Ever looked up at the sky and wondered: where exactly does Earth end and space begin? With Shubhranshu Shukla becoming the second Indian in space and the first to the International Space Station, the intrigue around ‘space’ has increased. 

So, what is space? It sounds like a simple enough question. But as it turns out, pinning down the edge of space is anything but.

Let’s start with the basics. We live wrapped in Earth’s atmosphere, a protective cocoon of air that gradually thins out the higher you go. At some point, it becomes so thin it’s practically non-existent. But when does that happen? And who decides where Earth ends?

Check out this view of space from the International Space Station:

The 100-Kilometre Altitude Karman Line

For decades, the go-to answer has been 100 kilometres above Earth’s surface. That imaginary boundary is called the Karman line, and it’s widely recognised as the official entry point to space. But here’s the kicker — it’s not based on a precise scientific definition. And if you dig a little deeper, it might not even be the best choice.

So, what’s the deal?

The Sky Isn’t a Jar With A Lid

To understand why this is such a fuzzy topic, it helps to think of Earth’s atmosphere like a fluid — one that flows and changes. Down here on the surface, the air is thick because it’s being compressed by the weight of everything above it. Go higher, and the pressure drops fast.

That’s why mountaintops feel like thin air — literally. Climbers on Everest need oxygen tanks not because it’s freezing (though it is), but because there’s just not enough air to breathe.

This thinning of the air also limits how high aeroplanes can fly. Planes rely on lift, created by differences in air pressure above and below the wings. Less air means less lift. At a certain height, there’s just not enough atmosphere to keep a plane airborne, no matter how fast it goes.

Enter The Karman Line

Back in the late 1950s, Hungarian-American scientist Theodore von Karman ran the numbers. He found that at about 84 kilometres, an aircraft would need to fly so fast to stay aloft that it would overheat and disintegrate — basically, turn into a shooting star.

He wasn’t trying to define where space starts; just figuring out how high a plane could go theoretically. But his work eventually became the foundation of the so-called edge of space, now dubbed the Kármán line.

Then came Robert Jastrow, an astronomer who flipped the logic. Instead of thinking bottom-up like von Kármán, Jastrow looked top-down. He proposed 160 kilometres as the boundary because that’s about as low as satellites can orbit before burning up.

Eventually, the World Air Sports Federation split the difference and set the boundary at 100 kilometres — a nice round number that became the international standard for records and astronaut bragging rights.

But Wait—New Research Says That’s Too High

Enter Jonathan McDowell, an astrophysicist and space historian who dove into decades of satellite data to answer the question once and for all: where does space really begin?

McDowell studied more than 40,000 satellite orbits and found that at least 50 satellites dipped below 100 km and still completed multiple orbits. Based on this and a deep dive into the physics of drag and lift, he concluded that 80 kilometres makes far more sense as a practical boundary.

Bonus: This altitude also happens to line up with the top of the mesosphere, the highest well-defined layer of our atmosphere. Above that? You’re into the ultra-thin air of the thermosphere and exosphere, where satellites soar through what’s essentially a vacuum, but Earth’s influence still lingers.

Why Should You Care?

You might be thinking, “Okay, cool! But why does this even matter?” Actually, it really does.

For starters, where a country’s airspace ends and where outer space begins affects international law. A plane in your skies? That’s a matter of national control. A satellite in orbit? A totally different ball game.

There are also issues around astronaut wings. Yes, seriously. Different organisations award “space traveller” status at different heights. On the same flight, a military passenger might get wings, while their civilian counterpart doesn’t, simply because they crossed different thresholds.

So, Where Do You Draw the Line?

Here’s the twist: there might not be a one-size-fits-all answer. McDowell himself argues that context matters. Engineers, historians, space lawyers — they might all draw the line in slightly different places, depending on what they’re trying to solve.

For simplicity, the Karman Line can be considered where Earth ends and ‘space’ starts.