Scientists Reveal Where Earth Ends and Space Begins
- NASA
But as our atmosphere rises in altitude, it thins. At a certain point, the air is too thin for traditional aircraft, and any craft above that altitude requires a propulsion system, such as a rocket, to remain aloft. That distinction is the Karman line.
The line is named for Theodore von Kármán, an engineer and physicist who was born in Hungary in 1881. He became a prominent expert in rockets during World War II and co-founded the United States’ Jet Propulsion Laboratory.
He is credited as being the first to calculate the altitude above which a craft would need to use a propulsion system to fly.
Von Kármán originally calculated the boundary to be roughly 50 miles above sea level. But, today, the Kármán line is commonly defined as an altitude of around 62 miles or 100 kilometers. The agency that keeps track of standards and records in air and space, the Federation Aeronautique Internationale, also uses this figure to define where space begins.
The thinking behind that round number of 100 kilometers, McDowell says, is that the boundary can’t be defined precisely because of the variability of the atmosphere.
But McDowell wasn’t so sure that was the case. So, he re-examined the history and calculations of the Kármán line in a paper published in the journal Acta Astronautica in 2018. He found that von Kármán’s original calculation was more accurate than previously thought, and with decades of advancement in atmospheric models, the variability is probably only within a few miles of the original 52-mile calculation.
Some scientists have proposed other characteristics to define the boundary between Earth and space, such as the region in a planet's orbit where satellites break apart upon reentry, McDowell remarked.
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