9
LIVING DANGEROUSLY
I recently took advantage of Harvard’s spring break to visit friends in Colorado and do some work and skiing there. The Rocky Mountains are an extraordinary place to sit and think, with the nights as dazzlingly inspirational as the days. On clear dry nights, the sky is illuminated by brilliant dots of light sporadically punctuated by “shooting stars”—those tiny ancient meteoroids that disintegrate overhead. One night a friend and I stood outside the house where I was staying, mesmerized by the breathtaking expanse of luminous objects ranged densely across the sky. I had already spotted a couple of meteors before my friend and I both noticed a big one that lasted a few seconds.
Although I’m a physicist, I’m often content in such a magnificent setting to stop deliberating and simply enjoy the view. But this time I reflected on what that object was and what its trajectory might signify. The meteor—the culmination of a four and a half billion year-long story—glowed for a few seconds, implying that the visible meteoroid above might have traveled fifty to one hundred kilometers before it vaporized and disappeared. The meteoroid was probably about the same number of kilometers above us, which is why we saw it as a big arc in the sky. There it was—a thing of beauty and something we can at least partially understand. When I commented on this dust or pebble-sized object that was so wonderful to watch streak across the firmament, my friend—who is not a scientist—expressed surprise, saying he had imagined the object having a breadth of at least a mile.
The conversation rapidly swerved from calm admiration of the gorgeous sky to contemplating the damage a mile-long object careening to Earth would create. The probability of such a big dangerous object striking the Earth is small, and the likelihood that an object of any significant size would hit a populated region where it could do substantial damage is smaller still. Even so, extrapolating from the Moon’s surface (too few craters survive on Earth to provide a useful guide) tells us that millions of objects bigger than a kilometer and ranging up to about a thousand kilometers across have hit the Earth over its lifetime. But most of those impacts happened billions of years ago during the Late Heavy Bombardment which, despite the name, occurred relatively soon after the Solar System was formed and before it settled into its more or less stable state.
As is essential to the survival of life, the big meteoroid hit rate is currently much lower, and has been since the bombardment episode ended. Even the recent impact in Siberia caught by dash-cams and videos—the Chelyabinsk meteoroid that burned brightly in the sky and on YouTube—was only about twenty meters across. The only recent encounter of an object as big as was envisioned by my friend was the 1994 event in which Comet Shoemaker-Levy 9’s mile-sized fragments crashed into Jupiter. The initial object was bigger still—probably a few miles across before it broke into pieces. Some indication of the damage that mile-wide fragments can create was the dark cloud as big as the Earth that we could observe on Jupiter’s surface. Twenty meters is big but a mile across is another thing altogether.
Bear in mind that the story of meteoroids is not solely about destruction. Some good has also come from the many meteoroids and micrometeoroids that have rained down on the Earth. Meteorites—the remaining fragments of meteoroids on Earth—might have been a source of amino acids essential to life and also of its water—another key ingredient of existence as we know it. Certainly most of the metals we mine here come from extraterrestrial impacts. And one can argue that humans would not have emerged without the rapid rise to dominance of mammals that occurred after a meteoroid impact—more on this in Chapter 12—killed the terrestrial dinosaurs, which, I’ll grant, is not always considered to have been a good thing.
But this major mass extinction 66 million years ago is one of many stories that tie life on Earth to the rest of the Solar System. This book is about the seemingly abstract stuff such as dark matter that I study, but it is also about the Earth’s relationships to its cosmic surroundings. I will now begin to explore some of what we know about asteroids and comets that have hit the Earth and the scars they’ve left behind. I’ll also consider what might hit our planet in the future, and how we might prevent these disruptive, uninvited guests.
OUT OF THE BLUE
A phenomenon as bizarre as objects from space hitting the Earth sounds incredible and, indeed, the scientific establishment didn’t initially accept the veracity of most such claims. Although people in the ancient world had believed that objects from space could reach the Earth’s surface—and rural residents in more recent times were convinced of it too—the more educated classes were suspicious of the idea until well into the nineteenth century. The unschooled shepherds who had seen such objects falling from the sky knew what they’d seen, but these witnesses lacked credibility since many of those with similar backgrounds had been known to report imaginary findings as well. Even the scientists who did eventually accept that objects fell onto our planet didn’t initially believe that such rocks had originated in space. They preferred for them to have an Earth-based explanation, such as the descent of material that had been ejected by volcanoes.
