The Tunguska event of 1908 is pretty much documented now as the aerial explosion of a small asteroid over central Siberia. It is deemed by many experts that this exploding object that laid waste to 2000 square kilometers of forest was only 30 to 50 meters wide - tiny by asteroid standards. The point is that even though the devastation was local and the rest of the world slept safely, this was a fortunate hit in that it was in an uninhabited region. It would be nice and cosy to think that impacts like the Tunguska event are separated by thousands of years, but I fear that this is not the case. So, how frequent are impacts of Tunguska-size objects, and what would be the effect if one hit in a populated area?

The answer to the first question is still not known for sure, but new detection technology by organisations such as LINEAR and NEAT is showing that many objects the size of the Tunguska asteroid buzz the Earth every year. NASA’s own web site provides a table (See http://neo.jpl.nasa.gov/cgi-bin/neo_ca) which shows all the objects detected that have made close approach to Earth. You can interact with this table and enter your own criteria, and if you apply the criterion that you want to display all those objects that approached within, say, 5 lunar-orbit radii (that is about 2 million kilometers), you find that there were 20 recorded for 2002. Then you find there were only 14 recorded for 2001, and only 6 for 2000.

This is a worry; that estimates of the likelihood of a hit are always based on the currently recorded detection rate, when it is quite obvious that more and more Near Earth Objects (NEOs, as they are called) are being detected as the technology improves (which is the obvious reason for the increased recorded rate for 2002, rather than there actually being more objects out there in 2002).

Another great worry is that for one of the closest approaches in 2002 (120,000 kms on June 14 – only about 8 times the diameter of Earth), the object was detected only AFTER it had made that closest approach; the day after, in fact. So, what is the true statistic; that is, how many asteroids would we find if we had the technology to detect all passing rocks?

What it boils down to is that those 20 near misses of 2002 could well be 60 or 100. There is a period in each day called "daylight" during which the observatories are unable to detect objects that small. If we are conservative and take a figure of 50 rocks greater than 50 meters wide, passing within 2 million kms of Earth every year, we can mathematically confirm that a Tunguska type event can happen a couple of times a century. That might lead some to declare "so what, these 50 to 200 meter rocks only produce local devastation – it is not the end of the world". That leads to the answer to the second question – what is the likely effect?

Firstly, what do we mean "end of the world". It would take the hit of a small moon to end all life. Some of the major extinction events of the past have proved in the long run to be more creative than destructive, in that the species that survived began a brand new order of life. None could be called "end of the world" events. The point is that the living world could go on happily without us. But it doesn’t require mankind to be obliterated completely for you and I to consider it the end of the world. Would we not call the end of our civilisation "the end of the world"?

What are we really afraid of? There are massive, almost moon-sized asteroids out there large enough to end all life within a few hours were they to hit us. But one of these has not hit our planet since the early days of its history; that is, billions of years ago. That scenario is not that scary in any case, as we wouldn’t know much about it if it did happen. At the other end of the scale are the hits that I think should scare us, as they are quite frequent (on an historic scale) and can leave us in chaos rather than wiping us out. I mean a Tunguska event on an urban area. What would be the effect?

We have to ask ourselves what kind of economic collapse would bring on anarchy. When you look at the something like the crash of 1929, which really started just as a "paper" crash, accelerated by panic selling of stock, you realise how fragile are our first nation infrastructures. I think that people have not given enough consideration to the threat of relatively minor cataclysms which do not destroy great swathes of the planet, but have enough effect to bring down civilisations. And there is evidence it has happened before; many times.

What has brought this issue to head for me, and taken it from just an interest in asteroids and comets to something I believe we should be concerned about, are two very recent reports of impact craters that are quite definitely modern, in historic terms.

