A web exclusive story from Astronomy magazine

Why the asteroid threat should be taken seriously

By David J. Eicher

It commenced with a press conference, streamed onto the Internet, featuring a rock star, a filmmaker, and a cosmologist. On December 3, 2014, at the Science Museum in London, Brian May, astrophysicist and Queen founder and guitarist, Grigorij Richters, producer and director of the film 51 Degrees North, and Lord Martin Rees, Astronomer Royal of England, made an announcement.

They asked for global participation in “Asteroid Day,” an event to be held June 30, 2015, the 107th anniversary of the Tunguska event, an explosion caused by an incoming asteroid or comet that flattened more than 2,000 square kilometers of forest along the Podkamennaya Tunguska River in central Siberia. Asteroid Day is thus intended to raise awareness over the threat from Earth-crossing asteroids. They read a declaration about the danger our planet faces from impacts by small solar system bodies, a document signed by 100 important scientists, astronaut-explorers, entrepreneurs, and celebrities. They described activities that will take place next June to further raise awareness.

Asteroid Day

Festival origins

The idea originated several weeks beforehand, at the Starmus Festival in the Canary Islands. There, Richters, German-born and a resident of London, screened his film, which portrays events leading up to an asteroid impact in London, a film that was enthusiastically received and featured musical contributions by May. Richters also had the idea, along with his friend, photographer Max Alexander, to assemble a movement that would lead to Asteroid Day.

In terms of disclosure, I was a speaker at the Starmus Festival, sat in the front row to watch 51 Degrees North, and enjoyed it very much. I was even present at a dinner on the summit of La Palma, during the festival, when Richters and Alexander raised the issue of an Asteroid Day and began talking about it as a hypothetical event. And that made what was to come even more absorbing.

The press response to the Asteroid Day announcement was spectacular — I think, fair to say, beyond anyone’s expectations. Although it should be said that whenever Brian May does something, it certainly attracts attention, and the same could also be said of Martin Rees, who is one of the most brilliant people on the planet. The announcement found itself plastered throughout numerous newspapers and online media the world over. The attention was explosive, and certainly was also helped from the inclusion of two ex-astronauts, Ed Lu and Rusty Schweickart, under whose guidance the B612 Foundation has tackled the asteroid threat. This forward-looking organization focuses on the asteroid impact danger and proposes a future Sentinel mission to thwart a potential large space rock with Earth’s name on it. They were also joined by the ubiquitous Bill Nye, president of the Planetary Society, who did an excellent job of explaining the realities of asteroid impact dangers.

The Asteroid Day crew, a loose assemblage of folks helping the hard-working Richters, established a website, www.asteroidday.org.

Get ready for ASTEROID DAY
June 30, 2015, is the 107th anniversary of the Tunguska event, when a small asteroid or comet flattened more than 2,000 square kilometers of forest in central Siberia. In marking the anniversary, scientists, astronauts, and celebrities will come together to help raise awareness about the dangers our planet faces from Earth-crossing asteroids. To learn more about the events planned for Asteroid Day, visit http://www.asteroidday.org.

As the mission of Asteroid Day moved toward producing educational content and fleshing out plans for the summer of 2015, reactions to the announcement and the subsequent publicity began trickling in from the community of astronomy enthusiasts. Strangely, I found the topic to be more polarizing than logic would have dictated. Massive support rolled in from many who love watching and studying the night skies — after all, protecting the planet from impact is a good thing. But another contingency struck out in social media posts, on blogs, and elsewhere, sometimes even angrily accusing the movement of exaggerating the possibilities of death from the skies. In a world increasingly dominated by 140-character tweets, I found lots of hearsay and accusations washing back and forth with little substance or real understanding.

The question arises, then: What exactly is our current best knowledge about the real danger of future impacts? To help answer this, I consulted a number of planetary scientists and read voluminous papers from others. Gradually, a clear picture of reality began to crystallize.


