Where Does Space Begin?

Practically everyone agrees that "outer space" begins where Earth's atmosphere ends. Unfortunately, figuring out where

Earth's atmosphere ends isn't so easy. After all, there's no sign along the skyway from Cape Canaveral that reads "Now leaving

Earth's atmosphere. Please re-enter soon!"

NASA calls anyone who flies higher than 50 miles (80 km) an "astronaut." But according to the Fédération Aéronautique Internationale (a European body that regulates competitive

aeronautics), space starts 62 miles (100 km) above the planet's surface. Fifty miles? One hundred kilometers? Is

everyone just using round numbers?

Space Case #1: Homosphere vs. Heterosphere

Actually, a line somewhere between 50 and 62 miles up makes scientific sense. Up to about that altitude, the atmosphere's composition remains fairly constant, with nitrogen accounting for 78 percent, oxygen accounting for 21 percent, argon accounting for a little less than 1 percent, and myriad other elements showing up in small amounts. Given that compositional consistency, the portion of sky below that altitude is sometimes called the "homosphere."

Above that altitude, air effectively begins to separate into its various elements. Nitrogen, which is heavier, stays closer to Earth, while lighter oxygen drifts upward. By 100 miles up (160 km), there's more oxygen around than nitrogen. By 400 miles up (640 km), helium is the most prevalent element. And by 650 miles up (1050 km), hydrogen becomes top elemental dog. Given all the diversity, this higher slice of the heavens is sometimes called the "heterosphere."

But is the heterosphere "space"? At 62 miles up, you're still below the lowest orbiting satellites (100 miles up)--and even further below the space shuttle's orbital path (185 miles up). In fact, even orbiting space shuttles aren't really flying in a vacuum. They still have to deal with residual atmospheric effects. But you have to draw the line somewhere, and rocket scientists generally put the heterosphere in space.

Space Case #2: Four (or Five) Layers of Insulation

Your local weatherman might disagree. Meteorologists often use a different atmospheric model based on thermal structure. In this model, at 62 miles up, you've only climbed to just inside the "thermosphere." And the thermosphere continues for hundreds of miles above that. The thermosphere works in conjunction with three other layers of gaseous insulation to keep Earth cozy:

1. Troposphere

The troposphere starts at Earth's surface and reaches up between 5 and 9 miles (8 to 14.5 km) depending on where you're standing (it's deepest at the equator, shallowest at the poles). Practically all of the world's weather happens here, as air and water rise and fall in a continual convection process. The troposphere gets colder as you go up, down to around -62 degrees Fahrenheit (–52 degrees Celsius).

2. Stratosphere

Starting at the top of the troposphere and reaching up to 31 miles (50 km), the stratosphere actually gets warmer as you go up--until, at the top, the weather outside is about the same as a cold day on the Earth's surface. Here you'll find the ozone layer, which protects the planet by absorbing and scattering ultraviolet solar radiation.

3. Mesosphere

Above the stratosphere, where only a few specially built planes can fly, is the mesosphere, which reaches up to 53 miles (85 km). The temperature starts to get colder again, dropping to below -150 degrees Fahrenheit (-100 degrees Celsius). Only rocket-powered craft can operate here.

4. Thermosphere

Sometimes called the "upper atmosphere," the thermosphere reaches up to 372 miles (600 km)--or even higher. The few molecules that exist in the ultra-thin air up this high get baked by solar radiation, which drives the temperature up to more than 2,000 degrees Fahrenheit (1,100 degrees Celsius).

But Wait, There's More . . .
5. Exosphere

In the end, the atmosphere doesn't so much end as it fades away, eventually becoming indistinguishable from interplanetary gases and space itself. Some experts continue to treat this in-between terrain, called the exosphere, as part of our atmosphere. Others consider it beyond the pale. But at least this much is certain: by somewhere out here, you've made it to space.

Want to learn more?
See the Earth from space at NASA's "Visible Earth" site
http://visibleearth.nasa.gov/