Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)

Chapter 6. Alien Attack!

MINDLESSLY—AT LEAST, LACKING WHAT WE WOULD call a mind as we know it—it examined the bright light of the star ahead of it. Employing a highly sophisticated complex of observational instrumentation, it patiently took data, examining each bit of information as it came in. After weeks of steadily staring at its target, the results were in.

The star was orbited by several gas giants. Each of these had icy moons with possible water under the surface. The star also had not just one but three smaller planets with the potential for liquid water as well. And on the second one of these out from the star were unmistakable signs of biotic life—free O2 in the atmosphere at large levels of disequilibrium. If it had been equipped with emotions it would have whooped with joy. Instead, it silently and efficiently began preparing for the next phase of its mission.

Using sophisticated engineering and technology the probe began to slow its approach to the solar system. Its fantastic speed—nearly that of light itself—gradually bled away over the course of nearly a year. Course corrections were made, angling the probe this way and that. All the while, it took observations, scouting for the target it needed. Finally, the target was acquired: a metallic asteroid over a mile wide. As the probe passed the asteroid, aiming carefully, it released a small package just a few meters across.

The package was a probe in its own right, and it used its onboard rocket to decelerate further and land on the surface of the asteroid. It immediately sent an “all clear” signal to the mothership, which did not respond, but instead sharply accelerated away, heading off to the next star on its list, a star it would not reach for decades.

On the asteroid’s surface, a hatch opened on the probe, and a small spider emerged . . . then another, and another. In all, a dozen such robots started crawling over the landscape. Composed of a sophisticated amalgam of metal, ceramics, and spun carbon fiber, they went to work: digging, smelting, manufacturing. They worked without fatigue, without emotion, tirelessly day and night (such as there was on the slowly rotating asteroid). After a month they were ready.

Like a fungus expelling spores, the asteroid erupted in thousands of tiny explosions. Each puff imparted velocity to a ball of metal a meter across, each of which headed toward one of the planets and moons initially targeted by the interstellar probe. Inside each ball were over a hundred of the spiders. The robotic arachnoids were possessed of sophisticated programming, but in the end the goal was simple: convert any and all available materials into more spiders. When enough were manufactured, build more mothership probes. Launch them, and repeat the cycle.

They needed metal of almost any sort, and what they couldn’t find they could create. Their programming was very sophisticated, honed over millions of years of such missions. And they weren’t picky about materials; almost anything would do. Each spider could create the components needed to replicate itself in just a few hours, and these would then move on to replicate themselves as well. Once the first spiders touched down, they could cover a planet in just a few days, converting everything—everything—into more spiders and more probes.

Being smaller and closer, the surface of Mars was destroyed first. The rocks were rich in iron, which made things easier. Within hours, that many more spiders went off in search of raw material.


Earth fell within days. The first spiders landed in Australia and consumed everything in their sight. Rock, metal, gas; all could be converted if needed. Water, plants, flesh—these would do as well. Humans never had a chance. Though the intense light of the interstellar probe’s engine had been tracked by Earthbound telescopes for months, it didn’t answer any hails, and there wasn’t enough time for any of the governments to react anyway. By the time the spiders landed it was already far too late. They swept over the planet, and after less than two weeks there were in essence no living creatures left on Earth. The entire surface of the globe had been converted into robotic factories. Within a year, bright flashes blossomed over the planet as more interstellar probes were launched, each an exact replica of that first one—which itself was generations removed from the first probe launched so many eons before. That first probe was long since dead, having expended all its packages. It was no longer useful. But its progeny “lived” on, sweeping across the galaxy.

And now, several thousand more headed out into deep space. When the original mission started, mankind didn’t exist; only hominids ambled across the African plains. Their descendants ruled the Earth, but their reign was brief. All those billions were now gone, converted into hordes of little metal spiders and more interstellar probes.

Man’s dream of reaching the stars was finally achieved, but not quite in the manner in which he thought.


No matter where you go on the Earth, you find life.

On the plains, on mountaintops, high in the air, down to the deepest ocean depths, life abounds. Even far underground, microscopic life has adapted to conditions we would consider lethal. Life is everywhere.

Earth seems marvelously tuned to support life, but that’s an illusion: we are the ones who are in fact tuned by evolution, as are all the other forms of life on, below, and above the Earth’s surface. As the Earth has changed over the eons, so has life. It seems almost inevitable that, once life first got its start on Earth, it would flourish.

We know there are other planets in our solar system, and even orbiting other stars. If life is so plentiful here, it stands to reason it may also be on those other worlds. They may teem with simple microorganisms, and it’s possible that there could also be more complex forms of life in space, things that we would recognize as intelligent.

If that were so, what would they think of us? Would they be a threat to us?

To understand that, we’ll start by taking a short journey backward in time—though my definition of “short” may differ a bit from yours.


Some 4.6 billion years ago, you were spread out over countless of cubic miles of space.

So was I. So was the book you’re holding now, and the clothes you’re wearing, and everyone and everything you’ve ever known, ever seen, ever touched, ever dreamt about. All your atoms were part of a vast disk, tens of billions of miles across and a million miles thick. The disk was almost entirely made up of hydrogen and helium, but it also contained scattered impurities of zinc, iron, calcium, phosphorus, and dozens of other elements. It rotated slowly, held together by its own gravity and the gravity of the lumpy swell at the center.

