Sacheverell leaned back in his chair. “As you’d expect, the big players are the Americans. We have two main civilian programmes, one in New Mexico, and one right here in Arizona. Lowell Observatory have a point six-metre Schmidt at Flagstaff, just a few mountains to the north of us, and the University of Arizona have Spacewatch Two on Kitt Peak, to the south. And the University of Hawaii are just starting a massive programme on Maui, one of the Hawaiian Islands. It’s a sixty-million-buck project, financed by the USAF.”
“Is that it?” Noordhof asked.
“There are photographic programmes but if you don’t have a CCD you’re not in the game. Put a charge-coupled device at the eyepiece of your telescope and you’ll get as much light in two minutes as you would with a two-hour exposure hour on a Kodak plate. In that two minutes Flagstaff can cover ten square degrees of sky down to magnitude twenty. Spacewatch Two covers only one square degree, but it gets down to twenty-one in half the time.”
“Sacheverell has overlooked the rest of the world,” Webb pointed out. “For example, the Japanese have a private network of amateurs and they’ve also started with a pair of one-metre class telescopes. The Italians have a small-scale network centred round their instruments in Campo Impera-tore, Asiago and Catania. The French and Germans have a one-metre Schmidt on the Côte d’Azur.”
Sacheverell waved his hand dismissively. “I don’t want the survival of America to depend on a bunch of Japanese amateurs. As for the Italians, they’re penniless. Half the time their telescopes are lying idle. We’re detecting three Earth-crossers a night.”
Kowalski said, “And we have our upstairs telescopes. We operate our Schmidt remotely, as a robotic telescope, from this room. Normally we feed in a pre-selected list of galaxies over there but we could just as easily scan the sky looking for a moving object.”
Noordhof tapped the table. “Like I said in New York, you people have every conceivable facility at your disposal.”
“You mean Pan-STARRS?” McNally asked, round-eyed. “The Hawaiian system?”
“Ay-firmative. With immediate effect, it’s yours.”
“What are their CCD chips in these systems like?” Webb asked.
Sacheverell waved sheets of paper. “While our token Brit is feeling fragile I’m downloading from Albuquerque, with the Colonel’s help. They’re large format, high quantum efficiency, fast readout. They perform close to the theoretical limit.”
Shafer was scribbling furiously on a yellow notepad. He had dispensed with his ponytail and his long grey hair was swept down over his shoulders. “What’s the sky coverage with these Pan-STARRS telescopes?”
Sacheverell said, “Nine square degree starfields, reaching mag twenty with twenty-second exposures. They don’t go as faint as Spacewatch Two but like I say their CCDs have fast readout. They can carry out a saturation search in half the time of Spacewatch. Spacewatch has depth; Albuquerque has breadth.”
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.”