[youtube]https://www.youtube.com/watch?v=yeQBzTkJNhs&feature=related[/youtube]
If this had happened over Melbourne or Sydney, or any major world city, it would still be making headlines.
On October 8, a mini-asteroid screamed into the upper atmosphere over the Indonesian coastal town of Bone, in the South Sulawesi region, and exploded with the force equivalent to two to three times that of the atom bombs that incinerated Hiroshima and Nagasaki.
It is now estimated to have been about 10 metres across and travelling at more than 20 kilometres per second.
Atmospheric pressure fiercely decelerated more than 1000 tonnes of stone to a near standstill almost instantly, converting its kinetic energy, or mass times velocity squared, into a briefly dazzling fireball.
By the time video cameras were pointed towards it, all that remained was a very high altitude trail of debris and the twisted expansion of the compressed air the object punched through the sky ahead of its disintegration.
The blast was detected by a network of infrasound monitoring stations maintained by the Comprehensive Nuclear test Ban Treaty Organisation at distances of up to 18,000 kilometres.
The Bone object wasn’t seen coming. It was well below the minimum 200-300 metres diameter range that the killer comet and asteroid warning observatories are likely to pick up, although objects this “small” have been found and tracked in the past.
As NASA and the authors of a detailed Canadian analysis of the event point out, the most common types of stony asteroids would not be expected to cause ground damage unless their diameters were about 25 meters wide or larger.
Objects the size of the Bone meteorite are currently estimated to collide with the outer atmosphere once every 2-12 years.
The Bone fireball is a sharp reminder of an often-made prediction by astronomers studying earth crossing objects.
That is: some time this century, as the human species expands into a larger target across the surface of the earth, a dangerously large rock or comet shard will burn a city, instead of explode over a wilderness such as the Tunguska meteorite of 1908, and millions will die.
Back of the envelope calculation. If a Bone sized meteorite arrived every 5 years that would be 20 a century. To have a 10% of having a major city underneath, we’d have to cover 0.5% of the surface of the earth with major cities. And a Bone sized meteorite isn’t large enough to reach the surface. Assuming the Bone meteorite was 10m in diameter, a 25 m diameter rock is 2.5 times bigger in every dimension, so about 15 times heavier. One assumes such rocks are less common. And they are definitely easier to spot.
I’d be curious to see the logic behind the last paragraph.
The logic merits an article but I will offer a quick executive summary.
In the last century there were two very dangerous fireballs. Tunguska in 1908 and a similar magnitude event half a continent away in eastern Siberia in 1948.
Both occurred in June and have been associated by researchers with the debris stream from Comet Encke, which earth passes through in that month. A third event is deduced from an impact at the same time of year by Apollo mission seismographs left on the moon in the 70s. The moon is a shield.
The Tunguska sized objects can be resolved with current technology, but they come from the direction of the sun, making them notably difficult to detect and track. Collision predictions require more than a single detection of an object, as without a sequence of observations nothing is know of the actual trajectory.
Both twentieth century mega fireballs occurred over uninhabited lands. Where cities and towns now exist.
The real value of the existing detection networks, which cost very little, is to pick up objects larger than 1000 metres that pose a severe risk on a global scale. The risk from such objects is very low in frequency but the consequences of a collision are catastrophic. As we move down scale, the risk of an impact event rises with a larger population of smaller objects, but the extend of damage diminishes to vanishing point for example for stony meteorites of less than about 25 metres diameter. The Tunguska object has been estimated at around 30 metres diameter, but moving more rapidly than 20 kilometres per second hence discharging far more kinetic energy.
The timely and reliable detection of Tunguska sized objects is problematical with today’s networks and capabilities. Objects larger than this will also generate damaging tsunami if there is a substantial oceanic impact. There are studies which review the possibility of object originated tsunami in the recent past in various locations, including Australia. These studies are controversial, but the physics involved in them is not.