Over millions of years matter accumulated in the center of the disk, gravity pulling in ever more material. As it compressed it got hotter, and eventually the core temperature reached 27 million degrees Fahrenheit. Hydrogen fusion triggered, and it was at that moment that our Sun became a real star. Light flooded out, followed by a wave of subatomic particles, a nascent solar wind.

In the meantime, the outer parts of the disk were busy accumulating matter as well. At first clumps only stuck together because of chemical processes. Ice crystals formed farther out from the Sun, where temperatures were low. Chunks of silicates smacked into each other and stuck. Over time, as the aggregations grew, so did their mass, and so did their gravity. These planetesimals started actively pulling in more matter in a runaway process—more mass, more gravity, more matter, more mass, and so on—that was finally only quenched when there was no more material to accrete. Some of these new planets were small, some large. Some decent-sized ones were ejected out of the system entirely when they passed too close to the larger ones.


Self-portrait, 4.6 billion years BC. This illustration shows what our solar system looked like when it was young. A disk of rock, gas, ices, and metals revolved around the newly born Sun, just starting to form the planets we know today.


The ones that survived all had rocky cores and thick atmospheres. Some had atmospheres thousands of miles deep, and no real surface to speak of. Others were smaller, with dense atmospheres to be sure. They also had molten surfaces, heat left over from the formation process.

When the Sun in the center turned on, its fierce light and solar wind hit the new planets. The pressure from the light and the ejected matter slammed into the thinned disk, blowing away the leftover detritus, clearing the space. Eventually, all that was left was a handful of planets, billions of asteroids, and trillions of icy comets, all orbiting a young, hot star.57

Our solar system was born.

It looked different then! Jupiter was farther out from the Sun than it is now, while Saturn, Uranus, and Neptune were closer in. Their mutual dance of gravity would eventually migrate them to their current distances. In the inner solar system, Mercury, Venus, Earth, and Mars all had thick, soupy atmospheres. Over time, as the inner planets solidified, they too would change. Mercury, so close to the Sun and with such low gravity, would have its atmosphere stripped away. Also, the tiny planet’s lack of a magnetic field left it open to the full brunt of the Sun’s solar wind, which aided in tearing Mercury’s atmosphere away atom by atom.

Venus would eventually lose its hydrogen and helium, but chemical processes over billions of years, including a runaway greenhouse effect, would give it a thick atmosphere of carbon dioxide. Trapping the heat from the Sun, the planet would become a forbidding desert of kilnlike heat. The surface rocks are always just a hairbreadth from being molten.

Earth too had a dense atmosphere, nothing at all like today’s—it looked more like Jupiter or Saturn back then, consisting mostly of hydrogen and helium left over from the disk from which it formed. At its distance from the Sun, incoming heat (plus the heat coming from the surface) puffed up the thick air, in a delicate balance with gravity. Over millions of years, the lighter elements were lost, leaving behind an atmosphere of carbon dioxide, water vapor, carbon monoxide, ammonia, methane, and other noxious gases, most of which leaked out from inside the Earth.

Eventually the surface cooled, forming a thick crust over the molten, semiplastic mantle of rock. The heavy elements like iron, iridium, and uranium sank to the center. The radioactive elements decayed, generating heat, adding to the heat trapped inside left over from the formation of the planet. Convection currents began, a magnetic dynamo ensued, and Earth became protected from the ravages of the solar wind.

Not that the young Earth was safe from threats from space. A lot of the material in the solar disk was swept up by the planets, but not all of it. A huge repository of asteroids still roamed the system, and their paths would sometimes intersect those of the planets. Shortly after the planets formed, they were all bombarded mercilessly. Nearly every solid surface in the solar system bears witness to this devastation; a quick glance at the heavily battered and cratered surface of the Moon will confirm it.

The Earth bore the brunt of its share of collisions too. It was hit significantly more than the Moon was, being bigger and having stronger gravity—in fact, the most commonly accepted theory on the formation of the Moon itself is that it coalesced from material ejected when the Earth was hit by a very large object, perhaps as big as Mars, an apocalyptic collision that is terrifying to imagine. But over billions of years plate tectonics and erosion have wiped out all the evidence of this early bombardment. Only the most recent craters are still around; even ones older than a few million years are nearly invisible. Still, this early pelting from space created an immensely hostile environment. Anytime things began to settle down, some fifty-mile-wide rock would come crashing down, resetting the geological clock.

Eventually, though, the rain of iron and rock ceased. As the Earth cooled, more complex molecules could form. The methane in Earth’s atmosphere was a source of hydrogen, as was ammonia, which also had nitrogen. Carbon dioxide provided the carbon, which, when liberated from the oxygen, could combine to form ever more complex chains of atoms. Amino acids—the building blocks of proteins—probably formed quite early, and started combining in new and interesting ways. Lightning from the atmosphere and ultraviolet light from the Sun may have provided energy needed to break up and reform the molecules. At some point—no one knows exactly when or how, but it was virtually simultaneous with the cessation of the bombardment from asteroids—the molecules formed into a pattern that had a fantastic property: it could reproduce itself. As things stand today, this molecule was probably incredibly simple, but it still possessed the amazing ability to gather up raw materials and construct them in such a way as to reproduce a copy. These then went forth and multiplied.

