It was exactly seven thirty when the lone rover set out, on that Martian spring morning in 2038. The sun had been above the high mesa wall to the east for the past hour, and had already vaporized the overnight frost lying on the valley floor. It was now imparting an unearthly glow to the red landscape, despite sharp rocks and dusty stones everywhere, casting long morning shadows westward.

Of the crew of thirteen at NASA's Kasei Valley landing site, only two were in the rover, which was casting its own long shadow out in front. The two were headed off on a one-day field trip to survey the next valley to the west, on the other side of a range of low orange hills. The hills lay about six miles away, easily visible from the landing site.

The huge rover, which pulled a loaded trailer, was light blue in color, with six large yellow wheels, both colors partly obscured by red dust. White sand bags covered the rover's roof, helping to protect the two inside from the higher level of radiation on Mars.

A NASA structural engineer and geologist, John Erway, was driving, and a strikingly red-haired Denise Lavoisier was sitting in the passenger seat nearest the side window. Denise was a French medical doctor and geneticist, with the European Space Agency.

"Getting under way, Max," John was saying, speaking casually into a dash microphone, in the clear accent of the Pacific Northwest. "Rover fully operational. We'll check back when we reach the survey site. Should be there by about eight thirty."

"Good luck," answered Max, in a strong German accent. "I hope you find something—it's about time you did."

Max Erlanger, with ESA, was on duty in the habitat's communications room that morning. Mostly he would be handling the daily flood of messages between the landing site and an ever vigilant Mission Control in Houston, Texas.

John and Denise's field trip was part of the mission's search for microbial fossils on the Martian surface, a search they had both begun to suspect was futile. The thirteen-man mission had landed eight weeks earlier, and had conducted many such field trips, but so far had found no convincing trace of primitive life forms. If there were traces of life on Mars, they were likely deep under ground.

All the same, the neighboring valley on the other side of the hills ahead was one more reasonable place to look for fossils at the surface. It was only about twelve miles away, and since it was a former outflow channel, the mission needed to investigate it. Long ago, geologically speaking, torrents of water had poured down it.

As the two pulled away from the base's long habitat that Monday morning, the mission's return rocket, straight ahead, was the only man-made object they could see through the rover's large front windows. They would need that rocket in sixteen months, to take the thirteen of them back up to an orbiting mother ship for the trip home. The return rocket's proper functioning was vital to the success of the mission, and it was under constant monitoring and maintenance.

The solitary space vehicle lay due west of the base, about half a mile from the habitat and farm building. The cylindrical top of the huge, metallic machine glistened in the morning sunshine, and stood out against both the pink sky and the range of orange hills beyond. Farther south, the hills ended in a conical mountain, orange and gray in the morning sun. John and Denise had a clear view of that mountain, in a direction a little to the left of the return rocket.

The two could see only the top part of the gleaming space vehicle as they approached it—most of it was hidden behind a low hump. It was sheltered behind the hump, and positioned well away from the habitat too, for safety reasons, for it sat fully fueled. There was nearly a hundred tons of fuel and oxidizer on board, which was enough explosive power to demolish a city block.

Three Americans with NASA, Ben Ledermann, Dave Young, and Sally Clark, maintained the return rocket. The three had been working on it all the previous day, removing a small nuclear-powered chemical plant.

The return rocket had landed two years earlier, well before the thirteen-man mission had landed. During those two years, the nuclear-powered chemical plant inside had slowly converted Martian air and some water into the one hundred tons of fuel and oxidizer now filling the tanks of the space vehicle.

"Do you still worry about an explosion?" asked Denise, as they drove past the launch pad. "I was talking to Sally last night. She said the removal had gone well yesterday. No sign of any problems."

"Yes, I do worry sometimes," conceded John—he had been concerned that the half-mile distance between habitat and return rocket was not enough. "I know it's too late now to move the habitat, but I'm still dead sure that if the unthinkable were to happen and the rocket ever did blow up... that hump wouldn't be enough to protect the base. It's just loose gravel and stones. And speaking of safety, we'd better stop talking for now, until we reach the survey site."

It was a violation of mission rules to talk while out driving the rover over the open, rock-strewn terrain of Mars. If you wanted to discuss something you were supposed to stop, and John always complied. His instinct was not to talk while driving anyway. He was an expert in assessing risk and never took chances on Mars. He was even inclined to follow his own safety rules, far more stringent than the mission's rules. Indeed, if the decision about where to position the habitat had been his, it would now be sitting much farther east of the return rocket.

"Nineteen point four kilometers due west, Max," reported John, a minute before eight thirty. "All systems normal. We're just about to go outside. We'll check in again about twelve thirty."

"Good, but remember, if you find Mars life, you let us know at once," answered Max. "You don't hold back news like that."

"Don't worry, Max. We'll bug you the minute we find something."

A few minutes later, John and Denise were outside, wearing light-blue pressure suits, and overgloves and overshoes. Each of them also wore a light-blue radiation umbrella, filled with three inches of sand and water. Each umbrella had two support poles, bent half way down. These fitted into slots on each side of a backpack, but also rested on the shoulders.

Radiation levels on Mars are higher than on Earth, high enough that if you spent five years there without protection, you would be at grave risk of cancer some time. NASA research had shown that radiation umbrellas, and sand bags on rover and habitat roofs, brought radiation doses down to levels comparable with those on Earth.

They were soon hard at work, unloading the trailer.

Suddenly, they felt the ground shake, in a single sharp jolt. Denise had never felt an Earth tremor before, although John, an American from Washington State, had experienced many.

