Space and time are two aspects of the same thing, dimensions of the universe. There was a keyhole in spacetime, or as the engineers of mission control phrased it, a window. The two Mars craft had to be launched out of Earth orbit through that keyhole, through that window, at a certain time and in a precise direction with exactly the proper velocity, if they were to reach the moving pinpoint of light that was their destination.

For twenty-three days the two dozen men and women of the Mars mission, plus their expedition commander, Dr. Li, checked and re-checked every piece of equipment stowed aboard the long sleek Mars spacecraft. While they did so, specialist teams of technicians and robots attached bulky ovoid tanks of propellants around the aft end of each craft. The spacecraft began to look like thin white pencils surrounded by clusters of matte-gray lozenges at their eraser end.

The propellants had been manufactured on the moon and catapulted from the airless lunar surface to rendezvous with the spacecraft waiting in Earth orbit. The mission to Mars required not only Earth’s resources, but the mining and processing centers on the moon as well.

On the twenty-fourth day the Mars-bound personnel left the assembly station for good and transferred their personal gear to the spacecraft. Twelve men and women aboard the habitat module of Mars 1, twelve plus Dr. Li in Mars 2. No one made a single mention of the fact that there would be thirteen aboard Mars 2. None of the scientists or pilots would admit to being superstitious; still, no one spoke the word "thirteen."

Space-suited technicians attached the long tethers that connected the two assembled spacecraft. Manufactured in the microgravity environment of a space station facility, the tethers had a tensile strength many times greater than that of any material that could be made on Earth.

Once they were on their way to Mars, tiny cold-gas thrusters would spurt in a precisely programmed order and the spacecraft would begin to spin up in a stately, graceful rotation. The tethers would stretch to their full five-kilometer length, and inside the connected Mars spacecraft a feeling of normal gravity would return, while the universe outside would start to revolve slowly past their observation ports.

A cluster of astronomical telescopes and high-energy radiation sensors was carefully placed at the midpoint of the long tethers, where they would be effectively weightless and could maintain precise pointing accuracy for the astronomers who would operate them remotely from Earth.

Other thrusters would later do-spin the spacecraft enough to reduce the internal gravity to the Martian level. By the time they arrived at Mars the explorers would be fully accustomed to the low Martian gravity. On the nine-month flight back home the spacecraft would spin up to a normal terrestrial g once again.

The interior of the habitat module was like the interior of every spacecraft Jamie had ever been in: a central corridor flanked either by the closed doors of privacy compartments or the open benches and equipment racks of workstations.

Up forward was the command section where a Russian cosmonaut and American astronaut copiloted the spacecraft. Just behind it was a sort of passenger compartment with seats for all the personnel, which could also serve as an informal lounge or conference room.

There was no need for acceleration couches. The rockets that would propel them to Mars produced very low levels of thrust; they would hardly feel as much acceleration as during a jet airliner’s takeoff. Lifting off from the ground and going into Earth orbit required a big jolt of thrust, several minutes of three g’s or more. That had all been done by space shuttles and unmanned rocket boosters carrying cargo. Once in orbit, though, the rest of the solar system could be reached gently.

One part of the habitat module was different. A section toward the rear was devoted to an oblong window made of thick quartz. Once they got to Mars, this observation port would be studded with cameras and other sensors. For now, though, it made a fine picture window.

The hour they were scheduled to depart, Jamie found himself at the observation port, hovering easily in zero g, his slippered feet dangling a few centimeters above the foot restraints set into the metal floor. He saw the Earth sliding past, an enormously massive curve of deep luminous blue, then the duller green-brown of land and the hard gray wrinkles of a mountain chain, dusted with clutching skeletal fingers of white snow. Another ocean slid into view, the immense swirl of a tropical storm’s seething clouds forming a gigantic gray-white comma over the water.

"Those are the Andes Mountains."

Joanna had come up beside him, floating noiselessly. He had not noticed, he had been staring at the world so intently.

"Come to say good-bye to Mother Earth?" Jamie asked her.

"Not good-bye," she whispered. "We will return."

"Adios, then."

She nodded absently as she slipped her feet into the floor loops, her eyes on the world they were about to leave.

"I still can hardly believe I’m here," Jamie said. "It’s kind of like a dream."

Joanna glanced up at him. "We have a long and difficult journey ahead of us. Hardly a dream."

"It is for me."

She almost smiled. "You are a romantic."

"Aren’t you?"

"No," Joanna said. "Women must be practical. Men can be the romantics. Women must think about the consequences."

"Departure in three minutes," came a Russian-accented voice over the speaker in the ceiling above them. "Please take your assigned seats in the forward lounge."

Jamie took Joanna by the shoulders and kissed her on the lips, lightly, swiftly.

"For luck," he said.

Joanna pushed free and floated away from him, her face frozen, unsmiling, her eyes wide and fearful. Without a word she turned and grabbed the edge of the hatch for purchase, then launched herself up the passageway toward the forward lounge.

Jamie waited a few moments, then went after her, moving more slowly. Then he saw Tony Reed hovering in the doorway to his cubicle, a sardonic smile on his lean face.

"I don’t think the direct approach will work with little Joanna," Reed said.

Jamie said nothing. He pushed past Reed, heading forward.

The Englishman followed him. "I may have told you too much about our little cabal to get rid of Hoffman. Remember, my impetuous friend, that she may have wanted to have you picked for the expedition, but she certainly did not want Hoffman to come with us."

Jamie looked over his shoulder and said, "White man speaks with forked tongue."

Reed laughed all the way to the forward lounge.

There were no windows in the compartment. If necessary, this entire forward section of the spacecraft could be detached and flown by the pilots up in the cockpit into a reentry trajectory and an ocean splashdown. The procedure was for emergency use only; the mission plan called for the spacecraft to return to Earth orbit, where the personnel would transfer to shuttles for the final ride to Earth’s surface. But a water landing was possible, if the need arose.

Jamie had barely floundered through the swimming course required by the mission planners. He wondered how the seven other scientists strapping themselves into their cushioned chairs would handle such an emergency. Or the four astronauts and cosmonauts in the cockpit, for that matter. It would be fine irony to go all the way to Mars and back and then drown.

"Departure in thirty seconds," came Vosnesensky’s voice from the cockpit. "I am putting an external camera view on the display screen."

The compartment had a small screen built into its forward bulkhead. It flickered briefly, then showed the curving bulk of the blue-and-white Earth sliding past. Jamie took the last remaining seat and clicked the safety belt across his lap to prevent himself from floating out of the chair. Reed had taken the chair beside Joanna.