Phil continued after him. "Well, maybe this will. When I was writing my dissertation on Soviet whale hunting, I went on a Greenpeace rescue mission to the Sea of Okhotsk. We weren't supposed to be there, but never mind. We found out that the Soviets had a way of creating false sonar images using sound transmitters at sea; we'd pick up an echo and rush off to protect a pod that wasn't even there, while the hunters were killing whales somewhere off our screen."

The two men entered Stoll's office.

"This isn't a sonar blip, Phil."

"No. And that's not the relevant part of the story. We started keeping video records of the images for future reference and found that whenever the transmitters were turned on there was an almost imperceptible burst of energy--"

"A start-up surge. That's common."

"Right. The point is, the signal had a fingerprint, a signature we could check before running off on a wild whale chase. The computers went down here for almost twenty seconds-- you called it a smokescreen, and you may be right. But as I was watching the countdown clock in the Tank, I realized that there's one eye that wouldn't have blinked."

Stoll stood beside his desk. "The computer clock."

"Right."

"How does that help us? We know from when to when the shutdown occurred."

"Think. The satellite continued to store images, even when it couldn't transmit them to Earth. If we could compare an image from the instant before with one taken an instant after, we might be able to figure out what was done to the system."

"Theoretically. You'd have to superimpose two at a time and compare them for subtle changes--"

"The same way astronomers search for asteroids moving against a star field."

"Right," said Stoll, "and it'd take a long time to compare the dozens of images pixel-by-pixel. We can't even trust the computer to compare them for us, since it may have been programmed to overlook certain artifacts."

"That's just it. We don't need the computer. All we have to do is study the one set of before-and-after shots. That's what I meant about the computer clock. It wouldn't have shut down, even if a virus crawled in. But it would have taken a fraction of a second for a false image to supplant a real one--"

"Yes, yes." Stoll said. "Shit, yes. And that would show up in the time-encoding on the photographs. Instead of coming in at-- what is it, every.89-odd seconds, there'd be an infinitesimal delay on the first bad egg."

"And that delay would show up right on the bottom of the photograph."

"Phil, you're brilliant." Stoll reached around and snatched his calculator from the desk. "Okay-- the photographs should advance in increments of.8955 seconds. When we find one that's.001 second late, we have the first of our fakes."

"You've got it. All we have to do is ask NRO to run a check backward until they hit the time discrepancy."

Stoll dove for his chair, got Steve Viens on the line, and explained the situation. While he waited for Viens to run his time check, Stoll unlocked his desk drawer, pulled out a tray full of diagnostics diskettes, and began his check of the inner workings of the system.