The document discusses various existential threats to humanity, including those of human origin like nuclear war and ozone depletion, and natural threats such as asteroid impacts, solar changes, gamma ray bursts, and the possibility of a hostile extraterrestrial civilization. It provides examples and potential solutions for each threat, noting that some threats like asteroid impacts and solar changes are inevitable but also occur over very long timescales, while others like nuclear war can be addressed through arms control and environmental protection. The document aims to raise awareness of existential risks while acknowledging that the likelihood of most scenarios is extremely low.

1. Jack Oughton Astronomy, Eschatology and Apocalypse: Threats From
Space
In the last century, the advance of human technology and industry has come far enough that we
can now, for the first time, threaten our own existence. But the continuing survival of humanity is
not dependent entirely upon our actions alone. There has always been, and will always be,
however small the chances are of occurrence, natural catastrophes with differing damage
potentials, from threatening small areas, to life itself. Planet earth is more resilient than
humanity, and has weathered countless events that if happening today, would annihilate all
creatures on the planet. But as scientific understanding grows, so does our awareness of
scenarios that threaten the continued existence of our tough planet. It goes without saying that
the chances of most of these scenarios actually happening are so remote as to be nearly
impossible, and many of the solutions offered aren’t even close to technologically viable within
the near future. However it would be foolish to discount such scenarios and solutions, nobody
can predict the future.
Threats of Human Origin
NUCLEAR EXCHANGE CAUSED BY MISINTERPRETATION OF ASTEROID AIRBURSTS
The stockpiling of nuclear weaponry since the mid 1940s has given the fragmented tribes of
humanity the tools to utterly destroy each other. In the 1960s, the Cuban Missile Crisis was an
example of two tribes almost ready to ruin the world. Although today the threat of nuclear war
has diminished, similar nuclear stalemates still exist, as many major countries possess these
weapons. An example of this is between India and Pakistan. The basic principle behind nuclear
stalemate is known as mutually assured destruction; in that, the need for self preservation
outstrips the need to destroy the enemy. If two nuclear powers engage in an exchange, it is a
zero sum game, both for them and probably for everyone else; nobody wins in a nuclear winter.
However, if a side perceives that it is receiving a nuclear attack, it now has nothing to lose and
could retaliate. Consider the possibility of an asteroid 80 ft. across striking the atmosphere. It
enters the atmosphere at 50,000 M.P.H., which suddenly creates a barrier of compressed air in
its path. The projectile is stopped dead by the friction created, converting kinetic energy a shock
wave, in this case equivalent to the blast of a one-megaton bomb. The misinterpretation of this
blast by a nuclear power could cause a nuclear exchange. (Shoemaker, 2001)
Solutions: Apart from a global nuclear disarmament, providing poorer nuclear powers access
to better data regarding Near Earth Object (NEO) activity should prevent any
‘misunderstandings’.
OZONE LAYER DEPLETION DUE TO HUMAN ACTIVITY
In this scenario, the threat is caused by the sun, but the problem caused by man. The ozone
(O3) layer is the shield which prevents 97-99% of the ultraviolet light from the sun reaching usi.
Ultraviolet light is categorized into UV-A, UV-B and UV-C. UV-C is utterly lethal to life, but is
completely screened out by the ozone layer, the others also have harmful effects on organic
matter, and progressive exposure to these forms can cause the formation of genetic mutations
and cancers. Ozone depletion allows more of the UV radiation, particularly the more harmful
wavelengths, to reach the surface. Much of the ozone depletion is caused by the release of
Chlorofluorocarbons into the ozone layer. In CFCs, the disassociation of chlorine caused by UV
light causes each chlorine atom to catalyse and remove tens of thousands of ozone molecules
before decaying in the stratosphere. One CFC molecule can stay in the ozone for a century,
destroying around 100,000 ozone molecules during that time (Columbia University , 2008).
Therefore this is a problem that will continue to manifest itself, even if we no longer add to it.
Solutions: This problem is already being proactively addressed. The 1987 the Montreal

2. Protocol, called for reducing CFC use by 50% by 2000. In 1992 an amendment of the treaty
called for the end of CFC production in industrial countries by 1996. (United Nations
Environment Programme, 2000). A way to address the residual problem that CFCs will cause in
future would be to remove them from the atmosphere, plans are already being made to use
powerful infrared lasers targeted at the troposphere, these beams would then selectively
destroy CFC molecules through multiphoton dissociation. (Stix, 1993) Currently, the costs of
such a project outweigh the potential merits, as the laser disassociation process is expensive
and inefficient.
