Alan Bond and Mark Hempsell have produced a thorough, scholarly book here which is centred around analysis of the cuneiform tablet K8538 (the "Planisphere") at the British Museum. The tablet seems to be a 700 BC copy of an astronomical clay 'notebook' dating back to the pre-sunrise of 29th June 3,123 BC in Kish (modern Iraq).
Most of the early chapters analyse the Planisphere a sector at a time, decoding the star signs and Sumerian commentary by matching constellations and planets against state-of-the-art programs which can reproduce the night sky from any location and at any time in the last few thousands of years. But it's what happened next which captures the interest. Just before dawn the astronomers see a bright object appearing from behind the clouds, moving faster than anything they have ever seen before. It's big enough to show phases as it traverses the sky from East to West (at this stage it is still exo-atmospheric) before it vanishes into the earth's shadow and then below the Sumerian horizon to the North-West.
Thirty seconds later (p. 71) the object enters the top of the atmosphere over Greece and an ionised shock-front becomes visible as a gigantic arrow head in the pre-dawn darkness. As the object traverses the coast of Albania it is seen as an enormous fireball, larger than the sun.
The asteroid dumps enormous amounts of energy into the atmosphere. As it reaches the southern Alps, it is releasing the equivalent of a one kiloton atomic explosion every metre while the shockwave over-pressure on the ground amounts to several atmospheres.
In its final second of flight, the asteroid gouges a 2 kilometre cut out of the mountain overlooking Austrian Köfels, at the 6 degree gradient of its very shallow trajectory. This glancing blow was sufficient to vaporise the asteroid.
The final 4 kilometre-wide, 850 million tonne fireball hit the mountain at Köfels at 14 km/sec. The energy released was 14,000 Megatons, producing not a crater, but the demolition of the mountain into the gigantic landslide we see today.
The aftermath was an ejecta plume which spread backwards, recapitulating the entry trajectory. This searingly hot mass would have launched into space, achieving a height of 900 km over the Mediterranean, and re-entering over northern Egypt and Syria. The astronomers of Kish saw the plume like a gigantic tree, low on the horizon, a few minutes after their strange star had vanished.
As it re-entered, the plume dumped 500 Megatons of energy into the atmosphere (= four simultaneous detonations of the largest H-bombs ever tested). This would have incinerated anyone beneath it. It also seems likely there were major climate changes post-impact.
Bond and Hempsell tell a thorough story although inevitably there is always a great deal of interpretation with such ancient and fragmentary material. But as they observe, it all seems to hang together.
