WE ALREADY KNOW what an asteroid strike did to the creatures who lived during the Cretaceous period. Though it may not have been the sole driver of the K-T mass extinction, the 6.2-mile-diameter bolide that landed off the coast of Mexico roughly 65 million years ago devastated the planet, radically altering the Earth’s climate for possibly a decade or more. Among the scientists who study impacts, that one would have been classed as a 10 out of 10 on the Torino scale, a kind of Richter scale used to quantify impact hazards. Such disasters, where the entire planet is affected, are likely to strike once every 100,000 years or so (though not necessarily with the destructiveness of the K-T impact). That means we are long overdue for another one.
Will we wake up tomorrow to a newscaster telling us that humanity has six months to live, so we’d better make the best of things before an asteroid wipes us out?
Not likely. Contrary to Hollywood myths, we’d probably see an asteroid like the one that hit during the K-T mass extinction coming many years before it smashed into us. Less than two decades after scientists discovered the role an asteroid played in the planet’s most recent mass extinction, NASA launched an asteroid-spotting program called Spaceguard. The goal of Spaceguard was to discover and track 90 percent of near-Earth objects larger than a kilometer. A near-Earth object, or NEO, refers to asteroids, meteoroids, comets, and other heavenly bodies whose orbits around the sun bring them close to our own orbit. Most NEOs are not dangerous—they’re either so small that our atmosphere would burn them up, or they zip past us millions of kilometers away. That being said, there is a class of NEO called potentially hazardous objects, or PHOs, and these are the ones we have to be worried about. To achieve PHO status, an object has to be larger than 1 km and its likely trajectory must take it closer than 7,402,982.4 km from Earth.
That sounds pretty far away, especially when you consider that we’ve had some near misses over the past two decades when sizable asteroids have come within thousands of kilometers of the planet (some would have caused explosions comparable to a nuclear bomb if they had hit). But our solar system is a constantly shifting set of gravitational fields, and the orbits of small objects shift a lot over time. If an asteroid zooms past Jupiter or another planet on its way to us, gravity from that other body could easily pull the asteroid into a new course, converting it from distant to deadly. That’s why astronomers want to keep a sharp eye on any large rocks or balls of ice that come within a few million kilometers of our orbit.
The good news is that over the past two decades, we’ve gotten pretty good at spotting and tracking NEOs and PHOs. The bad news is that, at least right now, nobody is quite sure what we’d do in what NASA astronomer and asteroid hunter Amy Mainzer calls one of the most hopeful scenarios. That would be when an astronomer—possibly Mainzer herself—verifies tomorrow that there’s a mile-diameter asteroid twenty years out, on a direct collision course with Earth.
Mainzer is obsessed with seeing into space. That’s why she’s worked on instrumentation for NASA spacecraft like the WISE (Wide-field Infrared Survey Explorer) satellite, whose sole job was to map as much of the sky as possible using an infrared telescope. Once the WISE mission was complete, Mainzer and her colleagues were able to reprogram the craft in 2010 to scan the sky for NEOs—they dubbed this mission NEOWISE. It was the NEOWISE mission that helped complete the Spaceguard project by identifying enough one-kilometer-or-bigger NEOs that we can say with confidence that we now know where 90 percent of them are. In all, we’ve located nearly 900 NEOs of that size. “That’s good for Earthlings,” Mainzer told me lightly by phone from her office at the Jet Propulsion Lab in California. But then, more seriously, she added, “We don’t know where most of the other ones are.” In her most recent work, Mainzer gathered data on PHOs among asteroids, and she and her colleagues estimate there might be as many as 4,700 of these potential impactors that are bigger than 100 meters. To give you a sense of what that means, a 100-meter asteroid wouldn’t cause a mass extinction, but it would easily flatten a city or a small country. If it landed in the ocean, the tsunamis it generated could do profound damage to coastal areas.
Given that our local volume of space is swarming with deadly rocks, why aren’t we bombarded all the time? The simple answer is that we are. Every day, we are hit by tiny NEOs, most of which we never notice because they flame out before reaching the Earth’s surface. “You know the video game Asteroids?” Mainzer asks. Of course I do. “Well, it’s actually pretty accurate. Asteroids break up and make more little pieces. And there are far more little pieces than big pieces.” Aside from the relative rarity of larger asteroids, there’s also the fact that our solar system is a dynamic, constantly shifting sea of debris. All the overlapping gravitational fields of the planets and their moons may send rocks spinning into our path, but they also send them spinning out of it, too. “If you put a particle in near-Earth space, it doesn’t stay stable,” Mainzer explained. “After about ten million years, it will go into the outer solar system, crash into the sun, or crash into the Earth.” Keep in mind that 10 million years is nothing to a planet like Earth, which has been around for 4.5 billion years. Essentially, there’s only a short time window for these NEOs to do any damage before they’re hurled elsewhere by gravity.
