The Four Horsemen of the Space Apocalypse

Asteroids. Solar storms. Supernova. Gamma-ray bursts. Space is trying to kill us, and it's time to take it seriously.

By Owen Lewis

The terms “existential risk,” “catastrophic risk,” and “risk to civilization” are used too casually, and often incorrectly: generally, as synonyms for “big risk.” This common misuse denudes the words of their meaning and import. At the Cosmopolitan Globalist, we adhere to a formal definition: It is a global catastrophic risk if it might, realistically, kill ten million people on two separate continents, or wreak US$10 trillion in damages. It is a risk to civilization if it threatens organized human life: When Albert Einstein said, “I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones,” that is what he meant. An existential risk—as the root word exist suggests—threatens the extinction of humanity.

An existential risk is necessarily a global catastrophic risk, but not vice-versa. Journalists, politicians, and even—too often—scientists sometimes describe climate change and pandemics as existential risks. They are unlikely to be. Climate change is a global catastrophic risk, to be sure; and the Covid19 pandemic a global catastrophe in progress. But they are not existential threats.

As we’ve observed previously, our minds for some reason have difficulty seriously imagining that we face more than one catastrophic risk at a time. These days, most people worry about climate change, with the occasional pause for pandemic panic. But let me direct your attention upward. I regret to tell you that space could swat H. Sapiens off the interstellar map as casually as we would despatch a fly. Of all the risks we discuss in this series, space is the best candidate for an existential threat. We know this not through computer models and projections—although there are plenty of these—but from the historical and geological record.


Deep Impact

The Earth has seen five mass extinctions. Only one of these may firmly be attributed to an asteroid; the rest were probably caused or assisted by volcanic activity. Volcanoes certainly have world-destroying potential. But probably this would happen over a period of hundreds of thousands of years, even millions. A clever species like ours may well be able to figure out how to mitigate that risk or adapt to it.

Asteroids and comets are a different story.

An asteroid the size of Manhattan could plough through our atmosphere in seconds. It would blow a massive hole in the crust of our planet, vaporizing water and stone alike in a flash, ejecting mountains of rock and debris from the impact site. Some of this material would break the Earth’s gravitational grip and sail out into the Solar System, but most would fall back to the ground. The force of the impact would throw thousands of cubic kilometers of rock into sub-orbital trajectories, and these meteors would return to Earth and set the world’s forests ablaze, devouring animal habitats and vegetation, causing food chains to collapse. The cloud of dust thrown up by the impact, along with smoke and soot from the fires, would blot out the Sun, perhaps for years.

We know this because this is exactly what happened, 66 million years ago, in Mexico’s Yucatán Peninsula. The Chicxulub impactor smashed into the Earth with the force of several billion Hiroshimas and brought the 180-million year reign of the dinosaurs to an end—along with much of the rest of life on earth.                                             

Impacts that big are rare, of course. They happen about once every 100 million years. Smaller collisions occur more frequently, though, and these could be devastating. An asteroid a tenth the size of the Chicxulub impactor could easily cause a global catastrophe, if not a mass extinction. These arrive about once every 100,000 years.

Mid-sized asteroids have left two craters under Greenland’s ice sheet. The Hiawatha impact crater was formed when an iron asteroid about the size of the Lower East Side smashed into the Hiawatha Glacier. A slightly larger asteroid formed the other one. No one’s sure exactly when these events happened: It may have been as long 2.6 million years ago or as recently as 11,700 years ago—right around the end of the last glaciation.

Smaller rocks hit even more often. Hundred-meter-long bad boys make it to Earth about once a millennium. Such an impact wouldn’t cause a global catastrophe, but that would be no consolation for the place it hit. Asteroids between 50 and 80 meters long hit us every few centuries. The Tunguska event was the most recent; the asteroid blew up in the atmosphere over Siberia in 1908, exploding with a thousand times the force of Hiroshima, flattening 2,000 square kilometers of forest. Fifty thousand years ago, a nickel-iron asteroid this size left a crater more than a kilometer wide in Arizona.

