What started as a tool for military surveillance is now essential to earth scientists examining changes in landscapes over time.

最初作为军事监视工具的东西现在对于地球科学家检查景观随时间的变化至关重要。

A huge military surveillance balloon that broke its mooring in the U.S. state of Maryland has descended in rural Pennsylvania, after sparking an air traffic alert.

美军防空监视气球在马里兰州失控，并引起了空中交通警报，最终坠落在了宾夕法尼亚州田园地区。