How high can you go?
Climb from the ground past the birds, the clouds, the planes and the edge of space — all the way to the Moon.
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Sea level · 0 m
You start here
Right now you're sitting at the bottom of an ocean of air — about five tonnes of it pressing on your shoulders. Look up, and start climbing.
You never feel that weight because the air pushes on you equally from every side at once.
The whole atmosphere weighs roughly 5.5 quadrillion tonnes, yet it's only a thin film: if Earth were an apple, the breathable air would be thinner than the skin.
Fair warning: space is much, much farther up than it looks. The climb is the point.
The tallest living things
A coast redwood named Hyperion rises about 116 m — the tallest known tree on Earth, taller than the Statue of Unity.
Trees can't pump water much higher than this; gravity and friction set a hard ceiling on how tall a tree can grow.
Hyperion's exact location is kept secret to protect it. It was already a sapling around the time of the Roman Empire.
The tallest building
The Burj Khalifa tops out at 828 m. From here the people below are specks — yet you've barely begun to climb.
You can watch the sunset from the bottom of the Burj Khalifa, take the lift up, and watch the same sunset a second time from the top.
Even so, you're not yet a thousandth of the way to where space begins.
Where clouds are born
Fluffy fair-weather clouds form their flat bottoms around here. A cloud is just the height at which rising air gets cool enough for its water to condense.
Fog is simply a cloud sitting on the ground. When you walk through mist, you're walking through a cloud.
A modest cumulus cloud can weigh as much as a hundred elephants — it just floats because that weight is spread across billions of tiny droplets.
Birds in the sky
You're now in the realm of birds. Bar-headed geese are the champions — they migrate straight over the Himalaya, flying as high as 7–8 km.
Their blood is specially tuned to grab the thin oxygen up high. A human dropped here without training would already feel light-headed.
They make the crossing in a single day-long push, flapping the whole way — there's too little air up here to glide on.
Hot-air balloons drift here
Hot-air balloons and small aircraft cruise through the low sky. The air is still thick and breathable — but it's already thinning fast.
Air pressure drops by about half for every 5.5 km you climb. The change is exponential: most of the atmosphere hugs the ground.
A hot-air balloon flies because the heated air inside is lighter than the cooler air around it — you're floating on a bubble of warmth.
Half the air is now below you
At about 5.5 km, half of the entire atmosphere's mass is already beneath your feet. You're breathing the thin top half now.
% of the atmosphere below you
This is roughly the altitude of the highest permanent human settlements, in the Andes. People who live there have more blood and bigger lungs.
Climb just another 5.5 km and you'll have three-quarters of the air below you. The top of the sky comes up fast.
The death zone
The summit of Everest
8,849 m — the highest point on land. Above about 8,000 m is the 'death zone', where there's too little oxygen to survive for long.
There's only about a third as much oxygen here as at sea level. Most climbers carry bottled air just to reach the top and get back down.
Jet streams scream across the summit at over 160 km/h, which is why the peak often wears a banner of blown snow.
The rock at the very top of Everest is limestone — full of fossils of sea creatures, lifted here from an ancient ocean floor.
Cruising with the airliners
Passenger jets cruise around 10–12 km. The cabin is pressurised because the air outside is now too thin to keep you conscious.
Outside the window it's about −55 °C, and you're already above most of the planet's weather.
Concorde flew higher still, near 18 km, fast enough that passengers could see the curve of the Earth and the sky turning dark blue overhead.
The highest bird ever
A Rüppell's griffon vulture once collided with an aircraft at 11,300 m — higher than airliners cruise, and the highest any bird has ever been recorded.
Its blood carries a special form of haemoglobin that grabs oxygen even where the air is far too thin for almost any other animal.
No human could stay conscious here for more than a minute or two without a pressure suit — yet this bird flies through it.
Stratosphere
Above the weather
You've left the troposphere — the thin layer where all weather happens. Above here the sky stops being turbulent and the air grows still.
Every cloud, every storm, every rainbow you've ever seen happened in the layer now entirely below you.
The biggest thunderstorms punch right up to this ceiling and then flatten out sideways into an anvil shape — they've hit the lid of the weather.
Breathable air, running out
By now there's almost nothing left to breathe. The oxygen your body needs has all but vanished, even though the sky still looks blue.
Breathable oxygen vs. sea level
Watch the gauge: usable oxygen has been collapsing since you left the ground, and from here on, no unprotected human can survive.
The Armstrong limit
Around 19 km, the pressure is so low that water boils at your body temperature. Your blood and saliva would literally bubble without a pressure suit.
This invisible line, not the edge of space, is where 'just add oxygen' stops working. Above it you need a sealed suit or cabin to live at all.
From a balloon up here the sky overhead is already inky black, even in daytime — but it is still 80 km below where space officially begins.
The planes that touch the edge
Spy planes like the SR-71 Blackbird cruised near 26 km — so high and fast that pilots wore full pressure suits, just like astronauts.
From the cockpit the sky was black and the Earth visibly curved. Pilots said they could see the difference between night and day on the planet below at once.
The Blackbird flew faster than 3,500 km/h. If a missile was fired at it, the plan was simply to accelerate and outrun it.
Weather balloons & ozone
Weather balloons drift up to ~30–40 km, into the ozone layer — the thin shield of gas that soaks up the Sun's harshest ultraviolet light.
