This painting is titled Massive Terrestrial Strike by noted space
artist Don Davis (URL
http://www.donaldedavis.com). The painting is uncopyrighted,
and was obtained from the NASA Web site URL
http://impact.arc.nasa.gov/gallery/index.html without any
endorsement, expresed or implied, by NASA for this project.
"A little before midnight on the evening of March 26th [2003]
an incoming meteoroid about the size of a car broke apart in the upper
atmosphere, showering debris all over the Chicago suburb of Park Forest
and surrounding areas and damaging at least six houses and three cars..."
"... the fact is that an estimated 200 tons of other worlds fall to Earth
every single day." - Sky and Telescope Magazine, August 2003, page 78
A glance at our Moon through a telescope shows that the history of
the inner Solar System included episodes of massive bombardment. There
is considerable evidence that an impact from space coincided with the
extinction of the dinosaurs 65 million years ago, and that possibly
other impacts coincided with previous extinctions. Craters here on
Earth were eroded away by wind, water, and tectonic activity over the
eons, erasing the record on Earth that is so plainly evident on the
Moon's surface. Recently, evidence of a prior impact some 250 million
years ago associated with a mass extinction of species was discovered
off the coast of Australia.
A thin layer of iridium can be found throughout many places in the
world at the K-T boundary. This is evidence of a massive impact that
occurred approximately 65 million years ago, because we know how far
down from surface corresponds to that date from various dating methods,
and iridium does not occur naturally on Earth in high concentrations,
but it does occur in higher concentrations in certain types of meteorites.
A widely-accepted theory is that an impact of an asteroid or comet
65 million years ago was associated with the extinction of the dinosaurs
and approximately 70% of the life forms on Earth. This cartoon by Jerry
Perichon clearly demonstrates the essential elements of the theory.
The Manacouagan Crater in Canada is approximately 100 kilometers
(62 miles) in diameter. It is typical of the several dozen large
craters found by satellite and aerial land surveys caused by impacts
from meteors.
In more recent times, on June 30, 1908, a body from space exploded
8 km high over the Stony Tunguska River in Siberia with a force of
several megatons, leveling everything over an area of 2150 square
kilometers. The Tunguska event is estimated to have been caused by
an object about 50 meters in diameter, and its explosion to have
released the equivalent of a 10-megaton thermonuclear weapon. This
is considered by experts in the field to be at the low end in size
and impact power of objects that merit our concern! The threat from
space is real, not from evil intelligent life, but from giant rocks
or ice balls, and it is something that could happen again in our
lifetimes.
Here are some quotes from a report titled "The Comet/Asteroid Impact Hazard:A Systems Approach" by Drs. Clark R. Chapman and Daniel D. Durda of the Office of Space Studies, Southwest Research Institute in Boulder, CO, and Dr. Robert E. Gold of the Space Engineering and Technology Branch, Johns Hopkins University Applied Physics Laboratory, Laurel, MD, published 24 February 2001:
"... we believe that international human society (and elements of it, like the U.S. government) needs to make an informed, formal judgment about the seriousness of the impact hazard and the degree to which resources should be spent toward taking steps to address, and plan for mitigation of, potential cosmic impacts. The existing unbalanced, haphazard responses to the impact hazard represent an implicit judgment; but that judgment does not responsibly address the extraordinary and unusual consequences to nations, or even civilization, that could result from leaving this hazard unaddressed in such an arbitrary, off-hand way. ... The dinosaurs could not evaluate and mitigate the natural forces that exterminated them, but human beings have the intelligence to do so."
"Clearly, in cases of impactors >1 km in diameter, we enter a realm never previously encountered by modern civilization. Even the great World Wars of the twentieth century left many nations relatively undamaged, and they were thus able to serve as nuclei for recovery. An unexpected impact by a 2 km asteroid might well destroy agriculture in both hemispheres and around the world, leading to mass starvation from which no nation would be immune."
"An ancillary concern is whether the astrometric follow-up capabilities (largely undertaken by amateur astronomers and poorly-funded professionals in several nations) will ensure that reliable orbits can be established for newly found NEOs so that they will not be lost."
Dr. Chapman appeared on a PBS Nova television program on the topic of asteroid impacts, and is a recognized authority on the subject. The book Hazards Due to Comets & Asteroids (Tom Gehrels, ed., University of Arizona Press, 1994) contains a paper by David Morrison, Clark R. Chapman, and Paul Slovic titled "The Impact Hazard" in which they estimate the potential human deaths per year from various size NEO's. Their table on page 70 estimates up to 3000 world deaths per year from NEO's in the 1-2 kilometer diameter range, dropping to 300 deaths per year for objects 300 m to 2 km landing in the ocean and 30 deaths per year if they hit land, and 55 deaths per year for Tunguska-like objects 50-300m in size.
Since no such objects are recorded by history to have hit Earth and killed people, how do they arrive at such estimates? They estimate the average number of years between such events and the resulting number of deaths, and divide the deaths by the number of years to obtain an annual death rate. Therefore, you have a greater chance in any given year of dying from an NEO impact than from an airplane crash (there were no commercial airline fatalities in 2002, and annual deaths from air crashes usually are far less than 3000). Please remember that if an event is calculated to occur on average every 10,000 years, it has equal probability of occurring every single year, at any time, not just with a 100 percent probability every 10,000 years. Remember the two "500-year" floods that occurred along the Mississippi River in the early 1990's, one right after the other!
To understand the effect of an asteroid's hitting the Earth, we have excerpted data below from Table II of Morrison et. al. When reading this table, it is important to remember that when an NEO hits a city the size of Washington, D.C., a million people die in a matter of minutes from a 10-100 megaton blast caused by the smallest of NEO's that the very best surveys cannot even detect. Even though the "per-year" numbers do not look large, all it takes is one event to kill millions or even billions of people. It is also important to remember that events do not happen like clockwork at periodic intervals, that the "Interval" column gives the probability of the event happening in any given year, and that all years have that same probability of the event's occurrence.
| Equivalent Yield (megatons) | Interval (years) | NEO Diameter (meters) | Consequences |
|---|---|---|---|
| less than 10 | Upper atmosphere detonation of stones and comets | ||
| 10-100 | 1000 | 75 | Meteor Crater, Tunguska; destroy Washington, DC |
| 100-111 | 4000 | 160 | Destroy large urban area, e.g., New York City |
| 1000-10,000 | 16,000 | 350 | Destroy small state, e.g., Delaware |
| 10,000-100,000 | 60,000 | 700 | Destroy moderate state, e.g., Virginia |
| 100,000-1 million | 250,000 | 1,700 | Affects climate, freezes crops, global destruction of ozone; destroy large state, e.g., California |
| 1 million-10 million | 1 million | 3,000 | Raise dust, change climate, widespread fires; destroy large nation, e.g., India |
| 10 million-100 million | 4 million | 7,000 | Prolonged climate effects, global conflagration, probable mass extinction; direct destruction approaches continental scale, e.g., Brazil, U.S., Australia |
| 100 million-1 billion | 16 million | 16,000 | Large mass extinction of the sort 65 million years ago |
| Greater than 1 billion | Threatens all advanced forms of life |
Last modified: January 3, 2008.