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That was too close... Five asteroid impacts that never were

By Andy Cooke

Five Asteroid Impact Disasters that never were...

You’ve all seen the diagrams – the Solar System, and Earth’s orbit around the Sun.

Mercury, Venus, Earth, Mars, and the like.

... and they're totally misleading.

This is realistic:

So - what am I going on about?

Notice what you can't see in the diagram above.

The planets themselves.

In virtually every diagram of the Solar System you'll have seen, you see a stylised Mercury propped somewhere in its orbit. A Venus, likewise. The Earth, with swirly clouds, and the Moon nearby. Mars all red and ochre.

We know, intellectually, they're invisible at this scale. In the image above, the Earth-Moon system - including the circle of the Moon's orbit around the Earth - would be smaller than the pixel-width used in the above image to show the orbits... and the Moon swings around Earth 384,000 kilometres from Earth's centre. And this is just a 2-D slice through a 3-D volume.

Okay, I've just impressed on you all the fact Douglas Adams summarised as "space is really big." So what?

The "so what?" is that although there are uncounted numbers of asteroids, ranging in size from small rocks to mini-planets like Ceres, there's a lot of space for them to wheel around in. For an asteroid to impact on the Earth, firstly, it has to cross the Earth's orbit somewhere. Come from outside the Earth's orbit to inside, or vice versa. Now, there are asteroids that do that, but they're a small fraction of the whole.

Then, they have to cross the Earth's orbit in such a way that they're crossing the plane of our orbit (imagine a flat sheet of paper on which the Earth's orbit is drawn) at exactly the distance of the Earth's orbit itself. If it crosses our orbital path above or below that plane - and there's a lot of "above" and "below", it can't hit us. It's physically impossible.

Then, it has to not only cross our orbit, not only cross through it exactly in the plane of the Earth's orbit around the Sun, but do so at the exact moment Earth itself is passing through that point on it's near-billion-kilometer year-long trudge around the Sun.

All of which isn't to say it's impossible - it has happened on multiple occasions - but that there's a lot of space to miss us. Basically, it helps put in perspective what exactly is a near-miss?

I'm going to define it as coming to within the orbit of the Moon - within one "Lunar Diameter." That's less than one pixel on the scale above. That is - the asteroid needs to go through the individual pixel on the image showing where the Earth was at the very moment it went through.

Near-misses on this scale during recent human history count, for this article, as "impacts that never were."

Hadean Era - plentiful impacts

During the very earliest days of the Solar System, in what is called the "Hadean Era", the then-cluttered Solar System had such outrageous amounts of debris that there were constant bombardments. In fact, we believe that one huge impact, by a no-longer-existing rogue planet called Theia, created the Moon.

But in these more sedate days, we expect to go millions of years between significant impacts. Oh, we have a near-constant rain of dust and pebbles, too small to reach the surface and burning up as meteors, and the occasional bigger chunk that makes it all the way down as a meteorite. But the bigger ones - they're a lot rarer.

(Of course, the energy of an impact could be increased dramatically for any given size of impactor by having it strike at a higher velocity. For the effects of an impacts on Earth by a comparatively small impactor at extraordinary speeds, take a look at my book The End and Afterwards)

It's been 64 million years since the Dinosaur Killer. We maybe due a big one - but that's not the subject of this article. After all, if a Dinosaur Killer were to hit, there wouldn't be any alternate history possible. It'd be an extinction event. An end of history

Even a smaller one, though, could be catastrophic, if it hit in the wrong place.

As soon as you get on the order of tens of metres of size, you're looking at the equivalent energy of megatonne-level nuclear explosions.

Hundreds of metres and you're looking at dozens to thousands of megatonnes.

When you get past a thousand metres, we're talking about impact energies so large that talking about megatonne equivalents is almost meaningless.

One of these dropping somewhere highly populated would make for a very big change to history. When you get to the larger sizes, you could see other effects triggered. (Incidentally, have you read Tom Anderson's The Twilight's Last Gleaming?)

So, without further ado, let's look at five asteroid impact disasters... that weren't.

5: Asteroid 458732 swings by Earth and World War One on 17th September 1918

The First World War has been raging for four years. The second Battle of the Somme has just concluded. Vladimir Lenin was shot and wounded 18 days ago. The Battle of Dobra Pole is under way in the Balkans, and General Allenby is about to launch the Battle of Megiddo - near the very site of Biblical Armageddon.

