下午两点，我穿好舱外活动服出了气闸舱。
裂缝位置在第七舱段迎风面——所谓的"迎风面"是指面向轨道前进方向的一面。天环以每秒7.8公里的速度绕地球运行，这个方向上微陨石撞击最频繁。裂缝很可能就是微陨石撞出来的。
我挂在安全绳上，飘到裂缝旁边。先用探伤仪测了一下——裂缝比我目测的更深，已经贯穿了舱壁外层的铝合金板，只剩内层钛合金板还撑着。
如果内层也裂了，四百人会在三分钟内失去意识。
我打开焊枪，开始工作。
太空焊接用的是电子束焊。在真空中，电子束可以穿透很深的金属，形成高质量的焊缝。但电子束需要精确控制功率——太大烧穿，太小焊不牢。
第一条焊缝。焊枪发出蓝色的光，在裂缝上缓缓移动。金属熔化的地方泛出橙红色，然后迅速冷却成银白色。
第二条焊缝，加固。
第三条——
焊枪灭了。
我按了几下开关，没反应。看了一眼腕上的显示屏：焊枪电池故障。
备用电池在工具箱里。工具箱挂在三米外的扶手上。三米，在太空中不远，但安全绳够不着。
我解开安全绳的快扣。
控制中心的声音在耳机里炸开："赵师傅！你干什么？"
"换电池。"
"你在安全绳范围外！"
"我知道。"
我蹬了一下舱壁，朝工具箱飘过去。没有重力，没有阻力，三米只需要两秒。但这两秒里，如果我的速度方向偏了一点，我就会永远飘进太空。
我的手指抓住了工具箱的把手。
换好电池，我又飘了回来。重新扣上安全绳。继续焊。
第三条焊缝完成。探伤仪检测：焊缝质量A+级。
我回到气闸舱，脱下舱外活动服。手心全是汗。
控制中心的人说我违规操作，要写检讨。我说行，写就写。
三个月后，天环管理委员会给我发了一枚奖章——"最佳安全贡献奖"。颁奖词写得很漂亮，说什么"以专业技能和敬业精神守护了三千人的生命安全"。
只有我知道那天下午发生了什么。只有我知道那条裂缝有多深，焊枪电池为什么偏偏在那时候坏了，以及我解开安全绳的那两秒里心里在想什么。
我不是英雄。我只是一个焊工，干了七年，走了两万公里，焊了三万条缝。
但那天晚上，当我站在第七舱段的舷窗前，看着窗外的地球从黑暗中升起，弧线边缘泛着蓝色的光——
我觉得这七年值了。
后来我把那枚奖章熔了，用电子束焊枪把它焊在了第七舱段的内壁上，就在裂缝的正对面。
没人知道那是奖章。它看起来就像一块普通的金属补丁。
但每次我路过那里，都会用手指摸一下它。冰凉的，光滑的，和周围的舱壁融为一体。
就像那条裂缝从未存在过。
The Welder
My name is Zhao Tiezhu, and I'm the only welder on Tianhuan Space Station.
Tianhuan is the largest space city in Earth orbit, home to three thousand people. It's assembled from twelve ring-shaped modules, each two hundred meters in diameter, rotating to generate artificial gravity. My job is maintaining the weld seams between modules.
Space welding is completely different from ground welding. No air means no oxidation, so weld quality is actually better. But no air also means no convective cooling — the heat from the welding torch can only dissipate through conduction and radiation, so after each segment you have to wait twenty minutes, or the surrounding metal will soften.
That day, I found a crack at the junction between Module 7 and Module 8. Less than one millimeter wide, five centimeters long. On Earth, such a crack would be nothing. In space, it's lethal.
I reported it immediately. Control center said: "Noted, maintenance scheduled for tomorrow."
I said: "It might not last until tomorrow."
Engineering assessed it as "within safe range." But their engineers had never been outside the airlock. They didn't understand: in space, a one-millimeter crack is a countdown.
I went out that afternoon on my rest time. The crack was deeper than I'd estimated — it had already penetrated the outer aluminum layer. Only the inner titanium layer held. If it cracked, four hundred people would lose consciousness in three minutes.
Halfway through welding, the torch battery died. The spare battery was in the toolbox, three meters away — beyond the safety tether's reach. I unclipped the tether.
Two seconds in vacuum. If my trajectory was off by a fraction, I'd drift into space forever.
I grabbed the toolbox handle. Changed the battery. Floated back. Finished the weld.
Three months later, the station committee gave me a "Best Safety Contribution" medal. The citation was eloquent — "professional skill and dedication protected three thousand lives." Only I knew what really happened that afternoon.
I melted the medal down and welded it to the inner wall of Module 7, directly opposite where the crack had been. Nobody knows it's a medal. It looks like an ordinary metal patch. But every time I pass it, I touch it with my fingers. Cold, smooth, indistinguishable from the surrounding wall.
As if the crack never existed.
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