You'll be asleep, or on the toilet, or standing in a queue impatiently waiting for a poncey kind of coffee. The amazing thing will happen, and you will carry on doing whatever you were doing before.
It's not just the most amazing thing mankind has done. It's the most amazing thing, so far as we know, that has ever happened. Better than a Mayor on a zipwire, better than a Queen parachuting into the Olympics, better even than a man with no legs climbing a flagpole.
We're leaving the solar system.
Go on, shrug. So what? We've been tinkering around in space for ages.
Except without you noticing it we've gone 11.2 billion miles from home using computers not very different to a ZX Spectrum. That's the same as travelling the 93 million miles between the Earth and the Sun 121 times, relying on the power of something less advanced than a modern digital watch.
There are two unmanned Voyager probes flying through space at 35,000mph, operating at temperatures so low they're 1/10,000th of those on Earth and surviving radiation doses 1,000 times worse than needed to kill you or I instantly.
They were launched in 1977 - confusingly Voyager 2 set off first - to have a peek at our nearest planets, which just happened to be lined up in a row so that it would take a minimum of power to use the gravitational pull of each to ping from one to the other.
They were only intended to last for five years but have worked seven times as long. They were going to look at just two planets, but NASA got some extra cash and the probes - by this point millions and millions of miles from Earth - were reprogrammed remotely via radio wave to go and look at the rest of the solar system as well.
(Think about that the next time the gonk from IT says it can't be fixed. He'll probably tell you deep space doesn't spill orange juice on the keyboard, but it's worth a try.)
The total cost of design, build, launch and 35 years of operation is £540m. It cost us 17 times more than that to put on the Olympics.
And what did we learn? Well, the big red splodge in the middle of Jupiter is a giant storm so huge Earth could fit inside it three times over. It has three moons we didn't know about, and one, called Europa, has a crust of water ice on its surface floating above an ocean 30 miles deep.
Jupiter also has a magnetic field which, when it passes over Io, strips it of heavy ions which create beautiful auroras more amazing than our own Northern Lights, and which generates 3million amperes of electricity - enough to boil 333,000 or so kettles with every pass.
One of Saturn's moons, Titan, has ethane oceans and loads of organic matter with an atmosphere just like Earth before life evolved. Uranus has a corkscrew-shaped magnetic field because it's on the yang after being knocked on its side somehow, and Neptune should have been named after Michael Fish rather than a god of the sea as it's the windiest place in the solar system, with gusts of 1,200mph.
The probes run on electricity produced by plutonium, and although it's about one-quarter the power needed to run the average American house, over time more of their equipment has been switched off to conserve it and keep the important bits going. Heaters, cameras and scientific instruments have been powered down and about all that's left is a gyro, an ultra-violet light meter and the digital tape recorder used to store data.
Yes, a tape recorder. Google it, kids.
This information is transmitted back to Earth in radio waves which take 17 hours to get here, and the signal is about 20billion times less powerful than your wristwatch.
One of the last pictures Voyager took is of Earth, from four billion miles away. We're that little speck in the right hand band of light.
It's more than twice as far away now, at the edge of our system where the solar winds have dropped to nothing and any day it will leave the protective bubble of charged particles called the heliosphere which covers our sun and all the planets which orbit it. We don't know quite when it will happen, because we haven't sent anything through this frontier before.
When the two Voyager probes pass out of the heliosphere they will be in the gap between stars - the interstellar medium. It is as different to our system as the Atlantic Ocean is to a paddling pool, filled with gas from the Big Bang, dust from smashed planets, stars dying and being reborn, and loads of stuff that is either spooky, scary or entirely unknown.
If ET finds them he's going to have a lot to wonder about, because on board is a gold record - we should be grateful it's not a TDK 60 I suppose, but do aliens prefer vinyl? - explaining a little about human life.
There are pictures of a foetus, a page from Newton, music by Bach and Beethoven. There are photographs of scientists and traffic jams, a handy map of the solar system and whereabouts in it we are, and people saying 'hello' in 55 languages including those now extinct.
There are beeps and clicks from human brain waves recorded while a lady read some philosophy aloud and thought about the man she was in love with, and handy instructions etched on the outside of how to use a stylus to play the record and what speed to do it at.
Once it passes into the turbulent unknown, Voyager has enough power for another eight years or so. Then the plutonium will be used up, and it will drift in the interstellar winds. Scientists reckon it will take about 40,000 years for it to reach the nearest planet, which of course might be inhabited by nothing but methane, bacteria or creatures who have evolved beyond CD players.
But there are two even more amazing things about all of this.
The first is that humans decided to do it at all - to see what is out there, for the hell of it, because it would be interesting, to wave at whoever might see, and just because. It's a lot more inspiring than going to Mars because it has minerals, or closing down bits of the space programme because it's all got too expensive.
We are leaving our own solar system with the technological equivalent of a piece of paper flicked by a rubber band, and whether it quietly disintegrates, sends us new things to amaze or is sucked into a black hole and made sentient by a race of robot beings so it tries to return home and destroy us all, it will always have been the most enlightening thing mankind ever did.
The second thing is that on that record are a series of grooves which, when played correctly by an alien who figures out how to do it and has recently found a hi-fi in some space junk and didn't know what it was for, will boom out Chuck Berry's Johnny B. Goode at full volume.
When summer's over, and you wave goodbye to the heroes of the Olympics and Paralympics while wondering what's left to feel good about, think about little green men bopping around to a 1958 twelve-bar blues riff in 'B' and know that you can be proud of yourselves for at least another 40,000 years or so.
Well done, humanity.