Asteroid impact study finds effects of collisions or explosions on smal asteroids may be hard to predict


by Barbara Cherry, Legislative Affairs Office


"Asteroids: Perils and Opportunities"

hearing before the Subcommittee on Space and Aeronautics, Committee on Science, May 21, 1998.

Rohrabacher , Chairman, (R-CA), Brown (D-CA), Cook (R-UT), Gordon (D-TN),

Bartlett (R-MD), Hall (D-TX), Roemer (D-IN), Weldon (D-FL), Luther (D-MN)

WITNESSES: Dr. Clark Chapman, Southwest Research Institute; Dr. William Ailor, The AerospaceCorporation; Dr. Gregory Canavan, Los Alamos National Laboratory; Dr. John Lewis, University of Arizona; Dr. Carl Pilcher, NASA.

Chairman Rohrabacher opened the hearing by commending Congressman Brown for his leadership and long track record of pushing the Executive Branch to deal with the issue of cataloging and characterizing asteroids. He stated that the potential impact of these hazardous objects is one of national security, economic as well as scientific interest. Congressman Rohrabacher noted that the potential to mine asteroids for metals, minerals and other resources that can be used to build large structures in space was an important aspect of the hearing. Mr. Rohrabacher chided NASA for not "walking the talk" by funding the Near Earth Object (NEO) search program at the levels suggested in the Shoemaker Report. He noted that NASA has no trouble finding $50 million for a program pushed by the Vice President to transmit pictures of Earth into everyone's living room and cannot find a few million dollars to increase the likelihood of cataloguing all of the potentially hazardous NEOs l km or larger.

Congressman Brown echoed his long-standing interest in this subject and the importance of addressing the issue of cataloging and characterizing NEOs even though the risk of impact is small because of the enormous potential catastrophic consequences.

Dr. Chapman discussed the "possibility that an asteroid or comet might strike Earth in our lifetime, perhaps destroying civilization as we know it." He presented a chart which illustrated the chances of dying from an asteroid impact against selected other causes (USA). Congressman Cook noted that the chances of dying from an airplane crash and from an asteroid impact were both l in 20,000.

Dr. Chapman noted that if a mile-wide asteroid hit earth, it would create a hole larger than Washington DC, it would be deeper that 20 Washington monuments stacked on top of each other, ruin agriculture production, and hundreds of millions to billions of people would die. He noted that the consequences were devastating and, therefore, it was prudent to implement the recommendations contained in the Shoemaker Report of cataloging 90% of all of the NEOs with diameters of 1 km or larger within a decade. This would reduce by a factor ten the uncertainty of knowing if an asteroid were headed toward Earth and would likely provide sufficient time to try and deal with the situation.

Dr. Pilcher testified that NASA is committed to the goal of cataloging 90% of all of the NEOs with diameters larger than 1 km within a decade and that we are on track to do so. He stated that NASA has a rich program of research on asteroids and comets which will provide essential information if the Nation were ever to divert an asteroid. Dr. Pilcher stated that the Space Science Strategic Plan includes as a objective, to catalog 90% of the NEOs with diameters larger than 1 km within 5-6 years and NASA has put into place a program to do this. Dr. Pilcher stated that the budget has been doubled to $3 million and NASA will maintain at least this level of funding in the future. Dr. Pilcher outlined all of the elements of the NEO Search program and where increases in the budget have enabled NASA to support new activities. He discussed the Partnership Council, a Council chaired by the Administrator and General Estes of the Air Force to discuss issues of mutual concern to both agencies - NEO detection is one issue which the Partnership Council is addressing. Dr. Pilcher stated that the only recommendation that NASA is not implementing from the Shoemaker Report was the recommendation to build a dedicated 2 meter telescope - -because planned upgrades to existing telescopes can do the job. Dr. Pilcher told the Committee that NASA would do what it takes to do the job right.

Dr. Canavan's testimony addressed several issues. He stated that new technology developed since the Shoemaker Report was issued has increased the detection rate. He emphasized that one area that has not been addressed is long period comets whose orbits intersect the Earth. He said there is no clear concept how to do this and it may constitute as high as 50% of the threat. Dr. Canavan also stated that characterization of asteroids is important if one were to try and alter the course of an asteroid or comet. He discussed the Clementine II mission, which he said represented excellent collaboration between NASA and DOD before it was canceled. Dr. Canavan closed by saying that the current level of funding for NEO searches is 1/3 to _ too low to adequately do the job.

