Webb said, “I’m impressed. Herb, impress me even more. Tell us what you’ve got in the southern hemisphere.”
Sacheverell hesitated. “Okay, we’re weak there.”
“What’s your point, Oliver?” Noordhof asked.
“We have almost no coverage of the southern sky. Nemesis could sneak up on us from south of the celestial equator when all our telescopes are scanning the sky to the north. Maui can look south to a limited extent, and the ESO Schmidt in Chile might have picked it up serendipitously if they hadn’t shut it down.”
“The British closed down the UK Schmidt in Coonabarabran,” Sacheverell accused Webb, pointing a skinny
finger in his direction. “Why did you guys leave yourselves with no asteroid-hunting capability?”
“The giggle factor. Our Minister for Science thought the impact hazard was a joke.”
“Are you telling me half the sky is uncovered?” Noordhof asked in dismay.
“It’s worse than that. I’m thinking of the Atens.”
“Excuse me?”
“I hate to add to our troubles, but there’s a blind spot about thirty degrees radius around the sun. Anything could be orbiting inside it. An Aten is an asteroid with an orbit which puts it inside the Earth’s orbit, and therefore in the blind spot, most of the time. Only a handful have been discovered but nobody knows how many there really are. Now say the Russians discovered one on a near-Earth orbit.”
Noordhof acquired a thoughtful look. Leclerc had been writing in a little red leather Filofax. He looked up and said, “The probability that we would independently discover it is remote. It would hide in sunlight until it pounced. An Aten makes a lot of sense as a weapon.”
Webb continued, “Sacheverell’s telescopes are all geared up to search the sky around opposition. They’re pointing high in the night sky, far from the sun. But if an Aten is coming at us, it won’t be there. It will come at us low in the sky, close to the sun. Most of Herb’s telescopes can’t even reach that low. If Nemesis is an Aten you might see it before dawn, or just after dusk, a few days before impact. Binoculars would do.”
Noordhof took a cigar from his top pocket. “I need a consensus on the detection issue. Can you people deliver or not?”
Shafer had finished his scribbling. Now he stood up and moved over to the blackboard. He picked up yellow chalk and started to write in a fast, practised scrawl. “The way these telescopes are operated, sure there’s a strong selection effect acting against the discovery of Atens. But I disagree with Ollie about Atens as weapons. For precision work the
Russians would need something they could track for a long time, maybe years, and you can’t do that with The Invisible Asteroid. I say Nemesis is reachable with Spacewatch and Pan-STARRS. There are 4π steradians of sky and each steradian is 180/π degrees on a side. That gives us forty-three thousand square degrees of sky over the whole celestial sphere. How much of that can we cover? For a start these things are faint, which means we have to go deep. But we can only do that in a pitch black sky. Okay, so there’s no moon this week. But to avoid twilight the sun has to be at least twelve degrees below the horizon, and to avoid atmospheric absorption the sky we’re searching has to be at least thirty degrees above it. I reckon we have maybe only five or six thousand searchable square degrees of sky on any one night.”
“Declining to zero if it’s cloudy,” Judy Whaler pointed out.
“The five-day local forecast is good,” Kowalski said. “Except for the last day.”
Shafer continued: “Okay, from Herb’s figures I reckon the whole of the world’s asteroid-hunting telescopes will cover no more than two or three hundred square degrees of sky an hour. That means say a month to cover the whole sky once.”
“And we’ve been given five days,” said Whaler. “Six to one against.”
“Not even remotely,” Shafer disagreed. “Look at square A on Monday, and by Murphy’s Law Nemesis is in square B. Look in B on Tuesday and it’s moved to A or C. Apart from which, most of the time it will be too faint to be seen, because it will be too far away, or hidden in sunlight like Ollie’s Atens, or camouflaged against the Milky Way.”
“So how long, Shafer?” Noordhof asked impatiently.
Shafer drew a graph. He measured off tick marks on the axes and labelled the horizontal one “diameter in km,” and the vertical one, “p % per decade.” Then he drew an S-shaped curve, copying carefully from his paper. Webb saw what the physicist had been calculating and was awestruck at the speed
with which he had done it. Shafer tapped at the blackboard. “Assume Nemesis is a kilometre across, with the reflectivity of charcoal. That gives it absolute magnitude eighteen at one AU from Earth and sun.” He drew a vertical line up from the 1-km tick mark on the x-axis to its point of intersection with the curve, and then moved horizontally across to the vertical axis, where he read off 0.85. “You want to discover Nemesis with eighty or ninety per cent probability, with all the world’s asteroid telescopes going flat out? Assuming it’s not an Aten? It will take us ten years.”