The arrival of meteorites from outer space became part of established thinking only in June 1794, after a fortuitous fall of stones over the Academy in Siena—where many educated Italians and British tourists could witness the event firsthand. The dramatic phenomenon began with a high, dark cloud that emitted smoke, sparks, and slow-moving, red lightning that was followed by stones that rained down to the ground. The Abbe Ambrogio Soldani in Siena found the fallen material interesting enough to gather eyewitness accounts and to send a sample to a chemist living in Naples—Guglielmo Thomson—the alias of a relocated William Thomson who had fled Oxford in disgrace over his activities with a servant boy. Thomson’s careful investigation indicated an extraterrestrial origin for the object, offering a more consistent explanation than the far-fetched proposals then in circulation involving a lunar origin or lightning hitting dust and one that was also better than the more credible competing proposition that it had originated in the then-active Vesuvius. Taking the source to be volcanic activity was understandable in that Vesuvius had coincidentally erupted only 18 hours earlier. But Vesuvius is located some 320 kilometers away and in the wrong direction, which ruled it out as an explanation.
The case for a meteoroid origin was finally settled by the chemist Edward Howard with the assistance of the French nobleman and scientist Jacques-Louis, Comte de Bournon, who had been exiled to London during the French Revolution in 1800. Howard and the Comte analyzed a meteorite that had fallen near Benares in India. They uncovered an amount of nickel that was much greater than would be expected on the Earth’s surface as well as stony materials that had been fused by high pressure. The chemical analyses that Thomson, Howard, and the Comte performed were precisely the sort of thing that the German scientist Ernst Florens Friedrich Chladni had suggested to confirm his own hypothesis that such objects hit the Earth at too great a speed to be consistent with other proposed explanations. In fact, the Siena fall occurred only two months after the publication of Chladni’s book, On the Origin of Ironmasses, which had—alas—received negative reviews and an unfavorable response before the Berlin newspapers eventually got round to reporting the Siena fall two years after it occurred.
More widely read in England was the short book that Edward King, a fellow of the Royal Society in England, published that year. King’s book reviewed the Siena event and a lot of Chladni’s book too. The case in England for meteorites had been further solidified even earlier as well when a 56-pound stone fell on December 13, 1795 at Wold Cottage in Yorkshire. With an increased appreciation of the methods of chemistry—just recently separated from alchemy—and with so much firsthand evidence, meteorites were finally recognized in the nineteenth century for what they were. Many objects with bona fide extraterrestrial credentials have fallen to Earth since that time.
MORE RECENT EVENTS
Headlines about meteoroids and meteorites are pretty much guaranteed to spark our interest. But even while avidly following these remarkable events, we shouldn’t forget that today we generally live in equilibrium with the Solar System, and we rarely encounter dramatic disruptions. Almost all meteoroids are small enough to disintegrate in the upper atmosphere where most of their solid material gets vaporized. Bigger objects arrive only infrequently. But small objects do visit us, and they do so all the time. Mostly micrometeoroids enter the atmosphere, and these particles are so small they don’t even burn up. Though less frequent, millimeter-sized objects enter the Earth’s environs pretty often too—perhaps once every 30 seconds—and those burn up with no significant consequences. Objects bigger than about two or three centimeters partially burn up in the atmosphere so fragments of these might make it to the ground, but those will be too small to be significant.
But every few thousand years, an explosion caused by a big object low in the atmosphere might occur. The largest such event ever recorded occurred in 1908 in Tunguska, Russia. Even without any surface impact, an explosion in the atmosphere can produce noticeable consequences on Earth. This particular asteroid or comet—we often don’t know which—burst in the sky near the Tunguska River in the forests of Siberia. The power of this roughly 50-meter-sized bolide—an object from space that disintegrates in the atmosphere—was the equivalent of about 10 to 15 megatons of TNT—1,000 times bigger than the Hiroshima explosion but not quite as big as the largest nuclear bomb ever detonated. The explosion destroyed 2,000 square kilometers of forest and produced a shock wave that would have measured about 5.0 on the Richter scale. Notably, the trees at what was almost certainly ground zero were left standing, while the surrounding ones were smashed down flat. The size of the zone of the upright trees—and the absence of a crater—meant that the impactor likely disintegrated about six to ten kilometers above the ground.