Threats of Natural Origin
NEAR EARTH ASTEROID (NEO) IMPACTS
It is believed that many extinction level events in the past were caused by impacts from large
asteroids, evidence is found in large craters which remain, despite the earth’s geographically
active surface. Objects 150m in diameter are dangerous enough to cause regional devastation
for a land impact or the threat of a major tsunami for an ocean impact. Such impact events are
predicted to occur once every 10,000 years or less. Larger objects possess the potential to do
even greater damage, a rocky asteroid 200 miles across impacting land at 22 k/ms a second
would cause a blast with a yield of 3.31 x 1012 megatons of TNT, an earthquake of 13 on the
Richter scale, and displace a layer of ejecta 3310 m thick. (Marcus, 2005). This would probably
doom all life, but the earth would not be significantly affected, apart from a large impact crater.
Solutions: Although the level of possible devastation is linked to the size of the impacting
object, thankfully larger objects are much easier to detect. In 1992 a US congressional mandate
called for NASA to locate 90% of threatening near-Earth asteroids within the decade. A number
of efforts which receive money through this mandate are all considered to be working on the
informally titled Project Spaceguard, and many other countries have their own Spaceguard
programs, including one in the UK, based in Powys. (Spaceguard UK Portal). There are
currently 928 NEOs which are classified as potentially hazardous asteroids; however none of
them come anywhere close to 200 miles in diameter. These asteroids do have an absolute
magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size, enough to
potentially cause localized destruction in their impact zone, depending upon their composition
and speed of impact. Being able to detect these objects is only the first phase of a solution. If an
object was found to be on an impact course we would have to find a way to either change its
direction or neutralize the threat it posed. Many solutions are being considered for both of these
options, including interception with nuclear ICBMS. However, this creates the dangerous
possibility of simply causing many smaller fragments to impact earth instead. Other proposed
ideas include redirection via “pushing” with reflected solar energy, or by utilizing the solar wind
by the attachment of kapton solar sails (Morrow, 2001)
Chixculub Crater Case Study (NEO)
It is now widely believed that the large-scale mass extinction of animal and plant species, known
as the Cretaceous–Tertiary extinction event approximately 65.5 million years ago was caused
by an asteroid impact in the region of the Yucatán Peninsula (Hildebrand, 1991). The Chicxulub
Crater is greater than 180 kilometres in diameter, one of the largest impact created structures in
the world. Evidence supporting this hypothesis comes from sedimentary rock layers found
globally at the Cretaceous–Tertiary boundary. These contain a concentration of iridium
hundreds of times greater than normal, and of a different isotopic composition of most terrestrial
occurring iridium. Iridium is extremely rare in the earth's crust due to its great density; most of it
sank into the earth's core while the earth was still molten. It is believed that the 10 kilometre
impactor was a chondrite, a type of asteroid that contains large proportion of its mass as iridium.
Correlating data from the estimates of the total amount of iridium in the K–T layer, and the size

3. of the crater at Chicxulub; the asteroid is thought to have impacted with a force of 100 trillion
tons of TNT. Other craters around the world have been dated to approximately the same age
as Chicxulub, most significantly Silverpit crater in the UK and the Boltysh crater in Ukraine. This
suggests that the asteroid that struck the earth could have been even larger, and fragmented
upon contact with the atmosphere.
The effects upon life would have been substantial, apart from obliterating everything in the blast
radiuses, the impacts would have displaced enough debris into the atmosphere to significantly
diminish the sunlight received on the earth’s surface. These would have caused a lowering
global temperature and inhibited photosynthesis, proliferating ecological food web disruptions
and the extinction of over 50% of species present before the event, in a ‘nuclear winter’. (N
Macleod, 1997). Global firestorms may have resulted from incendiary fragments from the blast
returning to Earth. Analyses of ancient amber suggest that the oxygen content of the
atmosphere was high (30–35%) during this period. A higher O2 level could have resulted in
atmospheric combustion. If widespread fires occurred, they would have increased the CO2
content of the atmosphere and caused a temporary greenhouse effect once the dust had
settled, exterminating the most vulnerable survivors of the "nuclear winter". Thankfully, disasters
of this scale are expected to occur only once every 100,000,000 years.
GRAVITATIONAL DISTURBANCES CAUSED BY PROXIMITY TO MASSIVE CELESTIAL
OBJECTS
The effects of this scenario vary in their extremity depending upon the mass of the object, the
proximity, and angle of passage that the object takes. The concept is that, an object with
enough mass could interfere with the gravitational equilibrium of the solar system. It is believed
many of the comets which we are able to observe (possible threats in themselves) originated
from a hypothetical region of space around 1 light-year away known as the Oort Cloud. The
great distance of this cloud from the sun means that although the objects contained within it
orbit the sun, they are more susceptible to the gravitational effects of temporarily passing
objects. This disruption can redirect their courses and some of them are propelled towards the
solar system, where their orbits can be trapped by the sun or the planets.
