What Is The Temperature Of Outer Space Close To Earth?

Next time you need a fun fact to impress your friends, try this one: the coldest temperature ever reached in the known universe occurred right here on Earth. In 2021, scientists at the University of Bremen in Germany brought the thermostat down to 38 trillionths of a degree above absolute zero. That's even colder than the coldest area of outer space ever recorded by telescopes; astronomers calculate the faraway Boomerang Nebula to have a temperature of 1 degree above absolute zero. With that in mind, the temperature of outer space close to the Earth is warm, at least from a cosmic perspective.

There is no physical boundary between the Earth's atmosphere and outer space, since "outer space" is just a concept. Instead, our atmosphere becomes thinner and thinner the farther away from the surface you travel, so geologists have come up with various criteria to define where Earth's atmosphere "ends" and outer space "begins." The most widely used definition of that boundary is the Kármán line. This imaginary border occurs at 100 kilometers (62 miles) above the Earth's surface, and it conveniently marks the height where the atmosphere is too thin for planes to fly. In a sense, geologists have defined the boundary of space as the region where only spacecraft can travel.

Determining temperatures at the Kármán line is far from straightforward. The Kármán line lies near the lowest part of Earth's penultimate atmospheric layer, the thermosphere. Because it's unshielded from the sun, the thermosphere has extreme temperature properties. For starters, temperatures in the thermosphere actually rise the higher you go. However, solar weather can disperse its gases unevenly, causing vast fluctuations in temperature. The coldest areas of the thermosphere can reach -184 degrees Fahrenheit, while other regions can reach over 3,600 degrees.

One reason the temperature of outer space closest to Earth varies so drastically is the volatile nature of the sun. The sun isn't like a lightbulb with a consistent, stable output. Instead, it releases its energy in fluctuating amounts, resulting in different solar weather patterns, such as solar flares and solar winds. In fact, temperatures can rise by 930 degrees Fahrenheit when the sun is particularly active. Such variations in energy blow the gases in the thermosphere around, causing their densities and temperatures to fluctuate. On average, the Earth's atmosphere gets thinner with height, but localized density fluctuations partly explain the variability in temperature of outer space near Earth. However, it's not the whole picture.

While lower regions of the thermosphere are denser on average, they aren't exposed to the same amount of the sun's energy. Gases in the highest levels of the thermosphere, thin as they may be, are bombarded by high-energy X-rays and UV rays. Upon absorbing these high-energy rays, the upper thermosphere can vary in temperature anywhere from 930 degrees to over 3,600 degrees Fahrenheit. This absorption of high-energy rays by the upper atmosphere creates a shielding effect for gases in the lower thermosphere, allowing temperatures near the Kármán line to reach as low as -184 degrees Fahrenheit.

Another major factor that influences the temperature of near-Earth outer space is the Earth's day-night cycle. The sun-facing thermosphere of the Earth is usually about 360 degrees hotter than the shadowed side, a temperature difference that changes over a 24-hour cycle. Therefore, the temperature of outer space closest to Earth can fluctuate hundreds of degrees Fahrenheit in a single day.

Human beings don't have very accurate internal thermometers. Our perception of hot and cold is a sense called thermoreception, and it's an adaptation of the nervous system to aid in thermoregulation. But factors like humidity, wind, and atmospheric composition can skew our perception of temperature. For example, when you turn on a fan, you may feel cooler, even though the actual temperature of the air stays the same. The temperature we experience, therefore, differs from the temperature measured by thermometers. That's why, in addition to providing the actual temperature outside, weather reports also include the "feels like" temperature and "wind chill factor" calculations in their broadcasts.

The thermosphere demonstrates our subjective perception of temperature perfectly. Although the temperatures in regions of outer space close to Earth can reach thousands of degrees, you would still feel extremely cold if you were to ever travel there. In the upper thermosphere, at an altitude of around 250 miles, the atmospheric density averages 2 grams per cubic kilometer (about 0.3 ounces per cubic mile). Gas particles at that height are so thinly distributed that they can't insulate our skin enough to stop body heat from escaping, nor can they transfer their heat energy at a high enough rate to cause noticeable feelings of warmth. Yet, although you would feel freezing-cold in near-Earth outer space, you'd be cooked alive at the same time. The pressure is so low in outer space that the fluids in your blood vessels and tissues would start to boil. Simultaneously, the skin on the side of your body exposed to the sun would burn, not from heat but from the radiation of high-energy solar rays. Clearly, outer space is a dangerous place, even when it's close to home.