The third time this transmission was repeated, she wrote it down: twelve characters in all, a basically random assortment of digits and letters. But she did notice that all the letters were in the range A through F. Which was a strong hint that this was a number expressed in hexadecimal notation: a system typically used by computer programmers.

The fact that it had twelve digits was also a clue. The network chips used by almost all computer systems had unique addresses in that format: twelve hexadecimal digits.

And here was where Dinah got a weird feeling on the back of her neck, because the first few digits in that string looked familiar to her. Network interface chips were produced in large batches, with unique addresses assigned to each chip in sequence. So, just as all Fords rolling off the assembly line in a given week might have serial numbers beginning with the same few characters, all the network chips in a given batch would start with the same few hex digits. Some of Dinah’s chips were cheap off-the-shelf hardware made for terrestrial use, but she also had some rad-hard ones, which she hoarded in a shielded box in a drawer beneath her workstation.

She opened that drawer, pulled out that box, and took out a little green PC board, about the size of a stick of gum, with an assortment of chips mounted to it. Printed in white capital letters directly on the board was its MAC address. And its first half-dozen digits matched those in the transmission coming from the Space Troll.

She reached for the key and coded in QSO, meaning, in this context, “yes, I can communicate with . . .” and then keyed in the full MAC address of the little board in her hand—different from the one in the original transmission. It was a way of saying, “no, I can’t communicate with the one you mentioned, but I can communicate with this other one.”

QSB, came the answer back. “Your signals are fading.” Then QTX 46, which she guessed meant something like “Will you be available on this frequency forty-six minutes from now?” As anyone on Izzy would understand, this meant “I will call you back when you have orbited around to the other side of the planet.”

QTX 46, she answered back. “Yes.”

They were passing over the terminator, currently dividing the Pacific into a day side and a night side.

WHO THE HELL ARE YOU TALKING TO?

This was a transmission from Rufus, loud and clear. She looked out the window to see the West Coast of North America creeping over the horizon toward them, identifiable as a pattern of lights delineating the conurbations of the Fraser Delta, Puget Sound, the Columbia River, San Francisco Bay. Which meant that Alaska had line of sight to Izzy.

“Knock knock!” came the voice of Dubois Harris through the curtain. He’d been waiting there a long time.

“Come in,” Dinah said, and keyed back a brief transmission to Rufus making a joke about the Space Troll and telling him she would be in touch later. She checked the world clock app on her computer screen. It was shortly before dot 7, therefore 7:00 A.M. in London, therefore ten in the evening for Rufus in Alaska.

A somewhat distracted and scattered conversation followed, Dinah trying to maintain a train of thought with Doob while fielding sporadic, peremptory interruptions from Rufus. “Something kinda weird just happened on the radio,” she said. “Do you want a drink? It’s evening for you, right?”

“I pretty much always want one,” Doob said. “Let’s not worry about it. What’s up?”

Dinah related the story. Doob looked distracted at first, perhaps because of all the ham radio jargon, but focused when she showed him the MAC addresses.

“The simplest explanation,” he pointed out, “is that it’s a troll, just messing with you.”

“But how would a troll know those MAC addresses? We don’t give those out—we don’t want our robots getting hacked from the ground.”

“The PR people have come through here, haven’t they? Taking pictures of you and your robot lab. Mightn’t it be the case that a picture got snapped when you had that box open, and some of those PC boards visible?”

“There’s no gravity in here, Doob. I can’t leave things lying around on my desk.”

“Because,” Doob said, “obviously what’s going on here is that someone wants to talk to you through a private channel—”

“And they are proving their identity by mentioning numbers that could be known only to a few people. I get it.”

“And all I’m saying is that a really sophisticated troll would look for some detail like that, in the background of a NASA publicity photo, as a way to fool you.”

“Noted,” Dinah said. “But I doubt it.”

“Who do you think it is, then?”

“Sean Probst,” Dinah said. “I think it’s the Ymir expedition.”

Doob got a distracted look. “Man, I haven’t thought about those guys in ages.”

IT WAS STRANGE THAT A STORY AS EPIC AND AS DRAMATIC AS THE voyage of Ymir could go forgotten, but those were the times they lived in.

The ship had stopped communicating and then disappeared against the backdrop of the sun about a month after its departure from low Earth orbit (LEO) around Day 126. A few sightings on optical telescopes had confirmed that it had transitioned into a heliocentric orbit, which might have happened accidentally or as the planned result of a controlled burn. Assuming it was following its original plan, Ymir should then have made almost two full loops around the sun. Since its orbit was well inside of Earth’s—the perihelion was halfway between the orbits of Venus and Mercury—it would have done this in just a little more than a year, grazing the orbit of Greg’s Skeleton—Comet Grigg-Skjellerup—a couple of hundred days ago. But this would have occurred when it was on the far side of the sun from the Earth, making it difficult to observe. The next event would have been a small matter of impregnating the comet’s core, or a piece of it, with an exposed nuclear reactor on the end of a stick, and then turning it on to generate thrust by blowing a plume of steam out the entry hole. They would have done a large “burn”—pulling out the reactor’s control blades, powering it up, and releasing a plume of steam—that would have altered the comet’s trajectory by about one kilometer per second, enough to put it on a collision course with Earth, or at least with L1, a couple of hundred days later. The timing was awkward, and many had griped about it, wondering why Sean hadn’t gone after some other comet, or plotted some other course that might have brought it home a little sooner. But people who knew their way around the solar system understood that it was near-miraculous good fortune for any comet core to be in a position to be grappled and moved in such a short span of time. The hasty shake-and-bake nature of the Ymir expedition, which had stirred up so much controversy, had been forced by the implacable timeline of celestial mechanics. Time, tide, and comets waited for no man. And even if it had been possible to bring a comet back sooner, it would have been reckless, and politically impossible. What if the calculations were wrong and the comet slammed into the Earth? So, the plan of the Ymir expedition was the only one that could have worked.

If, indeed, it were working at all. And since much of the action—the rendezvous with the comet and the “burn” of the nuke-powered, steam-fueled engine—had occurred while it was on the far side of the sun, this had been very much in doubt until a couple of months ago, when astronomical observations had proved conclusively that Comet Grigg-Skjellerup had changed its course—something that could only have happened as the result of human intervention. The comet was headed right for them. It would have triggered mass panic on Earth had Earth not already been doomed. Since then they had watched its orbit converging slowly with that of Earth, and plotted the time when it would disappear against the sun once more as it reached L1. The reactor would then have to be powered up again, as a huge “burn” would be needed to synchronize Ymir’s orbit with Earth’s and pilot it through L1 to a long ellipse that would bring it their way.