This gravitational process could be applied to Earth. An object that disrupted the orbit of earth
significantly enough could cause enough of a change in distance to alter climate dramatically.
An object with greater mass still could cause our planet to be ejected from the sun’s
gravitational pull. Life could not survive the travel through interstellar space as it has evolved
specifically in the sun’s warmth.
It is known that Stellar-mass black holes travel through the Milky Way. Therefore there is a
chance that one could pass by our solar system. One of these objects would have enough mass
to cause all sorts of gravitational havoc, and because of its invisible nature (unless it had
acquired an accretion disk), would be extremely hard for us to detect. However, significant
gravitational interactions between the Sun and any object of stellar mass are expected to occur
only once every 1019 years. It is believed extremely unlikely that we will endure the effects of a
black hole before the Sun exterminates life on Earth. (J. Binney, 1987). Much larger mass black
holes, former Quasars ejected from galactic mergers at speeds of around 10 million miles per
hour (Than, 2007) are also believed to exist, and because of their speed, mass and angle of
ejection, could orbit outside of the galactic plane, dipping in and out of the galaxy as their orbits
take them. Gravitational interaction between one of these former Quasars and our solar system
could be dramatic, enough to gravitationally capture our sun.
Interaction between stellar mass objects also produces another apocalyptic possibility.
Hypothetically, if a passing star where to collide directly with the sun, the merger would cause a

4. dramatic increase in the rate of fusion, if the colliding star had enough mass, the merger could
create a supernova as the sun collapsed under this new gravitational instability (Sanders, 1970),
this would definitely destroy the planet outright.
Solutions?: Die?
CHANGING SOLAR ENERGY OUTPUT
As far as we are aware today, this scenario is certain, but estimated to occur slowly, over
billions of years. Our Sun is currently in the Main Sequence of its life, which means that it is
fusing hydrogen to helium. Over the last 4.5billion years life has adapted to the conditions this
creates on earth. However, once Sol depletes its supply of hydrogen, it will begin to fuse helium,
and change in structure to a red giant star, brightening and increasing in size 100 times. Initially
the increased solar output would increase temperatures upon the earth’s surface, disrupting the
ecosystems and weather systems of the planet. Significant reduction of the amount of carbon
dioxide, caused by a planet that is getting drier, would not allow for photosynthesis, and plants
would die. This would cause a huge reduction in the amount of oxygen available; then animals
would die. As the sun’s output increases, the oceans would begin to evaporate into the
atmosphere, intensifying the greenhouse effect and absorbing more of the already significant
solar output. By now, temperature change will have rendered the planet completely
uninhabitable as conditions begin to mirror those found on Venus, which suffered the same fate
due to its proximity to the Sun. As solar output rises, the very surface of the earth itself would
begin to melt. Eventually, the earth would either be swallowed completely by the increasing size
of the sun, or would increase its orbital distance as the sun’s gravitational pull weakens, due to
the loss of solar mass.
Solutions: In this situation there are two options, to either move the species to a more
hospitable region, such as a mass migration to another planet. The more challenging solution to
would be to directly alter the sun itself, this could be done by changing the predominant fusion
process back to hydrogen, there would be no lack of fuel, as atomic hydrogen is the most
abundant element in the universe. The challenge would stem from the star itself, how could we
insert energy into a body that radiates energy so powerfully?
HOSTILE ETI
Humanity has been broadcasting radio waves indiscriminately into space since the end of the
19th century. Many SETI (Search For Extraterrestrial Intelligence) organizations have also made
efforts to communicate, by beaming concentrated microwave messages towards star systems
that appear suitable. A number of SETI scientists criticize this, claiming it unwise to transmit to a
species that we cannot know the motives of. Also, recent publications of US governmental
documents suggest that the alien species are taken as a credible threat. (Andrews, 2008). If we
are to assume that an alien civilization is capable of detecting our transmissions and travelling
the incredible distance to reach earth in a timely fashion, we can assume their technology is
vastly superior to ours. If this race for some reason chose to engage us in conflict, we could also
reasonably assume that their superior technology would prevail, as the lessons of our own
history have taught us.
