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"Ah." Madeline leaned forward, matching A.J.'s arms-on-table posture. "I get it. Phobos could be an accident. That base survived because it was in vacuum, not to mention microgravity. The Vault can't be an accident."



"No. Mind you, I'm not saying they planned for sixty-five million years. I suspect they didn't. But they planned for millions, may be a few tens of millions." A.J. glanced at Helen. "Somebody like Nick Glendale who specializes in probability analysis could demonstrate it, I'm pretty sure, just from the math alone."



"I'll ask him to, in fact," Helen said. "Once you and Joe put together the basic data."



"Millions of years . . ." Madeline said softly. "Millions . . . Okay, I get your point. If the Bemmies had a faster-than-light drive, there'd be no reason to create such a vault. Even with all of them dead in this solar system, they'd expect some other Bemmies to come along much sooner than that."



"Yep. They could travel between the stars, but even for them it was a slow business."



"A haphazard one, too," Rich said. He ran fingers through his thinning hair. "Without an FTL drive, there'd be no way to maintain any sort of transsolar political unity of any kind. It'd be hard enough to do, even with one. However the Bemmies were organized, politically, it would have started fragmenting the moment they spread beyond their home system. Give it a few millennia, certainly tens of millennia, and even the records would start getting lost. As if Shelley's poem Ozymandias was repeated over and over again, in one star system after another."



Helen had always loved that poem, to the point where she'd committed it to memory. She recited the closing lines now:

"My name is Ozymandias, King of Kings, Look on my Works, ye Mighty, and despair! Nothing beside remains. Round the decay Of that colossal Wreck, boundless and bare The lone and level sands stretch far away."





Silence filled Thoat, for a time, as they contemplated an alien civilization spreading across star systems over an immense span of time—and losing its memory as it went. The thought was majestic and melancholy at the same time.



Helen herself broke the silence. "I understand. They'd have no reason to expect any other Bemmies to come into our solar system at any given time."



"No, they wouldn't," Joe agreed. "A reactionless drive isn't magic. All it does is make sublight interstellar travel possible, where it really isn't with any kind of rocket drive."



"Why not?" Rich asked.



"Because you're basically driving yourself—any kind of rocket, chemical or nuclear-power, it doesn't matter—by throwing exhaust out the back end. That means the farther and longer you want to go, the more fuel you need to bring with you—but the more fuel you carry, the harder it is to increase your speed. We engineers call it the rocket equation, and it's been a paradox for us since the beginning of the space age."



"Simply put," A.J. elaborated, "the best speed a rocket can reach—relative to the velocity of the exhaust that's driving you forward—is proportional to the natural logarithm of the percentage of mass left after all the fuel is consumed."



Seeing the linguist's cross-eyed look, Joe chuckled. "Let me put it more simply still, Rich. Could you cross the Atlantic in a small boat with an outboard engine? Assume for a moment that the ocean is as still as a pond, and there are no weather problems. Just look at it as a straight fuel-and-engine problem."



"Well . . . no, not really. Oh, I suppose you could eventually get across—assuming, like you said, that we ignored the real conditions of an ocean. But, jeez, it'd take forever."



"Why?"



"Well, it's obvious. To keep the engine going, you'd have to haul a great big damn barge full of gas, and how fast could you possibly go if . . . Oh. I see."



"Yup. Welcome to the rocket equation. On Earth, on the oceans, we can get by just by making the ships big enough. That works, well enough, with speeds that low. But it really doesn't work, if you're trying to cross stellar distances with a rocket drive. That's because the mass ratio problem gets progressively worse, the faster you go. And with distances like that, you have to go very fast, or you'll spend . . . Oh, with chemical fuels, it would take thousands of years just to reach Alpha Centauri—and you'd need a fuel tank about the size of the Moon. Nuclear drives are better, but not that much better."



"What you're saying, in short," Helen came in, "is that a reactionless drive is the equivalent of using sails to cross the ocean. However the Bemmie system worked, they were able to use some sort of energy that they didn't need to carry with them."