It was hardly more than a simple chemical reaction, or really a long series of them. These reactions needed materials—the elements found in the air and surface—and emitted waste products. One of these waste products was oxygen. As oxygen built up in the air, the chemical processes started to change. Oxygen, to many of these very simple microbes, was toxic, as waste products sometimes are to the organisms that produce them. As the gas built up, it poisoned them. Some species of microbes adapted to the new environment (and their descendants still live today as blue-green algae and other forms of life), but those that couldn’t perished; they had thrived on Earth for millions of years, but their own waste killed them.58 However, a slightly different complex molecule was around at the same time. It was able to use the waste. Oxygen, when combined with other chemicals, can release a lot of energy, which in turn can be useful for reproduction and increased metabolism. This different microbe fed off the waste of the others, and when the oxygen levels got high enough that the first life-forms started dying, the oxygen-users were ready for the coup. They took over. Some oxygen-producers survived, mutating and adapting all the time as some fit into the environment better than others. Oxygen-users that were more adept at using the fuel flourished; others died off.59

Asteroid impacts, vast solar flares, and the random nearby supernova may have wiped out this process or culled it back to near-extinction levels many times over the millions of years the scenario played out, but eventually a toehold (or a pseudopod hold) too firm to shake off was established.

Earth became alive.

Now, this tale is but one way life may have arisen on Earth. We don’t know for sure how it happened. We’re not even sure where it happened: land, sea, air, in the deep ocean . . . or even on Earth. Our planet is only one of several where, in the early life of the solar system, conditions were ripe for the development of life.


Mars, though, was smaller than the Earth, and farther from the Sun. It cooled more rapidly than the Earth did, and may have gone through the same series of events, but on a shorter time scale. We don’t know this for a fact, of course, but it’s certainly possible that Mars had a thriving microbial ecosystem long before the Earth did. Unfortunately, it was doomed. Once Mars cooled down enough, it lost any magnetic field it might have had, so it became victim to the Sun’s solar wind. Its weaker gravity allowed almost its entire atmosphere to leak away to space, and now the pressure on the surface is a miserable 1 percent of Earth’s, thinner than at the top of Mount Everest.

Robotic explorers sent to Mars have shown us a clear history of a watery surface, however. Chemicals in the ground and rippled wave patterns indicate that in the past there were vast floods, perhaps as frozen water underground was heated by volcanism or impacts. There are also indications of ancient lakes, now desiccated, as big as the Great Lakes in the United States. Even today there are (still controversial) hints of transient, short-lived events where liquid water flows on the surface . . . only to quickly evaporate into the thin air.


The Mars rover Opportunity took this snapshot of the Red Planet on March 1, 2004. The presence of sulfates and other chemicals in the rocks indicates that water once flowed over the surface of Mars.


But four billion years ago it was a different story. Did ancient Martian oceans teem with simple life-forms, bacteria, protozoans? We don’t know, and we may never know. Still, future probes may yet find fossils in ancient Martian rocks.

But suppose for a moment that Mars once was alive. What does that have to do with Earth?

In 1984, an unusual meteorite was found in the Allan Hills region of Antarctica. Meteorites are relatively easy to find there; on the ice, a black rock sticks out pretty well. The dry air preserves the meteorites well too.

This particular meteorite, dubbed ALH84001, packed a huge surprise. The chemical composition of the rock itself was very similar to the known composition of the rocks on the surface of Mars. Inside it were small bubbles of gas, trapped within the rock when it formed. When the ratios of the various elements of the gases were measured, they matched the ratios found in the Martian atmosphere! Scientists checked this against the chemicals in other planets as well, but no other gas ratio in the solar system matched nearly as well as that of Mars.

Clearly, ALH84001 was an interplanetary interloper, a rock from the Red Planet. It wasn’t even the first discovered; many were found before their now more famous cousin. How these rocks got here is a matter of some irony.

The surface of Mars bears the marks of a violent history. Like every other planet in the solar system, Mars has been bombarded over its life by asteroids and comets. In contrast to Earth, with its thick atmosphere and watery surface, erosion works more slowly on Mars, so we still see the craters, scars from the ancient impacts.

When such a collision occurs, rock from the surface is launched into the air, of course. But some of that material can be given so much energy that it can actually leave the planet altogether and fly into space. From various studies of ALH84001, it has been found that it formed very early in the history of the solar system, cooling from volcanic lava some 4.5 billion years ago. It sat on or below the Martian surface for almost all the time since, and then suffered a terrible blow. An asteroid impact on Mars hurled the rock into space. It orbited the Sun for at least 16 million years, judging from the rate of cosmic-ray impacts on its surface. Its orbit got nudged this way and that every time it passed by its home planet, and eventually the orbit changed enough that it began to move closer to the Sun. Some 13,000 years ago, our planet got in the way. The rock fell to Earth, got lodged in the Antarctic ice, and sat patiently waiting to be found.

So the rocks from Mars that landed on Earth as meteorites were themselves launched into space by impacts from much larger rocks.60 This is a very violent way to get a rock into space, of course, and you’d expect that an impact like that would destroy any rocks, or certainly damage them considerably.

But recent studies have shown that that might not be the case. A low-angle impact, for example, can loft rocks relatively softly (though it’s not exactly the kind of ride for which you’d line up for tickets). There may be other factors involved as well, including pressure waves from the atmosphere and under the rock due to the impact that also combine to soften the blow.

