ASTR 1210 (O'Connell) Study Guide
22: IMPACTS AND BIO-EXTINCTIONS
Impact of a "planet buster" asteroid
"We can never anticipate
the unseen good or evil that may come
upon us suddenly out of
space." --- H. G. Wells
With the arrival of a scientific assessment of sky phenomena,
cataclysms imagined in earlier times were cast off as the
superstitions that they were. The cosmos became regarded as mostly
benign, apart from a few skeptics like H. G. Wells. It was hard to
conceive of Earth being threatened by any extraterrestrial event short
of a major change in the Sun. But recognition of a huge, unforeseen,
genuine cosmic threat from another quarter emerged in the mid-20th
Until that time, craters on the Moon and Earth were usually
interpreted as having a volcanic origin even if they were not
located in volcanically active regions. Recall that until the first
spacecraft reconnaissance in the mid-1960s's, there was no direct
evidence for craters on other planets (excepting the Moon).
Then, E. Shoemaker demonstrated (1960), by comparing the structure
of the Barringer Crater
in Arizona & others to nuclear bomb craters and discovering the
presence of shock-heated minerals
like coesite, that most isolated craters were formed in explosive
impacts by extraterrestrial bodies, not vulcanism.
This revealed that the history of the Earth and solar
system was even more violent than had been supposed. We now know that
craters cover the surfaces of
almost all solid bodies in the solar system and that most of these
were produced by a devastating rain of impactors.
The implications were serious because, although most impacts had occurred
in the distant past, there were still millions of interplanetary bodies
that could collide with the Earth in the future.
This threat was bad enough, but the main revolution in our view
of the importance of impact events for Earth was begun by the
publication by Alvarez et al. in Science Magazine (1980) of
"An Exterrestrial Cause for
the Cretaceous-Tertiary Extinction".
There has been great controversy over the impact/extinction
- Based on geological evidence described below,
this paper argued that the major extinction of lifeforms 65
million years ago, including dinosaurs, was produced by the
impact of a single moderate-sized asteroid.
- Although the dangers of extraterrestrial impacts had been
speculated about before, this study provided the first direct evidence
for a link to worldwide biological extinctions.
But the extraterrestrial proponents have indisputable facts on their
- Many geologists & paleontologists have argued for other,
non-astronomical, mechanisms for extinctions, such as the massive
volcanic outbreak at
Traps, which might have caused a sudden extreme greenhouse event.
- Earth moves continuously at high speed through
of asteroids, meteoroids, and comet nuclei, for example the "Apollo,"
"Aten," and other types
- The unambiguous conclusion is that major impacts are
inevitable! The question is not if?, but when?
- The areal density of lifetime impacts on the Earth's surface for
craters over 500-m diameter is higher than that on
the Moon. See this illustration
of the impact density.
- Finally, the energy deposited by big asteroids is large
enough that the destructive effects can be global.
The Barringer "Meteor Crater," near Winslow, AZ. 1 mile diameter.
Created by the impact of a 50-m diameter metallic meteoroid about 50,000 years ago.
B. Direct Evidence for Major Impacts on Earth
- Fossil Craters
- From spacecraft images, we are now aware of the ferocious impact
history of the surfaces of all other solid bodies investigated
so far in the Solar System (e.g. Mars,
Jupiter's moon Callisto, and of
course, our own Moon).
- Owing to weathering and crust recycling, older craters
have been mostly erased from Earth's surface.
- Nonetheless, we have been able to identify
over 150 large fossil
craters in regions where older geological strata are exposed.
E.g. at right is Manicouagan Crater, Canada. This Space Shuttle image
shows a 43 mile diameter annular lake (in winter), part of a 62 mile
diameter crater structure. Age: 210 million years.
- Impact geology. Shocked & melted rocks or other debris
characteristic of sudden high
temperatures or pressures which go beyond what is encountered
in volcanic eruptions can be identified even in the absence of a
definite crater nearby. In some cases, these can be found at
from the point of impact. Impact signatures have been identified at
many locations worldwide.
- Observed near misses (last 50 years)
Some large meteoroids have been seen passing through our atmosphere
without striking the surface. At right is a picture taken Aug 10,
1972 in Grand Teton National Park of a near-miss (click for
enlargement). The object is about 10-m diameter and at an altitude of
about 55 km, moving at 15 km/sec (33,000 MPH). It approached at such
a shallow angle that it skipped off the atmosphere. If it had hit the
Earth, it would have had H-bomb equivalent impact energy.
