Previous encounters with Heinlein left me quite underwhelmed, and The Rolling Stones again greeted me with generic characters and an unexciting opening. However, the silly humor grew on me about 100 pages in and I was pleasantly surprised to find that it incorporated some heavy concepts of science-fiction. A space adventure from an older and simpler time.

Heinlein's early work is famous for being addressed to an audience of teenage boys and providing them with some pep talk that they can achieve greatly in life. So, here too we have teenage twin brothers with exceptional talent and vision as the main protagonists. Castor and Pollux Stone have ideas of how to get rich by interstellar trade.

Throughout the book, they follow various variations of this project. In the first part, they try to acquire a spaceship. They succeed, but end up as crew members of a family trip rather than the commanders of their own freighter. So, with them on board are their mom and dad, their grandmother, and their older sister and toddler brother. At first, their plan is to sell bicycles on Mars, then they travel to a dangerous asteroid belt with the idea of selling luxurious goods to the prospectors digging there for precious metals. As it turns out, trading is a much more complex business than they anticipated. And there are other obstacles and dangers that need to be overcome.

For a modern audience, the adventures feel very old-fashioned. Occasionally, the events take unexpected turns (for instance, the talk about telepathy is totally a red herring) and some developments show a decent amount of ingenuity. Similarly, the humor is really from another time, but it's somewhat amusing. Like when at the end of a lengthy and eventful episode of selling their bikes to the Martian citizens, with already much less profit than they anticipated, the twins get to know about the steep taxes that Mars charges for this sort of transaction - and the two end up with hardly anything but experience. Or when their grandmother defends them in court, but in such a subpar way that even the judge asks them if they wouldn't rather drop her from the case.

There is barely any character development, but I have to say that I started to take a liking in the heroes of the story. When reading Stranger in a Strange Land I was frankly appalled by its portrayal of women. This is not an issue here. In fact, I was quite surprised at how progressive this novel is for its time.

The grandmother is the superior engineer and it's often her who solves problems in a brilliant way. She is easily my favorite character. The mother is a very successful doctor and becomes the hero in her own subplot that revolves around an epidemic in space. Actually, I would have liked to follow the plot thread more closely. The father is successful, too, but he is clearly the least gifted member of the family. Actually, he has many qualities more traditionally associated with the loving mother stereotype. There is an exception, though. The sister is apparently too stupid to do other things than cooking and to look out for a husband to settle down with.

I liked how the story addresses the intricacies of space travel. Most importantly, at many points it emphasizes the importance of movement (including astronomical objects) and distance. Minimal deviations (the decimal places in your calculations) in initial conditions or in regard to your destination may eventually lead to enormous deviations. As is pointed out, this also makes it more difficult to model the events, as it's difficult for reality to closely instantiate the theoretical pathways. It even discusses more basic aspects, like the impacts that zero gravity may have on the human body (space sickness as opposed to sea sickness).

Certain events within the story are explained in more scientific terms. For instance, it is explained how thrust of less g is necessary to take off from the moon (as compared to the Earth), because of its weaker force field. It also considers how flights may be much cheaper at certain times and planetary constellations and even explores the social consequences that these circumstances imply (much more people staying on a planet to wait for these events, an immensely increased chance of collision at other times). It's pointed to how they create a frame of reference or how they use radar reflectors to later regain goods they throw in the direction of Mars.

Along the way, it introduces concepts such as escape velocity (necessary to overcome the gravitational field of a planet), burnout (the point when to turn off fuel burning), the Titius–Bode law (describing the spacing between planets in a solar system), or the slingshot or swing-by maneuver (by which planets are used to increase the speed of smaller bodies on their way to other planets). I was really impressed how such explanations feel very natural even within a story that is so lightweight in spirit.

Interesting side note: In court the mother excuses herself by saying that she is a "stranger in a strange land". So, this is a theme that was on Heinlein's mind many years before the release of the novel by that name. There is one thing that I still don't understand, though. Why is this book called "Two Times to Pluto and Back" (in German)? Is Pluto even mentioned in the story?

Rating: 3/5