By Duncan Steel 30/06/2020


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It’s June 30th, marked as Asteroid Day by many people of an astronomical bent around the globe. On this date in 1908, early in the morning in a remote part of central Siberia, the sky fell in.

Well, not literally. What happened is a substantially-sized bit of cosmic detritus – a lump of rock and perhaps ice – arrived in the upper atmosphere at a speed of around 30 kilometres per second (108,000 kph). It was about 50 metres in size, based on a stone-like density and an energy release that has been estimated as being between 3 and 20 megatonnes of TNT equivalent. Compare that with the Hiroshima nuclear bomb, at 15 kilotonnes about a thousand times less energetic. This was some big bang.

This occurrence is generally known as the Tunguska Event (for the Podkamennaya Tunguska river) and it seems to have been the last time that an asteroid of any great size hit the Earth. Though personally I think it was a fragment of a comet. And actually there have been other impacts over the past century, usually in remote locations (look down from orbit and you’d see that much of our planet is yet uninhabited), although in 2013 a smaller lump of rock exploded in the atmosphere just south of the Russian city of Chelyabinsk, putting over twelve hundred people into hospital. That asteroid was maybe 20 metres across, yielding an explosive energy of around 500 kilotonnes (half a megatonne) of TNT equivalent.

Although the Tunguska Event has entered popular culture in a variety of ways, we know comparatively little about it. While much of the northern hemisphere had anomalously-bright night skies in the days following the event in 1908, it was almost two decades before the first scientific expedition (led by Leonid Kulik) reached the point directly below the explosion. Strange though it may be to imagine it, a lump of solid rock 50-100 metres in size entering the atmosphere at a typical asteroid speed (20-30 km/sec) will not reach the ground intact. Rather, it shatters as the ram pressure on the leading face exceeds the tensile strength of the rock, then the fragments quickly spread sideways, increasing the hypersonic drag they experience and so rapidly depositing their kinetic energy. As I wrote: a big bang.

This occurred at an altitude of about 10 km, the sort of height where jetliners fly. Down on the surface, the first thing that happens is that a flash of light far brighter than the Sun sets things on fire, so intense is the radiation. Thirty to ninety seconds later the blast wave reaches the ground, paradoxically blowing out some of the fires whilst levelling the vegetation for miles around. In the Tunguska Event about 2,000 square kilometres of the taiga forest were flattened, attested to by the photographs obtained by the first expedition to the area in 1927. No crater was there to be found: as I wrote, the object disintegrated far above the ground.

Obviously we know more about the Chelyabinsk asteroid just seven years ago. In that case the luminosity blinded many people. That flash came down at the speed of light, and so was almost-instantaneously seen. What most people did not realise was that a blast would be coming at the speed of sound, far slower. A couple of minutes later that blast arrived, damaging over 7,000 buildings. Factory roofs and garage doors were smashed in. Many of the injured were lacerated by flying shards of glass as windows were turned into high-speed ferociously-sharp projectiles. All praise the school teachers who had the good sense to get pupils under their desks after seeing the flash.

So much has been written about Tunguska – whole books, in fact – that there seems no need to re-hash too much of the easily-accessed information here. Indeed, I have written about it extensively in my own books:

 

 

Let me hasten to add that the asteroid impacts shown on those book covers are far bigger than the Tunguska projectile.

Of course, in writing a popular-level book one needs to use certain language and describe things in ways that can be quite distant from the reality of scientific research. We write peer-reviewed papers for a select bunch of potential readers, and that’s no way to achieve a best-selling book. Research is heavily-specialised. I note that because my own research that is connected with the Tunguska Event is indeed somewhat specialised. In reviewing my publication list I reminded myself of four restricted areas in which I might be regarded as having contributed to an understanding what happened in that remote part of Siberia, and what it might mean for humanity in the future.

First, from about 1988 I started to work on the possible genetic links between the Tunguska object and daytime meteor showers that occur in late June and early July, and are detected using VHF radar systems. This was a technology used in research at the University of Canterbury for over 60 years, latterly by my PhD thesis supervisor Professor Jack Baggaley, and it was from that knowledge base that I knew about such meteor showers. My reasoning (put simply) is that the Tunguska projectile might be just a larger lump in a complex of meteoroid streams orbiting the Sun and spawned by the gradual break-up of a comet. For some decades those mid-year daytime meteor showers, plus associated nighttime showers seen in the final two months of each year, have been known to have similar orbits to Comet Encke, a well-known comet in an unusually small orbit which results in it returning every 40 months. So, I thought that perhaps the Tunguska object was a sliver of Comet Encke. Or, perhaps, Comet Encke is just one large (around 5 km) remnant segment of a far larger comet, the Tunguska bolide being a far-smaller chunk.

Second, having become really rather interested in the peculiar orbit of Comet Encke and how it might have come about – its aphelion is well within Jupiter’s path, and so how it entered this cis-jovian orbit was a mystery – I started a long set of investigations in celestial mechanics with David Asher, Mark Bailey, Bill Napier and Victor Clube. This led to the concept of Coherent Catastrophism: fundamentally, that if a large comet gets itself trapped in the inner solar system (like Comet Encke) then there will be an extended period (tens of millennia) during which the comet undergoes a hierarchical disintegration, affecting the terrestrial environment in various ways (not just big bangs).

Third, I recognised (and I have no idea why no-one had noticed it earlier) that on the same date (30 June 1908) a meteorite was observed to fall at Kagarlyk, near Kiev. Very few meteorites are actually seen to land each year. The coincidence in terms of time and location seemed to be… well, astronomical.

Fourth, I came across various papers by Russian investigators of Tunguska in which they wrote of someone named ‘Mouson’ observing a very bright aurora in the Antarctic at about the time of the explosion over Siberia. They suggested this might be similar to the upper-atmosphere disturbances observed at conjugate geomagnetic points when large nuclear tests were conducted between 1958-62. At the time (the early 1990s) I was working at the University of Adelaide, and the Mawson Institute for Antarctic Research was on the same floor as my office in the Physics Department, so I soon recognised that ‘Mouson’ was ‘Mawson’ after transliteration into Cyrillic and back again. Indeed (Sir) Douglas Mawson was in the Antarctic over the winter of 1908, and in his book The Home of the Blizzard  he mentioned there being an exceptional auroral display at about that time. So, with colleague Richard Ferguson I dug Mawson’s original notebooks out of storage, and read what he had written (in pencil). Indeed on that date there was a bright aurora, though looking at other records as the winter progressed it was not clear that this was the most-spectacular display. We also had the problem of working out the time for Mawson’s notes: with a pendulum clock that was erratic, and no Sun to re-calibrate it, it was not clear precisely what time of ‘day’ it was during that long winter ‘night’. Our conclusion could only be a definite ‘maybe’.

Finally, for the centenary of the rude arrival of the Tunguska projectile in 2008, I reviewed for Nature the various bizarre suggestions made over the decades for the origin of the explosion: a UFO/flying saucer that lost control, a mini-black hole, an anti-matter meteor, and so on. It was just a small lump of rock, folks, maybe fifty metres in size. Though whether it should be regarded as being an asteroid or a comet is moot.