There are no alarm bells or announcements that go off when you go high enough in the sky to hit the stratosphere.
Even the crew don't react - they have been here before and they are just getting on with business.
But as a first timer, your body sure tells you soon enough.
NASA's SOFIA (stratospheric observatory for infrared astronomy) has been quietly visiting Christchurch for the last couple of years. It's a massive multi-million dollar telescope installed on a multi-million dollar converted 747. The Americans built the plane and the Germans built the telescope. It's parked out the back of Christchurch Airport where no one can see it.
On first impression, it looks like the love child of a regular commercial jet and a spaceship.
They've taken an old-school 747, ripped out the inside and packed in loads of extremely complex technology, so much so that a plane that could take hundreds of people at one point in time can now only take about 20.
Inside are loads of computers and screens and people working on them who, let's be frank, are the smartest smart people America and Germany can provide. They're the very best at what they do. They've left their families for two months to go and gather data for university and government scientists. They're friendly folk when you climb on board and one by one will introduce themselves clearly and with a strong handshake. Then they go back to their computers and look at long strings of numbers and lines that would take years for you or me to understand.
You know it's a special plane before you even take off, because the engines take a very long time to warm up. And then, as it takes off, it seems to really lumber up into the sky as it drags an immense amount of technology with it. You can feel all that machinery weighing the whole thing down at the back.
The craft rumbles obstinately over the now blacked-out eastern suburbs of Christchurch and rolls off into the sky. Overall this trip takes about 10 hours, and on this particularly journey we're effectively flying almost all the way to Antarctica, almost all the way to Tasmania and then back home.
At 8000 kilometres, it's a trip which could almost get you to America itself. Instead, it nearly takes you to two different destinations and then just flies home. You'd expect that to make it particularly frustrating and boring.
But it's probably the most interesting flight you could ever go on, short of going to the immense effort of becoming an astronaut.
The crew here want to see a number of things in the universe tonight.
Top of the agenda, around Antarctica, they want to check out something called the 'Arches' which just happen to lie right near the centre of the Milky Way. Astronomers only recently found out the surprising news that our galaxy's centre is in fact two black holes colliding - pretty terrifying if you think about it too much.
On the computer screens we can't see these massive balls of energy sucking in whole planets around them, because they're so strong they're sucking in light itself. But you can see the dark patch where they are. Seeing it in an image is disturbing. Seeing it in real time on a computer screen is jarring. Literally everything we know, including time and space themselves, end up in that waste disposal of the universe, the end of everything at the very heart of everything.
What the scientists are trying to figure out is why the 'Arches', right next door to this pit of despair, seems to be a breeding ground for new planets. Why this area isn't being completely decimated by the monster black holes is just a mystery. They'll collect the data and move on to the next project, somewhere deep in the Southern Ocean.
Apart from the fact that you're effectively travelling in a flying futuristic telescope, the first half hour in the air doesn't seem to feel too different to a standard commercial flight.
Then, all of a sudden, you stand up. Ugh! What was that?
Your whole body feels really, really weird. I asked the camera operator and he said the same thing happened to him. It's as if you've had too much to drink and you've reached that uncomfortably drunk stage when anything could happen.
And that's when your primal urges kick in - when you suddenly realise you're in completely new and unfamiliar territory. The stratosphere. At a stretch, inner space.
Suddenly you have that fear and excitement a kid gets when they go somewhere completely different they've never been before, like a foreign country. Except as a hardened adult you stopped having those feelings years ago, which makes it all the more interesting.
And all the things you've been ignoring on this trip up until this point suddenly make sense.
Like the copious amounts of water everyone seems to be drinking, because the air outside has pretty much no water vapour in it, at all, and it's sucking it all out of you. Regular international flights will dehydrate you but this is a particularly aggressive atmosphere. I drank at the very least 4 litres of water during the flight and only once did it barely come out the other end.
Or what about the tons of safety precautions you were gleefully ignoring until now? Like the full range of fire-fighting equipment that would kit out a small rural fire brigade, of which they have three whole sets on an aircraft for just 20 people? Equipment which everyone on board is expected to be able to not only locate immediately but use competently?
Or, even better, the little green pouch you're forced to carry everywhere containing something like a plastic bag with a little oxygen tank attached which you put over your head in case of an emergency to keep you alive for about fifteen minutes. I left mine on a counter once for a couple of minutes and was severely reprimanded. These guys aren't mucking around. How many times do they have to remind me this is an "experimental craft"?
This part of the sky is totally foreign to your average adult. And it'll stay that way, for the near future anyway. It's completely off limits to pretty much everyone. Even if you had stacks of money you can't just pay to go there.
But it's perfect for star-gazing. Up this far there's none of that pesky water vapour that messes with the way astronomers measure the stars. Canterbury in particular has a reputation among aeronautical astronomers for having an unusually low level of water vapour in the winter, making it that much easier for these sky people to get above.
After the Arches the scientists go after a mysterious star called 30 Doranis lodged somewhere deep in the Magellanic Cloud. By the time we've reached Aussie skies they're doing some checks on Mars, which comes up as a bright orb throwing out long rays on all the screens.
Unbelievably, that's all they've got time or fuel for on this trip. As the temperature in the cabin drops steadily, SOFIA edges her way back to New Zealand. When they get back, the airborne astronomers will send all their information to the scientists in the universities, who will convert it into information which may, now or in the future, change the way we think about everything.
But way up in the sky, where the Earth's atmosphere begins fading into space, it's easy to forget what's going on down below when you start to get a taste of what's going on above: that we're floating in a vast, beautiful, but utterly inhospitable universe.