The strong tremor came as a surprise to John nevertheless. Both of them stopped what they were doing and looked around. There was a light wind from the north, but nothing out of the ordinary.

"What in the world was that?" exclaimed Denise.

"At home it would have been just a small earthquake," explained John. "I guess it's not our lucky day. First time we have a tremor, and it happens before we even get the seismic recorder unloaded."

Setting up a seismic recorder to monitor tremors in the Martian crust was routine on field trips. There were two recorders permanently set up at the base too, but so far no seismic event of significance had ever been recorded. The Martian crust, unlike the Earth's mobile crust, did not seem to shake much.

"I see," said Denise. Then she added: "But won't the equipment at the base have recorded it?"

"Should do, but it would have been better if we could have recorded it too. This spot could easily be closer to the event. Anyway, it can't be helped. You figure out the survey grid, while I get the seismic gear set up. Maybe we'll get another tremor today."

Normal survey procedure was to select an area of typical terrain, one hundred by one hundred meters square, and break it down into a grid of ten-by-ten meter square sections. The day's work would consist of examining, drilling, sampling, measuring, and recording everything of significance in each of the one hundred grid squares.

"John Erway calling Kasei-75. Come in, Max," said John, just before twelve thirty, beginning his routine noon report to base. He was sitting in the rover cockpit, speaking into the dash microphone. Noon on Mars is not at twelve o'clock, but at twelve twenty, for a Martian day lasts nearly twenty-four hours and forty minutes.

Meanwhile, Denise was in the rover's tiny kitchen, preparing some canned food for lunch, a task she found relaxing. On rover trips so far, they had eaten only canned food. Back at the habitat though, they were now eating food produced at the mission's huge greenhouse: mostly grains, beans, and vegetables.

When Denise finished preparing the food, she brought it to a short, narrow table in the eating area, and sat down in a relaxed frame of mind, looking out the window beside the table. Fine sand and dust were now eddying around the rocks and stones outside: the light northerly wind earlier had strengthened in the previous hour.

Shortly after, John came back from the driver area.

As he sat down opposite her, he ran one hand through his short, dark brown hair. There was a worried expression on his face.

"That tremor this morning. Did they notice it at the base?" asked Denise, puzzled by his expression.

"I don't know. I couldn't get through."

"Oh," said Denise, pushing back a strand of red hair from her forehead, as a concerned expression appeared on her face too.

She hesitated, and then said: "That can easily happen when there's a range of hills in the way, can't it?"

"It can," agreed John, for whom communications was a back-up skill. "Radio communication on the surface of Mars can be really bad sometimes. It's close to line-of-sight only. So you can be sure those hills between us and base will be blocking quite a bit of the signal. And when you factor in midday solar radiation—you get a lot more electrical charge in the air at midday than early in the morning—the two things together could explain it. Or it could be just a system glitch at the hab. Hard to say."

"So we shouldn't take it too seriously?" said Denise, feeling somewhat reassured.

"Probably not. Pity we lost the comsats. If we still had them, we wouldn't have to depend on line-of-sight communications."

He looked even more worried for a moment, for his possibility-oriented imagination was telling him to drive up to the top of the range of hills five miles to the east, and try to get through from there. But that could well be an overreaction, his reason was countering, finally persuading him to put the idea out of his mind. They had serious scientific work to do that day, and he was conscientious. Instead he just said: "The food looks good. Let's eat. Looking for dead bugs on Mars is hungry work."

Over two years earlier, NASA had put three communications satellites into a synchronous orbit around Mars, to ensure very good communications between Mission Control and the landing site, day and night. Two of these comsats sat in the sky east and west of the landing site. Unluckily, five weeks before the mission landed, a micrometeor shower had badly damaged both comsats. The two damaged comsats had finally died, six weeks after the landing, rendering the remaining comsat on the other side of Mars useless.

This meant that a rover on a field trip could no longer communicate with the landing site by bouncing radio signals off a comsat. Instead, it had to fall back on the less reliable line-of-sight surface radio, which could be blocked by hills and mesas.

After lunch, they continued work outside, slowed only a little by dust eddying around rocks and stones. The wind was diminishing however, and by three o'clock had died away completely.

Just after three, they finished up and began loading the trailer. The day's survey had uncovered no sign of Martian life forms.

Once back inside the rover, John went straight to the driver area, and tried to get through to the landing site, but with no success.

A little before three fifteen, they set out on the trip back, both of them now very concerned. John was especially concerned, for the sun was now much lower in the sky, and he knew it could no longer explain the lack of radio communication.

The top of the low hills ahead was a series of ridges. When they got to the top of the first ridge, at three forty, John stopped the rover and tried once more to make contact with the landing site. Still nothing. Denise could see the mounting concern on his face.

"Something's wrong, isn't it?" she said.

"Sure looks that way. It makes no sense. We should easily be able to get through from here. We're high up, the sun's a lot lower down, and the base is only seven miles away."

He started up the rover again, and pressed on over the rough red terrain, as fast as he dared drive. He was now deeply concerned.

At ten to four, when they reached the top of the last ridge, John stopped again. From there they could make out the landing site in the distance. It now lay just five miles ahead, set against the spectacular backdrop of the escarpment wall of a high mesa. The wall, about twelve miles farther east, was lit up by the late afternoon sun.

Denise was already studying the landing site through her field glasses, as John reached for his.

"Oh, my God!" she cried suddenly. "The return rocket. It's... it's disappeared. But it can't have. I must be seeing things...."