See text of the Spaceguard Survey Action Plan requested by the Subcommittee at the hearings, submitted on 9 June 1998.

Statement on

The Threat of Impact by Near-Earth Asteroids


Dr. Clark R. Chapman

Southwest Research Institute

before the

Subcommittee on Space and Aeronautics

of the

Committee on Science

of the

U.S. House of Representatives

at its hearings on

"Asteroids: Perils and Opportunities"

May 21, 1998

Mr. Chairman and Members of the Subcommittee:

I am pleased to discuss with you the threat to our civilization from impacting asteroids. The threat is something I think we should all think about, but I am happy to report that I feel that we can still sleep well at night. I am from the Southwest Research Institute, of San Antonio, Texas, a large, diversified, non-profit research institute in its 52nd year of serving this nation. As a research scientist in the Boulder, Colorado, Space Studies Department, I am expert on asteroids and on studies of impact craters on planetary surfaces. I participate on the imaging team of NASA's Galileo mission that is currently orbiting Jupiter and studying its moon, Europa, which may have an ocean beneath its icy crust. Earlier this decade, Galileo made historic, first-ever observations of two asteroids, Gaspra and Ida, which orbit the Sun within the main asteroid belt, between the orbits of Mars and Jupiter.

I am also on the science team of the Near Earth Asteroid Rendezvous mission; that spacecraft, developed at Johns Hopkins Applied Physics Laboratory, will enter orbit around the asteroid Eros eight months from now. Nearly 25 miles long, Eros is one of the largest of the so-called Earth-approaching asteroids; it has a 5% to 10% chance of ending its existence, several million years from now, by crashing into the Earth. NEAR is studying Eros not because of its danger but for clues it may hold about the origin of the solar system. If Eros does crash into Earth, it will be even more devastating than the impact 65 million years ago that extinguished the dinosaurs, and made it possible for mammals and, eventually, homo sapiens to thrive on planet Earth.

o o o

The Impact Hazard

I wish to talk with you not about the probability of impacts millions of years from now, but about the slight possibility that an asteroid or comet might strike Earth in our lifetimes, perhaps destroying civilization as we know it. It takes a truly huge object like Eros, or like the comet in the movie "Deep Impact," to threaten mass extinctions of species. Fortunately, Eros cannot strike Earth in the near future. And impacts of such magnitude occur extremely rarely, once in perhaps 100 million years. That's only one chance in a million of happening during the 21st century: really unlikely! It is an appropriate topic for science fiction, but nothing to worry about. Such a body is so large, there's little we could do about an Extinction Level Event, anyway ("Deep Impact" notwithstanding).

A more serious problem, and one that we can do something about, is the chance that a smaller asteroid or comet, about a mile wide, might hit. The best calculations are that such an impact could threaten the future of modern civilization. It could literally kill billions and send us back into the Dark Ages. Such an impact would make a crater twenty times the size of Meteor Crater in Arizona. The gaping hole in the ground would be bigger than all of Washington, D.C., and deeper than 20 Washington Monuments stacked on top of each other. It would loft so much debris into the stratosphere, which would spread worldwide, that agricultural production around our globe would come to a virtual halt: the dust would dim the sunlight for months, perhaps a year. Especially if the asteroid struck without warning, there would be mass starvation. No nation would be unscathed, so no nation could assist others, unlike the aftermath of World War II.

Such civilization-threatening impacts happen hundreds of times more often than Extinction Level Events, perhaps once every few hundred thousand years...or one chance in a few hundred thousand that one will impact next year...or one chance in a few thousand during the next century -- during the lives of our grandchildren. Those chances are so small that they are difficult to comprehend. But it is more likely to happen than that the next poker hand you are dealt will be a Royal Flush. The chances are much greater than the chance that you will be the big winner in a state lottery, yet people buy lottery tickets all the time. Few people would board an airplane if they thought its chances of crashing were a chance in a few thousand. Indeed, the chance that your tombstone will read that you died from an asteroid impact holocaust is about the same as that of your tombstone saying that you died in an airliner crash. The Table shows some other comparative odds of death, to put the impact hazard into perspective. Should we do nothing in the face of the slight possibility that everything our forebears have created since the Renaissance might be undone?

