“No abduction. I just drove here from Albuquerque. The Pontiac is mine.”
“Oliver, how many objects are we dealing with out there?” Leclerc was red-faced.
“The known Earth-crossers? About a thousand over a kilometre across. And Spacewatch are finding new ones at the rate of two or three a night.”
“I didn’t know interplanetary space is so crowded. I’m surprised life on Earth has survived.”
“It nearly hasn’t. It was almost wiped out at the Permo-Triassic. Big Daddy’s a mouse compared with some of the stuff out there. Hephaistos and Sisyphus are ten kilometres across. They’d yield a hundred million megatons. But it’s not a simple impact thing.”
Judy was now well ahead. The men were gasping. “Bear track!” she called over her shoulder, and the men followed her off the road on to a narrow path through the trees.
“Not a simple impact, Ollie. Meaning?” The ground under the snow was a soft carpet of pine needles. They had adopted a loping motion and were descending at a fair pace, but it did leave Webb wondering about the return trip.
“Chances are the big ones come in as part of a swarm.” Webb was weaving through low, snow-covered branches. “It’s more in the nature of a bombardment episode, with supercomets disintegrating to dust and choking off sunlight for thousands of years at a time. We think the planetary system is surrounded by a huge cloud of comets, reaching nearly to the stars. The whole solar system, comet cloud included, orbits the Galaxy in a two-hundred-million-year cycle. But as it goes round and round it also goes up and down like a carousel. So, we go up and down through the plane of the Galaxy. Every thirty-six million years we hit the Galactic disc.”
“Which disc we see as the Milky Way,” Judy said, scarcely out of breath.
“I saw it last night. From here it’s brilliant. Anyway, because the Galactic disc has a concentration of stars and massive nebulae, every thirty-six million years when we go through it we get gravitational tides which perturb the comet cloud. The comets are thrown out of their old orbits, they come flooding into the planetary system, the Earth gets bombarded and we have great mass extinctions. Therefore life goes in thirty-six-million-year cycles. Old life is swept away to make way for the new.”
“Not so fast!” Leclerc shouted. They stopped. Leclerc leaned forward, resting his hands on his knees and taking big gulps of breath. Webb looked up. The observatory was out of sight. Their voices were muffled in the snowy woods.
“Alors! All those years you mock the astrologers, and now you tell us our fate does lie in the stars. Where are we now, Oliver, in this great cycle?”
“We’re slap bang in the disc now, and we’re due for another mass extinction.”
“This Galactic connection,” Judy mused. “Is it relevant to Nemesis?”
“Could be. Some of the Earth-crossers are just strays from the asteroid belt. Herb will tell you they all are, but there are also serious people who think that. However I reckon that, because we’re at a peak of the extinction cycle, maybe half are degassed comets. A comet comes sunwards and grows a nice tail so you can see it from a hundred million miles away. But after a time so much dust from its tail has fallen back on to the nucleus that it chokes off. The comet becomes blacker than soot and almost undetectable. It becomes a soft-centred asteroid.”
“I see the relevance,” said Judy, panting a little. “If it’s a main belt stray it’s a cannonball. If it’s a degassed comet it’s a snowball disguised as a cannonball. Get it wrong when you try to deflect it and we have ourselves a nice little mass extinction. If we have no time to drill holes in Nemesis, the big picture becomes part of the equation. Up or down?”
Leclerc pointed downhill, and they set off again, Judy still leading. After five minutes the snow began to thin and the Ponderosa pines were giving way to scrub oak, through which they caught glimpses of sunlit Arizona desert in the far distance.
“Oliver, how should we be short-listing for Nemesis?” Leclerc asked.
“Whatever asteroid the Russians used, it had to be reachable. What could they reach, André?”
“For deep space missions the Russians launch from Earth orbits two hundred kilometres high. Even with Proton boosters, their cosmonauts could not rendezvous with and return from any asteroid with an interception speed of more than”—Judy was leaping over a fallen tree, light as a gazelle “—say six kilometres a second.” The men took it together like a couple of Heavy Brigade chargers.
“That means we’re looking for asteroids in Earth-like orbits, that’s to say low eccentricities, low inclinations and semi-major axes close to the Earth — Sun distance. There are at least half a dozen Nemesis-class asteroids which interweave with the Earth’s orbit. They have plenty of launch windows with? δV in the range four to six kilometres a second, round trip times three months to a couple of years.”
“In energy terms they are surely easier to reach than the Moon,” Leclerc suggested.
“Much. We’ve already soft-landed on a couple. You know, we could check out the orbits of these in short order.”
“Maybe the cosmonauts weren’t bothered about returning,” Judy called back.
That hadn’t occurred to Webb. “A suicide mission?”
“Why not? Save on re-entry fuel, put it into reaching a more distant asteroid. Would you die for your country, Ollie?”
“My love of country is undying. André, say Judy is right. What δV will you give me?”
Leclerc exhaled, “For a one-way ticket? We must relax the criteria to twelve kilometres a second.”
“That means they could have reached anything in the inner planetary system.”
“Merde!” They pounded on down, exhaled breaths steaming.
“There’s another tack,” Webb said. “Very few kilometre-sized asteroids could be diverted on to us. It has to be a near-misser, a potentially hazardous asteroid that already passes between us and the Moon.”
“So what does that do to your list?” Judy asked. They were now half loping, half scrambling down the steep mountainside at speed; by unspoken consent they had abandoned thought of the return climb.
“Depends how big a punch the Russians could deliver and how long a start they had. If they had summoned up a hundred-megaton punch say five years ago they could have gone for quite a few hazardous objects in the kilometre class. There are plenty of asteroids which pass close by. Too many.”
“Like two trains going round intersecting tracks, Oliver,” suggested Leclerc, puffing. “You only have a collision when they reach the point of intersection at the same time.”
They slowed; Judy went down on her backside, and edged down some scree. Webb said, “What you and I ought to do, André, is match past Russian interplanetary probes to asteroids along their track. The further in the past they deflected it, the bigger the shift they could have achieved by now.”
“Good, Oliver. You draw up a hit list of near-missers and I will see whether any of the Phobos and Venera series could have passed close to them, maybe even with a side probe fired off.”
Now they were off the scree and running together down lightly wooded hillside. Inside his Eskimo suit, Leclerc was sweating, red-faced.
“Even a fast flyby,” Judy suggested. “Our kamikaze cosmonauts could have—” she raised her hand and they stopped, almost cannoning into each other. “Did you hear that?”
Webb strained his ears.
“Gunfire,” Leclerc said, and sure enough there was a crackle of shooting down and to their right. It seemed as if several weapons were being fired.