Lucifer's Hammer - Larry Niven [19]
"Both. Simple for the camera, as much as I can understand for me. If it's not too much trouble."
"Too much trouble?" Sharps laughed. "How could it be too much trouble? Your network tells NASA you want to do a documentary on space, and NASA sends up red rockets. Right, Charlene?"
The PR flack nodded. "They asked us to cooperate—"
"Cooperate." Sharps laughed again. "I'd jump through hoops if I thought it would help get a budget. When do we start?"
"Now, please," Harvey said. "The crew will set up while we chat. Just ignore them. I take it you're the resident expert on comets."
"I suppose so," Sharps said. "Actually I like asteroids, but somebody has to study comets. I gather you're interested mainly in Hamner-Brown."
"Right."
Charlie caught Harvey's eye. They were ready. Harvey gave them the nod. Manuel listened and watched the indicator, and said, "Speed."
Mark stepped in front of the camera. "Sharps interview, take one." The chalkboard came together with a loud clack! Sharps jumped. They always did, first time. Charlie busied himself with the camera. He kept it aimed at Sharps; they'd film Harvey asking the questions later, when Sharps wasn't around.
"Tell me, Dr. Sharps, will Hamner-Brown be visible to the naked eye?"
"Don't know," Sharps said. He sketched something unlikely on the IBM print-out in front of him. The sketch might have been of a pair of mating Loch Ness monsters. "A month from now we'll know much better. We already know it's going to get as close to the Sun as Venus, but—" He broke off and looked at the camera "What level do you want this at?"
"Anything you like," Harvey said. "Make me understand, then we can decide how to tell the public."
Sharps shrugged. "All right. So there's the solar system out there." He waved toward one wall. A big chart of the planets and their orbits hung next to the blackboard. "Planets and moons, always where they should be. They do a great complicated dance around each other. Every planet, every moon, every little rock in the asteroid belt, all dancing to Newton's song of gravity. Mercury got a little out of step and we had to revise the universe to make it fit."
"How's that?" Harvey asked. And I'd have preferred to do the poetry myself, but what the hell …
"Mercury. Orbit changes just a little every year. Not much, but more than Newton says it should. So a man named Einstein found a good explanation, and incidentally managed to make the universe a stranger place than it was before."
"Oh. I hope we don't need relativity to understand comets—"
"No, no. But there's more than gravity to a comet's orbit. That's surprising, isn't it?"
"Yes. Are we going to have to revise the universe again?"
"What? No, it's simpler than that. Look … " Sharps jumped to his feet and was at the blackboard. He looked for chalk and muttered
"Here you go." Mark took chalk from his pocket and handed it.
"Thanks." Sharps sketched a white blob, then a parabolic curve. "That's the comet. Now let's put in planets." He drew two circles. "Earth and Venus."
"I thought planets moved in elliptical orbits," Harvey said.
"So they do, but on any scale you could draw you can't see the difference. Now look at the comet's orbit. Both arms of the curve look just the same, coming in and going out. Textbook parabola, right?"
"Right."
"But here's what the comet really looks like when it falls away from the Sun. A dense nucleus, a coma of fine dust and gas"—he was drawing again—"and a plume of dusty gas streaming away from the Sun. Ahead of the comet, going out. The tail. A big tail, a hundred million miles long, sometimes. But it's nearly a vacuum. It has to be—if it were thick, there wouldn't be enough matter in the comet to fill that much space."
"Sure."
"Okay, and again like the textbooks. Material boils out of the head of the comet into the coma. It's a thin gas, tiny particles, so tiny that sunlight can push them around. Light pressure from the Sun makes them stream away, so the tail always faces away from the Sun.