There have now been many telescopically-observed near misses
within twice the distance of the Moon.
- A complete list of these, including predicted passages,
is given here.
- Some interesting examples:
2011 CQ1: a small 1-m diameter
meteoroid that came within 3,400 miles of Earth's surface on 4 Feb 2011.
2005 YU55: a 400-m asteroid
that passed within 202,000 miles (inside the Moon's orbit) on 8 November 2011.
Largest known near-miss object until 2028.
2012 DA14: a
passage of a 45-m meteoroid at a distance of only 17,000 miles, inside the
geosynchronous satellite belt(!), on 15 February 2013.
Twitter site routinely reports passages of asteroids within 0.2 AU
(19 million miles) of the Earth. (Note that this includes passages up
to four times farther than would be nominally considered "hazardous"
for an Earth impact.)
- Witnessed impacts.
- Tunguska, Siberia 1908:
the biggest recorded hit
- Energy release: equivalent to ~20 million tons of TNT. [One
million tons, or one "megaton," is the explosive energy of a typical hydrogen
bomb.] Tunguska was 1000 x the energy release of the Hiroshima atomic
bomb ("only" 20,000 tons).
- The explosion flattens an area the size of
- Likely cause: air detonation (altitude 10 km) of a stony meteoroid about 30 m
- Tunguska Redux (Chelyabinsk meteoroid, 15 February 2013)
- In a near-replay of the Tunguska event, a 20-m diameter meteoroid
(see picture at right) exploded over Chelyabinsk, Russia, at an altitude of
23 km (14 miles). The shock wave overpressure from the explosion
caused extensive structural damage and produced a number of injuries
(but no deaths).
- The explosive energy of the meteoroid was equivalent to about 450,000
tons of TNT. It was smaller than Tunguska because the incoming body was
smaller and had a lower velocity. It also exploded at a higher altitude,
reducing effects at the ground level. The event, however, was a sobering
reminder of the impact threat from space.
- The incoming meteoroid had not been detected before impact
because of its small size and the fact that it approached from the
direction of the Sun. It was unrelated to the (ironically) highly
publicized approach of 2012 DA14, which passed Earth harmlessly only 16
hours later. Mother Nature was just having a little fun raining on
the smartypants asteroid prediction parade.
- Continuous atmospheric detonations. Air Force infrared
monitoring satellites have discovered that several Hiroshima-sized
meteoroid explosions occur each year in the atmosphere.
- Context: imagine the
political firestorm if the Chinese government were doing this!
- Predicted and witnessed impacts: the first and (so far) only
one of these was
2008 TC3, a 5-m
diameter meteoroid that was detected only 20 hours before it hit the
Earth in northern Sudan on 7 October 2008. No damage or injuries. A
number of fragments were recovered.
Where does the tremendous explosive energy of impactors
originate? From their high velocities. In the theory of Newtonian
dynamics, there is an energy of motion, or kinetic energy,
associated with any moving object. When a fast moving object hits the
Earth's surface, this energy of motion is quickly converted into
heat and results in a concentrated explosion.
From Newton's dynamics, the kinetic energy (KE) of the
incoming object is given by:
Want to see a real impact on these gigantic scales?
The deposited impact energy then equals this kinetic energy. It is
traditional to express this energy in units of the equivalent energy
released by a ton of TNT, a powerful and widely-used chemical
explosive which serves as a standard of comparison for other kinds of
explosives. One ton of TNT releases 4 x 1016 ergs of
Combining and converting our expression for deposited kinetic energy,
we find that:
- KE = 1/2 M V2, where M is the mass of the
object and V is its velocity with respect to the Earth.
- V is large!
Earth's orbital velocity ~ 30 km/sec = 66,000 mph; this is a
typical speed for potential impactors moving near the Earth.
Maximum impactor velocities could be up to about 120,000 mph
- Mass is large!
- M = Density x Volume = Density x 1/6 π D3,
where D is the diameter of the object.
- For rocks, Density ~ 5 gr/cc
- ===> Mass ~ 2.5 D3 tons, where D = diameter in meters
===> Impact energy = 250 D3 equivalent tons of TNT, where D is in
Note the very strong dependence on size. A factor of 10 increase in diameter
results in 1000-fold increase in deposited energy.
A tiny object with D = 4 meters packs the explosive energy of the
Hiroshima bomb (20,000 tons of TNT).
