The headache was interfering, its pain preventing her from concentrating sufficiently to produce a coherent thought. She pushed the pain into a small, isolated part of her mind and built a mental wall around it. Then she tried again.

This time she felt equal amounts of both matter and antimatter feeding into the engine's reaction chamber, and instantly there was a tremendous release of energy. Yet it would take a few minutes for sufficient power to build, allowing her to lift from the lunar surface. During that time, the crawlers would continue their advance.

And, suddenly, Susan realized that by the time she finally could lift, the crawlers would be too near for their occupants to avoid the engine's lethal radiation.

She watched nervously as they came on-steadily, relentlessly-and all that time the power continued to build in Photon's engine. If the crawlers continued to advance, when she lifted they would be caught in a deadly storm of hard radiation produced by the ship's engine. Those driving the crawlers would die.

Unless she could do something to stop them. Unless she could turn them back before it was too late.

With a thought, she activated the transmitter, then shouted a single thought into its special psycho-electric circuits: Stop!

The crawlers staggered to a halt. After a few seconds, Clayton's voice again entered her thoughts.

"Captain Tanner, we know you are onboard that ship."

"The engine has been activated," Susan responded. "If you don't turn back immediately, I can't be held responsible for what will happen."

"We can't turn back, Captain," Clayton said. "You know that. We have orders to bring you in."

"And you know I can't allow that." They would never believe her; they could not. Any evidence she had once possessed was now gone. Even her LIN/C would be suspect-they would say she had somehow found a way to alter its contents. They simply could not believe the story she would be forced to tell. For their own sanity, they could not.

"You will be responsible for our deaths," came another voice into her thoughts. "Do you really want that, Susan?"

It was Karl! Karl drove one of those crawlers.

And instantly Susan again saw the apparition she had experienced in the briefing room in Luna City. Again she saw Karl, his flesh burned through by radiation-by the radiation from this ship!

No! she thought. She didn't want that. She knew now that she had not been responsible for those deaths ten years ago. She had done everything she could to save as many lives as possible. And she certainly did not wish to be responsible for these men's deaths now.

Yet, she knew she would be. If they did not turn back soon, they would be beyond the point of return before she was forced to lift. They would not be able to escape the hard radiation that would pour from Photon's engine as it rose from the lunar surface.

If she could only somehow override the engine's safety. If she could alter it so that it would lift before optimum power had been achieved.

Again she searched the computer's control points with her mind. There were myriad areas for control, yet there did not seem to be one for the process she needed. And, suddenly, she knew there was not. There was no control point to override the engine's safety circuits, allowing her to lift before sufficient power had been achieved.

There was simply no way around it; if the crawlers did not turn back soon, she would be forced to shut the engine down and give herself up. She did not want to do that, but she would have to. She had no choice.

But not quite yet. First, there was one final ploy she must try. They thought she had killed both Hyatt and Krueger. She might use that to her advantage.

"By now, you probably know I've killed twice already," she thought into the ship's transmitter. "I won't hesitate to kill again."

"You would kill me?" Karl asked.

"If I must."

Karl knew her history. He knew how she longed to get back into deep space, and he had to know she would do almost anything to make that dream come true.

She waited. Nothing more came over the ship's radio, and she transmitted nothing. The crawlers came on.

After a few seconds, the computer's thought entered Susan's mind: SUFFICIENT POWER BUILD-UP HAS BEEN ACHIEVED. AWAITING LIFT OFF COMMAND.

Still the crawlers advanced. Already the crawlers were probably too near. Already their drivers had been exposed to a lethal dose of radiation.

The power in Photon's engine continued to build, and suddenly she knew she either must lift from Luna's surface, or shut the engine down.

It was then that the computer's cold thought again knifed into her mind: WARNING. WARNING. SAFETY LIMITS ON ENGINE POWER GENERATION WILL BE EXCEEDED IN THREE MINUTES.

She knew what that meant. If she did not either lift or shut down the engine within three minutes, Photon's engine would detonate with devastating force-perhaps enough force to destroy Luna City, some sixty kilometers distant.

And instantly numerals began counting down in her thoughts: 2:59…2:58…2:57…

"What will be the consequences if I fail to lift within that time, and yet do not shut the engine down?" she thought at the computer.

DETONATION, the computer replied, WITH THE SUBSEQUENT DESTRUCTION OF EVERYTHING WITHIN A TWO HUNDRED KILOMETER RADIUS.

And still the crawlers came on.

2:55…

"I will lift in less than three minutes," Susan thought into the ship's transmitter, although she knew she would not.

She received no response. 2:53…2:52…2:51…

There must be a way out of this, she thought. She had to think of something. But the headache was again with her, throbbing behind her eyes, and she could not think coherently.

There was no way around it. She had to shut the engine down. And yet, she had visited herself on this very ship in deep space. If that had truly happened-and she did remember it happening-what could that mean?

2:47…2:46…2:45…

She watched through the ship's sensors as the three crawlers continued to advance toward her, realizing she could do nothing to stop them. If she didn't shut down the engine, Karl would become that charred apparition she had seen in the briefing room in Luna City. And not just Karl. Also Clayton, and whoever occupied the third crawler.

With a thought, she reached out to shut down the ship's engine.

Nothing happened.

Had she touched the wrong control point with her thought? she wondered.

She checked again. No, it had been the correct control point. Photon's engine should have shut down.

1:36…1:35…1:34…

Then, suddenly, she felt something. It was something strange, something alien.

At first she thought the alien presence inhabited the ship's computer. Then she realized it was not in the computer. The strange alien presence inhabited her own mind!

And instantly she realized it was this alien presence that had stopped her from shutting Photon's engine down.

What was it? How had it entered her mind? And how long had it been there?

Susan did not know.

Then, amazingly, she did know. The alien presence itself supplied the answers.

The presence came from an alien artifact. It came from the very pendant which, ten years ago, Susan had melted down and left for doctors to place in her head.

And it was this same presence that had told her before to visit her own past and melt down a pendant, so that it could be in her thoughts now.

But why had it stopped her from shutting down Photon's engine?

0:57…0:56…0:55…0:54…

She didn't have time to worry about that now. She had to get the engine shut down.

Again she reached out with a command thought. And again her thought was blocked.

How could she beat it? How could she get past this alien presence in her own mind?

She did not know. All she knew was that if she did not shut the ship's engine down soon, not only would those in the crawlers die, but so would everyone in Luna City.