This one of my favorites of the Heinlein juveniles. The book really was ahead of its time in many ways. Even though the book is aimed at 10-12 year old boys, it still contains:

  • A fractured family (dead mom, stepmother, stepsister, etc) and the problems and emotions that ensue
  • Several nice little discussions about spaceship design that are still mostly relevant today (eg, how thick to make the skin of the ship)
  • Considerable detail about ecological considerations when terraforming.
  • A credible explaination of the economics of new colonies; how they are financed, where the profit comes from and the mistakes that can be made along the way (and how they can lead to problems back on the mother planet).
  • Lots of descriptions of Jupiter's system, and what it would look like from Ganymede including a prescient mention of Jupiter's "false rings", ie, rings that weren't visible from Earth.
  • Tons of detail about farming in an unearthly setting where there even the dirt has to be manufactured from the materials at hand.

It really is good. Heinlein always treated his readers with respect, and expected them to be smart. As a kid, this really made me feel good.