Dear Daughters,

Sometimes the distances and the differences between people and places can seem immense. But looking from a different perspective can change that feeling. This week, I kept thinking about the fact that each and every one of us–indeed, all life as we know it–lives in a tiny sliver of the universe, a bubble around a rock that’s only 62 miles (100 km) deep.

Beyond that bubble is outer space, an alien cosmos that extends far beyond our imaginations in all directions. And no matter where on Earth you go, that great beyond is less than an hour’s drive away–if only you could drive straight up.

Outer Space Is Closer than You Think

Seriously. NASA calls anyone who flies higher than 50 miles (80 km) an “astronaut.” Meanwhile, the Fédération Aéronautique Internationale, a European body that regulates competitive aeronautics, says space begins 62 miles (100 km) above Earth’s surface. Beyond that, you’re outside our home bubble and into the alien universe.

“Wait a minute!” you say. “Fifty miles? One hundred kilometers? Is everyone just using round numbers?”

Actually, a line somewhere between 50 and 62 miles up makes decent scientific sense for the boundary between our world and everything else. Up to about that altitude, the composition of Earth’s atmosphere 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 the compositional consistency, this portion of sky is sometimes called the “homosphere.”

Above that, air effectively begins to separate into its various elements. Heavier gases stay closer to Earth, while lighter ones float upward. By 400 miles up (640 km), helium is the most common element. By 650 miles up (1,050 km), hydrogen is top elemental dog. Given 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 farther 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 the final frontier.

Bigger? Bubble?

Your local weatherman might disagree. Meteorologists often use an atmospheric model based on thermal structure. In this model, at 62 miles up, you’ve only climbed to the “thermosphere.”

Sometimes called the “upper atmosphere,” the thermosphere reaches up to 372 miles (600 km)–or even higher–above Earth’s surface. But it’s hardly an hospitable place. The few molecules that exist in the ultra-thin air up here get baked by solar radiation, which drives the temperature up to more than 3,000 degrees Fahrenheit (1,700 degrees Celsius). Beyond the thermosphere, the atmosphere doesn’t so much end as it fades away, eventually becoming indistinguishable from interplanetary gases and space itself.

Our “bubble,” in other words, isn’t really a bubble, with a definite edge and a propensity to pop (thankfully). Still, it’s worth bearing in mind just how slim our lifeworld is. Its total depth is less than the distances many of us travel to work each day. By way of comparison, the nearest celestial body to Earth, our moon, is roughly 240,000 miles away (385,000 km). To the sun, it’s roughly 93 million miles (150 million km).

No matter how distant and different we sometimes seem from each other, we all breathe the same air and survive in the same tiny sliver of livable space–every bit of which is precious in the grand scheme of things. This is the only livable sliver we know of. Anywhere. So we have to find ways to share it. And when the distances between us seem great, it’s often a good idea to look up to the heavens and consider just how close we really are.