Risk estimates vary, in part because of the changing estimate of the Tunguska object’s size—which have ranged from thirty to seventy meters. Something with size in this interval might hit at a rate ranging from once every few hundred years to once in every couple of thousand. Even so, most of the meteoroids that do hit or come near Earth approach relatively unpopulated regions, since the distribution of dense population centers is sparse.
The Tunguska meteoroid was no exception in this regard. It exploded over an unsettled area in Siberia where the closest trading station was seventy kilometers away and the nearest village—Nizhne-Karelinsk—was farther away still. Even so, the blast was sufficiently strong in this not-so-closeby village to knock out windows and topple pedestrians. Villagers had to turn away from the blinding flash in the sky. Twenty years after the explosion, scientists returned to the region to learn that some local herdsmen had experienced noise and shock trauma, with two of them actually killed by the impact. The consequences for the animal world were devastating, with perhaps a thousand reindeer killed by the fire that the impact left in its wake.
The event influenced a much larger region as well. The blast was heard by people living at a distance as far away as France is wide, and barometric pressure changed all over the Earth. The wave from the explosion circumnavigated the globe three times. In fact, many of the destructive consequences of the larger and better studied Chicxulub impact I’ll soon get to—the one that killed the dinosaurs—happened after the Tunguska event too, with winds, fires, climate change, and the disappearance of about half the ozone in the atmosphere.
Yet because the meteoroid exploded in a remote and unpopulated region and in a time and place where mass communication was minimal, most people barely paid attention to this tremendous blast until decades later, when an investigation finally revealed the full extent of the devastation. Tunguska was remote, and was isolated even further by the First World War and the Russian Revolution. Had the explosion happened a mere hour earlier or later, it might have hit a major population center, in which case atmospheric effects or an ocean tsunami would probably have killed thousands of people. Had this been true, the impact would have shaped not only the surface of the globe, but the history of the twentieth century—most likely with politics and science unfolding quite differently as a consequence.
Several smaller, but nonetheless newsworthy, celestial visitors have come to Earth in the hundred years since the Tunguska explosion. Though poorly documented, a bolide that burst in the atmosphere above the Amazon in Brazil in 1930 might have been among the larger ones. The net energy released was less than that of the Tunguska event, with estimates varying from 1/100 to 1/2 as big. Even so, the meteoroid mass was more than 1,000 tons, and might have been as massive as 25,000 tons—yielding an energy of about 100 kilotons of TNT. Risk estimates vary, but objects between 10 to 30 meters in size might hit at a rate ranging from roughly once a decade to once every few hundred centuries. The rate estimate strongly depends on the exact size of the object. An uncertainty in size by a factor of two can lead to estimates varying by up to a factor of ten.
A bolide that was similar in size to the one over the Amazon exploded about 15 kilometers above Spain a couple of years later, releasing the equivalent of about 200 kilotons of TNT. A number of explosions occurred in the next 50 years or so, though none were even as big as the Brazil event and I won’t list them all. One event of note was the Vela Incident of 1979, which occurred between the South Atlantic and the Indian Ocean and was named after the U.S. Vela defense satellite that observed it. Though initially considered a plausible meteoroid candidate, people now attribute it to a nuclear blast that was detonated here on Earth.
Of course sensors detect actual bolides too. Department of Defense infrared sensors and Department of Energy visible wavelength sensors picked up the signal from a 5 to 15 meter-wide meteoroid that exploded on February 1, 1994 over the Pacific Ocean near the Marshall Islands. Two fishermen off the coast of Kosrae, Micronesia, a few hundred kilometers from the impact, detected it too. Another even more recent explosion of a 10-meter-wide object occurred in 2002 over the Mediterranean Sea between Greece and Libya, releasing the energy equivalent of about 25 kilotons of TNT. More recent still was the October 8, 2009 event near Bone, Indonesia, which probably also originated from an object about 10 meters in diameter and which released up to 50 kilotons of energy.
Errant comets or asteroids can both be a source of meteoroids. The trajectories of distant comets are difficult to predict, but sufficiently large asteroids can be detected well before they arrive. An asteroid that made impact in 2008 in Sudan was significant in this respect. On October 6 of that year scientists calculated that the asteroid they had just found was about to hit the Earth the following morning. And indeed it did. It wasn’t a major impact and no one lived in the vicinity. But it demonstrated that some impacts can be predicted, though how much advance notice we’ll receive will depend on our detection sensitivity relative to the object’s size and speed.
