Simulation of Asteroid/Comet Impacts with Earth

 

Simulation 1053: Five million years, looking at worst event each millennium.
By Michael Paine 12 Apr 2001.

Update: 26 Apr 01 Graph of impacts and human evolution added.

This page provide further information about the simulation described in BBC: Asteroids 'affected human evolution' and Space.com: Reinventing Darwin Again: How Asteroids Impacted Human Evolution.

See also my Explorezone/Space.com article "SIMULATING ARMAGEDDON ON YOUR PC: ASTEROID IMPACTS WITH EARTH".

If you have a copy of the software check out, and contribute to, the Users Page.

The computer program is available on diskette distributed with the book "Comet and Asteroid Impact Hazards on a Populated Earth" by John S. Lewis, Academic Press. It was released late in 1999. See review.

Due to its random nature, each time the program is run it generates a completely different set of results. As described below, the overwhelming influence of a few horrendous events means that the total number of fatalities can vary considerably between successive runs.

As John Lewis points out in his book, the hazard from small Near Earth Objects (NEOs) has been underestimated in the past because 'average' properties were used in the estimates. His program simulates a mix of asteroid/comet types, speeds and entry angles. The strong iron asteroids are more likely to do damage than the 'average' stony asteroid. Also an object with a shallow entry angle is more likely to slow down without breaking up, and therefore reach a lower altitude where it is more destructive.

Set out below are the results of a total of five million years of simulation, looking at the worst event in each of 5,000 millennia. I want to stress that these are NOT predictions and that no known NEOs are on a collision course with Earth.

The original program uses a tsunami runup factor of 30 (the height of the wave at the shoreline was assumed to be 30 times the height of the wave in deep water). A more conservative runup factor of 5 was used in the simulation.

The program is mainly intended for runs covering a several thousand years. In these time periods impacts massive enough to cause global climatic trauma are extremely rare and the program did not fully account for these effects. Lewis and others suggest an explosion equivalent to one million megatons of TNT would be sufficient to cause over a billion human fatalities, mainly due to global starvation. A typical asteroid about 1 mile across would do the trick. Global climatic effects probably become insignificant for asteroids smaller than 500 yards across, with a typical explosion of 10,000 megatons of TNT (some 200 times larger than an H-bomb). The potential fatalities from these climatic effects have been included in the following fatality estimates.

Asteroid/comet diameter is estimated from mass (randomly generated by the program) and density (derived from the type of object - also randomly generated).
 

Simulation of 5 million years of cosmic impacts

by Michael Paine
 

Introduction

This analysis looks at the results of a simulation of 5 million years of
asteroid and comet impacts with the Earth. This period was chosen to
give an indication of the environmental disruption that may have
occurred during the evolution of our species.
The simulation is based on software developed by John Lewis from the
University of Arizona. It simulates the effects of impacts on current
human population densities. Since population sizes and distribution were
very different in the past the fatality estimates of the program have
been ignored and the analysis has concentrated on the likely
environmental consequences of impacts. These consequences are summarised
at http://ww4.tpg.com.au/users/tps-seti/climate.htm . In brief the
consequences can be categorised into:

• A. Local - devastation over a radius of tens of kilometres. No serious regional or global consequences
• B. Moderate regional - devastation over a radius of hundreds of kilometres (the size of a small country). Short term regional climaticproblems.
• C. Severe regional - devastation over a thousand kilometres (the size of a large country). Severe regional climatic disruption. Mild short-term global climatic disruption (year without summer).
• D. Moderate global - devastation over thousands of kilometres (continents). Severe global climate disruption for several years. Global food chain failures
• E. Severe global - global firestorms (ballistic re-entry of impact debris). Extreme global climate disruption for decades to centuries. Extinctions (demise of dinosaurs).

