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.

There is an 18-mile wide crater at the bottom of the Indian Ocean called Burckle crater. According to some researchers it was created by an asteroid impact only 4,800 years ago.

http://www.iht.com/articles/2006/11/14/healthscience/web.1114meteor.php?page=1

If such an impact were to happen today it might kill 250 million people and cause a tsunami wave 50 meters high out to a distance of almost 5,000 kilometers.

***************************************************************************

Burckle crater in the Indian Ocean
Age = 4,800 years
Latitude = 30.865 deg S
Longitude = 61.365 deg E
Width = 18 miles = 29 km
Depth of ocean = 12,500 feet = 3,810 meters

Target planet or moon = Earth
Diameter of the impactor = 900.0 meters
Density of the impactor = 7.80 gm/cm^3
Initial velocity of the impactor = 50.00 km/sec
Initial elevation angle of the impactor above horizon = 90.0 degrees
Density of the target surface = 1.00 gm/cm^3
Ocean strike: depth of water = 3810.0 meters
Distance of observer from surface impact point = 1000.0 km

Gravitational acceleration at surface of planet = 9.81 m/s^2
Minimum velocity of an impactor = 11.2 km/sec
Maximum velocity of an impactor = 71.9 km/sec
Atmospheric pressure at surface = 1.013E5 pascals
Atmospheric density at surface = 1.226E-3 gm/cm^3
Surface temperature = 15.0 deg C
Atmospheric scale height (isothermal) = 8.43 km

Initial kinetic energy of impactor in outer space =
3.72E21 joules = 8.89E5 Megatons TNT

An impact of this size occurs once every 3.37E5 years

Atmospheric entry:
Velocity of the impactor at the surface = 49.91 km/sec
Elevation angle of the impactor at the surface = 90.0 degrees
Amount of energy dissipated in the atmosphere = 0.38%
Duration of the atmospheric entry = 1.69 sec
Kinetic energy of impactor at the surface =
3.71E21 joules = 8.86E5 Megatons TNT

Ocean entry:
Velocity of the impactor at bottom of ocean = 32.09 km/sec
Elevation angle of the impactor at bottom of ocean = 90.0 degrees
Amount of energy dissipated in the ocean = 51.05%
Duration of the ocean passage = 0.10 sec
Kinetic energy of impactor at bottom of the ocean =
1.53E21 joules = 3.66E5 Megatons TNT = 8.25 Richter scale

Cratering effects:
Width of transient crater at bottom of ocean = 1.94E1 km
Depth of transient crater at bottom of ocean = 6.85E0 km
Volume of transient crater at bottom of ocean = 1.01E3 km^3
Width of final crater at bottom of ocean = 2.87E1 km
Depth of final crater at bottom of ocean = 1.09E0 km
Volume of final crater at bottom of ocean = 3.53E2 km^3

Thermal effects:
The fireball cannot be seen by the observer
Radius of the fireball generated by the impact = 3.10E1 km
Maximum distance that fireball can be seen = 6.27E2 km
Time for thermal radiation to reach its peak = 6.20E-1 sec
Duration of the thermal radiation = 4.02E2 sec
Distance out to which 1st degree burns happen = 522.0 km
Distance out to which 2nd degree burns happen = 475.0 km
Distance out to which 3rd degree burns happen = 429.0 km

Seismic effects:
Magnitude of ground shaking experienced by observer = 3.19 Richter scale
Time for the seismic waves to reach the observer = 2.00E2 sec
Observer’s experience: Shaking noticeably felt by people who are indoors
Distance out to which everyone feels ground shaking = 488.0 km
Distance out to which buildings partially collapse = 175.0 km
Distance out to which most private houses collapse = 43.0 km
Distance out to which most buildings collapse = 1.0 km

Air blast effects:
Peak overpressure of blast wave at the observer point =
8.20E3 pascals = 8.09E-2 atm
Time for the air blast wave to reach the observer = 2.94E3 sec
Peak wind velocity at the observer point =
1.90E1 m/s = 6.84E1 km/hr
Observer’s experience: Glass windows shatter
Distance out to which glass windows shatter = 1127.0 km
Distance out to which wood-framed houses collapse = 482.0 km
Distance out to which steel-framed buildings collapse = 149.0 km
Distance out to which highway girder bridges collapse = 128.0 km