Dr. Ailor discussed the risk the Leonid meteor shower will pose this November. He stated that in a normal year one see 10-15 meteors per hour and this November there will be as many as 200-5,000 meteors per hour traveling at a speed of approximately 155,000 miles per hour. He discussed the recommendations from a recent Conference that was held to address this issue: 1) During the period, satellite controllers should be on duty and check the health of the satellites frequently, 2) orient satellites so that sensitive components are shielded from the oncoming stream of particles and 3) recovery plans should be in place in the event of a system failure.

Dr. Lewis discussed the economic value of asteroids as a source to mine minerals and materials for earth or to produce materials in space for future space transportation. He noted the very low departure speed required to lift off from an asteroid for a return trip to Earth. Dr. Lewis stated that the keys to successful importation of materials from space are lower launch costs and careful choice of exploitation targets to favor those that are most accessible and have the richest resource concentrations.

Chairman Rohrabacher stated that NASA has not been spending adequate funding to search for NEOs as recommended in the Shoemaker Report. Dr. Pilcher noted that, the Office of Space Science has issued their Strategic Plan which includes a goal of cataloging 90% of the 1 km asteroids, that NASA funding wasn't adequate to accomplish this goal, and NASA had doubled the funding of the NEO program.

Mr. Rohrabacher asked Dr. Pilcher how many of the asteroid missions he discussed were actually in the budget. Dr. Pilcher replied that DS-1, DS-4, Contour, STARDUST, Comet Nucleus Sample Return, and Pluto Kuiper Express were all assumed in NASA's budget.

Congressman Gordon asked if NASA was the only Agency working on the problem. Dr. Pilcher responded that NASA supports researchers at Universities to address this issue and works closely with the Air Force. He stated that NASA is developing collaborations with the international community as well. Mr. Gordon asked if the Federal Government was coordinating adequately. Dr. Chapman responded that FEMA has little appreciation for the hazard of such an event. Dr. Canavan stated that interagency cooperation between NASA and the Air Force hasn't percolated down to the troops beyond the Administrator and General Estes.

Congressman Hall asked if we have to have a calamity before anyone takes something seriously and noted that it is an international problem and should have international participation. He asked how much NASA is spending on search activities. Dr. Pilcher responded that NASA is spending $3 million per year and approximately $1 billion over the next decade in asteroid/comet missions. Mr. Hall questioned if NASA is actually spending all of the money allocated for the purposes which the Congress appropriated the funds, indicating that Life and Microgravity funding was being spent for hardware and not research.

Mr. Roemer noted that Dr. Chapman had provided the sound bite for the evening news - that "a mile wide asteroid could hit the Earth tomorrow and we wouldn't know anything about it." He asked if the Federal Agencies had held discussions among themselves on what you would need to do to coordinate a response if an impact were imminent. The witnesses indicated that not much had been done.

Mr. Rohrabacher stated that one needs to put things in perspective and perhaps we aren't spending money wisely. He noted that Mission to Planet Earth is budgeted at $1.4 billion and perhaps it makes more sense to spend additional money on looking for NEOs and improving interagency coordination instead of spending money on Mission to Planet Earth. He stated that he hoped the Congress would move forward and lay the ground work for the Clementine II mission.

Congressman Hall said he thought the hearing was a waste of time unless "we arrive at some actions - if it is money we need to know how much." Both he and Congressman Rohrabacher charged each of the witnesses to draft a two page action plan on what is needed to address the NEO issue and what policies need to be developed to meet the challenges that a NEO impact threat poses.

Statement on The Threat of Impact by Near-Earth Asteroids by Dr. Clark R. Chapman, Southwest Research Institute (for a version of these remarks that includes the figures, see

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.

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 recommendations. 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 impact 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 despite 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 disasters. 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.

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 initial 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 characterize NEO's for possible utilization of their materials for space-construction, fuel, or life-support. Such an asteroid 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 character. 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.

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 LINEAR system), and operating them in conjunction 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 discussions over recent years about cooperation between 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.

Statement of Dr. Carl Pilcher, Science Director, Solar System Exploration, Office of Space Science, National Aeronautics and Space Administration

Mr. Chairman and Members of the Subcommittee:

I am pleased to have this opportunity to appear before the Subcommittee today to discuss NASA's current efforts and future plans to inventory and characterize the population of Near Earth Objects (NEOs).


