top of page

Where does space begin?

What exactly is it that makes space, “space”? Where does the atmosphere stop? At what point are you an astronaut? How high can you get before you can no longer survive the vacuum? What kind of Space does a Sent Into Space flight visit? We answer all of these questions in this article.

Panorama of Earth from space

Where does space begin? At multiple points...

You might be surprised to hear that the word ‘space’ by itself doesn’t have a firm scientific definition. Because the word predates our exploration of the regions above our planet, it isn’t actually very clear what constitutes space. That’s not to say there aren’t legally or scientifically defined regions of space, but the areas that do exist are categorized based on different variables which are more or less important depending on why you’re asking.

  • Near Space is the region over our planet where the pressure is below 6.3 kilopascals (about 1/16th of normal atmospheric pressure at sea level), known as the Armstrong Limit. This pressure is significant because, in an atmosphere of 6.3kPa, the boiling point of water is human body temperature, meaning a human being can’t survive without a pressurized suit. The exact altitude of the Armstrong Line depends on weather conditions in the stratosphere, but usually, it sits between 18 and 19km.

  • Outer Space starts at the Karman Line, exactly 100km above sea level. Originally it was hypothesised that at 100km the air becomes too thin to support aeronautic flight, meaning you need a rocket to get any higher. The maths behind this actually turned out to be pretty inaccurate, but the nice round number means it’s been adopted for various legal treatises and conventions as the boundary to Outer Space, even though it doesn’t really reflect a particularly important engineering constant.

  • Interstellar Space is defined as space outside the reach of the solar winds in a planetary system. Its edge is called the Heliopause and as yet isn’t definitively calculated — we’ve got probes working on it!

Okay, so different definitions of space are useful for different things, that makes sense. However, if you’re feeling unsatisfied by this answer, you’re not alone. We want to know where you have to go for it to feel like space. Well, what does space look and feel like?

Ask a primary school child and they’ll probably give you one of the following answers: the sky is black, there’s no air, you can see that the Earth is round and you float. Let’s take a look at each of these criteria for ‘spaciness’.

The sky is black

Why does this happen? To answer this, we have to answer another question first: why is the sky blue? I mean, why is it blue during the daytime (and yellow and orange and red and purple at dawn and dusk)?

When you shine white light through a prism, you can see a rainbow of colours as the prism spreads out the different frequencies of light — this is called refraction. When light shines through the air, the same thing happens, but on a much more subtle scale. It’s so subtle that over short distances you won’t see anything, but because there’s so much of it in the atmosphere, we see the effect in the sky. At different times of day, the sun is shining from different angles, its light passes through more or less air before getting to us. That distance determines what colour the sky appears to be.

As we get higher, though, the atmosphere gets thinner and thinner and gradually, the light from the sun gets refracted less and less. Eventually, the air is so thin that the light isn’t refracted at all. You can look down at the Earth and actually ‘see the sky', or at least the blue hue of the atmosphere that gives the sky its colour. Where there’s no air refracting the light, we see the true darkness of the empty space above.

Where does this happen? Well, it’s a gradual change, but you can start to really see the difference at about 20km as the horizon looks bluer and the sky directly above looks darker. By 25km, the atmosphere is reduced to a thin blue glow around the horizon.

There’s no air

A space, says the dictionary, is “a continuous area or expanse which is free, available or unoccupied”. Space in the astronomical sense matches this definition: it’s everywhere that stuff isn’t. More specifically, space is a vacuum, in that it’s devoid of matter. Simple!

Except it isn’t, really. A so-called perfect vacuum, where no particles of any kind exist within a set region, is impossible to achieve outside the realms of scientific models. Instead, we measure vacuum in relation to standard atmospheric pressure at sea level. A household vacuum cleaner creates a vacuum about 80% of the pressure of normal atmospheric pressure.

Well, we know the pressure at the Armstrong Limit is about 6.25% (1/16th) of normal atmospheric pressure, right? Somewhere around 28km, the air pressure drops to less than 1% of that at sea level. While you’ll never achieve a complete vacuum, <1% is a pretty good boundary to us!

The Earth looks round

The eagle-eyed traveller might be able to detect the curvature of the Earth by sight from the window of a plane, most of which have a modest cruising altitude of 10km. However, for a really spectacular view, you want to get as high as possible. The curvature begins to be very obvious from about 22km and becomes more pronounced as you rise further and further into Near Space.

You float

To truly escape the effects of gravity, you’d have to leave our solar system entirely and travel out into the depths of Interstellar Space. So why do people float on the International Space Station?

The feeling of having a weight isn’t caused by gravity alone, but rather by the opposition of two forces: gravity pulling you towards the Earth and the Earth pushing back, holding your body in place. When you don’t have a force resisting the pull of gravity — when you’re in freefall — you experience weightlessness. You can imitate this by going on a roller coaster with a sharp drop, or jumping in a lift (although I wouldn’t recommend this when travelling with passengers of a nervous disposition).

The International Space Station is pulled towards the Earth by gravity. So why doesn’t it crash into the Earth? Well, it’s also moving sideways incredibly fast. So fast, in fact, that as it moves, the Earth curves away underneath it, at exactly the same rate that it’s pulled towards the Earth, cancelling out and maintaining its distance. In other words, it’s in a stable orbit, just like the Earth around the Sun. The astronauts on board the ISS are in a perpetual state of freefall, not experiencing their weight because their forces are balanced out.


All of that adds up to the conclusion that to get to the bit of space that feels like space, you really don’t need to go to Outer Space. In fact, you can experience what we most commonly consider to be space-like conditions from about 25km up, just a few kilometres into Near Space!

Sent Into Space’s flights are designed to push the limits of high-altitude balloon technology, commonly reaching altitudes in excess of 35km. If you want to experience the wonder of taking a picture of the Earth from space, there’s no need to look for a rocket. Get in touch and find out how we can help you get there!


bottom of page