Halfway to Mars the sun suddenly turned deadly.

The mission to Mars had been timed for a period of low solar activity. Still, there was only the slimmest of chances that the spacecraft could carry their human crews through nine months in interplanetary space without running into a magnetic storm spawned by a solar flare.

Both on Earth and at the underground base on the moon, solar forecasters watched the sun in cramped narrow workrooms crowded with humming computers and video monitors. They saw a set of blotches take form on the shining surface of the sun, each of them bigger than the Earth itself. Their instruments detected weak radio emissions and bursts of soft X-rays from the sunspot group. Completely normal.

Then the flare erupted. Nothing spectacular, to the eye. Just a brief flash of light. But the incoming radiation grew swiftly, ominously, its intensity rising a hundred times above normal, a thousand, ten thousand, in the span of a few minutes. Ultraviolet and X-ray sensors aboard monitoring satellites went into overload. An intense burst of radio noise sizzled in astronomers’ receivers all around the Earth and shut down the radio telescope at the lunar base. It was a completely ordinary solar flare, no more powerful than a hundred billion hydrogen bombs all going off at once. Its total energy was less than a quarter second of the sun’s normal output.

But the cloud of subatomic particles it blew into space could kill unprotected humans in seconds.

The solar forecasters’ instruments automatically radioed a warning to the Mars spacecraft, more than seventy million kilometers away from Earth. The electromagnetic radiation from the flare, traveling at the speed of light just as the astronomers’ radio signals did, hit the spacecraft at the same instant that the warnings arrived.

Alarms hooted down the length of both ships, startling the men and women at their tasks, jolting those asleep into a terrified waking. The first moment of adrenaline-drenched shock gave way to the reactions drilled into the Mars teams by years of training. Every man and woman on each of the two spacecraft dashed, sprinted, raced for the radiation shelters.

For the first wave of electromagnetic energy from the flare was merely the precursor, the flash of lightning that warns of an approaching storm. Following it by a few minutes or perhaps even a few hours would be a vast expanding cloud of energetic protons and electrons, particles that could slice right through the skin of the ship and fry human flesh in seconds.

In low Earth orbit astronauts are protected from solar flare particles by the Earth’s magnetic field, which deflects the energetic protons and electrons flung off the sun and eventually pumps them down into the atmosphere at the north and south magnetic poles. Spectacular auroras can paint the skies for several nights in a row after a big solar flare. The geomagnetic field is bashed and buckled by the storm of incoming particles; for days it vibrates and twangs like banjo strings. Radio transmissions are garbled. Even underground telephone links can be scrambled.

On Earth itself the atmosphere absorbs any particles that power through the magnetic field, so that even the most energetic solar flare does not endanger life on the surface of the planet. On the airless moon, with its minuscule magnetic field, there is only one defense: go underground and stay underground until the storm blows over.

In interplanetary space the only defenses against a magnetic storm are those the spacecraft carry with them.

"Don’t sweat it," said Pete Connors. "We all knew we couldn’t make it all the way without running into a flare." He was trying to sound reassuring, but the expression on his long-jawed face looked quite serious, like a doctor discussing surgery with his patient.

"It’s more like the flare is running into us, isn’t it?" corrected George O’Hara, the Australian geologist.

The twelve men and women of the Mars 1 crew were crammed onto the benches that lined the walls of the spacecraft’s specially shielded radiation shelter. Everyone called it the "storm cellar." In this small compartment at the rear of the habitat module, the bulky propellant tanks attached to the spacecraft’s outer hull provided a measure of protection against the lethal radiation spawned from a solar flare.

The two Mars-bound craft used their half-depleted propellant tanks to absorb some of the high-energy particles streaming out from the sun. In addition, the crafts’ storm cellars were lined with thin filaments of superconducting wire. The first person to reach the radiation shelter — Pete Connors, as it turned out — punched the switch on the wall by the hatch to energize the shielding system.

The superconducting wire generated a strong magnetic field around the storm cellar, strong enough to deflect the lightweight electrons in the cloud of particles swarming past the spacecraft. But the heavier protons were the real danger, and the magnetic field was not nearly strong enough to deflect them.

Instead, the ship’s defenses included a set of electron guns that charged the outer skin of the spacecraft to millions of volts of positive charge. In theory, the incoming protons would be deflected from the spacecraft by its megavolt positive charge, while the craft’s magnetic field would keep electrons from reaching the skin and neutralizing the positive charge.

Small versions of the system had been tested aboard satellites flung into sun-centered orbits. Unmanned satellites.

"How long will we have to stay in here?" asked Ilona Malater. She was sitting between Tony Reed and the Greek biologist on the backup team, Dennis Xenophanes. Her long fingers clutched the edge of the bench so tightly her knuckles were white.

"Twelve hours or more," answered Ollie Zieman, the American astronaut who was Connors’s backup. "Maybe a couple of days."

"My god!"

"No sweat," Zieman replied, almost jovially. "Radiation level in here is almost normal."

The shelter already felt crowded and sweltering with the smell of suppressed fear. Jamie leaned his back against the bulkhead, wondering if the magnetic field being generated by the superconducting wires mere inches away from his flesh actually had no effect on their bodies. According to the system’s designers, the field was shaped so that the storm cellar was in the clear; the field extended outward in all directions, but the shelter itself was like a bubble in its middle.

Vosnesensky and his backup, Dmitri Ivshenko, were standing in front of the communications console built into the shelter’s forward bulkhead, by the hatch. Mikhail had clamped a communications headset over his curly hair.

"Radio communication is difficult," Vosnesensky announced loudly for everyone to hear, even though he kept his back to them. "We will use the laser system."

A magnetic storm can screw up radio waves, Jamie knew, but it shouldn’t have any effect on a laser’s beam of light. He felt a tightness in his chest, anxiety, even though they had trained for such emergencies. There’s a semi-infinite number of subatomic particles out there just dying to get in here and kill all twelve of us, he thought. Like a cloud of spirits of the dead scratching and moaning outside the door.

"Mars 2 is all right," Vosnesensky announced. "Everyone in the storm cellar with no trouble."

They’ve got the extra man, Jamie thought. Dr. Li makes it thirteen they have to squeeze into their shelter.

Pete Connors got up and went to stand between Vosnesensky and the other Russian. "All the ship’s systems are working okay?" he asked loudly.

"Yes, yes." Vosnesensky pointed to the panels of lights that showed the condition of the rest of the ship. Most of the lights were green. "The equipment was built to withstand radiation. It is only we fragile creatures of flesh and bone who need protection."