Not a global catastrophe, sure, but you wouldn’t want it to fall on your city. And as with so many risks, this one is rising.

That can’t be right, you’re thinking. Why would the risk be rising? It’s rising because there are more of us, and we’re urbanizing rapidly. A thousand years ago, a hundred-meter asteroid couldn’t smash into city of twenty million souls because no such city existed. The population of the world’s largest city, Constantinople, was barely a million, and only Hangzhou and Baghdad rivaled it in size. The odds of a city being wiped out by a mid-sized asteroid were astronomically small, so to speak. By 2030, according to UN projections, 706 cities around the world will be as big as medieval Constantinople, and there will probably be forty megacities of more than ten million people.

Mitigation is possible, fortunately, and it’s at hand. Every night, astronomers around the world search for dangerous rocks. There are programs, several led by NASA, dedicated to the search for dangerous extraterrestrial rocks whose orbits bring them close to Earth, or NEOs, as they’re called, short for near-earth objects. In 2005, NASA was instructed to look for NEOs wider than a kilometer, and they think they’ve found 90 percent of them. But they’ve only fingered 20 or 30 percent of the smaller ones.

But NASA’s new mission just received the go-ahead to launch in 2026. This time they’ll be looking for smaller rocks that previous searches may have missed: The Near-Earth Object Surveyor space telescope is moving into the design phase, and this will rapidly accelerate the pace. NASA hopes that within a decade of the launch, they’ll have found 90 percent of the NEOs that are wider than 140 meters.

So far, so good: None of the NEOs we’ve discovered so far seem to endanger us, thank goodness. If we found one, though, what would we do? It would depend how much time we have. If an asteroid on a collision course with the Earth was spotted months or years in advance, we might be able to send up a spacecraft to nudge it off its trajectory. Even a small push, at the right moment, would send it millions of kilometers away, sending it sailing safety past the Earth. If the timescale was shorter, though, we might be left with no option but to try to nuke it—just like in Deep Impact and Armageddon.

But it might not be so easy to destroy or redirect a rock that’s so close to hitting us. The debris might still be big enough to be dangerous, causing multiple impacts. The debris from a big asteroid might clump back together, even under the weak effects of gravity. We would obviously try, and we’d be right to do so. But it would be much safer to discover and divert an asteroid while it’s still far away.

So we’re going to test this idea this coming fall. The Double Asteroid Redirection Test  mission will deliberately crash into the smaller of a double-asteroid pair, slightly changing its motion. We’ll study this from the  Earth and use this knowledge for future planetary defence.

But we need to do a lot more work. Right now, we’re virtually helpless, with our only option some wild-eyed scheme we got from watching Deep Impact.

Even though truly catastrophic impacts are rare, they can change the course of life on a planet. Without the extinction of the dinosaurs, we wouldn’t be here now, at the top of the planet’s food chain. If we’d like to stay on our perch, we need to do more. Figuring out how to deal with the asteroids is an urgent human priority.

And all of this underscores the urgency of becoming a spacefaring civilization. Such a civilization would be far better equipped to deal with these threats. Asteroid mining would be commonplace, and we’d easily be able to redirect our mining technology to brush off the dangerous ones.


Short Circuit from the Skies

The next hazards from above are geomagnetic storms caused by coronal mass ejections, or CMEs. Often called solar storms, these are different from solar flares, which are sudden explosions of energy from the Sun that release into the solar system a blast of X-rays and other forms of electromagnetic radiation. Solar flares can cause radio blackouts, which may disrupt GPS systems, but by and large, they aren’t a serious problem. They’re absorbed by our atmosphere long before their energy hits the ground.

CMEs, on the other hand, are a different cosmic kettle of fish. They occur when the Sun’s often twisted magnetic fields build up so much energy that a portion of it snaps, often releasing over a billion tons of high energy plasma—composed of charged particles—at millions of kilometers per hour. Horrifying as that sounds, it’s not physically dangerous to us, nor to astronauts in the International Space Station, because the plasma is deflected by our magnetic field.