The most recent newsworthy event was the February 15, 2013 Chelyabinsk meteor, imprinted not just in pictures but also in living memories. This bolide explosion 20 to 50 kilometers above the southern Ural region of Russia generated about 500 kilotons of TNT’s worth of energy—most of which was absorbed by the atmosphere—though a shock wave carrying some of the energy hit the Earth several minutes later too. The event was triggered by an asteroid of about 15 to 20 meters across that weighed about 13,000 tons and descended with an estimated speed of 18 km/sec—about sixty times the speed of sound. Not only did people see the explosion—they felt the heat from its atmospheric entry too.
About 1,500 people were injured by the event—but mostly from secondary consequences such as blown-out panes of glass. The number of people affected was inflated by the many witnesses who had gone to their windows to see the blinding flash that—traveling at the speed of light—was the first sign of something odd. In an unfortunate twist—worthy of a good horror movie—the light in the sky had lured people to precarious locations right before the impact of the shock waves hit and did most of the damage.
Adding to the media frenzy, at the time the meteoroid hit, news reports had warned about a different asteroid that also appeared to be approaching Earth. The Chelyabinsk meteoroid snuck up undetected, while this other 30-meter object—which made its closest approach about 16 hours later—never made it to the Earth’s atmosphere. Many people speculated that the two asteroids had a common origin, but according to follow-up studies, this probably wasn’t true.
NEAR-EARTH OBJECTS
Like the predicted asteroid of February 2013, a number of close approaches by objects that never actually hit or entered the atmosphere have attracted plenty of attention. Other objects do arrive to Earth—but even among these, the overwhelming majority of them are harmless. Nonetheless, past collisions have influenced geology and biology on the planet, and could well do so again in the future. With the increasing appreciation of asteroids and the (possibly exaggerated) awareness of their potential danger, the search for asteroids with the potential to cross the Earth’s orbit has intensified.
The most frequent encounters—though not necessarily the biggest—come from what are known as near-Earth objects (NEOs)—stuff that is pretty close to Earth, with closest approach to the Sun no farther than 30 percent more than the Earth-Sun distance. About ten thousand near-Earth asteroids (NEAs) and a smaller number of comets meet this criterion, as do some large meteoroids within tracking range—and technically some solar-orbiting spacecraft too.
NEAs are divided into several categories. (See Figure 16.) The bodies that enter the Earth’s domain and come close without actually intersecting our orbit are called Amors—named after the 1932 asteroid that came within 16 million kilometers, or a mere 0.11AU. Although they don’t currently cross our path, the potential fear is that Jupiter or Mars-induced perturbations could increase these objects’ eccentricities so that they do ultimately intersect our orbit. The Apollos—again named after a particular asteroid—are ones that currently cross Earth’s orbit in the radial direction, though they can be above or below our ecliptic—the apparent path of the Sun in the sky that denotes the plane of the Earth’s orbit—so that they usually don’t actually intersect. The path can however change over time—again possibly making it deviate into a dangerous zone. A second category of Earth-crossing asteroids—distinguished from Apollos by their orbital domains, which are smaller than the Earth’s—are known as Atens. The Aten family is again named after an asteroid of its type. The final NEA category is Atiras—those asteroids whose orbital domains lie entirely within that of the Earth. They are hard to find, so only a few are known.
NEAs don’t last all that long on geological and cosmological scales. They hang around for only a few million years before they are thrown out of the Solar System or collide with the Sun or a planet. That means in order to populate the region close to the Earth’s orbit, new asteroids have to be in continuous supply. They are probably created by Jupiter’s perturbations to the asteroid belt.
Most of the NEAs are stony asteroids, but there are a fair number of carbon-containing carbonaceous asteroids as well. The only ones bigger than 10 kilometers wide are Amors—which don’t cross our path at present. However, there are a fair number of Apollos bigger than five kilometers across—certainly big enough to do a fair bit of damage should the trajectory prove to be infortuitous. The biggest NEA at 32 kilometers across is Ganymed, which is the German spelling of the Trojan prince whom the English call Ganymede. Ganymede, one of Jupiter’s moons, is a completely different object but also wins a size contest as it is the largest moon in the Solar System.
[FIGURE 16] The four categories of Near-Earth Asteroids. Amors’ orbits lies between that of the Earth and of Mars. Apollos’ and Atens’ paths cross the Earth’s orbit, but can extend beyond it for some fraction of the orbital period. Apollos have semi-major axes greater than the Earth’s, whereas Atens have semi-major axes that are smaller. The orbits of Atiras lie entirely within that of the Earth.