Still, Mainzer notes, there are probably “source regions” of the asteroid belt that are constantly resupplying the inner system with new NEOs. Possibly these source regions are shooting out new NEOs because of gravitational resonances with Mars and Jupiter, the two planets whose orbits sandwich the asteroid belt. “I like to think of it as a flipper on a pinball machine,” Mainzer said. “That’s what these resonances are like in the main belt—if an asteroid drops into one, it can get hurled very far from its original location.”
With the amount of data we’ve gathered from satellites like NEOWISE, it’s reasonable to hope we’d have twenty years to deal with an asteroid or other PHO big enough to cause destruction over the entire Earth. Knowing where most of the large NEOs are can help astronomers to track their movements and determine whether they’re on a collision course. That said, collision courses are always expressed in probabilities. We can’t predict precisely where gravity will tug one of these objects on its way to our cosmic neighborhood. Also, we’re still struggling to track objects that could cause tremendous damage without actually destroying humanity. “Your warning time depends on the design of your instruments,” Mainzer said. She’s currently working on plans for a new space telescope, dubbed NEOCam, designed to spot objects smaller than 100 meters and to find more of those PHOs. “We’re designing it to give us decades of warning,” she said. The goal for Mainzer and others in her field is to get 20 to 30 years of warning for a likely impact, so that we have as many options as possible for stopping it.
Most people who are serious about defending Earth from PHOs don’t talk about blowing things up. As Mainzer explained with the 8-bit game Asteroids, the problem is that asteroids tend to break down into smaller asteroids. Nuking an incoming object might not do much more than shower our planet with dozens of burning chunks rather than one big one. The damage would be roughly the same. The reason Mainzer’s data-gathering is so crucial is that the further away an asteroid is when we spot it, the easier it will be to nudge it out of the way. That’s right—our best bet is to nudge it. “Blowing up asteroids may be fun, but an Aikido move would be better,” Mainzer said, only half joking. “Having time gives you the ability to move its trajectory without a lot of energy.”
The question of how to finesse this Aikido move in space has been the longtime concern of a loose coalition of scientists, policy-makers, and government representatives associated with the Center for Orbital and Reentry Debris Studies. Run by aerospace engineer William Ailor, the Center has developed a series of suggestions over the past 15 years for how we’d deal with asteroid threats. An affable man with tidy gray hair and a touch of the South in his speech, Ailor sketched out how he thought an impact scenario might unfold. “Anyone can find these things,” he said. “There are amateur astronomers all over, as well as more formal programs in space agencies. Most likely, it would be spotted by that community.” If it’s a smaller object, we might have very little time to prepare. “People like to think we’ll have twenty years, but we might only have a few years.”
The next big hurdle wouldn’t be the question of how to divert the asteroid, though. Say, for example, Mainzer’s NEOCam is in orbit and her team spots an object bigger than 1 km that has a 1-in-50 probability of smashing into the Earth. “Should we spend money on that now?” Ailor asked. “Given the fact that it takes you years to build a new payload and fly a mission out to do something, you may have to start spending money before you’re certain it’s going to hit. And that’s the challenge for decision-makers.” The problem is that every PHO is a probability … until it isn’t. And the time to act decisively to push an incoming object out of the way is almost inevitably going to be long before we can establish that a collision is a certainty. Meanwhile, as the object hurtles closer to us in space, the less likely it is that we’ll be able to gently nudge it into a new orbit, out of our way.
So who would have to step up and push the world to launch anti-PHO spacecraft? The U.N. Committee on the Peaceful Uses of Outer Space has a group called Action Team-14 that deals with NEOs, and would likely be the first agency to coordinate Earth’s defense in this situation. Provided they can get buy-in from countries and corporations with the means to build spacecraft for the mission—and that’s a big if—the group would have to decide exactly what method of PHO deterrence would work best. Ailor’s company, the Aerospace Corporation, did a study in 2004 on what would be required to take a 200-meter object from a 1-in-100 probability of hitting Earth to 1 in 1,000,000. “You have to launch quite a few spacecraft,” Ailor said. “There’s a misconception that you would send up just one vehicle.” Redundancy would be crucial, in case one of the crafts fails—and besides, some techniques for moving the object require multiple spacecraft to work. Also, despite what we saw in the asteroid-nuking flick Armageddon, the vehicle would be a remote-controlled robotic craft. “If a human can get there, it’s way too close,” Ailor asserted.
If we have enough time, we’d want to try what Ailor called slow-push techniques. One would involve using a swarm of small spacecraft equipped with lasers designed to boil material off the surface of the object. As the PHO spat pieces of itself into space, enough thrust would be generated to gently move it out of its deadly path toward Earth. Another possibility would be to create a “gravity tractor” with one or more spacecraft. Parking bulky objects like other asteroids or big spacecraft near the distant object might generate enough gravitational pull to move it just enough. Many years later, this small perturbation would elegantly divert its course into a completely harmless orbit. Both of these techniques are untested. But as more spacecraft venture to NEOs and the asteroid belt over the next decade, we’re likely to see experiments to test whether these techniques could, in fact, jar a large object out of its current orbit.
