Lucifer's Hammer - Larry Niven [20]
"The stuff boils out unevenly. When the comet first falls into the system, it's a solid mass. We think. Nobody really knows. We have several models that fit the observations. Me, I like the dirty-snowball model. The comet's made of rocks and dust, the dirt, balled up with ices and frozen gases. Some water ice. Methane. Carbon dioxide—dry ice. Cyanogen and nitrogen, all kinds of stuff. Pockets of these gases thaw and blast out to one side or the other. Like jet propulsion, and it changes the orbit." Sharps was at work with the chalk, holding it sideways. When he finished, the incoming arm had jogs and jiggle in it, and the outgoing arm was blurred into a wide sweep not unlike the comet's tail. "So we don't know how close to Earth it's coming."
"I see. And you don't know how big the tail will be."
"Right. But this seems to be a new comet. Maybe it's never made the trip down close to the Sun before. Not like Halley's Comet, which comes every seventy years and gets smaller each time. Comets die a little every time they pass near the Sun. They lose all that tail material forever. So each time the tail's smaller, until eventually there's nothing left but the nucleus, and that comes as a handful of rocks. Meteor showers. Some of our best shooting stars are pieces of old comets falling onto Earth."
"But this one's new—"
"That's right. So it ought to have a spectacular tail."
"I seem to remember people said that about Kahoutek."
"And I seem to remember they were wrong. Wasn't there an outfit selling commemorative medals that would show Kahoutek exactly as it appeared? You see there's no way to know. But my guess is that Hamner-Brown will be quite a sight. And it ought to pass fairly close to Earth."
Sharps drew a dot within the blur of the comet's outgoing course. "There's where we'll be. Of course we won't see a lot until the comet passes the Earth, because until it gets by we'll be looking straight into the Sun to see it. Hard to observe then. But when it's passed us, it should be quite a sight. There have been comets with tails across half the sky. See them in daytime. We're overdue for a big comet this century."
"Hey, doc," Mark said. "You've got Earth right in that thing's path. Could it hit us?"
Harvey turned to look daggers at Mark.
Sharps was laughing. "Chances are zillions to one against it. You see the Earth as a dot on the blackboard. Actually, if I drew this to scale you wouldn't be able to see the Earth in the drawing. Or the comet nucleus either. So what's the chance that a couple of pinpoints will come together?" He frowned at the board. "Of course, the tail is likely to go where we do. We might be in it for weeks."
"What does that do?" Harvey asked.
"We went through the tail of Halley's Comet," Mark said. "Didn't hurt a thing. Pretty lights, and—"
This time Harvey's look was enough.
"Your friend's right," Sharps said.
"Dr. Sharps, why do all the astronomers get so excited about Hamner-Brown?" Harvey asked.
"Man, we can learn a lot from comets. Things like the origins of the solar system. They're older than Earth. Made out of primordial matter. This comet may have been out there way past Pluto for billions of years. Present theory says the solar system condensed from a cloud of dust and gas, an eddy in the interstellar medium. Most of that blew away when the Sun started to burn, but some is still in the comet. We can analyze the tail. The way we did with Kahoutek. Kahoutek was no disappointment to astronomers. We used tools we'd never had before. Skylab. Lots of things."
"And that was useful?" Harvey prompted.
"Useful? It was magnificent! We should do it again!" Sharps's hands waved around in dramatic gestures. Harvey glanced quickly at his crew. The camera was rolling, and Manuel had that contented look a sound man has when things are going well in his phones.
"Could we get something like Skylab up there in time?" Harvey asked.
"Skylab? No. But Rockwell's got an Apollo capsule we could use. And we've got the equipment here at the