He glanced around as we came in, and his eyes popped out on stalks. He half rose from his seat. "A--a message, Sparks?" he quavered. I shook my head. "No word yet," I reassured him. "I'll let you know. Meanwhile, what's the trouble around here?" "Trouble?" repeated Lancelot wonderingly. The Old Man groaned and pawed at what little remains of his hair. "Don't look now," he rasped, "but didn't Todd call me a couple of minutes ago with some wild-and-woolly tale about Themis disappearing?" "Oh--_that_!" smiled Biggs gently. "I thought for a second you meant there was something wrong. Why, yes, Dad. Themis _has_ disappeared--temporarily. Oddest thing--" "Talk sense!" I moaned. "Todd said something about there being a large body in our path, too. Did it--" I took a look at the central vision plate which reflected nothing between us and the far stars--"did it go away?" "Oh, no," drawled Biggs nonchalantly. "It's still there." "Still there!" I looked again, more closely, at the vision plate. It was as bare as a debutante's backbone at a ball. "What's still _where_, Lanse? Have you gone off your gravs, or are my optics myopic?" "The large body," said Biggs blandly. "It's Themis' moon. It's there. Three points to starboard, and one degree to loft." "Themis' _moon_!" croaked the Old Man. "What in Hades are you talkin' about, Lancelot! Themis _is_ a moon!" "I know," agreed Biggs. His larynx bobbled pleasantly. "That's the curious part of it. This is the first time in the solar system that any satellite has ever been found to have a satellite of its own! But we've located it, charted its trajectory, and cross-checked our calculations--haven't we, Dick?" Todd looked up from the plot table. "That's right," he said hollowly. "Themis _has_ got a moon of its own. An--an invisible moon!" "_Invisible moon!_" The skipper and I did a twin act. "Yes," said Biggs. "You know, I believe that's why Themis--er--'disappears' periodically. It is circled by a large, opaque satellite with the peculiar property of being able to bend light waves around itself. Consequently, every time the moon, revolving around its primary, comes between Themis and observers, Themis is occulted--and disappears!" * * * * * The Old Man looked at him like he had just grown a second head. "B-but that's impossible, son!" he gasped. "Oh, no," said Lanse quietly. "Unlikely, yes. But not impossible. Because--well, because the situation _does_ exist, you see." He clucked thoughtfully. "Strange, isn't it, that we should be the first to find it out? After these many years. But that's the Laws of Chance for you. Every other time a ship visited Themis, the invisible moon must have been on the far side." Hanson was fidgeting like he had wasps in his weskit. Now he broke in, "That's all very interestin'! But how about the chances of our crackin' up on this aforesaid moon-of-a-moon?"