And a 1074m diameter asteroid impacts the Earth.

Where? Well, in our timeline, it actually missed by 0.9 Lunar Diameters. Under a pixel in our diagram. But if it had hit the Earth itself while crossing through our pixel, it could have hit anywhere.

It would have hit with a force greater than a 50,000 megatonne explosion - more than a thousand times as powerful as the most powerful thermonuclear bomb ever tested.. A crater two-and-a-half miles deep and 7 miles wide is formed.

The world would have been very different indeed. And it was just pure random chance that governed whether it hit us or missed us.

4: Asteroid 3610024 zips by during the Cold War on 4th November 1982

The Cold War is at its height. Leonid Brezhnev lies dying in Russia. In May, the previous year, Yuri Andropov had confidently told the Politburo that the US were planning a surprise nuclear attack on the Soviet Union. American bombers had been repeatedly flying directly at Soviet airspace, peeling off at the last minute, while NATO Naval forces kept coming close to critical Soviet bases.

And an asteroid of up to 393m in diameter flew past the Earth within the orbit of the Moon, only 0.8 Lunar Diameters away. If it had impacted, it would have had the effect of a 3,000 Megatonne nuclear warhead - sixty times larger than the Tsar Bomb.

It doesn't take much imagination to see what could have happened next.

3: Asteroid 3780742 comes to within a sixth of the Moon's distance from the Earth on 14th August 2017 - unnoticed.

The Iraqi Civil War is raging, though ISIL are being pushed back and have recently lost the long Battle of Mosul. Seven months ago, Donald Trump was sworn in as President of the United States. Less than five months ago, the United Kingdom invoked Article 50 of the Lisbon Treaty. Twelve weeks ago, Islamic terrorists bombed the Ariana Grande concert in Manchester. Relations between Russia and the USA are terrible; Putin will expel 755 US diplomats in a couple of weeks.

A 40m asteroid hurtles past, a mere 56,000 km above our atmosphere. By 2017, we have got to the point where we expect to pick up dangerous asteroids before impact, but Asteroid 3780742 was only spotted two days after having passed by the Earth.

It would have hit with the impact of the Tunguska explosion, and was the closest significantly-sized asteroid never to have been spotted prior to closest approach.

An unexpected and unexplained 5-10 megatonne explosion at that time...

2 - Tunguska Impact, 30th June 1908 - didn't strike a few hours later and destroy the Russian Empire

Yes, this was an asteroid impact that didn't miss the Earth - but it impacted in a very remote location and could have been a lot worse. It struck the Earth at 60 degrees North in Siberia, and if it had impacted 4 hours and 47 minutes later, allowing the Earth to rotate another 71.6 degrees under its path, the Russian capital of St Petersburg would have been directly below. The Tsar and his court would have been eradicated by the 5-10 megatonne explosion and Russia would have been in turmoil. Just three years earlier, the failed 1905 Revolution had occurred. Vladimir Lenin was living in Western Europe.

As it happens, Sea Lion Press has explored an alternate history where the Tunguska asteroid struck elsewhere on that date - impacting on London...

1 - Eastern Mediterranean Event, 6th June 2002 - is not the Kashmir Border Event.

This was another actual impact, albeit of an asteroid significantly smaller than the Tunguska one. So why does it come in at number one on my list?

Because, like the Tunguska meteorite, if it had struck at a slightly different time (3 hours and 28 minutes earlier, so the Earth would have rotated 52 degrees less by the time of impact - and it would have struck in North Pakistan, 40 miles from Islamabad and on the disputed border with Jammu and Kashmir. Right on the Line of Control, actually.

Right in the middle of the 2001-2002 India-Pakistan standoff.

It may have been only an airburst of around 200-250 kilotonnes (more than ten times as powerful as the Hiroshima bomb), but an unexpected and unexplained explosion of that magnitude right in the middle of the massed troops, with both India and Pakistan in possession of nuclear warheads at the time...

Given the tensions elsewhere, less than nine months after 9/11, who knows what else might have happened? Of course, a sober and careful analysis of the explosion would have shown it wasn't nuclear in nature - but it's unlikely sober and careful actions would have been taken in the immediate aftermath.

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