Our electronics, however, are another story. If the magnetic field of Earth and the CME are aligned just right, they merge. The effect is an energy buildup up in the Earth’s magnetosphere, stretching out like a tail in the direction the CME is moving. If the event is strong enough, eventually the energy can’t be contained. The tail snaps backwards, releasing energy toward us and creating a geomagnetic storm. The incredible amounts of released energy induce strong electrical currents in wires and electronic devices, which can cause a surge to overload electrical grids or even destroy their transformers.  

This risk isn’t theoretical, either. In 1989, the largest solar storm in modern times knocked out power over all of Québec province. The biggest such storm we know about happened in 1859, at the dawn of the electronic age. The auroras from this storm, dubbed the Carrington Event, were so bright that the glow woke up gold miners in the Rocky Mountains. Thinking it must be morning, they began preparing breakfast.

The most modern technology at the time was the telegraph. The storm induced electric currents so strong that telegraph operators received shocks. It set off showers of sparks. It caused several fires. It was so powerful that telegraphs worked even when disconnected from their batteries, which must have spooked more than one operator thoroughly.

An event of that magnitude today would be devastating. Our lives and civilization are now powered, connected, and undergirded by wires and electronics. A storm of that size did occur, in 2012, but missed us by a hair. “If it had hit,” said physicist Daniel Baker of the University of Colorado, “we would still be picking up the pieces.”

Soon after the 2012 storm, Lloyds of London released a report estimating that its impact would have cost the US alone as much as U$ 2.6 trillion. It would have taken us four to ten years to clean up the mess.

Technical estimates place the odds of one hitting us for real at about 12 percent per decade. What’s worse, even though on the whole, stars like our Sun are fairly docile, we’ve recently learned, to our dismay, that every few thousand years, such stars are prone to throwing up superflares—events orders of magnitude larger than the Carrington Event. The risk is very real. Estimates of its probability—and the catastrophe it would represent—are as robust as the climate scenarios causing schoolgirls throughout Europe to wig out completely.

Again, the degree of risk is tied to the pace of technological change: If such a thing happened a thousand years ago, men would have seen a strange aurora in the sky and they would, perhaps, have been seized by the terrified thought that the End was nigh, at last. But it wouldn’t have been. Now? It sure could be, for many of us. There’s no way to keep a global population the size of today’s alive without electronics.

We know how to prevent geomagnetic storms from sending us back to the Middle Ages: Invest in upgrading our power generation and distribution systems to cope with massive surges. Unlike the ultraviolet radiation and X-rays from solar flares, which cross space at the speed of light, CMEs take hours, and in many cases days, to reach the Earth. We’ve got satellites between us and the Sun, so we’ll have at least a few hours of warning to implement countermeasures, such as temporarily shutting down transformers and substations, opening up extra lines to dump excess power, even shutting down much of the grid.

Grid operators today are aware of solar storms and their risks, but the politicians who determine our spending priorities aren’t. We’ve done some preparation, but we need much more. We need to upgrade all of these systems, build more grounding stations to dissipate surges, install more capacitors, and stockpile replacement transformers and other vital pieces of electrical grid infrastructure. It’s inevitable that a large solar storm will hit us. Sooner is more likely than later.

Though there’s some debate, the consensus is that at current levels of preparedness, we wouldn’t come out well. The loss of power in large parts of the world, for months, could easily kill millions. If this happened in the winter in the northern hemisphere, things would be even worse. If it took several years to restore electricity—let alone fix everything else—the human population could be decimated.

We’ve had ample warning. We’re more than capable of mitigating the worst effects. But the issue has taken a back seat to other concerns, particularly climate change. Considering how we’ve coped with a global pandemic—once thought to be a risk for which we were well-prepared—one isn’t filled with confidence.


Stellar Bombs and Pot Shots

Last on the list of ways space is trying to kill us are gamma-ray bursts, or GRBs, and supernovas. Unlike asteroids and solar storms, it’s impossible to protect ourselves against them. It’s just good luck (or by the grace of God) that we’re still around.