“We have five days,” Noordhof reminded Shafer in a flat tone.
“So consult a psychic,” said Shafer, going back to his chair.
The tense silence that followed was broken by the loud crackling of cellophane as Noordhof unwrapped his cigar.
“Willy, I think your calculation is flawed,” Webb said, knowing this was a rash thing to say to the mighty Shafer. “If it’s coming at us in a straight line out of a dark sky then it’s already close and bright. We don’t have to spend ten years looking.”
The physicist gave Webb a disconcertingly hard look. “Ollie, if it’s close and bright and coming at us in a straight line out of a dark sky, we’re about to be history.”
“It’s our only chance to find it.”
Sacheverell shook his head sadly. “It must be the jet lag. Willy has just told us that by the time it’s close enough to be found it’s too late to be stopped.”
“We can harden up on this.” Webb crossed to an empty bit of blackboard. “Say Nemesis is going to hit us in thirty days. There are 86,400 seconds in one day. If it’s coming in at fifteen kilometres a second then it’s only 30 × 86,400 × 15 = 39 million kilometres away now, a quarter of an AU, which makes it sixteen times brighter than it was at one AU. Herb, what’s the brightness of a one-kilometre asteroid at one AU?”
“Eighteen for a carbonaceous surface. Everybody knows that.”
Shafer was tapping at a pocket calculator. He said, “Inverse square brightness, forget phase angles. Yes, if Nemesis is a month from impact it could have magnitude fifteen. We should be able to pick it up now.”
Webb said, “Go for sixteen or seventeen visual and we cut the exposure times to seconds. We might even have a continuous scan. We could cover the sky in a week. It’s then down to bad luck, like coming at us out of the sun or approaching from the south.”
McNally’s slim fingers were agitatedly drumming on the table. “Can we inject some realism into this? If we’re a month from impact what am I supposed to do about it? Call up Superman? I need a year minimum, preferably two or three, to build some hardware.”
“But if Nemesis is a year from impact now, we’ll still only detect it in eleven months’ time, when it’s on the way in. A last-minute deflection is the only scenario you can work on.”
“Let me understand this,” Noordhof said. “If you guys are right, the chances are hundreds to one against our finding this thing in the next five days. Unless it’s so close that it’s maybe a month or two from impact. And even then maybe not if it’s coming at us out of the sun.”
There was a silent consensus around the table.
“Shit,” Noordhof added, looking worried. He turned to the Director of NASA. “McNally, you have to come up with a deflection strategy based on the month-from-impact scenario.”
“For Christ’s sake, that’s just off the wall.” The NASA director’s face was flushed.
Firmly: “You have no choice in the matter.” Noordhof was playing nervously with his unlit cigar. Webb had a momentary vision of Captain Queeg rolling little metal balls in his hand.
“Jim,” Shafer’s tone was conciliatory. “We’re the A team. Maybe you and I can come up with something.”
McNally shook his head angrily.
Leclerc asked, to break the tension, “What would happen if say somebody in Japan found an asteroid?”
“The whole astronomical community would know it within hours,” Sacheverell said. “Civilian discoveries go straight to the Minor Planet Center which has electronic distribution to all the major observatories. But look, forget Japan, Europe and Atens and crap like that. The action is at Lowell, Spacewatch and Hawaii”—Kowalski winced slightly, but said nothing—“and we’re linked in to these places here. We’ll see the exposures build up in real time.”
Webb said, “Detection isn’t enough. If we don’t follow it up, we lose it. We have to track it long enough to get a reliable orbit.”
Sacheverell said, “Follow-up means we come back to it every few hours, using the interval in between to search for other asteroids. An interval of a few hours gives you its drift against the stars enough to pick it up again the following night. To get a believable orbit, you need to track it for at least a week. To get decent precision, say to launch a probe at it, you have to update over months. There are follow-up telescopes in British Columbia, Oak Ridge Massachusetts and the Czech Republic. Also at Maui.”
Kowalski nodded. “We’re well placed for follow-up here. Our Grubb Parsons has a long focal length and its point spread function is small. On a good night we can do very high-precision astrometry.”