NEAs constitute another research area that has ripened in the last 50 years. Earlier on, no one even took the idea of impacts very seriously. Now people around the globe have begun to catalog and track NEAs wherever possible. Even on my recent visit to the Canary Islands, when I visited the Tenerife telescope, I found the director with a dozen students who were examining data to try to find them. The small old telescope there isn’t state-of-the-art, but I was impressed to see the motivated students and their appreciation of the methods used to search.
Today’s more advanced telescopes seek asteroids through the use of charge-coupled devices, which employ semiconductors to turn photons into charged electrons, leaving signals that pinpoint where the photons had hit. Automated readout systems have also helped escalate the discovery rate. The website of the International Astronomical Union’s Minor Planet Center of the Harvard Smithsonian Center for Astrophysics, http://www.minorplanetcenter.net/, reports the latest numbers of minor planets, comets, and near approaches that have been found.
For obvious reasons, the orbits that are near that of the Earth receive the most attention. The United States and the European Union collaborate on scanning for these in an enterprise called Spaceguard—coined as a shout-out to Arthur C. Clarke’s science fiction novel Rendezvous with Rama. The task of the first Spaceguard program was determined in a 1992 US Congressional survey report, which led to a mandate to categorize within a decade most near-Earth objects that are bigger than a kilometer. A kilometer is big—bigger than the smallest object with the potential to do harm—but was chosen because kilometer-sized objects can be found more readily and are sufficiently big to do world-wide damage. Fortunately, of the kilometer-sized objects we know about, most orbit between Mars and Jupiter in the asteroid belt. Until they change orbits to become NEOs, they certainly pose no threat.
By careful use of observations, projected orbits, and computer simulations, astronomers achieved Spaceguard’s goal of identifying most kilometer-sized NEOs in 2009, almost on schedule. Current findings suggest about 940 near-Earth asteroids of a kilometer or more in size. A committee convened by the National Academy of Sciences determined that even with uncertainties accounted for, this number is pretty accurate, with the total number expected to be less than 1100. These searches have also helped identify about 100,000 asteroids and approximately 10,000 NEAs that are smaller than a kilometer.
Most of the larger NEAs that were the target of the Spaceguard mandate come from the inner and central regions of the asteroid belt. The National Academy committee determined that about 20 percent of the orbits for which they have statistics pass within 0.05 AU of Earth. They called these more precariously located ones “potentially hazardous NEOs.” The Academy also determined that none of these objects pose a threat within the coming century, which is or course welcome news. The result is not all that surprising however since one-kilometer objects are expected to strike the Earth no more than once every few hundred thousand years.
In fact, there is only one known NEO with any measurable probability of hitting the Earth and doing damage in the near future. But the probability that it will come close is a mere 0.3 percent, and even that isn’t projected to happen until 2880. We are almost certainly very safe—at least for the time being—even with all the uncertainties accounted for. Some astronomers had earlier on raised concerns about a different asteroid—the demonically named, 300 meter-wide Apophis, which they had projected would miss the Earth in its close approach in 2029 but potentially return for an impact in 2036 or 2037. This was supposed to follow its passing through a “gravitational keyhole,” which they thought might have the potential to send it speeding off in our direction. However, further calculations revealed this to be a false alarm. Neither Apophis nor any known object should hit us in the foreseeable future.
But before breathing too big a sigh of relief, keep in mind that we still have smaller objects to worry about. Though objects smaller than the kilometer-size that Spaceguard originally targeted would do less damage, they should come close or strike more frequently. So Spaceguard was extended by a 2005 congressional mandate* to encourage the United States to track, catalog, and characterize at least 90 percent of potentially dangerous near-Earth objects bigger than 140 meters across. They almost certainly won’t find something truly catastrophic, but the catalog is nonetheless a worthwhile goal.
ASSESSING RISK
Clearly asteroids sometimes come close. Encounters will undoubtedly occur, but their expected frequency and magnitude remain subjects of debate. Whether or not something will hit and do damage on time scales that we should care about is not yet an entirely settled question.
Should we worry? It’s all a matter of scale, cost, our anxiety threshold, the decisions societies make about what is important, and what we think we can control. The physics in this book primarily concerns phenomena that occur over million or even billion year time scales. The model that I worked on, which the next part of the book will describe, might account for a 30 to 35 million year periodicity for large (few kilometer or so) meteoroid hits. None of these are time scales that are particularly worrisome or relevant to humanity. People have much more pressing concerns.
