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Scientists find metal molecules in the stratosphere, consistent with burnt up spacecraft. The impact of such pollution remains unclear.

MARK GARLICK/SCIENCE PHOTO LIBRARYGetty Images Earlier this month, part of China's Long March 5B rocket reentered Earth's atmosphere. As the spacecraft tumbled toward our planet's surface, experts raced to accurately calculate its trajectory. It wasn't an easy task. A number of variables—including Earth's gravity, weather conditions, and an object's surface finish—make reentry predictions difficult to calculate. In early May, the 23-ton core stage of a Chinese rocket, the Long March 5B, hurtled uncontrolled back toward Earth. The world held its breath as experts and amateurs alike toiled...

Now, don't panic. It's a big planet, and the odds of it hitting anyone are low… but not zero. To be more clear, this is a worrisome event but not one to be freaked out about. Why don't we know where it will hit? There are several factors here. For one, it's slowed by the very thin upper reaches of the Earth's atmosphere, and the density of the air is so low there that even a small change affects orbiting objects strongly (the density also changes with solar activity; space weather can make the upper atmosphere puff up, and it reaches higher).