However it worked, very small structures inside ALH84001 survived the ordeal. When the rock was examined by scientists, they found a wealth of intact structure inside, including some that indicated that the rock had been exposed to flowing water at some point in its past. When they took microscopic images, however, they got the shock of their lives.

Tiny wormlike formations were evident inside the Martian rock. They looked for all the world like fossilized bacteria! In 1996, a press conference was held, and the scientists who examined the rock announced what they had found, indicating that it could be, for the very first time in the history of the world, evidence of life outside the Earth. They were very careful to say that the evidence was not (pardon the expression) rock-solid, but very compelling. The “fossils” were actually the weakest of their evidence, and they took great pains to say that they might not be fossils at all. They might be natural formations caused by any number of processes.


A microphotograph of the “Mars rock” ALH84001 shows the presence of small, wormlike objects. While they look like primitive life-forms similar to those on Earth, their origin is unclear. They are far smaller than any similar terrestrial organism. The scale bar is 0.5 micron across; for comparison, a human hair is 50 microns in width.


Of course, the press ran with the fossil interpretation; a picture, after all, is worth a thousand words, and will sell a million issues of a newspaper. It was quite the media sensation. Over the years, however, one by one, the evidence for life in the rock has come under fire. At the moment, the best one can say is that the evidence is interesting, perhaps even still compelling in some ways, but everyone agrees we will need far better sources before we can talk clearly about ancient life on Mars.


But this brings up an intriguing possibility: if life did originate on Mars first, it could have been brought here by the same mechanism that brought us ALH84001. Is it possible that life on Earth actually came from Mars?

At first blush, this sounds like a dumb idea. Earth is flourishing with life—it’s actually quite hard to find any place on the planet that isn’t infected with it—but Mars is quite dead. Still, the steps needed to seed life on Earth are at the very least possible: life may have originated there first; a plausible mechanism exists for it to have gotten here; and conditions here eventually were good enough for that life to take hold.

The idea that life was brought to Earth from space is called panspermia. It’s a fascinating topic, fraught with one simple problem: how do you prove it?

Honestly, I’m not sure you can.61 But it’s very hard to rule it out. How do you perform experiments to test it? Re-creating conditions that haven’t existed in billions of years can be tough, and even then it doesn’t prove things one way or another because of uncertainties inherent in the experiments. But experiments along those lines can steer thinking in directions that may lead to further progress; in science, a good experiment is worth a thousand suppositions.

An interesting test would be to look for fossilized microbes on Mars that have a chemical tie to early life on Earth. A clear example of RNA-based or DNA-based fossil bacteria would be incredibly compelling evidence in favor of panspermia—either life started on Mars and came here, or both Mars and Earth were seeded from some third source.62

Until such evidence is found, we can only conjecture.

Still, in principle, it’s possible to examine the processes involved with panspermia.

The step after getting the life-laden rock off Mars (or some other body) is the journey here. ALH84001 spent at least 16 million years in space, and possibly more, where it was exposed to the hard vacuum of space, bombarded by high-energy subatomic particles, and bathed in killer ultraviolet rays from the Sun.

Anything that could survive that would have to be pretty tough.

Microbes can be tough hombres. Some bacteria can form protective spores around themselves, shielding them from the ravages of heat, cold, drought, and radiation. One type of bacterium—Deinococcus radiodurans—can survive intense doses of radiation, hundreds of times what’s needed to kill a human. It is rather like a computer with multiple file backups: it has many copies of its DNA that it can use in case some get destroyed by radiation, and the tools it uses to repair its own DNA (every cell nucleus has a repair kit) appear to be extremely adept at dealing with extreme conditions.

Of course, it would also help if any microscopic stowaway on board a meteoroid were gift-wrapped carefully too. A rock dislodged by an asteroid impact and flung into space would be irradiated by all manner of destructive sources. But if the rock were large enough, it might protect any microscopic cargo. Cosmic rays may not penetrate very far into the surface, for example. Other disruptive influences like ultraviolet light from the Sun, particle irradiation from the solar wind, and the odd solar flare or coronal mass ejection would have a hard time getting deep into the rock as well. Some early experiments in lofting bacteria samples into space and exposing them to the environmental hazards there indicate that some microbes could survive for a period of time in space.

If some Martian protovirus or bacterium were to wend its way deep into a rock that got blasted off Mars, then there is some chance—small, but finite—that it could survive the journey.

It would also have to survive the journey through our atmosphere. But again, if the rock were large enough only the outer layers would burn off as it plummeted through the Earth’s air. If the meteoroid disintegrated into smaller pieces above the ground, the individual impacts wouldn’t be so jarring to its biological stowaway either. A small rock would simply plop down, and if it fell into water or mud, which seeped into cracks, the microbe might suddenly find itself getting, in a literal sense, food delivery.

It’s important to note that Mars isn’t the only source of potential life. Comets are giant balls of rocks and ice that orbit the Sun, and are known to contain rather complex organic compounds, some of which are precursors (or have at least basic chemicals needed) for life. It’s possible that comets impacting the young Earth brought much of the water to our planet, and brought these chemicals as well. A meteorite that fell in Australia in the 1960s was also found to have amino acids in it, including glycine and alanine, both of which are commonly found in animal proteins. Even giant clouds of gas and dust in space are found to be rich sources of complex organic compounds. One study done by scientists even showed that DNA or RNA from bacteria, if shielded well, could actually survive being blown by the solar wind to another star.63 That work is pretty speculative, but it shows in principle that transport across large distances, even vast ones, is theoretically possible, even if very unlikely.