However, even if it’s a bit of a digression, I couldn’t very well write a book that touches on meteoroid strikes without at least giving some feel for established scientists’ conclusions about their potential impact on our world. The topic comes up in the news and conversation enough that it won’t hurt to share some current estimates. The projections are relevant to governments too when they consider how important asteroid detection and deflection would be.
In accordance with Congress’ 2008 Consolidated Appropriations Act, NASA asked the National Research Council of the prestigious National Academy of Sciences to study near-Earth objects. The goal was not to address any of the abstract impact questions, but to evaluate the risk posed by errant asteroids and whether something could be done to mitigate that risk.
The participants focused their study on smaller NEOs, which hit much more frequently and can potentially be diverted. Comets in short-period orbits are similar to asteroids in their trajectories, so they can be detected in a similar manner. Long-period comets, on the other hand, are virtually impossible to see in advance. They are also less likely to be in the equatorial plane of the Earth’s orbit—they come from all directions—so finding them is more difficult. In any case, though some of the recently observed events might have originated in comets, comets reach the Earth’s vicinity far less frequently. And it would be pretty much impossible to identify long-period comets in time to do anything—even if technological advances do eventually enable us to deflect asteroids. Since there is virtually no way at this point to make a complete catalog of hazardous long-period comets, current surveys focus only on asteroids and short-period comets.
But long-period comets—or at least those comets that arise from the outer Solar System—will be the ones of interest to us later. Objects arising from the outer Solar System are far more weakly bound, so disturbances—gravitational or otherwise—can more readily send one out of its orbit and into the inner Solar System or else out of the Solar System altogether. Though not the subject of the National Academy’s mitigation studies, they can still be the subject of scientific investigations.
THE SCIENTISTS’ CONCLUSIONS
In 2010, the National Academy of Sciences presented their results on asteroids and the threats they pose in a document titled Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies. I’ll present a few of the document’s more interesting conclusions, reproduce a few of the tables and charts that best summarize them, and add a few words and comments to explain what they mean.
When interpreting the numbers, remember to factor in the relatively low density of heavily populated urban areas, which the Global Urban Mapping Project estimates as approximately three percent. Though any destruction would of course be unwanted, the scariest threat would be to an urban area. The low density of cities on the Earth’s surface tells us that the frequency of any relatively small objects hitting and causing significant damage is about 30 times lower than their frequency of impact. For example, if a five to ten-meter-sized object is predicted to hit roughly once every century, something of this size would be expected to hit cities only about once in three millennia.
We should also take note of the large uncertainties in almost all of the projections, which scientists can estimate at best to within a factor of ten. One reason for the many news stories about distant threats that never materialize is that even for particular objects of particular sizes, minor errors in measuring trajectories can make a big difference to the predicted likelihood of a hit. We also don’t fully understand the effects and the damage that even known big objects can cause. Even with such uncertainties, the results from the National Academy study are fairly reliable and useful. So with the allowance of some degree of uncertainty, let’s now explore these fascinating state-of-the-art (circa 2010) statistics.
[FIGURE 17] NAS statistics for average worldwide fatalities per year from a variety of causes. Statistics based on data, models, and projections.
My favorite table is in Figure 17. According to these results, an average of 91 deaths by asteroid occur every year. Although asteroids are far behind most catastrophic causes of death—rates are comparable to those from fatal wheelchair-associated accidents (not listed)—the number 91 in the table next to asteroids is a little surprisingly and uncomfortably high. It is also ridiculously precise given all the uncertainties. Clearly, 91 deaths by asteroid do not occur every year. In fact we know of only a few such deaths in recorded history. The large number is deceptively high because it includes enormous hits that are predicted to occur only very rarely. Here’s an edifying graph (Figure 18) that helps explain.
[FIGURE 18] NAS estimates of average yearly fatalities caused by impacts from asteroids of various sizes based on data from an 85 percent complete Spaceguard Survey. This plot uses the now-revised near-Earth object size distribution and updated estimates for threats from tsunamis and airbursts. Older estimates are also shown for comparison.