Without that fragile layer of ozone, the surface would be sterilised by UV. Life on land exists partly because of this faint haze far overhead.
As a weather balloon rises, the thinning air lets it swell from a few metres wide at launch to the size of a house — until it bursts.
The highest skydive
In 2012 Felix Baumgartner jumped from a balloon at 38.97 km and fell faster than the speed of sound. Alan Eustace went higher still in 2014, from 41.4 km.
The sky above them was already black, but they were nowhere near space — barely a tenth of the way to the Kármán line you're climbing toward.
Baumgartner broke the sound barrier with his body alone, the first human ever to do so without a vehicle.
Stratopause · Mesosphere
Where the air gets warm again
Strangely, it's almost as warm as a freezer here, ~0 °C — much warmer than the air far below. The ozone layer soaks up sunlight and heats this region from above.
Temperature in the atmosphere doesn't just fall as you climb — it zigzags. It cools, then warms here, then plunges again higher up.
Mesosphere
Where shooting stars burn
Most meteors — the 'shooting stars' you wish on — flare and burn up right around here, 75–100 km up, scorched by friction with the thin air.
This layer is the hardest to study: too high for planes and balloons, too low for satellites. It's sometimes called the 'ignorosphere'.
Around 40,000 tonnes of space dust drifts down through here and settles onto Earth every year. Some of the dust in your home came from space.
The highest clouds on Earth
Eerie electric-blue 'noctilucent' clouds form right at the edge of space, around 82 km — so high they still catch sunlight long after night has fallen below.
They're made of ice crystals freezing onto the smoke of vaporised meteors. They are, almost literally, clouds made on the dust of shooting stars.
100 km · the Kármán line
Space starts here
This is it — the Kármán line, the official edge of space. Cross it and you're an astronaut. And yet, the Moon is still 3,800 times farther up.
There's no real 'edge', of course. The air just keeps thinning until there's effectively none. We drew a line at 100 km to have somewhere to call the start.
Everything you have ever known — every person, city, mountain and cloud — lives in the thin shell now entirely beneath you.
It's named after Theodore von Kármán, who worked out the height where the air gets too thin for wings to fly, so you'd have to go orbital instead.
The fire of re-entry
Spacecraft coming home hit the upper air around here at such speed that it glows white-hot — thousands of degrees — turning them into brief shooting stars.
It's not friction that heats them most, but the air being violently compressed in front of the craft, faster than it can get out of the way.
For a few minutes the glowing plasma blocks all radio, and mission control loses contact completely — the famous 're-entry blackout'.
Low Earth orbit
The Space Station
The ISS circles here, about 408 km up, at 28,000 km/h — fast enough to lap the Earth every 90 minutes. Its crew sees 16 sunrises a day.
Astronauts often describe the 'overview effect': seeing the whole planet as one fragile blue marble with no borders, and never thinking the same way again.
It's still so close to Earth that there's a whisper of atmosphere up here — enough drag that the station must be boosted higher now and then.
China's Tiangong station orbits at a similar height. Together they're the only permanently crewed outposts humans keep off the planet.
Hubble's high perch
The Hubble Space Telescope orbits a bit higher, around 535 km, above the blur of the atmosphere — which is how it sees billions of years into the past.
Looking far across space is looking back in time. Some galaxies Hubble photographed are showing us light that left them before the Sun even existed.
It once stared at a patch of 'empty' sky the size of a grain of sand held at arm's length — and found roughly 10,000 galaxies hiding in it.
Exosphere
The curtains of the aurora
The northern and southern lights shimmer between roughly 100 and 1,000 km, where particles from the Sun smash into the last wisps of air and make it glow.
Out here the atmosphere doesn't really 'end' — it just fades into space so gradually that individual air molecules can fly for kilometres without hitting another.
The colours are the air itself glowing: green and red from oxygen, blue and purple from nitrogen, lit up like a giant neon sign.
The radiation belts
Two vast doughnuts of charged particles — the Van Allen belts — surround Earth from ~1,000 km outward, trapped by the planet's magnetic field.
That magnetic field is a force field we can't see. Without it, the solar wind would have stripped away our air and oceans long ago.
Astronauts heading to the Moon had to punch through these belts fast to limit the dose of radiation they soaked up on the way.
The satellites that guide you
GPS satellites orbit way out at 20,200 km. Every map and ride you've ever followed was steered by clocks ticking in space, this high above you.
They orbit so high and fast that Einstein's relativity actually matters: their clocks run measurably faster than yours, and the system corrects for it.
Without that tiny relativity correction, your phone's map would drift off by about 10 km every single day.
The satellites that hover
At exactly 35,786 km, a satellite circles Earth once per day — so it appears to hang perfectly still in the sky. This is where TV and weather satellites live.
From up here, Earth is finally a whole sphere you could cover with a thumb. And the Moon is still ten times farther away.
Worn-out satellites are nudged a few hundred km higher into a 'graveyard orbit', where they'll drift, dead and silent, for millions of years.
384,400 km
The Moon
You made it to the Moon — the farthest any human has ever travelled. Its light takes 1.3 seconds to reach Earth; everything below you was the easy part.
Only 24 people have ever come this far, and just 12 have walked on it. No one has been back since 1972.
And we've barely left home: the Sun is still 390 times farther away, and the next nearest star is about 100 million times farther than the Moon.