At the Mercy of the Heavens

The cosmos is interesting rather than perfect, and everything is not part of some greater plan, nor is all necessarily under control. ~ Starhawk
At 7:17 am local time on June 30, 1908, a column of blue light as bright as the Sun split the sky of a remote area of Tunguska, Siberia. Ten minutes later the flash of blinding light was followed by a boom sounding like artillery fire. Windows were blown out of buildings and people were literally knocked off their feet over an area of hundreds of kilometers around. Across Europe and Asia, shock waves were felt in some areas that had the equivalent strength of a Richter five earthquake. For the next few days, night skies across Europe and Asia were lit with an eerie glow. What happened on that fateful morning?
Due to the remoteness of the event, there were few eyewitnesses and all accounts are from people that were removed from the site recalling only the sound and light show. Again due to the remoteness and the fact that Russia was in the midst of political turmoil, no expedition was undertaken until almost twenty years later in 1927. When the expedition arrived they looked out at a still devastated area looking as though there had been a massive explosion. They saw a zone about eight kilometers in diameter with scorched trees that were knocked down in a direction from what looked to be an epicenter of an explosive event. However, no crater was seen; today some scientists claim to have detected an impact crater, but it is not universally accepted. Later in 1960, it was established that the actual zone of leveled forest covered an area of 2,150 square kilometers.
Today we know that there was an explosion of something above the earth, possibly a meteor or more likely a comet. It had an estimated energy of the largest nuclear explosion ever at around ten to fifteen megatons of TNT, which is around one thousand times the energy released in the explosion of the bomb over Hiroshima.
If the Tunguska event had happened over a populated area, the history of the world may have been dramatically changed. If one of the world powers at the time had been struck in a population centre in Europe, World War I may never have happened. Then again, with a weakened opposition, another world power may have taken advantage of the situation and World War I may have happened before 1914. If whatever had happened in Tunguska had happened in the ocean, mega-tsunamis could have devastated coastlines around the world.
The Tunguska event has been hypothesized by some science fiction authors as being the explosion of such an invading alien craft. In Stanislaw Lem’s The Astronauts, the Tunguska event is explained as a crash of an interplanetary reconnaissance vessel from Venus. Soviet engineer Alexander Kazantsev theorized that the Tunguska event was caused by an alien craft. His theory on the origin of the explosive event was fictionalized in his short story, "A Visitor from Outer Space." He saw the Tunguska event as a crash of a nuclear powered Martian spacecraft seeking fresh water from Lake Baikal. Science fiction author Julian May’s Intervention depicts an out of control spacecraft as causing the Tunguska event.
Whatever the origin, the event should remind us all that in spite of our protective atmosphere that we can still fall victim to some event that happens in our skies, in some cases where origins are light years distant. We really are at the mercy of the heavens. A meteor or comet explosion is but one possibility. 
The universe around us, in spite of the apparent serenity that you see in the night sky on a clear summer evening, is an extremely violent place. Meteors and comets aside, there are black holes with a gravitational maw that will not even let light escape, supernova explosions that will appear brighter than an entire galaxy, gamma ray bursts which defy a good explanation of their massive energy output, neutron stars that exert magnetic fields millions of times greater than that of Earth and possibly alien civilizations that may not have peaceful intentions.

Alien Invasion
Science fiction authors have looked at alien invasions beyond the Tunguska event. In fact, one of the first true science fiction novels, written by H. G. Wells, War of the Worlds, depicts a Martian invasion. Wells shows the invader not being defeated by human technology, but by the forces of Earth microbes, benign to humans however fatal to the invaders. Wells’ novel may be the most famous of the alien invaders genre but, in spite of it being written in 1898, it was hardly the first.
French writer Voltaire, in 1752, wrote Micromegas, which has giant aliens from Sirius and Saturn. Their mission to Earth is out of curiosity only. Due to the differences between themselves and humans, initially they think that the Earth is uninhabited due to their differences in scale.
Jonathan Swift, in his 1772 Gulliver’s Travels, has in its pages a race of humanoid alien beings that live on a floating island called Laputa that shadows the Earth preventing rain and sun from reaching the surface unless tribute is paid to them.
In 1892, English clergyman, Robert Potter, in his The Germ Growers, describes a covert invasion in which aliens, who appear like humans, develop a virulent disease to assist them in their conquest of Earth.