What this graph tells you is that the majority of the number quoted above comes from larger objects, which are predicted to occur extremely infrequently. That’s the spike at a few kilometers. Such events are the “black swans” of asteroid hits. If you restrict your attention to objects less than 10 meters in size, the number goes down to less than a few per year, which is still probably on the high side. So what are the expectations for how often objects of different sizes will actually hit? Here’s one more graph (Figure 19) that should help. This one is a little busier, but bear with me. It’s actually a great summary of our current understanding.
Though harder to read, this plot contains a lot of information. It uses what is known as a logarithmic scale. This means that changes in size correspond to much greater time frame variations than you might have in mind. For example, a 10 meter sized object might arrive once per decade whereas a 25 meter object might impact the Earth once every 200 years. It also means that small changes in measured values could significantly affect predictions.
The top axis of this graph refers to how much energy an object of a given size will release, assuming it is traveling at 20 km/sec, as measured in megatons. So, for example, a 25 meter object would release about one megaton. The plot also tells how many objects of various sizes can be expected, and how bright they are likely to be—also related to how easy they are to track and find. Despite the larger number of smaller asteroids, their diminutiveness and their consequently lower brightness makes them more challenging to discover.
[FIGURE 19] Estimated number (left vertical axis) and approximate time between impacts (right-hand axis) of near-Earth objects as a function of diameter measured in kilometers. The top axis gives the expected impact energy in megatons of TNT for an object of a given size, assuming it was moving at 20 kilometers per second on impact. Also shown near the lower horizontal axis is a quantity related to the object’s intrinsic brightness. The different curves are based on older (solid) and newer (circles) estimates. The lower curve represents the number discovered prior to 2009.
The estimates of frequency of these events would be, for example, a 500 meter-sized object about every 100 millennia, kilometer-sized objects perhaps once in 500,000 years, and five kilometer objects on a scale closer to 20 million years. The graph also tells you that a dinosaur-killing-sized impactor of about 10 kilometers in size is expected only about once every ten to one hundred million years.
If you are solely interested in how often strikes occur, the information is clearer in the simpler plot in Figure 20. Notice that the vertical axis has the fewest years at the top and the most at the bottom, so big impacts occur much less frequently than small ones. Notice also the exponential numbers in the vertical column that tell the number of times 10 is multiplied by itself. For example, 101 is ten, 102 is one hundred and 100 is one.
[FIGURE 20] Average numbers of years between impacts on Earth of near-Earth objects of sizes ranging from about three meters to roughly nine kilometers across.
Finally, to get some idea of the extent of the danger from objects of various sizes, I’ll present one final diagram from the Academy study in Figure 21. This table tells us that for something a few kilometers in diameter, the entire globe would be affected. Large meteoroid hits don’t occur nearly as often as other natural disasters, so they almost certainly don’t pose any immediate threat. But if they were to occur, their impact in terms of energy and severity would be devastating. The table also shows, for example, that something 300 meters across might hit the Earth every hundred thousand years. This could increase the sulfur in the atmosphere to levels comparable to those caused by Krakatau, damaging life or at least agriculture on a large part of the planet. And, like the earlier plots, it shows us that a Tunguska-sized airburst might occur about once every thousand years. The full contours of any of these disaster scenarios would of course depend on the size and the location of the hit.
[FIGURE 21] Approximate average impact interval and impact energy for near-Earth objects of various sizes. Keep in mind that these quantities depend on the impactor’s velocity and physical and chemical characteristics.
WHAT TO DO
So what should we conclude from all this? First of all, it’s fascinating that all these objects in space orbit in the same general vicinity. We think of Earth as special and of course want to protect it. But in a larger picture it is just one of the inner planets in a Solar System orbiting around one particular star. However, even as we acknowledge the proximity of our neighbors, the second point to take away is that an asteroid isn’t the greatest threat to humanity. Impacts might occur and might even do some damage, but people aren’t actually in much imminent danger—at least in this regard.
Even so, the question of what to do should something dangerous appear is bound to come up. We’d feel pretty silly if we were watching an object on a dangerous Earth-bound trajectory for a few years but were impotent to do anything to improve our fate. The lack of grave danger doesn’t mean we should be entirely helpless to protect against any damage a meteoroid might do or that we should never think about mitigation.
Not surprisingly, a number of people have thought about the problem and many proposals—though no actual devices—for dealing with dangerous objects from space are under consideration. The two basic strategies are destruction or deflection. Destruction per se isn’t necessarily a great idea. If you blow up something in danger of hitting the Earth into a lot of pieces of rock hurtling in the same direction, you will most likely increase the odds of a hit. Though the damage from any particular piece should be less, a strategy that doesn’t encourage a greater number of hits would be better.
