It all depends on the size. Size matters. From what I’ve been able to gather the standard view on recurrence interval (time between impacts of a given size) from the Morrison, et al crowd is something like:

T-recur = (D / 4) ^ 2.35

T-recur is recurrence time in years
D is diameter of asteroid in meters

So let’s say you let D = 100 meters to represent an asteroid that can cause serious damage. So T-recur = 1,900 years.

I take it that some “radicals” in the Holocene Working Group think that number 4 should be replaced with something like 10. So a 100-meter impact happens much more often than once every 1,900 years, say once every 220 years.

David Morrison, who is really the acknowledged expert in this area, came up with a different number:

http://www.georgehoward.net/htmlfiles/David%20Morrison.htm

“Earth today in terms of probable casualties, noting in particular the existence of a threshold at about one million megatons of energy (corresponding to a two-kilometer asteroid) at which the global climate is severely affected and everyone is at risk, independent of proximity to the impact. One conclusion of such studies is that the statistical risk is greatest for impacts near the global threshold, amounting to an average risk of death for each individual on Earth of nearly one in a million per year, comparable to the risk of other more frequent (but less catastrophic) events such as earthquakes, severe storms, and volcanic eruptions.”

So Morrison’s risk of death per individual per year is 1 in 1,000,000. If you assume an average lifetime of 70 years then that’s a probability of 1 in 14,000 per individual per lifetime. This is roughly the same as my previous calculations in a previous post which assumed a maximum impactor size of some 10-20 km. So Morrison’s number is some 50 times more risky than the 1 in 700,000 amusement park accident rate being cited. He says that the risk is comparable to other natural disasters such as earthquakes and volcanoes. But unlike those other risks this one is potentially preventable which leads me to conclude that we should be spending at least some of NASA’s budget on this issue. How much? I’d vote for about 10 percent.

Slight correction. The chevrons found in Madagascar and Australia are from different impact events. The Madagascar chevron is thought to be related to the Burckle crater event of ~2,800 BCE. The Australian chevrons are from a different impact in the Gulf of Carpentaria that happened in 536 CE. Apparently there is some historical record of this second event – the Byzantine historian Procopius reported that the “sun gave forth its light without brightness” for the years 536 and 537 CE.

Oh, and as he hasn’t been mentioned for a while… Bruce Willis.

As I’ve shown in a previous post, that 1 in 700,000 figure is pretty iffy. It depends on many different parameters that are assumed such as the maximum size of the impactor, etc. Changing some of those parameters can give you figures as low as 1 in 15,000. So I wouldn’t put too much stock into that number.

@BA “If there isn’t, we may have to launch a rocket at the thing and simply smash into it – the energy of impact is actually far larger than the detonation of a nuclear weapon.”

Hmmm, that one made my eyes open a bit. I’m pretty sure that’s an incorrect statement but let’s see. Let’s assume you have some type of rocket with a mass similar to an Apollo command, service, and lunar module which was roughly 100,000 lbs (45,000 kg). Let’s assume it impacts the asteroid with a relative velocity of 30 km/sec (30,000 m/s). That’s a kinetic energy of 2.0E13 (20 trillion) joules.

1 kiloton of TNT = 1.0E12 calories = 4.184E12 joules

So the impact energy is about 4.8 kilotons TNT which roughly one third the yield of the Hiroshima bomb of 1945 and about 1.5 percent the yield of a modern nuclear warhead (e.g., W78 warhead of the Minuteman III ICBM) and 0.01 percent the yield of the largest nuclear weapon ever tested (the 50 Megaton Tsar Bomba detonated by the Soviet Union on October 30, 1961).

So unless the mass of the impacting rocket is much larger than the Apollo payload I’m going to have to call BUSTED on that statement.