This Committee has been a leader in focusing attention on the importance of cataloging and characterizing Earth-approaching asteroids and comets. In 1992, the Committee on Science directed that NASA sponsor two workshop studies, the NEO Detection Workshop, which was chaired by NASA, and the NEO Interception Workshop, which was chaired by the Department of Energy. In March 1993, the Science Committee held a hearing to review the results of these two workshops. In 1995, at the Committee's request, NASA conducted a follow-up study which was chaired by the late Dr. Gene Shoemaker. Each of these studies stressed the importance of characterizing and cataloging NEOs with diameters larger than 1 km within the next decade. We have taken steps to put us on a path to achieving this goal. I am here today to tell you about those steps, as well as to bring you up to date on the rich program of space missions to NEOs and related objects.

The NEO population is derived from a variety of scientifically interesting sources including planetessimal fragments and some Kuiper belt objects. Indeed, the Office of Space Science Strategic Plan includes as a specific goal " . . . to complete the inventory and characterize a sample of Near Earth Objects down to 1 km diameter." While the threat of a catastrophic collision is statistically small, NASA has a vigorous program of exploration of NEOs planned, including both asteroids and comets.

There has been much recent discussion about the potential threat posed by NEOs, but NASA has long been interested in them from a scientific standpoint. NEO investigations have had to compete for support against a number of other compelling science programs; funding selection criteria were based principally on scientific merit. This approach has led to the detection of over 400 NEOs, including more than 100 objects larger than 1 km and to a rapid advancement of the technologies necessary for NEO detection. In fact, this research effort has demonstrated that we can inventory the NEO population in a reasonable time, about a decade, with an achievable increase in funding from recent levels.

A little less than a year ago, NASA initiated a study of its existing NEO research to determine how well we were doing in terms of reaching our goal of inventorying the population of NEOs larger than 1 km and characterizing a sample of them. While we have made some impressive strides, it became apparent that the funding levels resulting from scientific competitive review ($1-1.5 M per year) was not sufficient to accomplish our goal. The detection of new NEOs in 1997, the last year for which we have statistics, is barely 10% of the rate needed to achieve the goal suggested in the Shoemaker report (detection of 90% of the NEO population larger than 1 km within a decade). In simple terms, we need to survey about 20,000 square degrees of sky a month for NEOs to a limiting brightness of approximately 20th magnitude to accomplish the inventory. To understand what this means, note that 20,000 square degrees is about half the sky and that magnitudes are a measure of apparent brightness-a 6th magnitude object is at the limit of detection for the human eye and 20th magnitude is almost 100,000 times fainter.

I would now like to describe briefly the existing search programs, NASA's plans to improve them, and some promising new research programs which we are considering. I will also comment on our joint activities with the Air Force Space Command. All of these efforts are directed toward increasing the rate of discovery of NEOs in order to reach our goal.


NASA's ground-based NEO program comprises three parts: Spacewatch, the Near-Earth Asteroid Tracking (NEAT) program, and the Lowell Near Earth Asteroid Survey (LONEOS).


Spacewatch is a program at the University of Arizona, led by Dr. Tom Gehrels, which has done much of the pioneering work in the field of NEO detection. This group is responsible for more NEO discoveries than any other. The current Spacewatch Program searches 200 square degrees of sky per month to a depth of 21st magnitude. This year NASA is funding a new state-of-the-art focal plane camera for Spacewatch, which will lead to an 8-fold increase in the area of sky that they search each month (to 1600 square degrees per month). We hope in the future to assist them in their efforts to bring their new 1.8 m telescope on line. This telescope will enable them to detect even fainter NEOs.


NEAT is a program headed by Dr. Eleanor Helin at the Jet Propulsion Laboratory. NEAT uses a specialized camera, which is based on a 4096x4096 CCD array for use on the 1 m GEODSS (Ground-based Electro-Optical Deep Space Surveillance) telescope, operated by United States Air Force Space Command (USAFSC) on Haleakala, Maui, Hawaii. This group is currently limited by the number of nights per month on which they can observe the sky using the GEODSS system. They are presently observing six nights per month on one of the seven GEODSS telescopes. With recent improvements they are now able to search 800 square degrees per night (4800 square degrees per month) to about 20th magnitude. We have funded the construction of 2 more cameras, which we hope to install on two other GEODSS telescopes. This increase in the level of effort for NEO detection is being discussed in the NASA-USAFSC Partnership Council co-chaired by NASA Administrator Daniel Goldin and AFSC Commander Gen. Howell Estes. It is in principle possible to scan 21,000 square degrees a month with three cameras and full access to three of the GEODSS telescopes. It is important to note that the GEODSS system includes one southern hemisphere site.