Table. Chances of dying from selected causes (USA)

               Cause of death                                            Chances

Motor vehicle accident                                                   1 in 100
Homicide                                                                 1 in 300
Fire                                                                     1 in 800
Firearms accident                                                        1 in 2,500
Electrocution                                                            1 in 5,000
Asteroid/comet impact                                                    1 in 20,000
Passenger aircraft crash                                                 1 in 20,000
Flood                                                                    1 in 30,000
Tornado                                                                  1 in 60,000
Venomous bite or sting                                                   1 in 100,000
Fireworks accident                                                       1 in 1 million
Food poisoning by botulism                                               1 in 3 million
Drinking water with EPA limit of tricholoethylene                        1 in 10 million

(From C.R. Chapman & D. Morrison, 1994, Nature 367, 33-40.)

Fortunately, unlike many disasters that threaten us about which we can do little, there are things we can do about the impact hazard. First, and most important, we can find out whether or not a mile-wide asteroid is actually headed toward us. By sampling the heavens, we can tell that there are at least 2,000 asteroids of the class that could strike the Earth which are more than a kilometer across; that's nearly 2/3rds of a mile across, and well within our uncertainties of what's big enough to cripple civilization. Of the 2,000, however, we have discovered and charted the paths of only about 245, or 12%. None of them, we have learned, are targeted towards Earth within the foreseeable future. But any one of the other 88% -- 1,755 potential killer rocks out there -- could strike at any time, even this afternoon, without warning. We simply haven't been looking hard enough.

Nothing is perfectly safe in this world. But if, ten years from now, we could say that we have reduced our worries by a factor of ten -- that the chances of an asteroid striking are ten times less, because we have discovered and certified 1800 of the 2000 potentially dangerous asteroids as safe, then we could sleep a little easier at night. Moreover, if -- by bad luck -- there really is an asteroid headed our way, there might, after ten years searching, be an excellent chance that we would have found it. And then, we could probably save ourselves. At the very least, we could evacuate ground- zero, and we could save up food supplies and try to weather the global environmental catastrophe. We even have the military technology, provided we have a decade's warning time or more (which is likely), to study the threatening object, to launch a rocket with powerful bombs, and explode a bomb in just the right place to give the object a little kick, causing its path to change ever-so-slightly so that, years hence, it misses the Earth instead of bringing catastrophe to our planet.

But we will not sleep easier, and we probably will not soon find the threatening object if it is there, if we keep doing just the meager, ineffective searches that we have been doing during the last few years. David Morrison, of NASA's Ames Research Center, and I published our book, "Cosmic Catastrophes" nine years ago, first calling to public attention the work of Gene Shoemaker's 1981 Spacewatch workshop. Dr. Morrison addressed the Congressional Space Caucus in 1989, telling them about the problem, and about the prospects. The Congress responded by calling on NASA to study the impact threat, which it has now done twice. There has been a lot of subsequent talk, but very little if anything has actually been done in response to the study's recommenda- tions. One of the chief projects searching the heavens, the Spacewatch program in Arizona, receives only about a quarter of its funding from NASA -- most of the rest is from private donations. Much of the NEO search effort has been assisted by volunteers.

Gene Shoemaker, who died tragically last year in Australia while studying im- pact craters in the remote Outback down under, worked tirelessly to help our nation, and the world, understand that the impact threat is real. He even co-discovered the comet, Shoemaker-Levy 9, that crashed onto Jupiter in 1994 creating zones of firestorm and devastation as large as the entire planet Earth. But de- spite Shoemaker's work, mine, and that of a few dozen other scientists around the world -- including today's witnesses John Lewis and Greg Canavan -- very little has been done to actually address the hazard that could end our civilization, or even our species.

At the current rate of discovery, it will take nearly a century to inventory 90% of the threatening asteroids. If an asteroid strikes during the next few decades, we will have failed our responsibilities "on our watch" to protect civilization, especially since we are the first generation with tools adequate for the job. To be sure, a century from now, technology will have inevitably advanced so that our great-grandchildren will be effectively searching the skies for threats. Unless, that is, civilization has been dealt a deadly blow before then, say in the next thirty years, in which case it will be our fault that we did next-to-nothing.