The largest nuclear weapons thought to be in the US or Russian
stockpiles yield about 10 million tons of TNT equivalent. That
corresponds to a 34-m diameter meteoroid, a size that is very difficult
to detect reliably near the Earth
If D = 1 kilometer, the size of a typical large
asteroid, the released energy is 250 billion tons of TNT!
Numerical calculations of impact physics have been made by
The picture on the left below is an infrared image of the
fireball from the impact of fragment "K" of Comet Shoemaker-Levy 9
on Jupiter in 1994. This deposited energy is equivalent to
about 5 trillion tons of TNT. The glowing atmospheric scars of
three earlier impacts can also be seen in the picture. As you look at
the scale of the impacts, remember that Jupiter is 11 times the
Earth's diameter, so the bright fireball is about the same size as
the Earth. Other images of the SL9 impacts on Jupiter can be
found here. On the
right is a 1998 movie poster showing an artist's concept of a much
smaller impact on Earth.
D. Potential Impact Scales
- City Buster: 15-m diam meteoroid ===> 106 tons
TNT = 1 Megaton (MT). Serious local consequences, if it
explodes at ground level. The atmosphere provides a partial
shield for objects of this size. Explosions will
be hydrogen-bomb scale but without the radioactivity. Both
witnessed Russian impactors (see above) were in this category, but by
virtue of atmospheric shielding they exploded at high altitude, doing
less damage than they could have if they reached the surface.
- People Buster: 1-km diam asteroid ===> 250,000 MT. No
atmospheric shield: atmospheric resistance has no effect on the
incoming velocity of objects this large. Would
produce hemispheric-scale effects; at the threshold for global
effects. A significant fraction of all humans would be killed.
Planet Buster: 10-km diam asteroid ===> 250 million MT.
Global effects. Ejected, vaporized rock and water fill the
atmosphere, producing a "global winter" and a consequent major
extinction of lifeforms, including virtually all humans.
E. Impact-Induced Bio-Extinctions
The fossil record (at right) shows that 5 great extinctions of
lifeforms on Earth occurred during the last 570 million years. These
are times where the fossil record abruptly changes character,
and many species vanish from more recent rocks. It is now
believed that most of these were probably induced by extraterrestrial
The last great extinction was 65 million yrs ago at
Cretaceous-Tertiary ("K-T") boundary in the fossil record.
- Half of all species (not individuals) of plants
and animals vanished.
- Dinosaurs were exterminated. This created an eco-niche for small
mammals (our forebears)
An Extraterrestrial Origin for the K-T Event
- As first shown by Alvarez et al. in 1980, the sedimentary rock
layers at the K-T
boundary are abnormally rich in platinum group metals like
iridium. Their composition is like meteorites/asteroids, not
Earth crustal rocks, which were stripped of these materials when they
settled to the Earth's core. Deposition of the anomalous layers is
worldwide. The extraterrestrial origin of this extinction is now
- The energy release involved would have required a 10-km diameter asteroid
- The site of the impact is now identified: Chicxulub
crater, N. Yucatan. See map at right. The crater diameter is 110
miles, and its age (via the radioisotope method) matches the
extinction event (65 Myr ago). Debris deposits are scattered
throughout the Caribbean area. The
image here shows gravity and magnetic
anomalies associated with the buried/undersea crater. See
this Washington Post
article for more details.
- Recently, more accurate dating and volume estimates for the lava
flows at the Deccan traps show that the major outbreak was essentially
coincident with the KT event and the Chicxulub impact.
This suggests that the impact (in what is now Mexico) may have
triggered massive lava floods (in what is now India) that contributed
to the extinction through global effects on the atmosphere.
Earlier Major Impacts
There is some evidence that the largest known
extinction, at the
Permian-Triassic boundary, 250 million years ago, which
extinguished 90% of all lifeforms on Earth, was also impact-related.
However, it is much harder to find impact tracers surviving from such
Evidence for the Late Heavy Bombardment
on the Moon suggests that the Earth was subjected to a similar intense
storm of large impacts about 4 Byr ago. This may have been sufficient
to sterilize Earth's surface of any primitive lifeforms which had emerged
in the first 500 Myr of terrestrial history.
F. Risk Level?
We can crudely estimate the frequency of large impacts from the
history of lunar cratering & bio-extinctions on Earth and our
knowledge of the number of potential impactors of different sizes:
Adjusting for the larger number of smaller impactors:
- E.g.: 5 great extinctions in 570 million years implies a rate of
about one global extinction (planet-buster) event per 100 million
years; the perpetrator would be a ~10-km diameter asteroid.
