Lucky searches for a traitor in the agrav propulsion project, who is leaking information to the Sirians.

Maybe because I figured out the mystery in one of the first chapters, but I found this book to be a little slow. Asimov spent a lot more time talking science without putting it in a true story format, which also slowed it down. Finally, Starr's obsession with humanoid robots is already wearing a little thin.

I like reading about science in a science fiction novel, but I much prefer that it be integrated into the story, rather than an aside by the author to try and teach us as in a lecture. That's exactly how Asimov treats Jupiter several times in this book. Eclipses are exciting to watch, and I'm sure it would be even more so when watching from Jupiter, but the description of it was not, unfortunately.

The book does carry the theme of the earlier books with it, that the Sirians (the Spacers from Aurora?) are planning to attack Earth, and they have spies all over the place. In this book, information about the secret agrav project, which can store gravitational energy for later use, is being sent beyond Earth's solar system. It's probably the immediate precursor for the efficient drives used in the Foundation years.

Several investigators have come to Jupiter Nine before Lucky, but none have found the spy. I wonder how much hostility they faced without Lucky's determination to deal with situations as he does. Regardless of Lucky's motivations, I agreed with his assessment of the commander in the first chapters, who can't seem to control his crew's violent tendencies and almost condones the fight that will ensue. Lucky gets into the tunnels and uses the agrav tube, which was pretty cool, but filled a little too much with technical jargon -more than previous novels, I think. A work crew is at his destination waiting for him, ready to give him an "initiation" to the station. Funny that they don't force Bigman to do the same. Of course, Bigman pulls a pistol on the gang when they cheat at the fight.

Asimov paints the gang leader, Summers, as an ex-criminal always willing to start a fight, but he is somewhat redeemed when it is explained that he befriended Norrich, who was blinded years ago in an accident on the station. Summers always looks out for Norrich and even got him a new seeing-eye dog when his died.

People always show a lot of hostility toward Lucky in these books. Bigman is even worse, as he jumps to conclusions and doesn't seem to know the basics about humanity -but maybe that's expected from someone who grew up in the backwater communities of Mars. But he does the same thing in all of these books, so I find it unoriginal. He even pulls a stunt on Norrich to "prove" that the man isn't blind like he says he is, shutting the light off and watching for Norrich's reaction. I liked the way Norrich explains away his reaction, as in reality, blind people do seem to hear extraordinarily well, and he knows his dog's routine of going to sleep as soon as the light is turned off.

What I don't like is Lucky's intuitive (and correct) guess that the spy is a robot. It is certainly plausible (as seen by the outcome), but it is not the only possibility. Why is it impossible that somebody could hide all their emotions perfectly, as a perfect spy could be trained to do? In a nice touch of continuity, Lucky has brought a V-frog from Venus to amplify the emotions of the people on the base, hoping to catch the surprise or anger (or something) of the spy. This is his only plan, and it is the only thing he can think of to try. So when somebody kills the V-frog, his plan goes astray, but he concludes that only a robot could have done it and been the spy (otherwise why stop there and not kill Lucky, too, he thinks). Not only that, but a robot that perfectly mimics a human being. I'm not sure what this is supposed to show. R. Daneel Olivaw had already made an appearance in The Caves of Steel by the time this book was published, and we saw the reaction to both Earth-humans and Spacers at that time, both of which were unfavorable. Additionally, Daneel was presented as the first truly humaniform robot, many many years after the Lucky Starr novels take place (even The Bicentennial Man took a long time to appear completely human). Does it showcase Lucky's ignorance that such a thing could be created at this time in "history"? Or is it supposed to be presented as a true development, given the revelation at the end of this novel? I'm not clear on the intention.

So with the agrav ship ready for testing, and no reliable method of detecting the spy, Lucky forces his way on board the test, possibly hoping to set up a situation that the robot could fail, though he is nowhere near as experienced as Susan Calvin, who destroyed robots psychologically in I, Robot or Robot Visions. One of the tenets he believes is that robots can show no emotion, so he dismisses anybody who gets angry with him, which is probably the entire crew!

The trip down into Jupiter's gravity well, which is presented as something ordinary ships of this sort could not do, was spent looking for a spy in the engine compartments. I, too, found it strange that the chief engineer would simply sit in his quarters as the ship made its first-ever shift from regular engines to agrav. This is where a lot of the science is given to us, in either a textbook or condescending manner, and which I didn't really enjoy.

The ship travels to Io, about which people of course knew nothing when this novel was written (just like naming the first men on the moon, which seem to be Russian, by their names). I didn't care, because it could have just as easily been another of Jupiter's moons, one not as volcanic as we now know Io to be. They set up camp for several days, watch the dance of the other moons, eclipse of the sun by Jupiter and various moons, and so on. Bigman displays a childish attitude of playing in the liquid ammonia that I don't recall his displaying previously. It gets him into trouble, as he falls into a small ammonia river and rips his oxygen tube. Fortunately, Mutt, Norrich's dog, comes to the rescue and gets him to safety.

When they leave Io, the ship is sabotaged, and Summers is found to be missing, proving him to be a traitor, but not the spy, according to Lucky, who illogically is still convinced the spy can have no emotions. They fall toward Jupiter, but Lucky takes charge, ordering the engineers to cannibalize for parts to replace those damaged, and setting a course for a moon even closer to Jupiter, where they easily land. I find it strange that there was no concern about their ability to land. When they do take off and get back to Io, Summers is still waiting for the Sirians to pick him up for his good work. Lucky is convinced they will never come for him.

But by now, Lucky has figured out what I did back when they first met Norrich, but he doesn't even tell Bigman. The robotic spy is Mutt, the seeing-eye dog. It was first obvious when the V-frog first registered emotion from the cat, but then the author didn't mention any emotion from the dog later when it was angry. When it rescued Bigman, I was absolutely certain. But Lucky was so obsessed with a humaniform robot that he overlooked this possibility. I still find it amazing that the spacers could create such a robot that was indistinguishable from a dog, which must be even more difficult than a human in some ways (especially since we cannot relate directly to what a dog is thinking, even if we can sympathize).

So Lucky sets up a situation where Bigman attacks Norrich, and the dog's positronic brain freezes. So the spy is found, the traitor Summers is killed (by falling off a cliff), and the project is saved. I expect, since the next book is the last in the series, that we will see more of the Sirians, and hopefully even a close to that theme.

While the story held my interest, the mystery did not, and this, combined with a bit too much tech talk and the obsession with robots, left me feeling a little cold toward this novel. We'll have to see what the next one holds.