Incidentally, it should be noted that a comet need not directly impact the Earth to transfer any contents. When a comet gets near the Sun, the frozen material sublimates (turns directly to a gas) and leaves the comet, forming the long tail. If the Earth sweeps through a comet’s tail, the cometary materials can mix with the Earth’s atmosphere. It’s still a somewhat violent process, since the velocities are so high, but in principle the comet stuff can reach the Earth relatively intact.

Let’s also be careful here and state explicitly that all of these are the building blocks of life, and not life itself. But the fact remains that the components needed for life as we know it not only exist in space but are relatively abundant—and these sources made it down to the ground intact enough to be studied by scientists. It’s entirely possible that space is simply buzzing with life, and it’s also possible that this is where terrestrial life got its start. If true, actually proving it would be one of the most colossal and fundamentally profound discoveries ever made.

But it also could mean trouble. If life exists out there as microbes, and some of them came to Earth now, could there be a less than happy ending? It’s all well and good for the surviving bacterium, and if panspermia is true, we owe our existence to space bugs. But that was more than three billion years ago.

What would happen if this event were to repeat itself today? We’ve all seen the movies The Blob and The Andromeda Strain. Could an interplanetary infection invade us, wiping out humanity (or mutating us into horrible nasty gooey things)?

To be honest, probably not. Life here is pretty tough, and anything coming down from space will have an uphill battle trying to take us over. It’s my opinion that they won’t be able to win. But the fight itself depends on what type of gooey thing is on the prowl.


Viruses, for example, are a favorite in science-fiction scenarios. There may be millions of types of viruses on Earth; we have only scratched the surface in investigating their diversity. Most are actually extremely simple structures: they are a snippet of DNA code wrapped in a protein shell called the capsid. They cannot reproduce on their own; instead, they invade a cell, inject their DNA into the cell’s nucleus, and then urge it to copy the viral DNA. Viruses are the stealthy ninjas of the submicroscopic world, sneakily invading the factory of the cell and turning it against itself.64

When enough viruses are created, they burst out of the cell, rupturing and destroying it. They then scurry off, seeking out more cells to subvert. If the body cannot fight off the infection, and the virus is virulent enough, the host can be killed. The tissue of the body literally liquefies.


There are other types of viruses too. Some use RNA, not DNA. Others attack bacteria and not tissue cells. And not to make you uncomfortable or anything, but your body is currently brimming with these viruses. Most are completely harmless. Some do cause a variety of issues—for example, they can throw off your body’s ability to regulate its systems, resulting in illnesses from mild to severe—but most don’t kill. They have to attack in ferocious numbers to do that, or be particularly virulent, like Marburg (which has a mortality rate of about 25 percent) and the more famous Ebola (with its truly terrifying 80 to 90 percent mortality rate).

The structural simplicity of viruses is both a blessing and a curse when it comes to an invasion from space.

Because they are such simple structures, viruses are resistant to many of the problems a more complicated microbe might have with exposure to space. Prolonged periods of vacuum, low temperatures, and even some radiation may not prove an obstacle to them. Embedded deep in a rock, they could fall to Earth intact, only to be opened like a cursed pharaoh’s tomb by a hapless scientist.

But if they got under his skin, they might starve to death.

That’s because viruses are generally adapted to attacking one specific kind of organism. A virus that can infect a plant can’t harm a butterfly, and one that is adapted to attacking bacteria (called bacteriophages) can’t hurt a human. Viruses are too simple to change radically, and the DNA or RNA snippet in the virus is like a key to a lock. A car key won’t work in a house door.

So even though any hypothetical space-borne virus might survive all the way into the lab of a scientist, it’s incredibly unlikely that it would swarm and multiply and turn us all into raging zombies.65

So in reality, viruses aren’t a big threat. They would find us completely incompatible for their purposes, and would quickly die out.66

Score one for life on Earth.


Interplanetary bacteria are another horror movie staple, and while they have some advantages as invaders over viruses, they’re also unlikely to be much trouble to us Earthbound creatures.

Unlike viruses, which are programmed to fit certain types of cells or proteins, bacteria are less choosy. And while viruses use our own cells’ machinery against us, bacteria consider us more as a flophouse. Like an unwanted guest, they can eat your food, mess up the place, and, of course, overstay their welcome.

The main difference between a virus and a bacterium is complexity. Bacteria are cells in their own right, and are considered alive. They can ingest food, excrete waste, and reproduce on their own. Give them a warm, wet environment with the nutrients they need, and they’ll do all three of these functions with abandon.

Our bodies make excellent sites for what bacteria need. Our bodies are loaded with bacteria, as with viruses. Bacteria are in your gut, in your skin, and living on your eyelashes. They’re everywhere, and in fact it’s been estimated that there are ten bacteria in your body for every single human cell!

You’re outnumbered.

The vast majority of bacteria inside you are benign. They either don’t do anything harmful or exist in numbers too small to do any damage. Many are beneficial to us; without them we’d die. They help us digest our food, for example; they also create vitamins and boost our immunity to more harmful types of bacteria. They even help us digest milk.

