Lucifer's Hammer - Larry Niven [42]
"I think we can ignore the rocks," Sharps said. "Do you see why, now?"
"Lord God, yes," Harvey said. He looked at the camera with a start. "Uh, yes, Dr. Sharps, it certainly makes sense to ignore the rocks."
"You're not going to show this, are you?" Tim Hamner sounded indignant.
"You're saying no?" Harvey asked.
"No … no … " Hamner doubled over and giggled.
"Now, she's coming at cometary speeds. Fast. Let's see, parabolic speed at Earth orbit is what, Dan?"
"Twenty-nine point seven kilometers per second. Times square root of two."
"Forty-two kilometers a second," Sharps announced. "And we've got Earth's orbital velocity to add. Depends on the geometry of the strike. Shall we say fifty kilometers a second as a reasonable closing velocity?"
"Sounds good," Forrester said. "Meteors go from twenty to maybe seventy. It's reasonable."
"Right. Call it fifty. Square that, times a half. Times mass in grams. Bit over two times ten to the twenty-eight ergs. That's for the vanilla ice cream. Now we can figure that most of the hot fudge boiled away, but understand, Harvey, at those speeds we're just not in the atmosphere very long. If we come in straight it's two seconds flat! Anyway, whatever mass you burn up, a lot of the energy just gets transferred to the earth's heat balance. That's a spectacular explosion all by itself. We'll figure twenty percent of the hot-fudge energy transfers to Earth, and"—more buttons pressed, and dramatic rise in voice—"our grand total is two point seven times ten to the twenty-eighth ergs. Okay, that's your strike."
"Doesn't mean much to me," Harvey said. "It sounds like a big number … "
"One followed by twenty-eight zeros," Mark muttered.
"Six hundred and forty thousand megatons, near enough ' Dan Forrester said gently. "It is a big number."
"Good God, pasteurized planet," Mark said.
"Not quite." Forrester had his own calculator out of the belt case. "About three thousand Krakatoas. Or three hundred Thera explosions, if they're right about Thera."
"Thera?" Harvey asked.
"Volcano in the Mediterranean," Mark said. "Bronze Age. Where the Atlantis legend comes from."
"Your friend's right," Sharps said. "I'm not sure about the energy, though. Look at it this way. All of mankind uses about ten to the twenty-ninth ergs in a year. That's everything: electric power, coal, nuclear energy, burning buffalo chips, cars—you name it. So our hot fudge sundae pops in with about thirty percent of the world's annual energy budget."
"Um. Not so bad, then," Harvey said.
"Not so bad. Not so bad as what? A year's energy in one minute," Sharps said. "It probably hits water. If it hits land, it's tough for anyone under it, but most of the energy radiates back out to space fairly quickly. But if it hits water, it vaporizes it. Let's see, ergs to calories … damn. I don't have that on my gadget."
"I do," Forrester said. "The strike would vaporize about sixty million cubic kilometers of water. Or fifty billion acrefeet, if you like that. Enough to cover the entire U.S.A. with two hundred and twelve feet of water."
"All right," Sharps said. "So sixty million cubic kilometers of water go into the atmosphere. Harvey, it's going to rain. A lot of that water is moving across polar areas. It freezes, falls as snow. Glaciers form fast … slide south … yeah. Harvey, the historians believe the Thera explosion changed the world's climate. We know that Tamboura, about as powerful as Krakatoa, caused what historians of the last century called 'the year without a summer'. Famine. Crop failure. Our hot fudge sundae will probably trigger an ice age. All those clouds. Clouds reflect heat. Less sunlight gets to Earth. Snow reflects heat too. Still less sunlight. It gets colder. More snow falls. Glaciers move south because they don't melt as fast. Positive feedback."
It had all turned dead serious. Harvey asked, "But what stops ice ages?"
Forrester and Sharps shrugged in unison.
"So," Hamner said, "my comet's going to bring about an ice age?" Now you could see the long lugubrious face of