Assessing the realities of doomsday

First, I turned to a presentation from a scientist whose work I have known for many years as being characterized by unimpeachable credibility, Paul Chodas of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. Chodas is a leading authority on the dynamics of asteroid orbits and the impact probabilities from small solar system bodies. He is the primary creator of the orbital calculation and impact probability software used by NASA, and specifically the near-Earth object office at JPL. Chodas is, along with his colleague Don Yeomans (who has just retired), also a co-developer of the Sentry impact monitoring system, automated software that continuously scans databases of the orbits of known asteroids, checking for potential future collisions. JPL’s Steve Chesley has also contributed substantial amounts of work to this project.

Chodas reminds us that just two years ago, we had two unrelated encounters with small bodies passing close to or striking Earth during the same day. On February 15, 2013, a small asteroid, perhaps measuring 20 meters across, came down over the southern Urals of Russia, barreling in at about 19 km/s, and exploded over Chelyabinsk Oblast, near the town of Chelyabinsk. With a mass greater than that of the Eiffel Tower, the asteroid exploded in an airburst, unleashing energy equal to about 500 kilotons of TNT, some 20 or 30 times the energy released in the Hiroshima atomic explosion. The enormous resulting shock wave shattered glass in the town’s buildings, injuring nearly 1,500 people. Eerily, within 24 hours, 2012 DA14, a space rock about 30 meters across, whizzed past Earth at a distance of some 27,700 kilometers, some 2.2 times Earth’s diameter.

Asteroid Day

Suddenly, the human race suffered its first known injuries from a small asteroid explosion, and a significant event from a small solar system body crossing Earth’s orbit. The two events, Chelyabinsk and 2012 DA14, which seemed intuitively connected due to timing, were not. They were separate objects on completely different orbital paths.

But these were just the latest events. Earth has a long history of impacts from other bodies in the solar system, one that is almost entirely hidden because of our planet’s continual resurfacing — from erosion, plate tectonics, volcanism, and more. In the early solar system, Earth was struck frequently and by large objects. Most planetary scientists believe the Moon formed as the result of a very early collision between Earth and a planetesimal some 4.53 billion years ago. During the so-called Late-Heavy Bombardment, about 4.1 to 3.8 billion years ago, numerous large objects impacted Earth. The Moon, which does not hide its scars so effectively, shows this impressive battering yet today.

Most travelers to northern Arizona are familiar with Meteor Crater near Winslow, and walking the perimeter of the 1-kilometer rim makes for an interesting hike. Some 50,000 years ago, a 30- to 50-meter iron meteorite, part of the core of an asteroid, hurtled into the desert plain, striking with the force of 15 megatons. More menacingly, however, is the story of the Chicxulub Crater, a subsurface scar lying underneath the Yucatán Peninsula in Mexico. In the late 1970s, two geophysicists working for the Mexican oil giant Pemex discovered a huge underwater arc in a ring some 40 kilometers across. They soon found another arc and then discovered the feature formed a circle, suggestive of an ancient impact crater.

Near-Earth asteroid 433 Eros spans some 33 kilometers in its longest dimension. This object poses more of a threat than most large asteroids because its orbit comes close to Earth’s.

At roughly the same time, Nobel Prize-winning physicist Luis Alvarez, along with his son Walter and other collaborators, had stumbled into a shock. They found evidence of a massive impact on Earth coinciding with the boundary between the Cretaceous and Paleogene geological eras, some 66 million years ago. The Alvarez team discovered high levels of iridium and osmium, and four years later scientists found shocked quartz and microdiamonds associated with an extraterrestrial impact. This coincided with the disappearance of the dinosaurs, and the K-Pg Impact (first called K-T before the redoing of geological nomenclature) was held responsible. Moreover, geological evidence ties the Chicxulub Crater with the impact, giving geologists a place on Earth where the impactor struck. This was no small rock, either, but a roughly 10-kilometer asteroid.

Two other recent events gave planetary scientists pause. In 2008, for the first time, astronomers discovered a small asteroid that was heading toward Earth, before it impacted. Designated 2008 TC3, the tiny space rock was a 4-meter-wide object weighing some 80 tons that Richard Kowalski of the Catalina Sky Survey near Tucson found on October 6 of that year. A day later, the small rock hurtled into Earth’s atmosphere and exploded over the Nubian Desert in Sudan. Enthusiasts and scientists recovered more than 600 meteorites collectively weighing some 10.5 kilograms and named the fall after the nearby desert railway station Almahatta Sitta.