Many modern novelists envisage a scenario similar to that of Wells with an actual invasion of highly advanced beings with a technological superiority to humans. Most Hollywood movies also see alien invasions this way. One modern classic alien invasion novel is by Larry Niven and Jerry Pournelle entitled Footfall. The aliens in this novel appear as human-sized elephants with multiple trunks. The culture of the aliens known as Fithp is well thought out; this is what separates it from many other alien invasion novels in which the aliens could be easily exchanged, by just changing the setting, with terrorists, enemy agents or even Indians in an old western.
The Fithp are herding beings and their concept of warfare and conflict is very different from that of humans; when two herds meet one will prevail over the other and be absorbed into the herd. When humans attempt to negotiate a peace it only confuses the invaders.
How likely is an alien invasion? We can never know until it actually happens, though there are some people that feel it is a distinct possibility. Stephen Hawking, the world famous physicist, who has altered our way of looking at the universe around us, feels that we should not advertise our planet through programs such as SETI or the Voyager spacecraft.
In fact, if we believe some science fiction authors, an alien invasion may already be happening. Some authors have looked at alien invasions as being more clandestine. Jack Finney’s Invasion of the Body Snatchers, William Tenn’s short story "The Liberation of the Earth," Robert Heinlein’s The Puppet Masters and John Campbell Jr.’s short story "Who Goes There?" disguised the paranoia of Cold War America as science fiction. John Wyndham in The Midwich Cuckoos has humanoid aliens but they come in the form not of adults, but of children.
Some authors have looked to the past as a possible setting for an alien invasion. Michael Flynn in Eifelheim wrote of a peaceful invasion in Europe during the scourge of the Black Death.
Poul Anderson in his The High Crusade depicts technologically advanced aliens that invade during the Middle Ages. Unfortunately for the overconfident invaders, they realize too late that they are vulnerable to the relatively primitive weaponry of the day allowing the humans to take over the invaders’ vessel.
The ever innovative Arthur C. Clarke, in his classic Childhood’s End, dealt with an alien invasion in a very unique manner. It was a peaceful invasion in which the citizens are assimilated into the next stage of human evolution.

Dark Nebula
A lesser known fact about the famous astronomer Fred Hoyle is that he was also a science fiction author. In perhaps his most famous novel, The Black Cloud, he envisioned a world which is threatened by an approaching dark cloud of gas. The cloud of gas turns out to be an intelligent being that is just as surprised to find that there is intelligence that can exist on a solid planet as the characters in the novel are that there is life in outer space. As unlikely as such as doomsday event may be when compared with other doomsday scenarios in terms of probability, it is not zero.
If Earth were to move, in its endless cycles of the galaxy-every 230 million years, the solar system makes an entire traverse around the Milky Way- into the path of a dark nebula, the world would be end in eternal night.
Dark nebulae are a type of interstellar cloud that is so dense that light from background emissions is obscured. There is one example that you can see in most astronomy textbooks, the Horsehead Nebula, which is found about 1500 light years from the Earth. The light is obscured by the fact that a dark nebula is made up of submicrometre dust particles which effectively block light transmission. Much of the Sun’s life-giving radiation would effectively be blocked from reaching the Earth if we ever found ourselves in a dark nebula.
We can take solace, however, if our planet ever should wander into such a cloud of eternal darkness, to realize that it is the true birthplace of future stars. Out of the destruction of one civilization will perhaps spring a new one.

Solar Flares
We can look up into the day sky and we can often see the Sun. Even in its visual absence due to a cloudy day, we will feel its evidence in the form of the life-giving warmth that it provides. Its steady light helps our plants grow which, in turn, feed an entire food chain. Only the life at the deep sea vents of the Earth is not dependent on the stability of our stellar partner. However, all is not so peaceful on the Sun.
Several times every day, the Sun emits solar flares which are essentially large magnetic outbursts containing high speed particles. Radiation across the entire electromagnetic spectrum accompanies the particle burst, especially in the invisible ranges such as radio and higher energy X and gamma rays. Flares reach a peak about every eleven years, related with the sunspot cycle.
The flares can affect our communications satellites upon which civilization is becoming more and more dependent, used in everything from Global Positioning System devices to television. Shielding is provided on satellites to limit the impact. Here on Earth, we are protected by our atmosphere and the Earth’s magnetic field.
We see everyday evidence of the solar flares from the earth’s surface. When the particles interact with the earth’s magnetic field we can see, especially in the northern and southern polar regions the Aurora Borealis and Aurora Australis. Possibly the most powerful flare on record happened on September 1, 1859. Not only was the site visible as white light, it also produced aurora-like effects as far down as the tropics and even set telegraph systems ablaze. Imagine if this had happened in 2012.