While we certainly hope to increase our surveying ability using the GEODSS system, we are aware that it has other vital missions. NASA's FY 1999 budget request includes sufficient funding for the construction of four more NEAT cameras, which will enable us to equip all seven GEODSS telescopes. The final application of the funds will depend on the demonstration that the NEAT camera can support the existing mission of the GEODSS system as well as the search for NEOs. This matter is currently being reviewed by the Partnership Council on NEOs.


LONEOS is led by Dr. Ted Bowell at Lowell Observatory in Flagstaff, Arizona. This group has great potential (capability to observe 4,300 square degrees a month down to 20th magnitude); however, they have not yet reached this level of performance. We are funding an augmentation to buy a second focal plane CCD and to support additional software development in order to allow them to reach their performance objective.


The increased interest in the search for NEOs has led to several recent proposals from new groups:

Catalina NEO Survey

We are supporting a new search program at the University of Arizona, which is headed by Mr. Steven Larson, to refurbish an existing telescope on Mount Lemon. When fully operational, this system will survey 8,000 square degrees of sky per month to a depth of 19th magnitude. This program will be fully operational within a year.


NASA is evaluating a proposal for support of a very promising search program from the MIT Lincoln Labs. This effort called LINEAR (Lincoln Near Earth Asteroid Research program) uses a state-of-the-art camera which was developed as a possible prototype for the next generation GEODSS detector. They are proposing to use a 1 m telescope at their Experimental Test Site near Socorro, New Mexico, to survey 10,000 square degrees down to 21st magnitude each month.

With coordination of these different observational programs, NASA believes it is possible to obtain the level of sky coverage to the appropriate limiting magnitude required to complete the survey. NASA has already committed over $3M this year, much of it to fund improvements to focal plane detectors, software, and electronics. NASA is committed to funding both existing and new search programs at, at least, the FY1998 level. We believe this is close to the level required to achieve our objective.


The study of the physical characteristics of NEOs is a major focus of both ground-based research and space missions. The ground-based work includes NASA-supported radar imaging of NEOs utilizing the Arecibo Radio Telescope and spectroscopy of NEOs from optical/IR telescopes to determine their composition.

Several NASA missions will travel to asteroids and/or comets to provide us with exciting new scientific insights about these objects at the same time this information is valuable for any future effort to respond to an impact threat. Over the next decade NASA will invest approximately $1B in these missions. Missions in flight or in development are:

Missions soon to enter development are:


The issues and challenges posed by NEOs are inherently international, and any comprehensive approach to addressing them must be international as well. Central areas of concern include coordination among NEO observers and orbit calculators around the globe and public notification should an object posing a significant hazard to Earth be discovered. NASA has begun discussing, with the international community, convening an international workshop to address these issues. The workshop will likely be held during the first half of 1999. The goal of this workshop will be to develop international procedures and lines of communication to ensure that the best available accurate information about any potentially hazardous object is assembled and disseminated to the public in the shortest possible time.

To facilitate coordination among NASA-supported researchers, other agencies and scientists, and the international community, NASA is establishing an NEO Program Office. This Office will coordinate ground-based observations, ensure that calculated orbital elements for NEOs are based on the best available data and support NASA Headquarters in the continuing development of strategies for the exploration and characterization of NEOs. In the unlikely event that a potentially hazardous object is detected, the Office would coordinate the notification of both the observing community and the public of any potentially hazardous objects discovered.

NASA is committed to achieving the goal of detecting and cataloging 90% of NEOs larger than 1 km in diameter within 10 years, and to characterizing a sample of these objects. We are developing and building instruments, and developing partnerships -- particularly with the Air Force -- which should lead to the necessary detection and cataloging capability being in place in 1-2 years. This capability will also allow us to detect and characterize many NEOs smaller than 1 km.

In summary, NASA's obligation and commitment is to ensure that we have the information necessary to understand the hazards posed by NEOs.

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