So it is time to move nearer the equator to avoid the incoming Ice Age?

side comment- the entire MASS of the tail of that comet is damn near nothing- Its mostly water vapor and dust- and not all that much of either. You need to worry about the ice cream and the nuts, not the whip cream on the outside ( I LOVED THAT BOOK - Lucifer's Hammer

QUOTE FROM THE BOOK- online at Baen.com
Hot fudge sundae," said Forrester.

"Hah?"

Forrester's grin was wide through his beard. "A cubic mile of hot fudge sundae. Cometary speeds."

Sharps's eyes lit up. "I like it! Let's hit Earth with a cubic mile of hot fudge sundae."

Lord God, they've gone bonkers, Harvey thought. The two men raced each other to the blackboard. Sharps began to draw. "Okay. Hot fudge sundae. Let's see: We'll put the vanilla ice cream in the center with a layer of fudge over it . . ."

He ignored the strangled sound behind him. Tim Hamner hadn't said a word during the whole interview. Now he was doubled over, holding himself, trying to hold in the laughter. He looked up, choked, got his face straight, said, "I can't stand it!" and brayed like a jackass. "My comet! A cubic mile of hot . . . fudge . . . sun . . . dae . . ."

"With the fudge as the outer shell," Forrester amplified, "so the fudge will heat up when the Hammer rounds the Sun."

"That's Hamner-Brown," Tim said, straight-faced.

"No, my child, that's a cubic mile of hot fudge sundae. And the ice cream will still be frozen inside the shell," said Sharps.

Harvey said, "But you forgot the—"

"We put the cherry at one pole and say that pole was in shadow at perihelion." Sharps sketched to show that when the comet rounded the Sun, the cherry at the oblate spheroid's axis would be on the side away from Sol. "We don't want it scorched. And we'll put crushed nuts all through it, to represent rocks. Say a two-hundred-foot cherry?"

"Carried by the Royal Canadian Air Force," Mark said.

"Stan Freberg! Right!" Forrester whooped. "Shhhh . . . plop! Let's see you do that on television!"

"And now, as the comet rounds the Sun, trailing a luminous froth of fake whipped cream, and aims itself down our throats . . . Dan, what's the density of vanilla ice cream?"

Forrester shrugged. "It floats. Say two-thirds."

"Right. Point six six six it is." Sharps seized a pocket calculator from the desk and punched frantically. "I love these things. Used to use slide rules. Never could figure out where the decimal point went.

"A cubic mile to play with. Five thousand two hundred and eighty feet, times twelve for inches, times two point five four for centimeters, cube that . . . We have two point seven seven six times ten to the fifteenth cubic centimeters of vanilla ice cream. It would take a while to eat it all. Times the density, and lo, we have about two times ten to the fifteenth grams. Couple of billion tons. Now for the fudge . . ." Sharps punched away.

Happy as a clam, Harvey thought. A very voluble clam equipped with Texas Instruments' latest pocket marvel.

"What do you like for the density of hot fudge?" Sharps asked.

"Call it point nine," Forrester said.

"Haven't any of you made fudge?" Charlene demanded. "It doesn't float. You test it by dripping it into a cup of cold water. Or at least my mother did."

"Say one point two, then," Forrester said.

"Another billion and a half tons of hot fudge," Sharps said. Behind him Hamner made more strangled noises.

"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."