So deflection is probably the more sensible approach. The most efficient deflection strategies involve increasing or decreasing the speed of an incoming object—not a sideways push. The Earth is pretty small and moves pretty quickly around the Sun—at about 30 km/sec. According to the direction from which the incoming object approaches us, changing its path so that it arrives earlier or later by a mere seven minutes—the time it takes for the Earth to move a distance of its radius—can be the difference between a collision and an exciting but harmless flyby. That’s not a huge change in orbit. If something is detected early enough—perhaps a few years in advance—even a small change in velocity should suffice.
None of the suggestions for deflection or destruction would save us from an object bigger than several kilometers in size that is capable of doing global damage. Fortunately, such an impact probably won’t occur for at least another million years. For smaller objects for which we can in principle save ourselves, the most effective deflectors would be nuclear explosives, which could perhaps prevent an impact for something as big as a kilometer across. However, laws forbid nuclear explosions in space, at least for now, so that technology is not being developed. Also possible, though not nearly as powerful, would be a collision of some object with an incoming asteroid so that it transfers kinetic energy, which is its energy of motion. If there is sufficient advanced notice, and especially with the possibility of multiple impacts, this strategy might work for incoming objects several hundred kilometers across. Other suggestions for deflectors include solar panels, satellites acting as gravitational tug boats, rocket engines—anything that could potentially create enough force. Technology along these lines might ultimately be effective for objects as big as a hundred meters, but only with a few decades advanced warning. All of these methods (and asteroids themselves) require further study so it is likely too soon to say with certainty what will work.
Such proposals, though interesting and worthy of consideration, are currently only possible visions for the future. No such technology currently exists. However, one project, the Asteroid Impact and Deflection Assessment mission—designed to test the feasibility of kinematic impact on an asteroid—is relatively far along in its planning. Also in the works is another related project—the Asteroid Redirect Mission—which would deflect an asteroid or piece of one into orbit around the Moon and perhaps host an astronaut visit later on. However, no actual construction on any of these projects has begun.
Some people would argue against building anti-asteroid technology on the grounds that it could be harmful in a broader sense. Some are afraid, for example, that such technology would be used for military purposes rather than for saving the Earth—though I find this highly unlikely in light of the required long lead time for any mitigation device to be effective. Others raise the potential psychological and sociological danger of finding an asteroid on an Earth-intercept trajectory when it is too late or beyond our technological ability to do anything about it anyway—which strikes me as a delaying tactic that can be used against a lot of potentially constructive proposals.
Such spurious concerns aside, we can nevertheless ask if we should make any preparations and, if so, when. This is really a cost and resource allocation issue. The International Academy of Astronautics holds meetings to address precisely these sort of questions and identify the best strategy. A colleague who attended the 2013 Planetary Defense Conference in Flagstaff, Arizona told me about an exercise in which the attendees were supposed to consider a fake approaching asteroid and ask themselves how best to address the simulated threat. They were asked to respond to questions such as “how to deal with uncertainties in its size and orbit that get updated over time,” “when is it appropriate to act,” “at which point should you call the President?,” (the meeting was in America after all) “when is the time to evacuate a region,” and “when would you launch a nuclear missile to deter a potential tragedy?” These questions—though at some level to my mind rather entertaining—make clear that even well-intentioned, well-informed astronomers can have very different attitudes and responses to an object approaching from space.
I hope I’ve convinced you that such threats are not overly pressing, even if some potential damage is possible. Though it’s possible that an infelicitously directed one could hit and wipe out a major population center, the odds of that happening anytime in the foreseeable future are extremely remote. The scientist in me is all for cataloguing and understanding the trajectories of as many objects as we can. And the geek in me thinks a spacecraft that can escort a potentially dangerous NEO into a safe orbit that won’t ever hit the Earth would be very cool. But really, no one knows for certain how best to proceed.
The ultimate issues for society, as with all scientific and engineering efforts, are what we value, what we learn, and what the ancillary benefits might be. You can now consider yourself armed with some basic facts if and when you choose to weigh in with an opinion. Current numbers help, but they are not complete. As with many policy choices, we need to combine educated guesses with practical considerations and moral imperatives. My feeling is that even without any threat, the science is sufficiently interesting to merit the relatively minor investment needed to find more asteroids and study them further. But only time will tell what society—and private industry—ultimately decides.