Where Does Space Actually Start?

Not long ago, I got in an argument on twitter about whether the International Space Station

is in space or in the atmosphere…

I mean, it’s called the International Space Station -- so of course it is!


Or… maybe… maybe it’s not.

It seems like a simple question, space starts at 100 kilometers.

That’s the Karman Line.

The Fédération Aéronautique Internationale’s demarcation of where space begins.

Then, I reached out to NASA and they confirmed it.

Yes, internationally, the Karman line equals space!

I was right!

The ISS is in space.

But, my twitter fighter was also right.

See, the Karman line is actually still in the atmosphere.

In the thermosphere, actually.

See, NASA spokesperson Dan Huot told me from a physical science’s perspective, [quote]

“there is no hard-definable point where space begins… the atmosphere just gradually

decreases the higher you get.”

So, where does space start, really?

Let’s do this graphically: We are at the bottom of Earth’s atmosphere


In the troposphere.

This is where regular clouds appear, where weather happens… mount everest, everything.

The troposphere contains about 75 to 80 percent of all the atoms that make up our blanket

of air.

Above that, practically speaking, there are three more layers of atmosphere.

The stratosphere, mesosphere, and thermosphere.

The stratosphere is where the ozone layer is, where planes fly, and where we sent that

360 cam on a weather balloon last year (it’s awesome).

At the top of that layer the pressure is less than 1 percent of ground level.

You’re above more than 99 percent of the atmosphere and we’re only at layer TWO -- we’re

still hundreds of miles from the International Space Station.

The mesosphere is next this is where meteors burn up -- they’re moving ridiculously fast,

so even though there’s almost no actual air -- they hit a lot of it in a short time.

But there’s still the thermosphere, where x-rays and UV are absorbed and where aurora


The air is hot, but there’s so little of it you’d still feel cold.

Molecules of air so rarely meet when they do, they naturally sort themselves by type!

Even though we’re calling these things atmosphere -- there’s not much atmo at all.

The thermosphere extends hundreds and hundreds of miles.

And is why this argument started.

The space shuttle, ISS, and some low-orbit satellites do fly in the thermosphere!

But they’re in a near vacuum.

There’s almost no air at all… less than point-0001 percent of the atmosphere exists


And yet, we’re not done.There’s another layer.The exosphere.

That same NASA spokesperson told me the shuttle missions started pre-atmospheric entry at

122km (400,000 ft).

That’s not even in the thermosphere -- but a FIFTH LAYER: THE EXOSPHERE.


Even though the thermosphere is a vacuum, it’s definitely space, the exosphere is

considered an atmospheric layer by NASA and some scientists.

Basically, because it’s where the sun’s rays overpower Earth’s gravity and move

particles around.

But it is huge, extending 190,000 km or halfway to the moon. On top of that, when the Earth

is in between the moon and sun, the solar wind pushes particles from our atmosphere

onto the lunar surface along a tail -- called the plasma sheet.

Bits of Earth’s atmosphere can be thrown so far it is on the moon.

It’s pretty insane.

Okay, to wrap it up.

In nature, things aren’t binary.

Where does the ocean end…?

At the beach?

The beach is a gradient.

The water goes back and forth, there are tides and weather…

So, where does our atmosphere end?

Not sure.

Where does space start?

The Karman Line.

100 Kilometers.


Because it had to start somewhere.

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Space is part of nature, which means it’s complicated and doesn’t always fit into

clean little boxes.

We have to tweak it.

For example, do you know what space actually looks like?

Chances are, you don’t, because space pics are usually fake.

Find out more with Amy, here!

What have you twitter-fought about lately?

Any good science ideas?

Let me know in the comments, or on twitter @tracedominguez.

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