Just five years later, on New Year’s Day 2014, Kowalski again discovered a small asteroid, 2014 AA, some 2 to 4 meters across, bound for Earth. Some 21 hours after discovery, the small rock entered Earth’s atmosphere somewhere along a line between northern South America and western Africa, quite probably into the ocean. The small number of observations didn’t allow calculating a precise point of impact.

Catalina Sky Survey/University of Arizona

Tracking space rocks

Close passages of asteroids to the Earth-Moon system occur frequently. The latest by a large asteroid, that of 2004 BL86, took place January 26, 2015, when this 300-meter space rock, a binary system, passed 1.2 million kilometers from Earth, about three times the distance between Earth and the Moon. According to physicist Mark Boslough of Sandia National Laboratories in New Mexico, an asteroid the size of the Chelyabinsk impactor, around 20 meters across, passes within geosynchronous orbit every two years, and within the Moon’s orbit nearly once a week. A Tunguska-sized (40-meter) object passes within the lunar distance from Earth several times a year.

This illustration shows the orbits of all of the so-called Potentially Hazardous Asteroids — those bigger than 140 meters across that come close to Earth — known in early 2013.

Asteroid impact expert Alan Harris, now retired from JPL, estimates that 200 million objects equal to or greater than 6 meters across are in Earth-crossing orbits. Harris, in fact, is the one who has produced population studies, most recently in 2012 and 2014, that have been quoted and used by Chodas and others. According to Harris, objects 6 meters or larger across strike Earth about once every two years. Roughly 10 million Chelyabinsk-sized objects are in Earth-crossing orbits and the impact interval is closer to 50 years.

The system of discovery used by Kowalski and his colleagues, the Catalina Sky Survey, is one of the primary tools employed by NASA to search for near-Earth objects and to create a list of so-called Potentially Hazardous Asteroids that could impact Earth. The first stage in assessing the threat of asteroids to Earth is to create a full inventory of near-Earth objects so that astronomers know what’s out there and can understand their orbits as carefully as possible. In 1998 the U.S. Congress issued a directive to NASA to discover and track at least 90 percent of near-Earth objects of 1 kilometer or larger in diameter. A further directive in 2005 ordered NASA to identify potential impactors of 140 meters or larger.

The Catalina Sky Survey is headed by Staff Scientist Eric Christensen and Senior Staff Scientist Steve Larson of the University of Arizona’s Lunar and Planetary Laboratory. The survey telescope is a 0.8-meter Schmidt camera located on Mt. Bigelow in the Catalina Mountains just north of Tucson. Further, the 1.5-meter telescope on Mt. Lemmon, also in the Catalina Mountains north of Tucson, is used as both a discovery and a follow-up instrument.

The Japanese spacecraft Hayabusa landed on the near-Earth asteroid 25143 Itokawa in 2005 and returned samples to Earth in 2010. Itokawa measures just 0.5 by 0.3 by 0.2 kilometers across, but such a space rock still could cause continent-wide devastation.

The Catalina Sky Survey is not alone. In Hawaii, the Pan-Starrs 1 telescope is also actively involved with near-Earth object discovery, as are the Darpa Space Surveillance Telescope and NEOWISE, a study using the Wide-Field Infrared Survey Explorer spacecraft. Additionally, the Lincoln near-Earth asteroid Research project has been a collaboration between the U.S. Air Force, NASA, and the Massachusetts Institute of Technology. Like the Catalina Survey, the Spacewatch program is hosted at the University of Arizona and uses two telescopes on Kitt Peak, Arizona, to help survey near-Earth objects. The NEOWISE survey, headed by planetary scientist Amy Mainzer at JPL, has been very productive, detecting more than 400 near-Earth objects in a relatively short period, including some 170 discoveries. Mainzer also leads a team that has proposed NEOCam, a space-based infrared telescope designed to discover and characterize perhaps the majority of potentially hazardous asteroids near Earth.