Now, I don't think the chances are great that this disaster will happen. The chances are, in fact, very small. But the consequences are so great that the simple probabilistic calculation of deaths per year is similar to that of many natural disasters, like earthquakes, hurricanes, or floods. Many more people die of war and disease than from natural disas- ters. But if you think earthquakes are a matter of concern, you might well think of impacts as of concern. As shown in the Figure 1, all natural hazards combined kill only about ten times as many people as would die, on average, from impacts. Of course, few people, if any at all, have died from impacts in recorded history. But we're playing the odds: just as we sometimes make a small investment in a high-risk chance of winning big in the stock market, we can make a comparatively small national investment in protecting civilization from the small chance of a global catastrophe.

o o o

The Spaceguard Survey

The visionary science fiction writer Arthur C. Clarke is widely credited with foreseeing communications satellites half-a-century ago. In the 1970's he wrote a novel that introduced the "Spaceguard Survey," a project that would search the heavens for threatening asteroids. (A more recent Clarke novel is the basis for the current movie, "Deep Impact.") Astronomers trying to scan the skies for dangerous near-Earth objects (NEO's) have adopted the name "Spaceguard Survey" to describe the proposed international array of telescopes that could find most of the celestial bodies that threaten us.

In 1992, the first Congressionally mandated Spaceguard Survey report was written by a NASA committee chaired by David Morrison, outlining the survey. The report was filed, but little was done. Following the spectacular portent of the Shoemaker-Levy 9 comet crashes in 1994, NASA formed another committee at Congress' behest, chaired by the late Gene Shoemaker. I was a consultant to, and participant in the deliberations of, this "Near-Earth Object Survey Working Group." Its updated plan and budget for the Spaceguard Survey was published in June 1995. In response to one of the questions of the Space and Aeronautics Subcommittee, I want to describe its recommendations.

The goal adopted by the Committee was to find 90% of the near-Earth asteroids and short-period comets larger than 1 km diameter within 15 years, or within 10 years if the recommended efforts by NASA could be augmented significantly by the Air Force and by other nations. Figure 2 shows the fraction of completeness (1.0 = 100%) that can be achieved for objects of different sizes (the x-axis is a logarithmic scale from 100 meters to 10 km diameter) for five different survey systems studied, ranging from the Palomar telescope once used by Eleanor Helin and the Shoemakers through an enhanced Spaceguard system.

The recommended approach was to build two 2-meter aperture (diameter of the primary mirror) telescopes, designed and dedicated for NEO discovery. These, and additional, existing 1-meter telescopes would be equipped with state-of-the-art detectors and electronics to search for NEO's and to make the crucial follow-up observations of ini- tial discoveries. Additional funds were proposed for coordinating the program and handling the massive load of data, and for half-time use of an existing larger telescope to study the physical properties of a representative sample of threatening objects.

The start-up costs were estimated to total $24 million for the first 5 years, followed by annual operations costs of about $3.5 million for a 15-year total of about $60 million, not including funding for the augmented Air Force or international facilities.

There are other desirable features of the Spaceguard Survey, discussed in the Shoemaker report. For example, radar observations of NEO's have unprecedented capabilities to pinpoint their orbits, as well as to assess their generic composition (metal, rock, ice). Scientific studies, which would inevitably result from the Survey, would shed light on the origin of planets as well as char- acterize NEO's for possi- ble utilization of their materials for space-con- struction, fuel, or life- support. Such an aster- oid may even serve as astronauts' "stepping stones" to exploration of Mars. I am sure that Prof. Lewis will amplify on these possibilities.

An integral part of the Spaceguard Survey is its international charac- ter. All nations are threatened by a globally destructive impact. So, naturally, there has been international interest in addressing the threat. Interest has been especially high in Russia, which -- due both to its vast area and to bad luck -- has been the target of two of the worst impacts of the twentieth century. In 1908, a 15-megaton TNT-equivalent blast occurred over a remote portion of Siberia, flattening the forests for tens of miles in every direction. This was due to the impact of a stony asteroid, which exploded less than 10 km up in the atmosphere over the Tunguska river valley. In 1947, another cosmic impact in the Sikhote-Alin region of Siberia formed more than 90 craters between 1 and 27 meters in diameter across the landscape. Not surprisingly, there has been interest among Russian astronomers and military technologists alike to respond to the cosmic threat. However, economic circumstances in the former Soviet Union make it unlikely that an initiative to start the Spaceguard Survey will begin in Russia. Another country, Australia, has actually backed away from its fledgling telescopic program, which -- until the past couple of years -- played a fundamental role by following-up on NEO's discovered elsewhere from its special location in the southern hemisphere. International attempts to encourage the Australian government to bring the telescopic program back into operation have been to no avail.

Clearly, other nations are awaiting America's leadership to jump-start the Spaceguard Survey. There are promising signs that the work is about to begin. NASA recently adopted as an as-yet-unfunded element of its scientific strategic plan the goal of finding 90% of the globally threatening asteroids in the next 10 years. I am sure that NASA's Dr. Pilcher will elaborate.