Periodically there are massive flares that can knock out electrical grids even for days at a time. They are also a concern for present and future space travelers. A massive flare could not only disable a spacecraft’s electronics but would expose future astronauts to lethal levels of radiation.
What if the flares became even bigger and were able to penetrate our atmosphere and magnetic field? It is not out of the question either when we look at other sun-like stars. It is well known that there are sun-like stars where there are flares that are around twenty times that of the most massive flares seen emitted from our Sun. Such superflares are estimated to be millions of times more powerful than a common solar flare. If one of these superflares were to be ejected by our Sun, it would literally fry the Earth. We would be helpless if we found ourselves in the path of one of them.
Larry Niven's short story "Inconstant Moon" details a scenario where a giant solar flare wreaks havoc on civilization. Arthur C. Clarke, in his "The Wind from the Sun," offers a unique perspective about the effect of a solar flare on a solar wind “sailing race.”

False Vacuum
How about a really bizarre demise for the Earth? According to a leading cosmological theory on the formation of the universe, it began as a false vacuum of empty space filled with energy. The model shows that our universe is part of a multiverse that has not decayed to its ground state or a true vacuum. The universe that we live in was created as a result of a bubble of lower energy that formed out of the larger false vacuum of the multiverse.
Even though our universe is currently at a lower energy state, it is still not a true vacuum. Therefore, it is not inconceivable that it could strive for a lower energy level. If a low energy bubble nucleates in the false vacuum of our present universe, it would expand at the speed of light, changing our universe again. If Earth were caught in the path of such a bubble nucleation, it would be instantaneously destroyed. It is probably fortuitous that we would not even be aware it was happening. All that we know would be gone in an instant. 
As bizarre as such an event appears to be, science fiction has looked at its impact as a theoretical doomsday event. Geoffrey Landis in his short story "Vacuum States," Stephen Baxter in "Time" and Greg Egan in "Schild’s Ladder" all explored the possibility of a false vacuum event as being the ultimate doomsday scenario.
In Schild’s "Ladder" a physicist works to test the fundamental equations of Quantum Graph Theory which states that reality is made up of a complex of mathematical graphs. During the experiment, he creates a bubble of something that is more stable than ordinary vacuum. As a result, it expands at the speed of light, threatening worlds in its path.
Maybe we do not have to worry so much about this particular scenario though. In the Many-Worlds Interpretation of quantum mechanics, the universe cannot end this way. In that interpretation, every time the false vacuum collapses to a real vacuum, the universe will split into several new worlds. In some of those worlds the universe decays and in others it continues as if nothing happened.
Then again, it is only an interpretation.
Black Holes
Physicists Albert Jackson and Michael Ryan of the University of Texas proposed that the Tunguska Event was the result of a black hole passing through the Earth. One of the major weaknesses of the theory is that there is no exit event.
Black holes were first proposed independently as a theoretical concept by French mathematician Pierre Simon Laplace in 1796 and by English geologist John Mitchell in 1783 as objects where the gravitational field was so great that light could not escape. It really did not gain any traction as anything more than a mathematical anomaly because the best minds of the day felt that there was no way that gravity could have any effect on a mass-less unit as a wave of light.
With Einstein’s Theory of General Relativity, first proposed in 1915, a whole new world of the cosmos opened up. Light, indeed, was affected by gravity. Within a few months of Einstein’s theory, German physicist, Karl Schwartzschild, in 1916, found, with the first modern solution of general relativity, that there can be regions of space from which nothing can escape including light
How are black holes created? The favored hypothesis is that they arise from the collapse of a massive star when its nuclear fuel runs out. With no radiation pressure to counterbalance the inward drag of gravity, the star will collapse in on itself creating a black hole. These types of black holes are predictable and easily detected.
Far more sinister are the mini black holes which arose in the conditions that were found when the universe was formed. They have been wandering the cosmos ever since. What would happen if one of these mini black holes were to enter our Earth? What if scientists who try to recreate the conditions of the birth of the universe created a mini black hole? It was a genuine fear when the new CERN large hadron accelerator was put on line in 2008.
 Science fiction authors have looked at the possibility of such mini black holes being artificially constructed. David Brin in his epic Earth describes an artificially created mini black hole that falls to the Earth’s core and subsequent attempts to retrieve it before the Earth is destroyed. In John Craig Wheeler’s technothriller The Krone Experiment a series of artificially created mini black holes threaten the Earth.
