Tolbukhin nodded reluctantly.

"There is something more," Li told them. "The medical staff at Kaliningrad has brought up the question of quarantine."

"Quarantine?"

Li felt miserable as he said, "Since we do not know what is infecting the ground team, they fear that whatever it is might infect us here in orbit if we return the ground team here."

"Holy shit," Klein muttered. "They want us to leave them down there?"

Tolbukhin grasped the larger implication. "That means they will not allow us to return to Earth if we have not found the source of the disease."

"Yes," admitted Li. "We ourselves might be quarantined in Earth orbit."

"If we live long enough to get back that far," the Russian said.

"The alternative is to leave the ground team and return to Earth without them."

"That’d kill them!" Klein snapped.

"Yes. But to rescue them and bring them back up to orbit with us might kill us all."

For long moments neither the astronaut nor the cosmonaut spoke a word.

Finally Klein said, "Well, you’ve got to do something."

Li knew he was right. The weight of responsibility was squarely on his shoulders. Let the four in the rover die, or risk the lives of everyone — including those in orbit — by allowing the last of their pilots to ride to the rescue on the last of the landing/ascent vehicles. Abandon the ground team altogether or risk catching their disease and killing everyone.

Li felt the weight of two dozen lives on him. The weight of two worlds.

When the last of the physicals was finished Tony Reed asked Yang Meilin, "What do you expect to find?"

She shot him a sharp glance from the chair on which she sat. "The cause of this epidemic."

Reed had barely budged from the corner of the infirmary where he had watched her examine all the people in the dome. Now he made a puzzled shrug.

"Vosnesensky thinks it might be Martian dust that we’re inhaling," he said.

Yang’s almond eyes watched him unblinkingly from beneath her straight bangs. "Do you believe that?"

"No, I don’t. We’ve tested the air here in the dome. It’s cleaner than the air in London, by far."

She got up from the chair, a tiny Chinese woman with a nondescript figure and an utterly forgettable round, flat-featured face — except for those eyes. Reed thought they looked at him accusingly. Why not? Why shouldn’t she blame me for this calamity? It is my fault, my responsibility. I was put here to protect the health of these men and women. Some protector!

"Well," he asked, "what do you think?"

She shook her head slightly. "I cannot tell. All the data from the tests we have just done are being analyzed by the medical computer aboard Mars 2. Until we get its results I cannot go further."

Reed gave an exasperated sigh. "It won’t do any good, you know. The first thing I did when they started coming down with this malady was to run all the medical records through the computer diagnostic program. It just burped out nonsense."

"Perhaps now, with more data…"

"I doubt it. The computer can only tell you what it already knows, and we’re facing something new and unprecedented here."

"Perhaps not. It may be something ordinary but unexpected. That is the great strength of the computer: it is not clouded by human expectations or emotions. It analyzes all the symptoms and reports which medical conditions fit the data."

"Yes," Reed sniffed, feeling real anger surging up inside him. "I’ll tell you what the damned computer will give us. It will suggest that the malady might be a variation of influenza — which it isn’t, because we’ve found no influenza viruses in the blood workups; or malaria — which is ridiculous because the nearest mosquito is two hundred million kilometers from here; or radiation poisoning — which it can’t be, because the dosimeters show that every member of the team is well within tolerable limits; or a vitamin deficiency — which is ludicrous because I see to it that everyone takes their bloody vitamin supplements."

Yang said, "Perhaps a slow virus? Perhaps an infection such as Legionnaires’ disease?"

"I thought of that," Reed snapped. "The symptoms don’t match."

The Chinese doctor murmured something too low for Reed to hear. Ignoring her, he went on:

"The marvelous computer analysis will also suggest the possibility of salmonellosis, tuberculosis, or typhoid fever — in decreasing probabilities, of course."

He stopped, out of breath, seething with a rage that he had not realized was in him.

"Why are you angry with me?" Yang asked, her mask of impassivity gone. She looked shocked, hurt.

Tony stared at her, his insides jumping, his hands clenching into fists. He took a deep breath, then stepped back to his desk.

"I’m sorry. I apologize. It’s not you. I suppose I’m angry at myself, really. This thing — I can’t for the life of me figure out what it is!" He banged a fist on the flimsy desktop.

"That is why we need the help of the computer program."

Reed cast her a cynical smile.

"Not to tell us what the disease may be," Yang explained, "but to rule out definitely what it is not."

"I don’t believe it can even do that."

Yang tried to smile. "Was it not one of your English writers who said that once you have ruled out the impossible, then whatever is left-however improbable it may seem-must be the truth?"

Reed blinked at her. "Arthur Clarke?"

As politely as she could, Yang replied, "I believe it was Conan Doyle."

KALININGRAD: In a windowless conference room in the mission control complex, twenty men and women from six nations thrashed out the problem that assailed them from nearly two hundred million kilometers away.

The oblong conference table was littered with scribbled sheets of paper, crusts from sandwiches, charts and viewgraphs, Styrofoam beverage cups, ashtrays heaped high with smoldering butts. Some of the people around the table slouched miserably, heads in their hands, jackets long pulled off and shirtsleeves rolled high. A few paced pointlessly along the length of the stuffy, smoky room.

They had long ago shouted themselves hoarse without arriving at a conclusion.

At the head of the table sat the chief of mission control, a lean red-headed Russian with a saturnine pointed beard and red eyebrows like inverted vees. He tapped a long fingernail on the imitation wood of the tabletop. In the exhausted silence of the room every head swiveled toward him.

"We cannot merely sit here without making a decision. Human lives are at stake. The success of the entire mission is at stake!"

One of the women, a Swede, coughed slightly, cleared her throat, then said: "Our alternatives are clear — allow the traverse team to die or take the risk of killing more members of the expedition in an attempt to save them."

"We can’t just let them die!" said another woman.

"But a rescue attempt might fail and there will be more deaths," countered a Japanese male.

"Half the reporters in the world are pounding on our doors," someone commented sourly. "We’ve got to do something, and do it now!"

"We should never have permitted an excursion into the canyon," a Frenchman complained. "Not on the very first mission. It was not in our original plan. We bowed to blatant American political pressure. That is what has put us in this chamber pot."

"But Brumado’s daughter is one of the people who are stranded. We can’t let her die! Who’s going to face him and say that we decided to let his daughter die?"

"I am convinced," said a chubby, balding Russian, "that the only thing we can do is to bring up the people in the dome right now, get them up to safety in the orbiting ships, and then send the last lander down to the canyon to take up the four in the rover."