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.

I’d be curious to see the logic behind the last paragraph.