Three years after publication of the Shoemaker Committee report, its basic conclusions remain sound, yet there are some new insights about how the Spaceguard Survey should be conducted. Furthermore, technological advances envisioned by the Shoemaker Committee have now been implemented, in several test cases: the Spacewatch Program in Arizona; the Near Earth Asteroid Tracking (NEAT) program -- a joint venture of the Jet Propulsion Laboratory (JPL) and the Air Force in Maui; the Lowell Observatory Near-Earth Object Survey (LONEOS); and the Lincoln Laboratory LINEAR program operating for the last few months in New Mexico have all helped to demonstrate that the Shoemaker Committee recommendations are robust. LINEAR, for example, with advanced electronics controlling its large charge-coupled device (CCD) array, is already discovering nearly twice as many potentially hazardous asteroids as the other programs combined. But the programs are not all fully operational. NEAT, for example, is allocated only 6 nights a month on its telescope on the rim of Haleakala Crater in Maui.

Let me turn to how the goals of the Spaceguard Survey are being addressed right now, in May 1998, and what the prospects are for the future.

o o o

How Are We Doing?

The bald truth is that we are not conducting the Spaceguard Survey...not yet, anyway. At the present rate of discovery, it would take nearly a century to meet the goal of finding 90% of NEO's larger than 1 kilometer across. If, indeed, a kilometer-wide asteroid were actually going to hit us in the year 2028 (not the false report headlined around the world in March, to which I will return), the current search effort might well miss it before it suddenly struck "out-of-the-blue".

Figure 3 shows how current efforts are slowly pushing up the numbers of discovered NEO's. The straight, slanting line shows the estimated population of Earth- orbit-crossing asteroids. Today, the survey is complete only for objects brighter than absolute magnitude (H) of 15. We need to survey to at least H = 18, for which it is estimated that there are 2,000 asteroids. The two curves, plotted for all discoveries through the end of 1995, and for discoveries through last month, show that we are inching up very slowly. (Note that the vertical scale is in equal powers of ten.)

The backbone of implementing Spaceguard would be to place more telescopes into operation. We cannot requisition existing telescopes for the task. Nearly all telescopes at major observatories are designed to peer, at high magnification, at extremely distant stars and galaxies in a tiny portion of the sky. Neither they, nor orbiting telescopes like the Hubble, are designed to survey asteroids. Spaceguard requires only modest-sized telescopes, but with a special design that can cover broad regions of the sky for objects down to about 20th magnitude (about a million times fainter than the faintest stars you can see on a clear, moonless night from metropolitan D.C.) According to an analysis by Dr. Alan Harris, of the Jet Propulsion Laboratory, about half the improvement in the current effort will be achieved by searching broader areas of the sky each month. The remainder will come from upgrading the telescopes so that they detect asteroids about a magnitude fainter than is currently achieved.

The Shoemaker Committee recommended achieving these goals by building and putting on-line a couple new, larger telescopes about 2 meters in aperture. But there is an alternative, or at least complementary, approach. That is to take existing mothballed Air Force telescopes, part of the so-called GEODSS program (Ground-based Electro-Optical Deep Space Surveillance), installing them, equipping them with the finest detectors and electronics (perhaps modelled on the LIN- EAR system), and oper- ating them in conjunc- tion with the other search efforts currently underway. Perhaps four to six of the one-meter GEODSS telescopes, appropriately deployed around the Earth, would suffice. However, while there have been discus- sions over recent years about cooperation be- tween NASA and the Air Force on the impact hazard, nothing has yet materialized, so far as I am aware.

There have been recent press reports of NASA augmenting its funding of search efforts to several million dollars a year. Such funding should bring the existing projects up to speed, but will be inadequate for meeting Spaceguard goals. It will be necessary either to build more, larger telescopes, or to bring quite a few GEODSS telescopes out of their crates in order for the survey to approach Spaceguard goals. These major efforts must also be factored into the cost estimates.

And that is not all, not by a long shot. Finding new Earth-approaching asteroids is just the beginning, not the end, of a responsible program for understanding the implications of the new discoveries, for properly alerting government officials and the public, and for establishing a framework in which mitigation -- should it prove necessary -- can proceed responsibly. Let me remind you of the sobering case of ten weeks ago. Headlines around the world screamed that a 1-mile-wide asteroid might strike the Earth in the year 2028. The next day, astronomers claimed that newly found data showed that the disaster wouldn't happen after all.

