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-likeorbits, 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 δ
V
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
“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-likeorbits, 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 δ
V
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
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