If you want to follow up an interest in this subject, required reading would be Clube and Napier’s "The Cosmic Winter". The authors are astronomers, and their work is endorsed by many other scientists (except those that will never admit that impacts have shaped Earth’s history, which is as blatantly ridiculous as the sages who clung to the notion that the Earth is the centre of the universe). They cite researchers, some using tree-ring sampling and others analysing peat bogs in north-west Europe, who have confirmed dates of around 7000 BC, 3000 BC, 2300 BC, 1700 BC, 1000 BC, 500 BC, 550 AD, 850 AD, 1300 AD for effects likely to have been caused by multiple impacts from space. Many of these dates can be related to vast changes in early civilised societies. The 3000 BC or 2300 BC dates could match the Iraqi crater, and those eras are the very ones where great turmoil is reported in the Bible and other ancient scriptures, including Noah’s flood, which is a common theme of several scriptures from different sources. The 550 AD date coincides with the dark ages of Europe and also with that crater newly discovered in Italy.

Also, all experts make the point that small craters, like those found in Iraq and Italy, could well be part of swarms that might have pelted many parts of the world in their respected eras. In the case of the 2300 BC date, this could easily explain the flood stories from around the world, and would not require there to have been a global flood, for which there is no geologic evidence.

So, it can be shown quite clearly that the threat from a small asteroid or comet is quite real, and can create localised damage on a scale that could destroy cities or even provinces. This has not been perceived as a major problem simply because people approach the problem of impacts with an "extinction" frame of mind, whereas it is easily possible with smaller impacts, for civilisations to be destroyed while species remain intact.

I pose the question: what if a nation like USA was to face a 100 trillion dollar bill for cleanup after a small asteroid hits one of its larger cities? What would be the effect? Could their infrastructure sustain itself? The scenario that scares me and should scare others, is the "looter" effect. We all know that when a catastrophe like an earthquake or severe hurricane strikes a modern city, the looters take advantage of the temporary loss of control; services like police and military are directed to the catastrophe, which leaves a hole for the some of the underprivileged to go in and raid unguarded facilities. Multiply that a thousand times, and what happens? After a city is hit by an asteroid, the rot may not happen at once. Failing infrastructure would follow from inability to finance the police, fire department or military (who would do those dangerous jobs without pay?), and without the full force of those services, you can be sure that many of the underprivileged would take advantage of the situation. If you are a realist, you recognise that there are millions of people in the world who would love to see the fall of the west. With the fall of USA, most of the smaller countries of the world who rely on trade with that nation, would likely follow into financial ruin.

These are not huge objects that require the likes of Bruce Willis and his team to go drill nukes into their cores (as depicted in the movie "Armageddon"). These are rocks that we could feasibly destroy with our current technology. All we need is better detection facilities, good enough to recognise the threat several million kilometers away, and also the unmanned transport to take the necessary weaponry quickly out to that distance. The money that is spent on armament to fight amongst ourselves would easily cover the cost. It would definitely cost less than the moonshots, which were massively expensive because most of the cost involved maintaining life-support systems.

The very real possibility of impacts from society-disrupting objects needs to be taken seriously because it is almost certain that the day will come when one of these things ravages a major urban area, and nobody could say for certain that the world could survive the financial shock. We are definitely not safe from a slide into the barbaric state, if financial institutions collapsed worldwide. To believe otherwise is to really bury one’s head in the sand.

To summarise:

1. It is not the big rocks like the one that destroyed the dinosaurs that should worry us, as they are rare in terms of their approach to Earth.

2. The smaller rocks, from 50-200 meters impact at speeds that can produce explosions tens or hundreds of times greater than an atomic bomb.

4. Most of these will hit in unpopulated areas, but there is obviously a risk of a hit in urban areas.

5. It does not take total devastation to bring down society. An urban hit that bankrupts a country like USA, could bring down civilisation as we know it. We can be sure that many of the world’s burgeoning poor would seize the opportunity of our failing infrastructures to create anarchy.

6. There is mounting evidence that this has been the reason for the fall of a number of civilisations in the past.

7. In terms of fragility, our civilisation is perhaps more vulnerable to collapse than, say, the Roman Empire. Due to the ever widening gap between the rich and the poor, there is a now a world of 4 or 5 billion potential barbarians ready to overrun any disabled first nations. And they are much more mobile than the hordes that overran Europe after the fall of Rome.

8. For the first time in history, we have the technology to prevent the next impact. An impact is inevitable, and apparently pending. So, why aren’t we taking the necessary steps to deal with the threat?

That last point is my main point. In a way, it could bring the world together. It could provide a common enemy; a real enemy.