Tsunami effects:
Inner diameter of initial cavity in the ocean = 3.77E1 km
Outer diameter of initial cavity in the ocean = 5.33E1 km
Depth of initial cavity in the ocean = 1.26E1 km
Wavelength of the peak amplitude = 3.98E1 km
Maximum height of tsunami at obs. without shoaling = 7.52E1 meters
Maximum height of tsunami at obs. with shoaling = 2.26E2 meters
Velocity of the tsunami for its maximum amplitude =
1.83E2 m/s = 6.58E2 km/hr
Period of the peak amplitude = 3.63E0 min
Time for the tsunami to reach the observer = 9.12E1 min
Distance out to which tsunami is 2 meters high = 20015.0 km
Distance out to which tsunami is 5 meters high = 20015.0 km
Distance out to which tsunami is 10 meters high = 20015.0 km
Distance out to which tsunami is 20 meters high = 11955.0 km
Distance out to which tsunami is 50 meters high = 4698.0 km
Distance out to which tsunami is 100 meters high = 2313.0 km

Estimated human death toll:
Estimated non-tsunami death radius = 482.0 km
Estimated tsunami death radius = 11955.0 km
Population density = Australia avg : number of deaths = 1.45E7
Population density = Russia avg: number of deaths = 4.67E7
Population density = USA avg: number of deaths = 1.72E8
Population density = World avg: number of deaths = 2.49E8
Population density = China avg: number of deaths = 7.68E8
Population density = UK avg: number of deaths = 1.37E9
Population density = India avg: number of deaths = 1.87E9
Population density = Bangladesh avg: number of deaths = 5.81E9
Percent of deaths caused by cratering effects = 0.00%
Percent of deaths caused by thermal effects = 9.39%
Percent of deaths caused by seismic effects = 0.09%
Percent of deaths caused by air blast effects = 11.85%
Percent of deaths caused by tsunami effects = 78.67%

Arizona was hit 20000-50000 years ago, leaving a 1200 meter wide crater. The chunk that hit there is only believed to be an 80ft rock. Still, if something like that were to hit a populated area, it would be a major disaster.
Speculations are also going into a hypothesis that the Indian Ocean was hit 5000 years ago. Traces of a huge tsunami in both Madagascar and Australia points to such an event, but it is still disputed.

I can’t think of any other recent events of that scale, but I wonder how many traces we would find of a Tunguska-like event if it happened in a remote area even a few century’s ago?! If it exploded in the air over e.g. the Himalayas, Antarctica or other areas where you wouldn’t find remnants of fallen trees, it would be hard to tell anything had happened without actively looking for it.

Also, maybe we are pretty good about tracking asteroids from the main belt. But I’d be a little more worried about some rogue comet that could come screaming in from deep space.

Since either a comet or an asteroid could (potentially) be an extinction level event I’m totally baffled by the fact that we don’t have an internationally funded and coordinated program in place to track these objects. But we do now have beer that was made from wheat that was grown on the ISS so I think we can have a relax over it (couldn’t resist another jab at the ISS- don’t get me going on the spider experiment up there).

I’m not so sure about that — Phil addresses the issues involved in deflecting/blowing up/etc. an incoming projectile and it’s not about the technological level… the laws of physics don’t care about technology…

nonetheless, I’d like to think we could figure something out if someone yelled “incoming!”

Here’s what I’ve been wondering about that: assuming asteroids of this size hit us every couple of centuries, can you give us some other examples? Even taking into account that most of these would hit in remote areas or the sea, wouldn’t there be geographical evidence at the very least? Like relatively recent craters? These things are big enough to leave a significant mark, aren’t they?

To be honest, though, I think the odds of being killed by an asteroid, at least a sizeable one we can see coming, are zero. I think that we are at the technological level that if we did see one coming that is still twenty years off, we would get off our backsides and actually get the job done. Since that threat really is upon us yet, we don’t feel like we have to be so proactive.

callmeclint, as I recall, McCarthy never really specifies the cause of the disaster. I too think it was an asteroid, but there are numerous debates about the subject. And yes, it’s a pretty cool book.