Exploration of Jupiter disadvantages for colonization because of the severe radiation conditions prevailing in Jupiter's magnetosphere and the planet's particularly deep gravitational well. Jupiter would deliver about 36 Sv (3600 rem) per day to unshielded colonists at Io and about 5.4 Sv (540 rems) per day to unshielded colonists at Europa, which is a decisive aspect due to the fact that already an exposure to about 0.75 Sv over a period of a few days is enough to cause radiation poisoning, and about 5 Sv over a few days is fatal. Ganymede is the Solar System's largest moon and the Solar System's only
Swords of Mars allies Jat Or and Zanda in the other. The passage shrinks the travelers down until to them the tiny moon is the size of a planet. There, all are captured by the sun-worshipping Tarids, white-skinned and blue-haired natives of Thuria (which they call Ladan) whose mental powers render them invisible to the spacefarers. To win freedom from their jeweled prison, the antagonists must join forces with each other, aided by another captive, the one-eyed and two-mouthed chameleon-like "cat-man" Umka. Amid betrayal and heroic sacrifice, the parties from Mars eventually return to their home planet with Carter and Dejah still separated