"So maybe the decryption key is something that was in one of those messages," he said. "I mean, I know you worked very hard on it, but maybe the Dracons weren’t interested in the official SETI-team response. Whoever they intended to have read their latest message might already have done so."

Sarah shook her head. "No, no. The current Dracon message is a response to our official reply. I’m sure of it."

"That might just be wishful thinking," he said gently.

"No, it’s not. We put a special header at the top of the official reply — a long numeric string, to identify that message. That’s one of the reasons we didn’t post the entire reply we sent on the web. If we had, everyone would have the header, which would have defeated its purpose. The header was like an official letterhead, uniquely identifying the response we sent on behalf of the whole planet. And this reply to our response references that header."

"You mean it quotes it?" he asked. "But, then, doesn’t everybody have it now? Any Tom, Dick, or Harry could send a new message to the Dracons and have it look official."

Her wrinkled features shifted in the cold glow as she spoke. "No. The Dracons understood that we were trying to provide a way to distinguish official responses from unofficial ones. They obviously grasped that we didn’t want everyone who managed to detect their latest message to know what the header was. So the Dracons quoted every other digit from it, making clear to us that they were responding to the official reply, but without giving away what had distinguished the official reply in the first place."

"Well, there’s your answer," Don said, quite pleased with himself. "The decryption key must be the other digits from the header, the ones the Dracons didn’t echo back."

Sarah smiled. "First thing we tried. It didn’t work."

"Oh," he said. "It was just a thought. Are you coming to bed?"

She looked at the clock. "No, I—" She stopped herself, and Don’s stomach knotted. Perhaps she’d been about to say I don’t have time to waste on sleeping.

"I’m going to struggle with this some more," she finished. "I’ll be along in a bit. You go ahead."

Don called McGavin’s office four more times without any luck, but finally his datacom rang. His ring tone was the five notes from a forgotten film called Close Encounters of the Third Kind, the sort of aliens-come-to-Earth story that seemed quaintly passe now. He looked at the caller ID. It said "McGavin, Cody" — not "McGavin Robotics," but the actual man’s name.

"Hello?" Don said eagerly, as soon as he’d flipped his datacom open.

"Don!" said McGavin. He was somewhere noisy and was shouting. "Sorry to be so long getting back to you."

"That’s all right, Mr. McGavin. I need to talk to you about Sarah."

"Yes," said McGavin, still shouting. "I’m sorry, Don. I’ve been briefed on all this.

It’s just awful. How is Sarah holding up?"

"Physically, she’s okay. But it’s tearing us both apart."

His tone was as gentle as one’s could be when shouting. "I’m sure."

"I was hoping you could speak to the people at Rejuvenex."

"I already have, repeatedly and at length. They tell me there’s nothing that can be done."

"But there must be. I mean, sure, Rejuvenex has tried all the standard things, but there’s got to be a way to make the rollback work for Sarah if you—"

He stopped talking, which was probably just as well. He’d been about to say, "if you just throw enough money at it." But McGavin wasn’t listening. Don could hear him saying something to someone else; from the sounds of it, he’d placed a fingertip over his datacom’s mike and was talking to a flunky standing beside him. At last McGavin came back on. "They’re working on it, Don, and I’ve told them to spare no expense. But they’re totally stumped."

"They thought maybe an experimental cancer drug was the culprit."

"Yes, they told me that. I’ve authorized them to spend whatever is necessary to try to get hold of a supply of it, or to synthesize it from scratch. But the researchers I’ve spoken to think the damage is irreversible."

"They’ve got to keep trying. They can’t give up."

"They won’t, Don. Believe me, this is a huge problem for them. It’s going to affect their stock price, if word gets out, unless they can find a solution."

"If you hear anything," Don said, "please, let me know at once."

"Of course," said McGavin. "But…"

But don’t have unrealistic hopes ; that was the implicit comment. McGavin had probably seen only an executive summary of the longer report Don had now pried out of Rejuvenex, but the bottom line would have been the same: no solution likely in the near future.

"Anyway," continued McGavin, "if there’s anything Sarah needs to help with the decryption work, or if there’s anything either you or she needs for anything else, just let me know."

"She needs to be rolled back."

"I am sorry, Don," McGavin said. "Look, I’ve got to get on a plane. But we’ll keep in touch, okay?"

Back in 2009, those who were part of the formal SETI endeavor had set up a newsgroup to share their progress in figuring out what the various parts of that first, original alien radio message said. It was rumored that the Vatican astronomers were working full-time on trying to translate the message, too, as was, supposedly, a team at the Pentagon. Hundreds of thousands of amateurs were taking a crack at it, as well.

Besides the symbolic-math stuff, parts of the original message turned out to be bitmap diagrams; a researcher in Calcutta was the first to realize that. Someone in Tokyo chimed in shortly thereafter, demonstrating that many of the block-graphic diagrams were actually frames in short animated movies. A new symbol in the last frame of each movie was presumably the word to be used henceforth for the concept that had been illustrated: "growth," "attraction," and so on.

The message also contained a lot about DNA — and, yes, there was no doubt that that was what it was, for its specific chemical formula was given. Apparently it was also the hereditary molecule on Sigma Draconis II — which immediately revived old debates about panspermia, the notion that life on Earth had begun when microorganisms from outer space had chanced to land here. The Dracons, some said, might be our very distant cousins.

The message also contained a discussion of chromosomes, although it took a biologist — in Beijing, as it happened — to recognize that that’s what was being talked about, since the chromosomes were shown as rings, rather than long strings.

Apparently, Sarah had learned, bacteria had circular chromosomes, and were essentially immortal, being able to divide forever. The innovation of breaking the circle to make shoelace-like chromosomes had led to the development, at least on Earth, of telomeres, the protective endcaps that diminished each time a cell divided, leading to programmed cell death. No one could say whether the senders had ringlike chromosomes themselves, or whether they were just depicting what they guessed to be either the universal ancestral or most-common kind. On Earth, in terms of biomass and number of individual organisms, chromosomal rings outnumbered the shoelace kind by orders of magnitude.

Once that piece of the puzzle was solved, a bunch of people simultaneously posted that the next set of symbols outlined various stages of life: separate gametes, conception, pre-birth growth, birth, post-birth growth, sexual maturity, the end of reproductive capability, old age, and death.

Lots of fascinating stuff, to be sure, but all of it seemed to be prologue, just a language lesson establishing a vocabulary. None of those early bits, except the tantalizing sample phrase that good was much greater than bad, seemed to actually say anything of substance.