That's what was reported in the press, but it is not exactly what happened. We now realize that data were already collected two-and-a-half months before March 11th, and published on the Internet, which were sufficient to demonstrate that the asteroid called 1997 XF11 was certifiably safe: it simply could not, realistically, impact the Earth. But months went by and the few astronomers who are funded, part-time if at all, to study all the new asteroid discoveries never had a chance to examine the data in detail. When one underfunded astronomer suddenly noticed quirky data about 1997 XF11 in early March, his hasty response was to announce a possible impact. Within hours, his colleagues finally looked at the data and concluded -- as they just as well could have done months earlier -- that the object could not possibly strike Earth in 2028.

There are several lessons to be drawn from this example. First, the Spaceguard Survey needs more than telescopes and observers. It needs to support enough people to keep track of the factor of ten higher discovery rate, to make carefully researched orbital calculations, and to report scrupulously doublechecked findings to the public in ways that place discoveries in a rational, unhyped framework. I look forward, for example, to the further development of an Impact Hazard Scale, somewhat analogous to categories of hurricanes or to the Richter scale of earthquakes, so that the scientific community, policy makers, and the public will have a common language for discussing new discoveries. A preliminary scale has already been devised by Dr. Richard Binzel of M.I.T.

An element of a discovery program is follow-up. Once an object is discovered, it must be observed from time to time, so that it isn't lost and so that its future orbit may be charted accurately. Currently, much of the follow-up work is done by amateur astronomers or by professional astronomers at small observatories. Little of this work is supported by NASA; indeed popular groups like The Planetary Society have invested their members' dues and contributions for such efforts. A serious program, however, must seriously address follow-up; it must also use non-search telescopes to measure the physical properties of potentially threatening objects. Are they made of iron? Are they dead comets, perhaps with the consistency of snowballs? Do they have swarms of moonlets circling around them? If we are ever going to have to divert a threatening asteroid, we will need a better understanding of what Earth-approaching asteroids are really like.

I want to comment on asteroids smaller than the 1-kilometer or 1-mile wide bodies with which we are mostly concerned (because of their potential to destroy civilization). For every kilometer-sized body there are a thousand others capable of 15-megaton impacts like the one that formed Meteor Crater 50,000 years ago or the 15-megaton blast over Tunguska in 1908. Dozens of those are larger than average -- large enough to cause a devastating tsunami, or tidal wave, capable of destroying cities around the entire coastline of an ocean or, if one was to hit land, capable of destroying a small state or nation. On the one hand, I worry less about these smaller cosmic projectiles. Whereas one just might, disastrously, kill hundreds of thousands of people, other kinds of natural disasters like great floods or magnitude 8 earthquakes are 100 times more likely to kill such multitudes than is an asteroid impact.

On the other hand, even while the Spaceguard Survey is targeting asteroids larger than 1 km in diameter, it will be finding perhaps ten thousand smaller Earth-crossing asteroids. We won't know immediately just how big they are. There's an excellent chance that objects capable of causing a Tunguska-like explosion will, a couple times a decade, pass within the 30,000-mile distance from the Earth that 1997 XF11 was originally predicted to pass. One of them might well hit during the next century. And even smaller objects, can cause frightening blasts in the atmosphere, which might even be falsely mistaken (e.g. in a location like the Indian subcontinent) for a nuclear attack. The White House was reportedly alerted on Feb. 1, 1994, following impact of an object only the size of a small house (tens of kilotons TNT equivalent energy), observed by a couple of fishermen in the South Pacific but also recorded by downward-looking surveillance satellites.

As the rates of discovery, of objects both large and small, goes up and the public becomes more aware of the danger from the skies, it will be essential that planetary protection be elevated from a sideline activity of a few astronomers, and some passionate amateurs, and be put on a sound, appropriately funded footing. The cost is not large. I believe that "Deep Impact" has already taken in more money at the box office than the cost of the entire Spaceguard Survey, from beginning to end. Astronomical programs are comparatively cheap. The really large expenses involve implementing mitigation hardware -- rockets and bombs. Fortunately that won't be necessary until a threatening, mile-wide object is found to be headed toward Earth... and then, surely, there will be no debate about using nuclear weapons in space -- just once -- to save civilization from catastrophe. The chances, however, are truly excellent that Spaceguard will find no threatening asteroid headed our way, and we can all feel a little more secure about our lives on what Carl Sagan called this "pale blue dot" -- planet Earth.

For more information about the impact hazard, how scientists came to understand its significance, and an evaluation of the "impact scare" of mid-March 1998, see my "Case Study" on the Web at:

Clark R. Chapman's Home Page

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