For everything except the last category the effects on early human
populations depend proximity to the impact - a matter of luck.
In addition to climate disruption (mainly darkness and cooling), the
larger impacts could lead to global warming due to the greenhouse effect
(water and carbon dioxide), lose of the ozone layer (particularly with
ocean impacts that propel chlorine into the upper atmosphere), acid rain
and toxins.
The simulation looked at the worst event in each of 5000 millennia. It
therefore gives an underestimate of the total number of impacts.
 

Results

The program recognises 5 outcomes of an asteroid or comet colliding with
the Earth:

•  The object skims the atmosphere and flies harmlessly back into space. This happened in 2% of the millennia.
• The object explodes above land in an airburst similar to an atomic explosion. This happened in 17% of the millennia.
•  The object impacts the land and forms a crater. This happened in 11% of the millennia.
• The object explodes in an airburst above an ocean. This was the most frequent outcome, accounting for 41% of millennia. Fortunately, until recently, most of these impacts would have been harmless to land dwelling creatures.
•  The object impacts the ocean, forming tsunami and, possibly, ejecting vast quantities of water and salt into the atmosphere. This happened in 28% of the millennia. (Larger impacts may also reach the ocean bottom and cause similar effects to a land impact)

Over the period of the simulation some 57% of millennia suffered an
impact that would potentially have consequences for land-dwelling
creatures. In most cases they would only be affected in they were close
to the impact site. The situation is different now with significant
human populations living in low-lying coastal areas.
 

Size of impactor

(The letters refer to the typical environmental consequences.)
500 to 900m (C)  - 108 events
1km to 1.5km (C/D) - 24 events
1.6km+  (D/E) - 13 events
 

Craters

(The letters refer to the typical environmental consequences.)
Over the 5000 millennia a total of 552 craters were formed on land. Of
these:
* 477 were less than 5km in diameter (A to B)
* 64 were between 5 and 10km in diameter (B to C)
* 9 were between 10 and 20 km in diameter (C) and
* 2 were more than 20km (D).
There were also 6 ocean impacts that could be expected to produce
moderate to severe global climate disruption (D/E), particularly
destruction of the ozone layer. Three of these involved transient
craters more than 50km in diameter and would probably have reached the
ocean floor.
Therefore during this simulation severe climate disruption occurred, on
average, every million years (2 land impacts and 3 ocean impacts).
 

Comparison with the actual impact record

The Canadian NRC crater database lists 32 craters that are younger than
5 million years. One is 52km in diameter, 3 are between 10 and 20 km, 1
is between 5 and 10km and 25 are less than 5km in diameter. Note that it
takes very unusual conditions to preserve craters of this size for more
than a few hundred thousand years.
At least two other significant impact events have occurred during the
past 5 million years:
* "Eltanin" - Just over 2 million years ago an asteroid estimated to be
2km in diameter struck the Southern Ocean, south west of Chile. Had it
struck land the environmental consequences might have been much worse.
If the collision had occurred a few hours early southern Africa might
have been wiped out, along with our ancestors.
* 800,000 years ago an asteroid or comet probably struck the Indochina
region. No crater has yet been found but it is claimed that the
"Australasian Tektite Strewn Field" (impact debris) was produced by an
impact that caused a crater 90km in diameter. This suggests an asteroid
or comet around 4km in diameter and severe global climate disruption.

It appears that neither of these events shows up in ancient climate
records but it is almost certain that, for several years at least,
creatures on Earth had to endure very severe conditions. Although the
physical Earth heals quickly from impacts its inhabitants might not do
so well and the course of evolution may well have changed due to these
and similar impact events.
 
 

References

Lewis J.S. (1999). Comet and Asteroid Impact Hazards on a Populated
Earth. Academic Press, San Diego.

Paine M. (1999) 'Asteroid Impacts: the Extra Hazard Due to Tsunami', The
Science of Tsunami Hazards 17-3, 155-166. The Tsunami Society, Hawaii.
 

Graph of impacts and human evolution

Human evolution data from Space.com: Reinventing Darwin Again: How Asteroids Impacted Human Evolution. For crater data see Impact craters and mass extinctions.

This is a draft version and is subject to change.