高空飞行：航空学的极限

The stratosphere is the highest layer of Earth's atmosphere where aircraft can still fly. The density of the air is just high enough here to generate lift on a wing or buoyancy on a balloon, so designing any stratospheric aircraft is a delicate technological balancing act for the engineer. Designing and operating an aircraft capable of conveying humans to the stratosphere is more challenging still: biologically, we simply do not belong up there. Temperatures often as low as -80C (-112F) and an ambient pressure rapidly diminishing with altitude make for an extremely forbidding environment. In fact, as we pass 50 000 feet (the lower end of Concorde's cruising altitude range), we enter the space equivalent zone - from a physiological point of view we might as well be in low Earth orbit.

The fact that stratospheric flight is possible at all - moreover, even safe and economical, at least in the lower stratosphere - is made possible by some relatively recent advances in our understanding of the science of high altitude flight. This book charts some of these developments; at the same time, it is a catalog of ways in which the stratosphere can catch out even the well-prepared flyer. Naturally, the failures of early explorers have signposted many of these dangers, but, as regular news headlines and the series of vignettes that punctuate the book illustrate, the learning curve has not levelled off, it has merely become shallower. Stratospheric flight is still aviation at the limit.

平流层是地球大气的最高层，飞机仍然可以在其中飞行。这里的空气密度刚好足以在机翼上产生升力或在气球中产生浮力，因此对于工程师来说设计任何一种平流层飞机都是一项微妙的技术平衡。

设计并操作有能力将人类送往平流层的飞机更是极具挑战性：从生物学角度来看，我们根本不适合那里。

温度常常低至-80°C（-112°F），随着高度增加大气压迅速下降，这构成了一个极其严苛的环境。事实上，在我们达到5万英尺的高度（协和客机巡航高度范围的下限）时，我们就进入了太空等效区域——从生理学角度来看，我们几乎可以等同于在低地球轨道上。

平流层飞行不仅可能而且至少在较低层是安全且经济的这一事实——得益于最近对我们理解高空飞行科学的一系列相对近期的进步——是可能的。这本书记录了一些这些发展；同时，它也是一部描述了平流层如何让即使是准备充分的飞行员也难以应付的方式的目录。

当然，早期探险者的失败已经指明了许多这些危险点，但正如书中的新闻标题和贯穿全文的故事插图所显示的一样，学习曲线并未停滞，而是变得更为浅显。平流层飞行仍然是航空技术的极限。

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