Larry Niven saw in the mini black hole a potential weapon. In his short story "The Hole Man" the antagonist commits murder with a mini black hole. In another short story, "The Borderland of Sol," space pirates use mini black holes as weapons. 
John Varley took the concept beyond Earth in The Black Hole Passes. Here a mini black hole threatens two deep space outposts.
Impact Event
Another theory to explain the Tunguska event is that it was a midair explosion of an extraterrestrial body such as a comet, meteorite or asteroid. Some scientists also believe that there was an actual impact event. Regardless of the origin or whether it was an impact or midair explosion, finding ourselves directly in the path of an extraterrestrial body is one of the more probable doomsday scenarios. When the Shoemaker-Levy comet slammed right into Jupiter between July 16 and 22, 1994, the human race was witness to an actual impact event. Even on a world the size of Jupiter it was a spectacular event. If it had happened on Earth, it would have had a devastating effect on our civilization, more likely ending it entirely.
An object ten kilometers in diameter last struck the Earth 65 million years ago creating the Chicxulub Crater near the modern day Yucatan Peninsula and more importantly, causing the extinction of the dinosaurs. This opened the opportunity for mammals, including humans, to evolve. 
A lesser known impact event but one that paled the dinosaur extinction was the Permian-Triassic extinction that happened approximately 250 million years ago. Ninety percent of the species of that time were exterminated.
Our Moon, a fixture in our night sky, was not always there. It has been theorized that it arose as a result of an impact event between Earth and a Mars-sized body billions of years ago.
Though impact events are highly destructive, they are also seen as givers of life. It has been theorized that an impact event or events may have been responsible for providing the water that fill our oceans. In another theory known as panspermia, it has been suggested that the building blocks of life may also have come from an extraterrestrial object.
What would happen if we were ever struck by a large extraterrestrial object today? Even before the event, what would be the effect be on society? If you knew that an object that would destroy civilization was going to hit the Earth in eight months, how would you and how would society react?
Even though nothing appears destined to impact with the Earth right now, we should not be complacent. A star or other large mass passing our solar system may cause a disturbance of the objects in the gravitational dance of the solar system. Even a small disturbance may be enough to move a large object onto a collision course with the Earth.
Science fiction authors have embraced this doomsday scenario. Arthur C. Clarke, in his classic Rendezvous with Rama, begins with the impact of an object in northern Italy which causes extensive damage. It also prompts in the novel the creation of the Spaceguard Program, which in turn, discovers the mysterious Rama spacecraft. In 1992 when the United States Congress authorized NASA to be responsible to find potential Earth-impacting asteroids, they called it Spaceguard, drawing their inspiration for the name from Clarke’s novel.
The novel also provided the seed for a subsequent Clarke novel, Hammer of God, which is about Earth’s efforts to divert an asteroid on a collision course with the Earth. Other authors have also looked at the impact of an imminent extraterrestrial collision with Earth. Gregory Benford and William Rostler in Shiva Descending look at the events leading up to the impact event. John Gribbin and Marcus Chown also wrote of a comet on a trajectory with the Earth in Double Planet
Larry Niven and Jerry Pournelle teamed up again to create Lucifer’s Hammer which took an impact event a step further. In addition to dealing with the events leading up to the collision, it follows a cast of characters in the aftermath of the impact event. Jack McDevitt took a slight detour from a direct hit on Earth by an extraterrestrial body and wrote Moonfall about an impact event on our nearest cosmic neighbour, the Moon.
Robert Heinlein looked at meteors as weapons. In his novel The Moon is a Harsh Mistress, lunar rebels launch rock filled shipping containers at the Earth. In Starship Troopers, it is the alien Bugs that launch meteor missiles at the Earth.

Gamma Ray Bursts
Perhaps an even scarier scenario than an impact event is a gamma ray burst. It is hard to imagine that something thousands of light years away could actually destroy the biosphere of Earth. A long time to await your eventual doom. How would society react if we learned such a burst was headed our way? Would we strive for the stars to escape our doomed world, spiral into a societal breakdown or just do nothing at all?
Gamma ray bursts were only discovered in the late 1960’s by United States Vela satellites which were initially launched to detect gamma ray pulses from nuclear weapons. On July 2, 1967, the satellites detected a flash of gamma radiation unlike anything that would be the signature of a nuclear weapon detonation. Scientists from Los Alamos were quickly recruited to find the source. The conclusion: Extraterrestrial in origin.
