By the time most of us think of “space” as a vast, cold void, we are in fact looking at a region of the atmosphere that can be hotter than a furnace and colder than outer space. Recent laboratory experiments have even pushed temperatures below the coldest region ever observed in the universe.
The International Astronautical Federation and the United States Air Force have long used the Kármán line—located 100 km (62 mi) above sea level—as the practical demarcation between the Earth’s atmosphere and outer space. At this altitude, the air is too tenuous to support aerodynamic lift, so only spacecraft can orbit.
The thermosphere, the outermost layer of Earth’s atmosphere, sits just below the Kármán line. Unlike the troposphere below, temperatures rise with altitude here because high‑energy solar photons are absorbed by sparse atmospheric gases. Depending on solar activity and geomagnetic conditions, the thermosphere can range from a chilling ‑184 °F (−118 °C) to an extreme 3 600 °F (2 000 °C) or more.
Solar output is not steady; flares and coronal mass ejections can increase ultraviolet and X‑ray flux by orders of magnitude. During solar maximum, the upper thermosphere can heat by up to 1 000 °F (≈ 560 °C). Meanwhile, the day‑side of the planet experiences far higher temperatures than the night‑side, contributing a 360 °F (≈ 200 °C) difference over a 24‑hour cycle.
At roughly 250 mi (400 km) altitude, the air density is about 2 g km⁻³—roughly 0.3 ounces per cubic mile. The sparse gas cannot transfer heat to a human body, so you would feel extremely cold. Yet the vacuum would cause bodily fluids to boil at normal body temperature, and the high‑energy radiation would ignite exposed skin. These hostile conditions illustrate why space suits are essential for astronauts.
For further reading, see the study on thermospheric temperature variability and the Boomerang Nebula article.