An excellent example of such a bacterium is Escherichia coli, more commonly known as E. coli. This little ovoid bug lives in huge numbers inside your intestines, and has the underappreciated job of helping you process your waste matter.67 Normally, they live happily in your gut, doing whatever it is they do. But that’s not always the case. E. coli eats and poops too, and some strains of the bacterium exude a toxic chemical brew. In low doses your body can handle it. But if you get too much in your system, it can make you quite ill. Food poisoning, for example, can be caused by eating food that has been contaminated by E. coli. If the infection is bad enough, it can be fatal. E. colican also get out of your intestines (through a hole or herniated region) and into your abdomen, causing peritonitis.

The list of possible problems bacteria can invoke is lengthy (diarrhea, vomiting, nerve damage, cramps, fever . . . you get the picture), but usually we live in an uneasy truce with the bugs inside us.

The bacteria inside us have, of course, evolved along with us so they can maintain this symbiotic relationship. A hypothetical bacterium that evolved on Mars, say, or some other planet would not enjoy this luxury. Still, the effect of an alien bacterium on us really depends on what it needs, and, pardon the expression, what it excretes.

If all it needs is a warm place with water and some nutrients, then any port in a storm, as they say. Your intestines will look just as good as any other place. And if the bacterium multiplies, and the colony emits a toxin, then that can be trouble.

But is that likely?

In reality, almost certainly not. The very complexity that makes bacteria more versatile and therefore more adaptable than viruses is also their Achilles’ heel: it makes them more fragile. Their internal machinery is unlikely to survive the journey through space, and their entry into our atmosphere.

Moreover, the conditions alien bacteria need to survive would make Earth look pretty unfriendly. The chemistry of the surface of Mars, for example, is very different from Earth’s. Its thin atmosphere means it’s hit by a high level of UV light from the Sun. There is very little if any water on most of the surface, and some readings indicate rather high levels of hydrogen peroxide, a chemical that tends to destroy terrestrial bacteria (which is why it’s used to clean wounds, though it should be noted that it is produced by some forms of life on Earth—notably the bombardier beetle, which uses it to ward off predators). Any bacterium that evolved to survive on Mars would most likely find Earth to be a very difficult environment—too wet, too hot, too alien.

Of course, not all life on Earth likes the same things we do. Some bacteria like extreme cold, some like it hot, others eat sulfur, and some like the extreme pressures found deep underwater or underground. These extremophiles are abundant on Earth, and may exist on Mars as well. But even if they are there, deep under the Martian surface, it’s unlikely they’d get scooped up and carried away by an asteroid impact.

Looking to other worlds in the solar system for potential bacterial breeding grounds is even more futile. Europa, a moon of Jupiter that is covered in ice, may harbor a vast water ocean under its surface. It’s an excellent candidate to look for life beyond Earth, but it’s a low-probability location for anything that’ll think our environment is cozy. The ice on Europa is probably ten miles or more thick; any impact that could loft a subsurface ocean-dwelling microbe into space would also be powerful enough to vaporize said microbe.

Another potential home for life is Titan, one of Saturn’s moons. Titan is aptly named: it’s over 3,000 miles in diameter (about the size of Mercury) and sports a thick atmosphere of nitrogen, argon, and methane. It rains there, but the drops are liquid methane! It’s cold on Titan, about −300 degrees Fahrenheit. Any water on the surface is frozen into a solid harder than terrestrial rocks. And while biochemists have speculated that life could arise in such a weird environment, it would be utterly alien to us. Any bug capable of living there would find itself in the equivalent of a blast furnace on Earth.

It seems that as incubators go, we’ve struck out of potential bugs in the solar system. Any alien microbes that would have evolved for Earthlike conditions almost certainly wouldn’t survive the trip.

Of course, this assumes that any form of life Out There is just sitting back and waiting for a ride. Maybe, though, the more sophisticated types would prefer to drive.


The question was asked so succinctly by the physicist Enrico Fermi in the early 1950s, over lunch with some other scientists. They were discussing the recent spate of flying saucer sightings and considering interstellar travel, human or otherwise. When the topic turned to aliens, Fermi asked, “Where are they?”68

The question, simple though it is, has a rich backstory. The basic idea is that by now either we should have detected intelligent life in our galaxy or it should have come visiting. Since neither has occurred,69 asking where the aliens are is a reasonable thing to do.

Let’s assume that for aliens to come knocking, their circumstances must be something like ours: Sunlike star, Earthlike planet, development and evolution of life over billions of years, discovery of technology, then the capability to travel between the stars. How likely is all this to happen?

For that we can turn to the Drake Equation. Named for the astronomer Frank Drake, it categorizes all the necessities of advanced life and assigns probabilities to them. If you fill in all the terms correctly what pops out is the number of advanced civilizations in the galaxy (where “advanced” is defined as being able to send signals into space—which is how we’d know they’re out there).

For example, the Milky Way Galaxy has roughly 200 billion stars in it. About 10 percent of these stars are like the Sun: similar mass, size, and so on. That gives us 20 billion stars to work with. We’re just now learning how planets form around other stars—the first planet around a Sunlike star was discovered in 1995—but we’re finding that stars like the Sun are rather likely to have planets. Even if we assign a ridiculously low probability of there being planets around another star (say, 1 percent), there are still hundreds of millions of stars out there with planets. If we assign a ridiculously low probability of these planets being Earthlike (again, say, 1 percent), then there are still millions of Earthlike planets. You can continue to play this game, estimating how many planets can support life, how many have life, how many have life capable of technology . . . each step in the chain is a little less firm than the last, but even the most pessimistic view of this series indicates we shouldn’t be alone in the galaxy. The estimates of the number of aliens out there vary widely, literally from zero to millions.