We have come a long way since that initial discovery, but still there is no model that is agreed upon to explain the source of these intense energy bursts. Gamma ray bursts are classified into either short or long term bursts. A long term burst is one that lasts two seconds or more and is most often found in the vicinity of stellar formation. A short term burst is naturally one that is less than two seconds in duration.
It is speculated that a long burst is the result of some aspect of stellar formation, since they are often found in areas of stellar formation.
A short burst is far more mysterious. The best theory is that they arise out of the merger of two neutron stars or a neutron star colliding with a black hole. A neutron star is the result of a supernova event where a massive star has collapsed to a diameter of around twelve kilometers; in effect, the entire mass of a star collapsed into the size of small city.
A gamma ray burst thousands of light years away headed in our direction would have a devastating impact on our planet. Scary to think that it has probably happened before too. The Ordovician Extinction Event which happened about 450 million years ago, in which sixty percent of species died out, is speculated to have happened as a result of the Earth being in the path of gamma ray burst.
If we found ourselves in the path of such an energy burst, it would rip apart the ozone layer exposing us to lethal cosmic and solar radiation. Plants and animals would start to die off as a result of this disruption of the atmosphere. If that were not enough, the molecular nitrogen that makes up a majority of our atmosphere would be converted into nitrous oxides which are a major component of photochemical smog. Quite a burden for any survivors.
One possible source of gamma ray bursts, at least the long ones, appear to be related to stellar formation such as a supernova event which provides the raw materials for future stellar formation. Supernovae are classified by the spectral absorption lines of different elements. If we see a hydrogen line in the spectrum, it is a Type II supernova. Otherwise it is a Type 1 supernova. Within the types, there are further subcategories based on the presence of other absorption lines.
A Type II supernova event is fairly predictable. In all stars both massive and smaller ones, as the hydrogen at the stellar core is fused into helium, the resultant thermal energy creates an outward pressure that holds the star in an equilibrium with the inward pull of gravity. Once the outward radiation pressure is gone as the star exhausts its fuel (there are several more fusions in larger stars that happen thus creating heavier atoms, before the star has only a nickel-iron core in which further fusion is impossible), the star can only support its outer shell through the energy created by electron degeneration in the core. If the star is sufficiently massive, at least nine times as massive as our Sun, the balance will fail and it does so in an explosive way. The core suddenly implodes in on itself while the stellar material is expelled at speeds up to ten percent of the speed of light driving a shock wave into interstellar space. The light given off by such an event can outshine an entire galaxy. In addition to a brilliant light show, there is also lethal radiation being released including gamma rays. Fortunately, there are no such stars in our cosmic vicinity and unlikely to present a threat.
Before we become complacent however, there is another more sinister way that supernovae could happen. A white dwarf in a binary star system could set off a Type 1a supernova. A white dwarf is an earth-sized star that is the final stage in the life of a star with a mass similar to that of our Sun. They are very common in our galaxy. Since there is no longer any nuclear reaction in a white dwarf, only the electron degeneration in the core holds its overlying plasma at bay.
In a binary system, it is possible for the white dwarf to accumulate enough material from its stellar neighbour to set off events that produce a supernova event. It does this by accumulating material that eventually exceeds the ability of electron degeneracy of the white dwarf’s core to support its weight thus commencing the implosion process.
In Supernova by Roger Allen and Eric Kotani, about an exploding star that threatens the Earth. For the real hard science fiction fan, there is an actual Hertzsprung-Russell Diagram which is a diagram used by astronomers to describe stars by relating luminosity to temperature.
Alastair Reynolds, in his short story "Angel of Ashes," discusses a nearby supernova that is just asymmetric enough that its radiation misses the inner solar system and spares life on Earth, thus spawning a new religion.
In a similar vein, Arthur C. Clarke wrote a short story called "The Star" from an extraterrestrial perspective. Future space travelers are present to witness a star in a distant solar system goes supernova destroying a civilization. The stellar explosion that was being witnessed was the Star of Bethlehem, the symbol of Jesus’ divine birth. Out of death comes a new life.
Charles Sheffield wrote Aftermath which is about Alpha Centauri going supernova, even though it is not supposed to according to our best theories. The novel, however, offers a really good description about the electromagnetic pulse from the exploding star plays havoc on our electronic civilization.

Even though it happened over one hundred years ago, the Tunguska Event still serves as a potent reminder of what a violent universe we live in. Perhaps the memory will fade with time into obscurity, but fate befalls those that forget. Science fiction is providing us with entertaining tales that also serve as a reminder that there is a broader universe out there, one that we should be aware of and also to prepare for a future in which one of these “unlikely” events befalls us.

Further Reading
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