In this image by Ron Miller, we see a probe pushing an NEO out of Earth’s path. (illustration credit ill.18)
What if the asteroid were heading toward us today, and we hadn’t had a chance to test the slow-push systems? “We don’t have anything off the shelf other than a kinetic impactor,” Ailor said casually, as if he were talking about computer parts. A kinetic impactor is “basically hitting it with a rock,” he explained. We’ve already tried this method on a comet with NASA’s Deep Impact mission, when a probe hit the Tempel 1 comet with a giant copper slug, dislodging huge amounts of dust and ice. Tempel 1’s orbit was perturbed slightly. So we know for certain that if we hit an incoming object with slugs or rocks, we have a good chance of redirecting it. “If you have one that gets too close or is bigger, you might have to use a nuke to move it,” Ailor conceded. That’s a last resort, and also untested.
The problem is that even our “off the shelf” kinetic-impactor solution would be tough. “You’d have to pull a craft together, grab the right kind of payload to do what you wanted, and find a launch pad,” Ailor said, seeming to be mentally ticking off a list he’d pondered many times. On top of that, there would be the issue of how to inform the public without causing either mass panic or denial. It’s easy to imagine people voting against an expensive anti-PHO program if there were only a 1-in-500 chance of mass extinction. Still, it’s possible we might band together as a civilization to deal with this existential threat, and fail anyway. As Ailor put it, “Of course, you might miss.”
If we’re facing an impact that’s a 10 on the Torino scale—that is, from an asteroid comparable to the one that hit at the K-T—we are certainly facing a mass extinction. The world would be wrapped in fires, and cities would be shaken by quakes, broiled by volcanic eruptions, and flooded by tsunami waters. Over the long term, the climate would be transformed by aerosols thrown into the stratosphere. How would we survive?
Initially, our survival would depend on retreating to the kinds of underground cities we discussed in chapter 17. The immediate aftereffects of the hit would be similar to a massive nuclear war, minus the radioactive fallout. Underground, we would be relatively safe from the worst of the firestorms and other disasters. Aboveground, temperatures and fires would die down relatively quickly. Within weeks, we’d be able to poke our heads back up and see the roiling clouds of dust that had replaced our sky. And that’s when our real troubles would begin. We’d likely suffer through something like a nuclear winter. Alan Robock, the atmospheric scientist who warned against solar-management geoengineering with particles in the stratosphere, was among the first scientists to suggest that supermassive explosions would result in planetary cooling. And the cold would likely intensify for several years. In an early paper about nuclear winter, Robock outlines a scenario that sounds like a mild icehouse. The first year after the explosion—in this case, an asteroid strike—we’d see a global buildup of ice and snow and lowering of temperatures by about two degrees. But as the cold deepened, the planet’s snowy surface would reflect even more light—creating a runaway effect that would cool us down possibly as much as 15 or 16 degrees in the following several years.
Without sunlight, agriculture would grind to a halt and wild plants would die back. Herbivores would die, and then the carnivores who fed on them would die out, too. Creatures who dwelled near the surface of the water would suffer in the immediate effects of the hit. Then, over time, runoff from the decimated land would fill the oceans with carbon and create deadly pockets of anoxic waters. Humans would have to rely on greenhouses for food, as well as whatever we could cultivate with little sunlight. Mushrooms, fungus, and insects would play a much bigger role in our diets than they do today.
There is also the distinct possibility that enough people would be killed in the strike that it would be impossible to maintain our civilization at its current level of development and energy needs. Megacities and high-tech societies require many people with specialized knowledge to make them function, and if only a few million people are left alive on the planet, it’s unlikely that we’ll have the right combination of skills to resurrect New York or Tokyo. What would we do if we had to rebuild human civilization from scratch? This is the kind of question that dogs apocalyptic science fiction, but preoccupies people in the real world, too. Alex Weir, a software developer based in Zimbabwe, is part of a small group that maintains the CD3WD database, a relatively small set of computer files that contain as much human knowledge as possible about what amounts to a pre-technological civilization. There are sections devoted to basic medicine, agriculture, town building, and power generation. At 13 gigabytes, it’s easily stored on a few DVDs, or (ideally) printed out as a thick sheaf of papers and stored in a three-ring binder. The idea is to keep the CD3WD database in your survival kit, a backup copy of everything history has taught us about creating an early industrial society. It is one of the simplest and most profound examples of how survival requires us to remember what has come before. If people need guidance with rebuilding the world after the icehouse is over, CD3WD and similar projects can help us restart civilization as quickly as possible.
It is inevitable that the Earth will be on a collision course with a PHO at some point. Obviously, our first duty is to keep mapping the skies, tracking NEOs, and perfecting our asteroid-nudging technologies. But we also need to accept that the Earth isn’t the safest place for us if we want to survive for another million years. We need to scatter to other planets and moons, building structures in space so that even if Earth is wiped out, humanity will survive. That’s why one of the keys to long-term existence involves creating devices that will help us escape the planet. One such device is the subject of the next chapter.