With these surveys and others underway, astronomers have discovered a large number of near-Earth objects, with more than 12,000 currently known. (Nearly all such objects are known to be asteroids, but about 1 percent are comets.) How many of these objects are relatively large? Some 868 are near-Earth asteroids larger than 1 kilometer across, and they would produce a global catastrophe if they struck Earth. Planetary scientists currently estimate that some 980 such objects ought to exist, and therefore that they know of just under 90 percent of them. Chodas and other planetary scientists stress that new telescopes with larger apertures and greater sensitivities, both on the ground and in space, will be needed to find the majority of the smaller asteroids, objects between 100 and 300 meters across.


How real is the asteroid threat?

From all that astronomers have learned about asteroids over the past generation, they know that the danger from near-Earth objects is very real. On average, they estimate a Tunguska-sized asteroid will strike Earth every 500 years. An asteroid the size of the object that created Meteor Crater will enter Earth’s atmosphere on average every few thousand years. It should be said that the object that created Meteor Crater was an iron asteroid, and that composition enabled it to survive until it struck the ground. It was only a little larger than Tunguska in total mass. But the fraction of iron objects relative to rocky objects is small.

A civilization killer like the 10-kilometer asteroid of the K-Pg impact, the extinction event that did away with the dinosaurs, will strike on average every 100 million years. But these are averages; the next big impact could happen next year, or 100 years from now. Or 300 million years from now. Averages are numbers games and don’t particularly care when the last event occurred.

The Galileo spacecraft captured this view of asteroid 951 Gaspra in 1991. Its dimensions (about 19 by 12 by 11 kilometers) make it only slightly larger than the kind of asteroid that could wipe out civilization.
NASA/JPL-Caltech/Arecibo Observatory/USRA/NSF

The K-Pg impact created a mass extinction event because a 10-kilometer asteroid unleashes enough energy to cause global catastrophe. A small asteroid impact from an object a few meters to a few tens of meters across would cause a localized problem; a 10-meter object might cause a local or regional crisis. A very small rock does you no good if you’re standing underneath it when it lands. An asteroid like the one that scooped out Meteor Crater or flattened the Siberian forest would cause a disaster of epic proportion if it struck a city. No one knows, and current research is investigating, whether a space rock of this size that struck the ocean would cause a far-ranging tsunami. But an asteroid of 1 to 2 kilometers in diameter — though it is smaller than the dinosaur killer — packs a sinister and devastating punch.

A 1- to 2-kilometer asteroid not only causes local and regional devastation, but it also strikes with such force and delivers so much energy that it casts a large amount of material far up into the atmosphere such that it comes down globally. Modelers of the resulting nuclear winter scenario believe such an impact ignites widespread catastrophic fires and blots out sunlight, permanently altering the planet’s ecosystem. It is this problem that wiped out the dinosaurs, who otherwise by rights should exist still today, and enabled small mammalian survivors to carry on, in need of only modest amounts of food, to evolve 66 million years later into human beings.

The 12,000 near-Earth objects now known by scientists are not the end of the story. Using work from a variety of sources and projects, Chodas estimates that something like 20,000 such objects in the range of 100 meters or larger must exist in the space surrounding Earth. In late 2014, NASA scientists released a bolide map showing 556 separate events between 1994 and 2013 when small asteroids entered Earth’s atmosphere, unleashing energy and resulting in a bright fireball in earthly skies. The range of sizes of these objects is believed to be from about 1 meter to 20 meters.

These 20 radar images reveal the 400-meter-wide asteroid 2014 HQ124, which passed within 1.25 million kilometers (about three times the Moon’s distance from Earth) of our planet June 8, 2014.

And the effects of an asteroid impact on Earth vary wildly with the size of the impactor, so the data about what’s out there, which is still partially unknown, becomes critical. According to Chodas, a 5-meter asteroid entering Earth’s atmosphere will produce a bolide with little other effect, unleashing about 10 kilotons of energy, and this type of event will happen on average every couple of years. An incoming 25-meter asteroid will produce an airburst event, unleashing 1 megaton of energy, and this will happen on average every 200 years. A 50-meter asteroid will strike Earth on average once every 2,000 years and will cause local scale devastation as it hits with 10 megatons of energy.

