In a broad sense, a spacecraft's propellant consists of two parts. One is the fuel itself—Xiao Yu used liquid hydrogen. The other is the oxidizer—liquid oxygen.
Fuel cannot burn, and therefore cannot produce thrust, without an oxidizer. Now, the fuel aboard the ship was nearly exhausted, while the oxidizer remained in relatively ample supply.
Under these circumstances, if Xiao Yu could somehow maneuver the ship to Titan, the moon's virtually limitless methane reserves could temporarily replace liquid hydrogen as his energy source. And once controlled nuclear fusion was successfully developed, Saturn itself—right next door—would become an inexhaustible reservoir of fusion fuel.
At that point, Xiao Yu would have energy to repair the ship, energy to run his systems at full power and restore his formidable computational capacity, even energy to mine minerals and manufacture new equipment—new ships, if necessary.
Reality, however, was merciless. Titan was right there, yet Xiao Yu simply couldn't reach it. Infinite energy lay within arm's reach, yet he lacked the very energy needed to get there.
"Liquid hydrogen… liquid hydrogen… I need liquid hydrogen!"
Xiao Yu was so frustrated he almost wanted to roar.
Strictly speaking, if he scraped together every last drop, the remaining fuel might barely be enough to reach Mars' escape velocity and break free of its orbit. But doing so would leave only enough energy to power the central computer for nine months.
Nine months sounded long—until compared with the vast timescales of interplanetary travel. In that context, it was little more than a blink of an eye.
Even if Xiao Yu recklessly escaped Mars' gravity, the ship's maximum cruising speed would be no more than six kilometers per second. At that speed, reaching Titan would take roughly seven years. In other words, he would die halfway there.
That outcome was absolutely unacceptable.
Xiao Yu began calculating in earnest, searching for another way.
"If I don't fly directly toward Titan, but instead head toward the Sun," he reasoned, "and use gravitational assists from Earth, Venus, and the Sun itself, I could reach about eight kilometers per second. The closer I get to the Sun, the faster I go. That way, I'd need only about two months to reach Earth's orbit. Wait—yes! The region near Earth's former orbit is full of comets right now. There's a huge amount of water being released. If I can collect some of it, then near the Sun, I can use solar ionization to split the water into hydrogen and oxygen. I'll have fuel again!"
His spirits lifted instantly. In the vast darkness, Xiao Yu finally glimpsed a sliver of hope.
"At closest approach to the Sun, my speed could reach twenty-six kilometers per second… then one full orbit around the Sun, another gravitational boost from Mercury. Venus should be on the far side of the Sun by then—fine, skip it. After that, Mars' gravity… and Jupiter—no, better keep my distance from Jupiter. Still, even passing tens of millions of kilometers away, Jupiter could add another three or four kilometers per second…"
The more he calculated, the more excited he became.
"If I head sunward, orbit the Sun once, then go to Saturn, the total distance increases—from about 1.3 billion kilometers to roughly 1.9 billion. But my speed increases even more. I could save at least five years! And near Earth's orbit, I can replenish liquid oxygen and hydrogen. Closer to the Sun, solar energy will be abundant. Fuel solved!"
"There's just one serious concern," Xiao Yu thought soberly. "Earth's orbit is utter chaos right now. I'll have to watch out for meteor impacts. And the Sun—there's no doubt it was affected by Jupiter's catastrophe. Solar wind strength must have increased by several orders of magnitude. That's another hidden danger."
"All right. When I reach Earth's orbit, I'll temporarily leave the ecliptic plane. I'll collect less water if I must—safety comes first. As for the Sun, I won't go inside Mercury's orbit. I'll lose some acceleration, but the safety margin will be much higher, and the thermal shielding won't be pushed to its limits."
"Good. That's the plan."
After one final systems check, confirming there were no glaring flaws, Xiao Yu clenched down and ignited the engines.
This was a gamble.
No one knew what dangers lay ahead. But gambling meant roughly a thirty percent chance of survival. Not gambling meant certain death. Xiao Yu refused to die just as controlled nuclear fusion had broken through, just as a brilliant future beckoned. No matter what, he had to take this chance.