“The Grubb Parsons is vital,” Webb agreed. “Without it follow-up would double the load on the discovery telescopes.”
Leclerc, pen hovering over his Filofax, asked again: “Suppose you find an asteroid and follow it up. What then?”
Webb said, “Nearly every one we find will be harmless. We’re looking for a needle in a field of haystacks. Old Spacewatch could pick up six hundred moving objects on a clear winter’s night, and overall twenty-five thousand asteroids a year. Out of all that, fewer than twenty-five were Earth-crossers. The rest were main belt. Now with all the
new systems combined the detection rates are fifty times higher. But that means we have also fifty times more junk to be sifted through. With the CCD mosaics you people are talking about I reckon we need to interrogate about a billion pixels every ten seconds. We have nothing like enough computing power on site to handle the data.”
Noordhof attempted a smile. “We have the Intel Teraflop at Sandia. It makes your hair stand on end. That too is yours, a personal gift from a grateful nation.”
Judy said, “That’s Wow, but how do we transfer the data over? Ordinary cable transmission can’t handle the flow.”
“We have satellites that will.”
“In that case,” she replied, “problem solved. I’ll download the CCD processing software from Spacewatch and transfer it over to our magic machine.”
“We have orbit calculation packages at the Sorel,” Sacheverell said. “I’ll pull them over to your computers. I presume it’s all Unix-based?”
Noordhof nodded. “All communication between here and Albuquerque must be secure. I’ll get a key encryption package installed when I’m fixing access to our computers. Judy, work at it through the night. Let’s be operational by dark tomorrow. Herb, when can you give me a damage profile for Nemesis?”
“Two or three days, if I can access the Sorel.”
“Have you been listening, Herb? At that rate we might as well wait for the field trial. I want a report over breakfast. O seven hundred sharp, all present, and nobody feeling fragile.”
Webb said, “We’re doing this all wrong.”
Shafer said, “Oliver, I was joking about a psychic.”
The smell of scrambled eggs and coffee drifted into Webb’s room, and sunlight had found weak spots in the heavy curtains’ defences. He reached for his watch with an arm made of lead, focused on the little hands, and knew he was in for another of Noordhof’s special looks. He rolled on to his stomach and looked longingly at the laptop computer and the crumpled sheets of paper scattered over the floor, which had shared his journey from Glen Etive. But there was no time. He skipped shaving and made it with minus two minutes to spare.
Breakfast things were laid out on the kitchen table and Shafer was dithering around the microwave oven. Judy was in an easy chair; she was into a severe white blouse and black skirt, a plate on her lap, and she was using her sharp, red-painted nails to carefully peel a hard-boiled egg. Sacheverell sat next to her with a plate on his lap. He was also pouring her a coffee and she flashed him a smile. McNally, Leclerc and Kowalski were at the window, sipping coffee and looking out over an expanse of desert from which the occasional tree-covered mountain protruded like an island in the sea.
Noordhof was busy on a croissant. A row of cigars protruded from a shirt pocket. He made a show of looking at his watch as Webb entered.
Webb poured himself coffee from a big percolator, heaped a plate with sausage and scrambled egg, and settled down at the farmhouse table. “You’re giving me a hard stare, Colonel.”
“Please God, deliver this man unto my sergeant,” Noordhof prayed.
A screen on a tripod had been set up and an overhead projector on the end of the kitchen table was throwing white light at it. The soldier nodded to Sacheverell, who had a stubble and looked a bit ragged.
Sacheverell put fork and plate aside, wiped his fingers with a handkerchief, took a pile of transparent overlays to the projector and moved them on and off the machine as he spoke. The first one showed three teddy bears of different sizes, with bubble text coming from the mouth of each, like a comic. One bear was saying
10
4
Mt
, another
10
5
Mt
and the third
10
6
Mt
. “I examined three scenarios which straddle the likely energy range. I’m calling them Baby Bear, Mummy Bear and Daddy Bear. As you see Baby Bear is ten thousand megatons, Mummy Bear a hundred thousand and Big Daddy is a million.
“First I had a look at Baby Bear, deep ocean impact. I had the idea that maybe the aggressors—the Russians?—might want to take out the UK or Japan while they were about it. Anyway, the Atlantic and the Pacific are big, easy targets. Okay. So half a minute into impact we have a ring of water three or four hundred metres high. Wave amplitude falls as it moves out but you’re still looking at a fifteen-metre wave a thousand kilometres from the impact site.”