What If Apophis Hit Your City? The Math, Then the Miss: 19,700 Miles

This is Vishala Space. What if Apophis hit your city? Start with the miss. On the thirteenth of April, twenty twenty-nine, this rock, about eleven hundred feet wide, passes nineteen thousand seven hundred miles above the Earth. That is inside the ring of TV and weather satellites. And it misses. But for four days in December two thousand and four, the odds of a hit climbed to two point seven percent.

So here is the math of that hit, placed over your city, with the error bars left on. First, the ruler. Shrink the Earth to a marble one inch wide. The Moon sits thirty inches away. The ring of geostationary satellites circles three point three inches from the marble's centre. On that night, Apophis passes just two and a half inches above the marble, inside the ring. And Apophis itself, on this ruler, is less than a thousandth of a millimetre.

About the size of a bacterium. Its real size is the first error bar. Radar from Goldstone and Arecibo gives about eleven hundred feet across, three hundred and forty metres, give or take forty. That is the figure NASA uses. Its heat glow in infrared gives about three hundred and seventy-five metres. And in twenty twenty-one, observers timed its shadow as it slid across distant stars. That method gives about fourteen hundred feet, four hundred and twenty metres.

A quarter wider does not sound like much. But a rock has three dimensions. A quarter wider is almost twice the rock. So keep both numbers. Every result in this video comes as a pair. Apophis was found in June two thousand and four, from Kitt Peak in Arizona, then lost, then found again in December. On the twenty-third of December, NASA's impact monitoring put the odds of a hit on the thirteenth of April, twenty twenty-nine, at about one in three hundred.

The next day: one point six percent. That earned a four on the Torino scale, the zero-to-ten scale for impact danger. It is still the highest rating any object has ever been given. Each new night of measurements should have shrunk the odds. Instead they rose. By the twenty-seventh of December: two point seven percent. About one in thirty-seven.

Why rise? Because the cloud of possible orbits was shrinking around a path that still touched the Earth. Then, that same day, astronomers found the asteroid in old pictures, taken nine months earlier, in March. Three more months of orbit. The cloud shrank again, and the Earth fell out of it. Twenty twenty-nine was ruled out on the day the odds peaked. The later scares, for twenty thirty-six and twenty sixty-eight, went the same way.

In twenty twenty-one, radar measured its distance to within about five hundred feet, from more than ten million miles away. NASA's verdict: no impact risk for at least the next hundred years. Now the what-if. Had it been on a collision course, it would have reached the top of the air at about twenty-eight thousand miles an hour: its own speed, plus everything Earth's gravity adds on the way down. Energy is half the mass times the speed squared. And the mass is the second error bar, because nobody has weighed Apophis. Its colour says stony, like the meteorites called L L chondrites: solid rock, about three point three grams per cubic centimetre.

But many asteroids are rubble piles, full of gaps. Itokawa, a similar stony asteroid, is only one point nine. Take three, close to solid rock, and the NASA size. That is about sixty-eight million tons of rock. At that speed, the energy that reaches the ground is about eleven hundred megatons of TNT. The Hiroshima bomb was about fifteen kilotons. So stack them.

More than seventy thousand Hiroshimas. The largest bomb ever tested, the Tsar Bomba, was fifty megatons. Apophis is about twenty-two of them. One viral video quotes about nine hundred and twenty megatons. That is the same math with a lighter rock. Now use the newer size: about twenty-one hundred megatons. Across both sizes and the density range, anywhere from about seven hundred to about twenty-three hundred.

Second by second. About thirty miles up, the air ahead of it pushes harder than the rock can hold, and it breaks apart. A small rock would explode high in the sky, like the one over Chelyabinsk in twenty thirteen. This one does not. The pieces stay together in a cloud about two thousand feet wide, and the air takes only about seven percent of the energy. It reaches the ground as one blow. The ground answers with a crater about three miles wide and about a third of a mile deep.

