to make younger readers advance from Harry Potter to science fiction, I've yet to find out about it.