Space. We all know it’s the “final frontier.” But when you look up at the night sky from Planet Earth, where exactly does outer space begin?
To answer this question, we can first consider the dictionary definition of space: the region beyond Earth’s atmosphere. When you look up, you’re looking through many miles of the protective envelope of gases we call atmosphere. Earth’s atmosphere is composed primarily of elemental gases: more than 75% nitrogen, less than 25% oxygen, and about 1% argon. The remainder is an assortment of trace gases, as well as molecules such as carbon dioxide, ozone, and water.
Earth’s atmosphere is typically divided into five layers, and the demarcation of the layers is based upon whether temperature increases or decreases with altitude within the layer. The altitude ranges shown for the layers are approximate; they’re not precise or fixed measurements, because these altitudes can vary somewhat according to the season and the latitude of your location on Earth.
Looking up, your gaze crosses these layers, from lowest to highest:
#1 Troposphere - first 10 miles above sea level. Nearly all weather and clouds are found here, and most commercial aircraft fly in the upper troposphere. Although we stargazers normally revile clouds, this is a case where spotting one can orient you on your journey to outer space. Or simply look for the lights of a high-flying jet.
#2 Stratosphere - 10 to 30 miles. The stratosphere contains the important ozone layer, a protective band of specialized oxygen molecules that absorbs UV radiation from the Sun. The stratosphere is the upper limit of high-altitude weather balloons.
#3 Mesosphere - 30 to 50 miles. This is the layer where most meteors burn up, so look for a “shooting star” to locate the mesosphere. Strange clouds called noctilucent clouds,
and oddball types of lightning such as sprites and elves are also spotted in this layer.
#4 Thermosphere - 50 to 400 miles. The International Space Station (ISS) and the Hubble Space Telescope orbit high in the thermosphere. Consult this website to find out when you can watch the ISS pass overhead in your area; it’s very easy to spot since it’s so bright.
If you’re so lucky as to live far enough north to see the colorful spectacle of an aurora, aka the Northern Lights, you’re seeing solar particles colliding with atmospheric gases in the thermosphere.
#5 Exosphere - 400 to 800 miles. This layer is where stray atoms and molecules from Earth’s outer atmosphere escape into space. Hydrogen and helium, the two most common elements in the universe, are the main ingredients of the exosphere.
So, where does space begin? Unfortunately, there is no well-placed “Welcome to Outer Space” sign up there. The Earth’s atmosphere simply gets thinner and thinner (that is, less dense) with increasing altitude until it gradually merges with the cold expanse of space. However, since around the middle of the Twentieth Century, space has commonly been considered to begin at 62 miles (100 km) above sea level, just slightly into the thermosphere layer. This is where the atmosphere becomes too thin for aircraft to maintain altitude, in other words, where astronauts must replace aeronauts.
Can you see it? Somewhere between shooting stars and the Space Station, the final frontier begins.