That’s not terribly satisfying, of course. The lower estimate is sobering. Maybe, just maybe, we really are alone. In all the galaxy, in all the vast trillions of cubic light-years of emptiness, ours is the very first planet to harbor creatures that can ponder their own existence.70 This is a humbling and in some ways frightening possibility. And it’s possibly true.

Another possibility is that life might be common, but “advanced” life is rare. Books have been written on this topic, and it makes for an interesting argument. Maybe once life gets to a certain stage, it tends to go navel-gazing and never develop or care about technology (alien psychology is a difficult topic to get too deeply into). And I hope that by the time you get to this point in this book, I’ve made it clear that civilization-ending events occur uncomfortably often over geologic time scales. Maybe every civilization eventually gets wiped out by some natural event before it can develop space travel advanced enough to prevent it.

I don’t think that’s a good answer, actually. We are within years of being able to prevent devastating asteroid impacts on Earth. We know we can properly shield ourselves from solar events. Our astronomy is good enough to pick out nearby stars that might explode, so if we saw one ticking away we could devote ourselves to getting away from it. All of these advances are quite recent, happening in a blink of the eye compared to how long life has existed on Earth. It’s almost impossible for me to imagine a civilization intelligent enough to explore the heavens yet not advanced enough to preserve its own existence.


I’m suspicious of the other end of the estimates of the Drake Equation as well, that there are millions of aliens out there as advanced as we are or more. If that were true, I think we’d have unequivocal evidence of them by now.

Remember, besides being vast, the galaxy is old. The Milky Way is at least 12 billion years old, and the Sun only 4.6 billion. If we imagine a star like the Sun forming just 100 million years earlier—a drop in the bucket compared to the age of the galaxy—then it’s not hard to imagine an alien civilization rising many millions of years before humans did. We know that life arose easily enough on Earth; it got started as soon as the bombardment period ended and the surface of the Earth calmed down enough for long-term growth of life to occur. This implies strongly that life takes hold given the smallest opportunity, which in turn means it should be abundant in our galaxy. And, despite a list of disasters epic and sweeping, life on Earth has managed to get this far. We are intelligent, we are technologically advanced, and we are a space-faring species. Where will we be in a hundred million years?

Given that stretch of time and space, an alien species really should have knocked on our door by now.

They should have at least placed a call. Communicating across the vastness of space is easier than actually going there. We’ve been sending signals into space since the 1930s. These are relatively faint, and an alien would have a hard time hearing them from more than a few light-years away, but we’ve leaked out stronger signals as time has gone on. If we wanted to target a specific star, it’s not hard to focus an easily detectable radio signal to any star in the galaxy.

The reverse is true as well: any alien race with a strong urge to chat with us could do so without too much effort. The Search for Extraterrestrial Intelligence (SETI) is banking on just that. This group of engineers and astronomers is combing the sky, scanning for radio-wave signals. They are almost literally listening for aliens. The technology is getting so good that the astronomer Seth Shostak estimates that within the next two dozen years, we’ll be able to examine the million or two interesting star systems within a thousand light-years of Earth. This will go a long way toward our discovering whether we are alone or not.

The one drawback with SETI is that the conversations will tend to be a bit boring. If we detect a signal from a star that is really close on the galactic scale, say, a thousand light-years away, the dialogue will really be a monologue. We’d receive the signal, reply, and have to wait two thousand years for them to get back to us (the time it takes for our signal to reach them plus theirs to come to us again). While SETI is a great and worthwhile endeavor—and if they find a signal it will be one of the most important events in the history of science—we’re still used to thinking of aliens actually coming here. Face-to-face, as it were, assuming they have faces.

But a thousand light-years is a long way (6,000,000,000,000,000 miles). That’s quite a hike, yet it’s practically in our laps compared to the size of the Milky Way.

Is that why we haven’t been visited? Maybe the distances are simply too great!

Actually, not so much. A trip to the stars wouldn’t take that long at all, if you maintain a sense of scale.


Let’s assume that we humans suddenly decide to fund the space program. And fund it really well: we want to send probes to other stars. That’s no easy feat! The nearest star system, Alpha Centauri (which has a Sunlike star and is worth a look-see), is 26 trillion miles away. The fastest space probe ever built would take thousands of years to get there, so we couldn’t really expect a payoff in the form of pretty pictures anytime soon.

However, that’s the fastest probe ever built so far. There are ideas out there on the drawing board that would make much faster unmanned probes, even ones that can move at a goodly fraction of the speed of light. Some of these include fusion power, ion drives (which start off slowly but accelerate continuously over years, building up ferocious speeds), and even a ship that explodes nuclear bombs behind it to provide a huge impulse in speed.71 These methods can drop the trip time from millennia to mere decades.

This might be worth doing. It’s expensive, sure. But there are no technological barriers to this idea, just social ones (funding, politics, etc.). Let me be clear: if we had the will, we could build spaceships like these right now. In less than a century we could be sending dozens of interstellar emissaries to other stars, investigating our own neighborhood in the galaxy.

