Lieutenant MacDonald smiled thinly. "It's not the Martians we're worried about, Flick. It's those damned moons." "What about them?" "Well, we want to make sure we clear them, that's all. You see, Mars has two moons, Deimos and Phobos. They're tricky little gadgets to calculate when you're plotting a landing on the mother planet. Both of them travel like bats out of hell. The inner one, Phobos, takes only seven hours and thirty-nine minutes to make a complete revolution. Deimos scoots along even faster. Though it's three times as far from its primary as Phobos, it gallops through its orbit in thirty hours and twenty minutes." "So," Muldoon said, "What? You're not afraid of one of them hitting us, are you? We're traveling faster than they are. And if you know where they're going to be at any given moment--" "No, we don't expect one of them to hit us. The thing we have to guard against is _our_ hitting one of _them_. You see, those satellites have peculiarities. One of them is that every once in a while, for no known reason, they suddenly cease being tiny balls of inert matter hurtling about their primary, and for a brief period become tremendously potent magnets. "Technicians have been studying the problem for a long time, but so far haven't discovered the solution. All we know is that the oddity exists. And so long as it does, Deimos and Phobos remain a constant hazard to spacecraft approaching Mars." "Magnets?" said Muldoon. "You mean they exert force on us? Drag us down to them like--" "Like," interrupted Captain Warren with a sudden bellow of dismay, "_this! Lark, throw clear!_" * * * * * For in the split of a second a change had marred the smooth, even flight of the _Liberty_. There came upon Muldoon a swift and sickening sensation of increased weight. Despite himself he lurched and tumbled forward to his knees aware that the ship's nose had spun dizzily off course, and that the cruiser itself was streaking at increased speed in a direction unplotted by the pilot. Then everything happened at once. Great beads of perspiration springing from his brow, Lark O'Day began pounding his controls like a master organist playing the keyboard of a delicate instrument. Captain Warren leaped to the audiophone, barked sharp commands to the men in the engine room below. And over the intercommunicating system MacDonald was crying hurried instructions to crew and passengers alike. "_Go to emergency quarters immediately! Hammock yourselves for crash landing!_" "Crash landing!" gasped Muldoon. "Here!" Warren grasped his arm, threw him into one of the well-padded percussion chairs of the control turret. "Lock your safety belt and relax. Everything's going to be all right ... I hope." He turned questioning eyes to Lark O'Day. The one-time privateer took time from his labors for an encouraging grunt. "I think so. We're hooked, but I think I can bounce her down on a slant. Hold tight, everybody."