A Chelyabinsk-level event (20-m diameter impactor) will
occur about once every 80 years
A people-buster event (1-km diameter impactor killing 1/4 of
the human race) will occur about once per 150,000 years.
for a plot of the predicted frequency of impacts as function of
The estimated risk to an individual (e.g. you) of asteroid impacts has
declined by a factor of about 30 in the last decade due to improved
surveys of potential impactors. The estimated net fatality risk (all impactor
sizes) is now a 1/700,000 chance per person per lifetime
- In human terms, a people-buster event is very rare but very
- Homo sapiens has existed for about 250,000 years and so
has probably experienced, and survived, one or two people-buster
events. However, if the most recent of these occurred before about
15,000 years ago, there would be no records of it because humans
had not yet developed written languages.
Laughably small and forgettable, yes?
Well.....no. Many people actively worry about events with
comparable or smaller statistical probabilities: death from sharks,
nuclear power plant meltdowns, tidal waves, or poisonous snakes, for
Risk ranking: Astronomers have created
the "Torino Scale" (a
combination of estimated impact energy with probability of a strike on
Earth) to provide a threat index for potential Earth impactors.
And, just to remind you of how low probability events can
become manifest, the massive 2011
Tohoku earthquake off Japan resulted in not only a deadly tidal
wave but also three nuclear reactor meltdowns!
I don't recommend that you add asteroid impacts to your list of
serious personal anxieties --- but you certainly shouldn't worry any
less about them than you do about those other more familiar
risks. And, of course, the real problem isn't the personal risk, it's
the devastation that a 1-km class impactor would inflict on the race
and on other higher lifeforms on Earth.
Various government agencies and private groups are consideration
approaches to mitigating the danger of impacts on Earth.
- First, we must identify threatening "Near Earth Objects"
(NEO's). A number of ground-based and space-based surveys,
have identified over 10,000 NEO's. The census is effectively complete
for the brighter, larger objects, but is deficient for those smaller
but still dangerous objects under 500-m in diameter.
- "Planet-busting" large asteroids (10-km) are relatively
bright and we believe
all have already been identified. There are relatively few,
and none pose a foreseeable threat.
- "People-busting" medium objects (1-km): it is estimated
there are about 980 objects larger than 1 km in Earth-crossing orbits,
of which 90% are now identified. There is no obvious near-term
threat in this category.
- "City-busting" small objects (10--500-m): bad news. Too many
(perhaps a million), too faint; search too expensive. We will
have to "live" with these. (Tunguska and Chelyabinsk are in this
class.) Over 5,000 with diameters larger than 100-m are known, but
another 15,000 are expected in that range. A
recent study finds that the number of potential city-busting impactors
may have been underestimated by as much as a factor of 10.
- A sobering footnote. The objects found in NEO surveys are
asteroids and comets with relatively small semi-major axes (i.e. with
orbits near the Sun). Comet nuclei plunging in from the edge
of the solar system could easily be in the dangerous impactor category
(as was Comet
Shoemaker-Levy 9), but we would have no way of detecting them
until late in their approach (say at the distance of Saturn). Making
detection even harder is the fact that comets can come from all
directions in space, whereas asteroids are more confined to the
- Second, we must develop technologies to eliminate them
- Best method: a gentle velocity deflection when they are
still at large distance from Earth, enough to change their orbit such
that they never intersect with Earth's orbit. Trying to break up
potential impactors with explosives, as in the movie "Armageddon,"
could easily create more danger than it prevents.
- Requires new, though feasible, space technologies, e.g. an
- Following the asteroid 1997 XF11 public relations debacle, NASA established a Near Earth Object Program to oversee
studies of potentially hazardous objects.
H. Summing Up
Impacts from fragments of interplanetary material are not only
dangerous to us, but they have also had dramatic effects on the
development of life on Earth. Based on our (still incomplete) surveys
to date, impacts do not present a looming threat to human welfare.
However, we must learn to live with them in the long run, and this
will require a large investment of money, energy, and brainpower. The
sooner we begin to make that, the better.
Reading for this lecture:
Study Guide 22
Bennett textbook, Chapter 12
Reading for next (last) lecture:
Study Guide 23
Bennett textbook, Chapter 24
May 2017 by rwo
Opening painting copyright © 1998, Don Davis. Tunguska
areal map from Clark Chapman/John Pike. Impact
frequency plot copyright © Prentice-Hall. Chicxulub map copyright
© 2001 Athena Publications. Text copyright © 1998-2017 Robert
W. O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 1210 at
the University of Virginia.