Solutions: This scenario allows for possibly the most unpopular solution humanity could ever
devise, which would be to stop using technologies that transmit microwave and radio waves as
a by-product of their use, this information blackout would silence our planet, but would not stop
the continued passage of messages transmitted in years past, which will have travelled further
than 90 light years. Despite the fact that alien life has not been found to exist, initial plans for
planetary defence have been considered should the possibility of interstellar war emerge, based
on a detailed analysis of modern warfare today and projected weapons developments over the
coming decades. (Taylor, 2006)

5. GAMMA RAY BURSTS
GRBs are created from the collapse of a massive star into a black hole, while inner portion of
the star collapses to form the singularity; the outer layers erupt into space. Due to the Godlike
power of this process, two beams of energy erupt from the polar regions of the dying star.
Composed mostly of high-energy gamma rays, they can carry more energy in them than the
Sun will put out in its entire lifetime. These are so energetic enough to be detectable across the
universe, and have been proposed to have caused extinction level events in the past, most
specifically the Ordovician-Silurian extinction event of 450 million years ago (Mellot, 2004). The
effects of the earth receiving a GRB burst vary, depending on the distance from the emission,
but simulations at the Goddard Space Centre suggest that a concentrated source of gamma
rays would create nitrogen oxide compounds in the ozone layer, which would remove around
50% of its protective effects within a week. (Thomas, 2006 ) Apart from the excess UV exposure
caused by a loss of ozone, the gamma ray dosage would be expected to be far in excess of a
lethal for all life. These bursts are hypothesized to be able affect us even at distances five times
the Milky Way’s width. (Golante, 2005)
Solutions: Due to the nature of these bursts we could have no advance warning of their arrival.
Theoretically the creation of some sort of planetary shield which would not interrupt the normal
input of solar radiation would be required, but how it could be created is not known.
CONCLUSION
It seems that although many of these scenarios are highly improbable or hypothetical, the most
dangerous natural threat comes from NEOs at this time. Much more pressing are threats of
human causation, specifically the stockpiling of nuclear arms and the possible atrophy of the
Ozone layer. Before watching the skies in terror we should look to our back yard.
Of course, the only way to really completely ensure the survival of mankind is to disperse it as
far as possible across the universe, colonizing wherever we can. This would mean, barring an
unforeseen event that brought an end to the physical universe, the species would endure in
some way. Although this at the moment is just a far off idea, proactive steps can be taken to
deal with the more probable threats which face us today.

6. i
http://www.nas.nasa.gov/About/Education/Ozone/ozonelayer.html
Andrews, J. (2008). Galactic Diplomacy and Negative Governmental Responses. The Exopolitics Journal ,
207-220.
Columbia University . (2008). Columbia Encyclopedia. Gale Group.
Golante, H. (2005). Biological Effects of Gamma-Ray Bursts: distances for severe damage on the biota. eprint
arXiv:astro-ph/0512013 .
Hildebrand, P. K. (1991). Chicxulub Crater; a possible Cretaceous/Tertiary boundary impact crater on the
Yucatan Peninsula. Geology , 867-871.
J. Binney, S. T. (1987). Galactic Dynamics. New Jersey: Princeton University Press.
Marcus, M. C. (2005). Earth Impact Effects Program. Retrieved April 18th, 2008, from
http://www.spaceref.com/news/viewpr.html?pid=6163
Mellot, L. L. (2004). Did a gamma-ray burst initiate the late Ordovician mass extinction? International Journal of
Astrobiology , 55-61.
Morrow, E. (2001). Solar Sail Orbit Operations at Asteroids. AIAA , 279-286.
N Macleod, P. F. (1997). Cretaceous-Tertiary biotic transition, The. Journal of the Geological Society , 413-419.
Sanders, R. (1970). The Effects of Stellar Collisions in Dense Stellar Systems. Astrophysical Journal , 791-810.
Shoemaker. (2001). Baltimore: American Geophysical Union.
Stix. (1993). Removal of chlorofluorocarbons from the troposphere. IEEE International Conference on Volume
(p. 135). IEEE Conference Record.
Taylor, B. A. (2006). An Introduction to Planetary Defense: A Study of Modern Warfare Applied to Extra-
Terrestrial Invasion. Brown Walker Press.
Than. (2007, May 29). Wandering Black Holes Found to Pack Their Own Lunches. Retrieved April 18, 2008, from
Fox News: http://www.foxnews.com/story/0,2933,275452,00.html
The Spaceguard UK Portal. (n.d.). Retrieved April 19, 2008, from http://www.spaceguarduk.com/
Thomas, M. ( 2006 ). Gamma-ray bursts and terrestrial planetary atmospheres. New Journal of Physics .
United Nations Environment Programme. (2000). The Montreal Protocol on Substances that Deplete The Ozone
Layer. Nairobi, Kenya: UNON.