Most supernovas occur when a massive star, between eight and fifty times the mass of our Sun, explodes. Our Sun will fuse a few elements on the periodic table, but massive stars gradually fuse heavier and heavier elements until they try to fuse iron and nickel. Unlike lighter elements that produce energy when fused together, however, when they undergo fusion, these heavier elements take more energy than they produce. This disrupts the delicate balance in a star’s core between pressure (supplied by energy generation) pushing outwards, and gravity, which acts to compress the star. As soon as iron and nickel are reached, fusion in the core quickly shuts off. At this point, gravity wins. The outer layers of the star rapidly collapse inward and rebound off the core in a massive explosion. A supernova is born. The shockwave from the star travels outwards at enormous speeds, from 50 to 150 million kilometers per hour, or 5 to 13 percent the speed of light.

The danger to life on Earth depends how close the explosion is. It’s hard to know for sure, but current estimates peg the boundary of the danger zone between forty and a hundred light years away. A nearby supernova would flood us with gamma and X-rays, damaging—or if it was close enough, destroying—our ozone layer. This could have deadly effects on many organisms, but it would be especially lethal to plankton, the base of our food chain. The massive increase in cosmic rays (charged particles) emanating from the supernova would also cause particle showers as they hit our atmosphere, increasing the levels of radiation to which life on the ground would be exposed.

Thankfully, we probably don’t need to worry about a supernova anytime soon. Betelgeuse, a red giant in the constellation of Orion, about 642 light years away, is the nearest star scheduled for explosion. When it goes, which could be anytime from tonight to 100,000 years from now, we’ll be perfectly safe. We’ll be treated to a spectacular show when the stellar detonation causes the star to become bright as the full moon. The closest supernova event that we know of took place between 1.7 and 3.2 million years ago, when a star about 195 light years from us blew up. Many species went extinct, but there was no mass extinction. We don’t know for sure whether ozone degradation and elevated radiation from the supernova were to blame. Perhaps it was something else, like a changing climate. Our planet’s location in the relatively quiet suburbs of the Milky Way has probably spared life on Earth the worst consequences of close calls with supernova.

Supernova, or other high energy phenomenon like the merger or two neutron stars into a black hole, can give birth to yet another deadly threat—gamma ray bursts, or GRBs. These massive beams of high-energy light are the most energetic electromagnetic events we know of, releasing more energy than our Sun will over its entire lifetime. On the bright side, they’re only dangerous if pointed directly at us. But they can be deadly at far greater distances, up to 10,000 light years away. So far, astronomers have never detected one inside the Milky Way. But that doesn’t rule one out. If one hit us from our cosmic backyard, it would burn away the ozone layer, exposing us to deadly solar radiation, and at the very least causing a mass extinction.

GRBs are the only space event on this list from which there is absolutely no way of protecting ourselves, even potentially. Since gamma rays travel through the universe at the speed of light, our first warning would be when it hit us. Betelgeuse, incidentally, will be the wrong sort of supernova to produce a gamma ray burst. GRBs are thought to occur in our galaxy around once every ten millennia, so while chances are pretty good that it will be too far away, or not aimed at us, we really could be fried by one at any time. There could, in fact, be one on its way right now. We just have no way of knowing.


These four horsemen of the space apocalypse—asteroids, solar storms, supernova, and gamma ray bursts—all have the potential to seriously damage our civilization. At worst, they could wipe us out. Many have speculated that disasters like these are the solution to the Fermi Paradox. Perhaps these cosmic cataclysms regularly wipe planets clean of civilizations, or even complex life. Perhaps they’ve left us to ask these questions alone.

Whatever the case, we’ve been strangely fortunate: We haven’t been wiped out. We can take steps now to reduce the risks posed by asteroids and solar storms. We should. We probably don’t need to worry about supernovas anytime soon. And there’s really nothing we can do about gamma ray bursts. As Keith Green sang,   

You just keep doing your best  

And pray that it’s blessed

And Jesus takes care of the rest

Against some of the monsters in the dark, there’s precious little we can do but pray and get on with our lives.

Owen Lewis (@is_fusion) is a science and space enthusiast and Christian humanist with degrees in geology and psychology. He is the founder of Fusion is the Future.


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