There were simply too many unfavorable factors. Not only could new malfunctions occur during the long journey, and not only might he be struck by meteors or fail to collect enough water—but even the ship's heat shield was a concern. It had been designed to withstand atmospheric reentry, not the raw fury of the Sun.
Yet at this point, there was no choice but to press forward. Even if it seemed impossible, struggling onward still meant hope. Xiao Yu had no intention of dying after countless trials had fused his consciousness with the central computer, granting him near-infinite life, immense computational power, and inexhaustible stamina—only to perish before his interstellar voyage had truly begun.
As Mars slowly receded into the distance, Xiao Yu spoke silently:
"Brother Mars, thank you for shielding me from such terrible storms. When my technology advances, I'll come back and give you a proper makeover—turn you green, release animals on your surface, make you a true paradise."
After making this quiet vow, Xiao Yu resolutely shut down the telescope. In his current situation, every bit of saved energy mattered.
Next, he powered down most nonessential functions of the main computer, leaving only navigation, radar collision avoidance, and a handful of critical systems. Thus began his bleak, solitary journey.
Mars lay about eighty million kilometers from Earth's orbit. It would take two months before Xiao Yu even approached that region. After such a long voyage, only to return to his starting point—the irony left him deeply frustrated.
There was also the issue of relative velocity. While orbiting Mars, Xiao Yu was also moving with Mars around the Sun at about twenty-five kilometers per second. When he left Mars and headed toward the Sun, from Mars' perspective he flew off along a straight tangent. But from the Sun's frame of reference, his path was a parabola. The full explanation was complex, but the result was simple: Xiao Yu approached the Sun at roughly eight kilometers per second, accelerating gradually under solar gravity, and would reach Earth's orbit in about two months.
Aside from instructing the onboard robot to prepare for water collection, Xiao Yu reduced energy consumption to the bare minimum. He couldn't even afford to process the data gathered from Jupiter. With nothing to do, and only a tiny fraction of CPU power allocated to the robot, Xiao Yu spent his days staring into the pitch-black void of space.
Without an atmosphere to blur it, the starfield was razor-sharp. Among countless stars, Xiao Yu found the faint glimmer of Tau Ceti.
Tau Ceti was the nearest known star system to the Sun confirmed to host planets, about 10.5 light-years away. It was also the next destination in Xiao Yu's grand interstellar plan. Tau Ceti possessed a gas giant roughly one and a half times the mass of Jupiter—plenty of fusion fuel—and two asteroid belts that could provide abundant construction materials.
"Tau Ceti… humanity's first interstellar voyage," Xiao Yu murmured, staring fixedly at the star, unable to stop himself from imagining what awaited him there.
After a long while, he pulled his thoughts back and sighed bitterly.
"If this solar run fails, then forget Tau Ceti—I won't even know if I can survive."
More than a month passed in the long journey. Meteor density in interplanetary space began to rise noticeably.
By now, the ship's trajectory had left the ecliptic plane. Most planetary orbits lay within the ecliptic, and even though Earth had exploded, the majority of its debris remained near that plane as well.
Leaving the ecliptic minimized the risk of collision.
During this month, Xiao Yu directed the robot to use the remaining materials to construct a crude filter net, designed to capture ice particles, dust, and other matter drifting through space.
Due to conservation of angular momentum, even after Earth's destruction, most fragments continued to orbit the Sun along roughly the same paths and speeds. Xiao Yu had deliberately adjusted his trajectory closer to Earth's former orbit, so the relative speed between his ship and the drifting ice and dust was modest—only one or two hundred meters per second. The filter net was strong enough to capture them, little by little.
The results, however, were discouraging.
Twenty-four hours after deploying the net, the total mass collected was less than one kilogram. Water accounted for less than thirty percent of it.
"A mosquito's leg is still meat," Xiao Yu muttered. "Slow and steady."
And so, while continuing to edge closer to Earth's orbit, Xiao Yu took on a new role—
that of a cosmic scavenger.