There were three hundred thousand NEOs a hundred yards across or bigger, and some two thousand half a mile across or bigger. Some were rocky, some metallic, others rich in organics.

Earth sat at the bottom of the deepest gravity well between sun and Jupiter. Over billions of years, twenty percent of all the NEOs would impact the planet.

It was undoubtedly true, she had learned, that if humanity was to have a long-term future, leaving the planet and dispersing was the only option. Space travel was no leisure luxury for a rich world, but essential for the survival of the species.

Perhaps, she comforted herself, her mission — successful or otherwise — might spur a greater awareness of the hazardous environment within which humanity had, perforce, made its home; perhaps, in some ultimate long term, she might actually prove the savior of humanity.

Perhaps she might be remembered as mankind’s greatest hero.

Rather than as its supreme villain.

Three days from her closest approach, with the asteroid grown massive in her window, she saw the flashes of the drone warheads which had preceded her.

The weapons had emplaced themselves close to the asteroid, though away from its surface. The flashes looked like miniature dawns. They were immediately surrounded by surges of debris from the asteroid’s pulverized surface, which rocketed out in well-defined jets, some fragments glowing white-hot.

There was, of course, no heat, no sound, no concussion wave transmitted from the massive explosions — though her heart quailed as she looked into the fusion light.

She carefully observed the explosions, and prepared to compute their consequences.

The mission philosophy was simple. The smallest impulse required to deflect that rocky body was more than could feasibly be delivered by a single weapon. Too large an explosion, besides, could shatter the object, removing its usefulness.

Therefore a string of automated weapons had been launched by Long March boosters over the days preceding Jiang’s own launch. The necessary impulse would be applied, not by one large detonation, but by a series of smaller ones. Tianming was distinguished only in carrying the last — albeit the largest — of the weapon set, to deliver the final tweak to the asteroid’s new trajectory.

The mission design offered a chance for more accuracy, besides. The asteroid still had to travel many millions of miles along its new path. The successive explosions could be used to herd the asteroid closer towards the required final trajectory. The last detonation was, of course, the final opportunity for adjustment.

It was explained to Jiang, carefully, that China lacked the facility for sufficiently precise deep space tracking of either the asteroid or an unmanned spacecraft, and the robotic expertise to enable such a craft to navigate itself, sufficiently precisely. Only a human navigator — such as herself — using optical techniques on the spot could make the precise measurements of the deflection achieved of the asteroid by the unmanned probes, and then emplace the final weapon sufficiently precisely to achieve the last elements of the required deflection.

Such was her purpose.

It was also explained, equally carefully, that time, resource deficiencies and mission constraints were such that it had not been possible to provide a separable delivery system for the weapon itself. Nor any return or reentry provisions for the crew. That is, it was necessary for Tianming to remain in place during the explosion.

This was a factor which she took into account, in the course of her decision to accept the mission.

To fly in space:to venture once again beyond the atmosphere, to become the first Chinese to venture beyond low Earth orbit, the first Chinese to spend a hundred days in space — for that, she had, to her own surprise, been willing to exchange everything. Her life.

Even her place in history.

Jiang Ling was a spaceflight junkie.

It was even possible, she mused, that had this flight been offered even to some of the one hundred frustrated American astronauts, dispersing slowly from Houston, some of them may have accepted, so great was the lure of returning to the secret place, to space.

And having made her bargain, she would, of course, complete her mission.

It was conceivable that the detonations would be observed from the United States itself, and elsewhere. If they were, the Party had a further plausible cover story, she knew: that the explosions were being used in a scientific analysis of the asteroid’s structure.

The light faded rapidly. The debris cloud dispersed quickly — or rather, the asteroid’s new orbit took it away from the fragments blown out of it by the weapons.

The dosimeter aboard Tianming indicated that the radiation dose she had already taken exceeded nominal safety limits.

Jiang Ling smiled.

She picked up her optical navigation gear — a sextant with a simple telescope — and began to study the new position of the asteroid.

On the last day before closest approach, she found it more difficult than before to comply with the order to complete her three hours’ cycling.

For her final broadcast she chose to feature the aquarium. She positioned the camera so that it focused on the apparatus, and then moved so that her own face was in the shot, close to the aquarium. For the benefit of the video camera, she made a show of peering into the microscope; her vision filled with blue water light. She spoke in her broadcast about these little creatures being her fellow passengers aboard Tianming.

It was a little corny, but it contained the essential truth. Somewhere in the milky-blue images of squirming sea urchins and eerily human starfish embryos, somewhere in this drop of the primeval sea which she had carried with her, so far from Earth, there was a sense of unity with all life, a hope of salvation.

She was not alone, even here, so far from the planet which had spawned her; she was still as one with all the creatures of the world.

Her greatest regret, in fact — which grew as 2002OA loomed — was that the thousands of creatures in this aquarium could not hope to survive the events to come.

Jiang Ling could no longer see Earth, Moon or sun.

On this, the hundredth day, the dark hide of 2002OA slid past the small window of Tianming.

It was as if she was flying over some miniature Moon, she thought. The surface was so pierced and broken by craters of all sizes that it was impossible to tell, by eye, how far away it was; she might have been in an Apollo spacecraft sailing over the surface of the Moon, sixty miles below, or peering through some camera at a plaster mockup, just out of arm’s reach.

The spacecraft was in the shadow of the asteroid now, and only the spotlights of Tianming illuminated the surface, less than a mile from the craft: she fired her camera through the window, and the digitized photographs of churned regolith were sent immediately to the ground stations.

She heard the clatter of solenoids, felt the judder of the craft as it was pushed by squirts of the automatic reaction control system.

She was beyond the useful reach of her optical navigation; now, the automatic systems of the spacecraft had come into their own — particularly the radar, which would determine Tianming’s distance from the asteroid surface, and match it to the ground-based calculations using her astronomical observations of the asteroid’s new path.

For optimal yield, the warhead required a standoff detonation, with the warhead placed forty percent of the object’s radius above the surface. There, the weapon could irradiate an ideal thirty percent of the surface of 2002OA.