Q&A: How We Knew Space Was a Vacuum

 

Sky Lights is a weekly blog about things you see in the sky (and some you can’t see). It covers a wide range of disciplines including: astronomy, meteorology, climatology, chemistry, physics, optics, earth & space science, and more. Back in June they covered the q uestion At what point during the history of science did we discover outer space was a vacuum? Va Sky-lights.org

Question: At what point during the history of science did we discover outer space was a vacuum? I’m pretty sure it was before we actually went there, but when did we know for certain? — MC, Algoma, WI

Answer: Excellent question MC. And the answer is a great example of how science builds on existing knowledge, sometimes in circuitous paths, sometimes to dead ends, but often to new discoveries. The idea that space is a vacuum took centuries to evolve from hypothesis to fact.

But if you’re looking for “absolute proof” it was in the mid-1940s when Germany fired V-2 rockets across the English Channel from Peenemünde. These rockets were the first to reach outer space (top graphic). Near the apex of their flight they were observed to move along a ballistic trajectory — motion in which gravity is the sole acting force. Ballistic motion can only happen in a vacuum.

The chart below shows how both atmospheric pressure and density decrease exponentially with altitude. At 100 km most of the atmosphere (99.9999%) is below you. 100 km also marks the Karman Line (shown in the main graphic), generally considered the “official” start of outer space. Above this altitude aerodynamic flight is no longer possible. Wings are useless. Only rocket thrusters and gravity can affect your motion.

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