With the newer size, about three and a half miles. The shock runs through the rock like a magnitude six point six earthquake. That number carries its own error bar: nobody knows exactly how much of an impact turns into shaking, so it could be most of a magnitude either way. Near the rim, broken rock falls back a yard deep, out to about eight miles. And one thing some impact maps draw, the model itself does not: a ring of fire. In these equations, a fireball needs more than about thirty-three thousand miles an hour. Apophis would arrive slower than that.

So there is no fire ring here. Here is the twist. The hole is the small part. The killer is the air. The impact drives out a blast wave, and its danger is measured in pressure above normal. At five pounds per square inch, most buildings collapse. How far out does that reach?

Two versions of the same model disagree. The two thousand and five paper treats the impact like a bomb on the surface: about twenty-eight miles. The calculator from the same team, in its current version, gives about fourteen. So the honest answer is a band: about fourteen to twenty-eight miles. Windows break out to about forty to seventy-five miles. Twenty miles out, the blast arrives in under two minutes. That band is ten to twenty times the crater's radius.

Now drop it in the deep ocean instead. The water takes the punch: a hole in the sea about four miles wide, and the sea floor barely feels it. And the wave? The models disagree again. Sixty miles away, the older theory says about a hundred and eighty feet. The calculator's code says anywhere from a few feet to about ninety. A NASA and NOAA workshop in twenty sixteen found that impact waves are short, break early, and fade faster than the old models said.

Both lines stay on the screen. So, your city. Same rock, same rings, every city treated the same, the hit placed at its centre. This is a calculation, not a forecast. Dubai. Sharjah, thirteen and a half miles away, sits inside the band where most buildings fall, in both versions of the model. Abu Dhabi, seventy-six miles away, is right at the outer edge where windows break.

London. Heathrow, fourteen miles out, is just inside the band. Gatwick, twenty-five miles out, is inside it. Brighton, forty-seven miles out, is in the window zone. Houston. Galveston, forty-seven miles out, the same. Mumbai.

Thane and Navi Mumbai, twelve and ten miles out, are well inside the band. Pune, seventy-five miles away, is at the edge. One rock, about eleven hundred feet wide, and a circle twenty-eight to fifty-six miles across where most buildings come down. That is why astronomers hunt for rocks this size. And it is why Apophis was taken off the risk list only when the numbers said so. Now the real night. Put a camera three miles from its path, with the Earth behind it, and wait.

At its closest, Apophis moves at about sixteen thousand six hundred miles an hour. Here it is, in real time. About two thousand six hundred miles inside the satellite ring, nineteen thousand seven hundred miles above the Atlantic, at twenty-one forty-five universal time. NASA calls it the closest approach by an asteroid this size that scientists have known about in advance. Something like it happens only every few thousand years. Earth's gravity bends its path by about twenty-seven degrees. Before the pass, Apophis goes round the Sun in a little under a year.

After it, a little over a year. It moves from one family of asteroids to another, the Atens to the Apollos. Two spacecraft are going to watch. The European Space Agency's Ramses plans to launch in spring twenty twenty-eight and arrive before the pass. NASA's OSIRIS-APEX, the spacecraft that brought back samples of the asteroid Bennu, arrives in spring twenty twenty-nine and stays about eighteen months. When they are done, the error bars in this video will shrink. And you can see it with your own eyes.

The pass happens over the Atlantic, with the Americas in daylight, so Houston sees nothing. London gets the best seat of the four: around a quarter to ten at night, high in the south. In Dubai, look west just after midnight. In Mumbai, between half past one and two in the morning. It will shine like a modest star, easy from a dark field, hard from a bright street, and it moves against the stars as you watch. That is the math of a day that will not happen. A rock eleven hundred feet wide, maybe fourteen hundred, made the world hold its breath for four days, and then it simply missed.

Next time, the other direction, all the way out: one continuous shot, from a marble to the edge of everything we can ever see. How far does that edge go? Next: the edge of everything we can see.