Of course, the trip times and the actual construction of the fleet make it difficult to explore much real estate. The galaxy has billions upon billions of stars, and building that many starships is impossible. Sending one probe per star isn’t cost-effective. Even if we let the probe simply sweep through a star system on a fly-by, moving on to the next star, exploring the galaxy would take forever. Space is big.

But there’s a solution: self-replicating probes.

Picture this: an unmanned spaceship from Earth arrives at the star Tau Ceti after a journey of fifty years. It finds a series of small planets and begins its scientific observations. This includes a census of sorts—taking measure of all the bodies in the system, including planets, comets, moons, and asteroids. After some months of surveying, the probe will move on to the next star on its docket, but before it leaves, it sends a package down to a particularly promising nickel-iron asteroid. This package is in fact a self-starting factory. Once it lands, it mines the asteroid, smelts the metal, refines out the necessary substances, and then autonomously builds more probes. Let’s say it builds just one probe that, after a few years of construction and testing, blasts off for another star system. Now we have two probes. A few decades later they arrive at their destinations, find appropriate accommodations, and then go forth and multiply again. Now we have four probes, and the process repeats.

The number of robot ambassadors builds very rapidly; it’s exponential. If this takes exactly one hundred years per probe, by the end of a millennium we have 210 = 1,024 probes. In two millennia there are a million probes. In three thousand years there will be more than a billion. Now, in reality, it’s not that simple, of course, but even a pessimistic approach shows that we can explore every single star in this galaxy in something like 50 million years, maybe a bit less.

Well, that sounds like a long wait! And we’re still a long way off from being able to do it. The technology is formidable.

But hang on—remember that civilization we considered, just 100 million years in advance of us? Given that much time, they could easily have examined every single star in the Milky Way, looking for life. If they saw our warm, blue world, one would think they’d make some note of it. Still, it’s possible they came here 50 million years ago and missed us humans (mining the Moon for a monolith à la 2001: A Space Odyssey maybe isn’t as silly as it sounds), or maybe they just haven’t gotten here yet.

But given the time scales, that seems unlikely. It just doesn’t take that long to map out a whole galaxy and visit the appropriate planets. That’s why I don’t think the “millions of civilizations” number from the Drake Equation is correct. We’d have seen them by now, or at least heard from them.72


But sometimes I wonder. Given all this information: the likelihood of life, the relative ease of galactic exploration, the time spans involved . . . and the fact that we have not detected any other life in our galaxy at all, there is another possibility that is worth mulling over.

Consider: what spurs technological advancement more than anything else?


The first Cro-Magnon who beat an opponent over the head with a tree branch was also the one who was most likely to live a bit longer, and be able to reproduce. The army with rifles will (in general) beat the one with spears. The country with missiles will (in general) beat the one with cannons. The ones with electronic remote-controlled drones, spy satellites, and instantaneous communication will outmaneuver the ones without.

Nothing advances technology like good old-fashioned aggression. Even one of the most noble events in human history—man walking on the surface of the Moon—was initiated because of the cold war, the space race with an enemy vast and powerful. Americans imagined Soviet missile bases in orbit and on the Moon, and the motivation to beat them was in place.

When I was a kid, it was fashionable in more intelligent science-fiction stories to assume that any aliens we meet were bound to be friendly—no warlike race would be able to get their act together long enough to reach the stars.

Humans are on the verge of falsifying that statement.

Putting the pieces together, we find that warlike races are perhaps more likely to achieve space travel. The ones with a history of victories will have the best technology, and will be most motivated to be at the very least wary of outsiders, if not openly hostile. This case can certainly be made for our own provincial example.

This hypothetical advanced civilization will be xenophobic, fearful of aliens. We’ve already seen that it’s technologically possible to create interstellar starships, and it’s also possible to engineer them to create duplicates of themselves, to speed up the time it takes to comb over the whole galaxy.

What happens when you take a paranoid species and give them the ability to build such spaceships?


The scenario plays out in the vignette at the start of this chapter. It makes a creepy kind of sense to me: any aliens that are that aggressive would want to wipe out potential enemies before they got sophisticated enough to pose a threat. The easy way to do it is to create space probes like the ones described, and use them to ruthlessly wipe out all life they find.

Death from the skies, indeed.

I’ve wrestled with this idea, wondering if it’s possible. One potential saving grace is the same as before: exploring the whole galaxy in this way doesn’t take long compared with the age of the galaxy itself. Therefore, according to the same logic above, it’s likely that if such a xenophobic civilization were to evolve, it would have been here by now.

Yet we’re still here. We know life has been around for billions of years. There have been the odd interruptions, but we’ve never been sterilized back down to the microscopic level. Like so many of the natural disasters we’ve seen, this puts a pretty good damper on the odds of being wiped out by nasty aliens. Simply put, if they were out there, we wouldn’t be here.73

I honestly don’t know if we’re alone in the Universe; no one does. However, given the immensity of space, and the grand depth of time, it sure seems unlikely. And if we do get out there, it also seems unlikely we’ll meet any nasty races like Klingons, Romulans, Vogons, Reavers, Daleks, or Kzinti. Natural disasters will still probably be our biggest worry.

But the galaxy is big, with room enough for lots of things. I may not know if we’re alone, but I’d love the chance to find out.