When asteroids are larger yet, the potential for widespread damage and deaths on Earth rises significantly. A 140-meter asteroid will impact Earth on average every 20,000 years, according to Harris, and will unleash 300 megatons of energy, causing regional scale devastation. A 300-meter asteroid will impact Earth roughly every 70,000 years, unleashing 2,000 megatons of energy and creating continent-wide devastation. A space rock twice that size, a 600-meter rock will impact Earth about every 200,000 years, impacting with 20,000 megatons of energy, and creating widespread but not global devastation.

It is the largest potential impactors, of course, that could create the biggest trouble. A 1-kilometer asteroid will impact Earth once every 700,000 years, on average, according to Chodas, impacting with the force of 100,000 megatons and causing a possible global catastrophe. Every 30 million years, on average, a 5-kilometer space rock will impact Earth, unleashing 10 million megatons and causing an event above the threshold of a global catastrophe. And as we’ve seen, once every 100 million years, on average, a 10-kilometer asteroid like the one that did in the dinosaurs will strike Earth, unleashing 100 million megatons of energy and causing a mass extinction.

The bottom line? A 1- or 2-kilometer asteroid will impact Earth, on average, about once every million years, and could produce a global catastrophe.

NASA/Hubble Space Telescope Comet Team

Protecting Earth’s future habitability

Over the past generation, physicists, astronomers, and planetary scientists have come to grips with the long-term future of Earth’s habitability. Once a hazy unknown, the distant future of life on Earth has now become relatively clear. The Sun is a slowly varying star and is gradually increasing its radiation as time rolls on. Set the current threat of global warming aside: If humans can survive all the other perils we face as inhabitants of a planet, increased solar radiation will ultimately kill off the human race, on planet Earth, a billion years or less from now. By that time, the Sun’s radiation will increase to the point where Earth’s oceans will boil away, and it will be game over.

But as we have just seen, many catastrophic asteroid impacts likely will occur within that time frame. Are we worried about a catastrophic event in the next 5 or 10 years? Or 1,000 years? Or 5,000? Perhaps not. But what we know about the near-Earth object population, and about the law of averages, says there is plenty to prepare for over the span of a billion years, in terms of defending our planet and our lives. We might have as many as 10 more impacts like the one that killed the dinosaurs. We might have as many as 1,500 impacts by a 1-kilometer asteroid over the next billion years, any of which could cause a global catastrophe.

The inventory of large near-Earth objects is pretty close to complete. Planetary scientists know of only 20 near-Earth asteroids larger than 5 kilometers in diameter, and it’s likely they’ve found them all. They have found only two larger than 10 kilometers, and according to Boslough, scientists are “98 percent sure” there are no others. Earth is effectively at zero risk for an impact by a 10-kilometer body, at least anytime soon, and they effectively shouldn’t enter the equation.

The boulder-strewn surface of the large main-belt asteroid 21 Lutetia stands out in this view taken in 2010.

However, the inventory of near-Earth asteroids is not entirely complete. Chodas estimates that planetary scientists know of about 90 percent of such objects larger than 1 kilometer. They have probably discovered more than 50 percent of the near-Earth objects a few hundred meters across. The space rocks measuring between 100 and 300 meters in our neighborhood? We probably know of roughly 15 percent of them. And the smaller objects, those of a few dozen meters or smaller? Planetary scientists know of 1 percent of those or less.

So the cataloging and analysis of orbits must continue. But the near-Earth object population doesn’t make up the whole story. The asteroids and comets close to Earth’s orbital space are not a static population. Over time, on the scales of several hundred thousand years, asteroids can migrate into near-Earth space from the more distant main belt of asteroids, the well-stocked group of space rocks orbiting between Mars and Jupiter. And far beyond the main belt, out in the vicinity of Neptune and Pluto, lies the Kuiper Belt, another huge population of icy asteroids and comets. And of course far beyond the Kuiper Belt, at the periphery of our solar system, is the Oort Cloud, an icy reservoir of perhaps as many as 2 trillion comets. Objects from the Kuiper Belt or beyond, be they comets or asteroids on peculiar orbits, could pass into the inner solar system and be on a collision course with Earth’s orbit, too.

The risk to Earth from impacts is clearly significant from the near-Earth object population, present but much less likely from the main belt of asteroids, and possible but unlikely from the Kuiper Belt and beyond. The risk certainly lessens greatly with greater distance from Earth. According to planetary scientists Hal Levison and Luke Dones of the Southwest Research Institute, the risk from the Kuiper Belt or the Oort Cloud is an order of magnitude, and possibly two orders of magnitude, less than from closer asteroids.

Moreover, Boslough raises the question of a particularly menacing population of small objects. Many amateur astronomers recall the exciting days in 1994 when backyard telescopes revealed dark blotches in the cloudtops of Jupiter, caused by the infalling pieces of Comet Shoemaker-Levy 9. Small objects whose orbits have evolved can fall into Earth with little or no warning, as was the case with Chelyabinsk. Boslough calls these objects “death plunge” asteroids and warns that we need a much better system of detecting potentially large numbers of these objects that could strike Earth more quickly than humans could devise a way to deflect them. Surveys should be extended to find all such objects like 2008 TC3 and 2014 AA, he suggests.

A first strike at such an early warning system will go live this year when ATLAS, the Asteroid Terrestrial-impact Last Alert System, comes on line. This project is being developed by the University of Hawaii and funded by NASA, and will consist of two telescopes, separated by 160 kilometers, designed to provide a one-day warning of a 30-kiloton “town killer” asteroid, a week’s warning of a 5-megaton “city killer,” and three weeks’ warning of a 100-megaton “county killer.” 

The best-studied asteroid is the main-belt object 4 Vesta, which NASA’s Dawn spacecraft orbited for more than a year in 2011–12. Although Vesta likely will remain in place throughout the solar system’s history, some main-belt objects can be nudged onto Earth-crossing orbits.

The risk from asteroids impacting Earth and causing widespread damage, death, and catastrophe is real, and is present every day of our lives. But it is to a degree a counterintuitive threat, which makes it hard for some people to take seriously. The risk at any given moment is almost nonexistent, but given enough time, a catastrophic event will happen again. Do we need to worry about an asteroid strike during our next foray out to lunch? Probably not. But someday a large enough asteroid with Earth’s name on it will enter the picture, causing horror and mayhem for humanity. Unless we do something about it, that is.

Large asteroid impacts affect the entire planet, whereas smaller ones have a more localized effect. To answer the question, “How often will asteroid death come to your town?,” the answer is more often from a global event than from a local one. If you multiply the impact frequency by the area affected, the larger events are more frequent. That balance is changing as planetary scientists discover more bodies, but the fact remains that the risk is still slightly greater from the remaining undiscovered big objects than from the small ones.

Understanding the risks from asteroid impacts on Earth is a pretty young exercise, as is the case with much of astronomy and planetary science. We now know that future dangerous impacts will happen, though they may be many years away. From a planetary scientist’s view, however, it would be grossly negligent to avoid completing as thorough a survey as possible of all the space rocks in Earth-crossing orbits and understanding other small bodies farther out in the solar system that could come our way.

It is an insurance policy for planet Earth. We should not be alarmed as concerned human beings. But we should be determined, informed, and on the clock, keeping track of the solar system and its movements. One day they will interact again in a big way with our planet. Perhaps we will discover incoming asteroids and be able to divert their orbit before disaster strikes. We damn sure will want to be ready when that day comes. Anything less would be a reckless misuse of the knowledge our species has worked so hard to gain.

David J. Eicher is Editor-in-Chief of Astronomy magazine. He is author of 18 books on science and history, including Comets: Visitors from Deep Space (Cambridge University Press, 2013) and the forthcoming The New Cosmos: Answering Astronomy’s Big Questions (Cambridge University Press, 2015). Follow him on Twitter: www.Twitter.com/deicherstar.