Figure 2 10 Long Period Comets example essay topic

Disclaimer The views expressed in this academic research paper are those of the authors and do not reflect the official policy or position of the US Government or the Department of Defense... Preface Interest in developing an asteroid defense system, intensified by the impact of comet Shoemaker-Levy 9 with Jupiter in 1994, continues to grow by leaps and bounds. Many major US publications such as Newsweek, Time, Ad Astra, Technology Review, Nature and even The Economist have run extensive articles on the subject. However, the interest goes well beyond the United States and the press. Russia, Italy, and Australia have recently hosted conferences on the asteroid threat and the United Nations will host one of its own in April of this year. Because of public interest, and at the urging of scientists and astronomers, the US Congress commissioned the Spaceguard survey to examine the asteroid threat.

Though no major decisions were made as a result of the survey (briefed in 1992), all agreed that the subject warranted continued discussion. In January of 1996, a NASA sponsored follow-on committee, headed by Dr. Eugene Shoemaker, will present recommendations for asteroid defense to Congress. Most expect the Shoemaker Committee to recommend an asteroid search program much like the one proposed in the original Spaceguard report. While all of this is going on, it appears that the US military, specifically the Air Force, has declined to participate in these surveys and the subsequent Congressional briefs.

The reluctance is somewhat understandable since scientists are just beginning to understand and quantify the threat. Planetary Defense, if undertaken, would be a new challenge, but one that clearly falls in the realm of military responsibility. The US Military has organizations, equipment and talent that could be invaluable to an asteroid defense program. We hope that this study will convince our leadership that the Air Force has both. iv the capability and responsibility to participate in the defense of Earth (and our space assets) from natural space debris. We would like to express our sincere gratitude to the following people for their assistance. We literally couldn't have done it without them.

Of the multitude of people and agencies who have assisted us in the preparation of this study, we owe special thanks to the Institute for National Security Studies, Colorado Springs for seeing the potential in our proposal and funding our research. Dr. Peter Brown of the University of Western Ontario, Canada for sharing his expertise and sound advice in the study of the meteor stream threat. Peter went out of his way many times to help us find information and make the contacts that made the paper possible. Ellen Carey and Jean Gilbert of William Beaumont Hospital Medical Library, Royal Oak, Michigan for their assistance in obtaining documents pertaining to risk assessment. David Morton, Hazard Center Library, University of Colorado-Boulder for their hospitality and assistance in locating sources relating to disaster planning. We are also indebted to the Hazard House for the long-term use of many excellent references during our studies.

John Darragh, Chief Scientist, Sandy Sutherland, and Lt Col Shirley Hamilton from Headquarters, Air Force Space Command for providing materials, insight, and contacts to the numerous agencies involved in asteroid research. Lt Col Neil McCasland for providing a sanity check, playing devil's advocate and generally making us think. Our lengthy discussions helped solidify many of the ideas expressed in the paper. Bob and Patsy Hancock of Colorado Springs for making our trip enjoyable and memorable (thanks for dinner, too). Dr. Jack Hills of Los Alamos National Labs for his technical advice and many tremendously useful papers on the subject. Dr. Gregory Canavan of the Los Alamos National Labs for his advice and technical expertise, as well as his insight into the asteroid "community" and the preparation of the Spaceguard reports... vs. Dr. William Wiesel, Dept of Astronautical Engineering, Air Force Institute of Technology for taking time out of a busy schedule (even canceling class) to discuss and advise on the project.

Maj Karl Johnson, our faculty research advisor for his incredible support throughout the year. He kept our research flowing smoothly and relieved us of many administrative burdens. Most of all, we would like to thank our friends and families for motivating us when we were beleaguered, and for bearing with us when we were gung-ho. This research project would have been very difficult without the assistance and encouragement of these individuals. However, the conclusions and recommendations, as well as any errors, are entirely our own... vi Table of Contents Page PREFACE... ii LIST OF TABLES... xi LIST OF FIGURES... xii LIST OF EQUATIONS... xv ABSTRACT... xvi CHAPTER 1: Introduction... 1 The Threat...

2 Roles and Responsibilities of the United States Military Regarding NSD Defense... 3 Notes... 4 CHAPTER 2: Natural Space Debris... 5 Definition of Natural Space Debris (NSD... 5 Sources of Natural Space Debris...

5 The Comets... 6 The Origin of Comets... 7 Types of Comets... 8 Short-Period Comets... 9 Long-Period Comets... 10 The Asteroids...

11 Main Belt Asteroids... 12 Extra-Belt Asteroids... 15 Earth-crossing Asteroids... 18 The A AAO Orbits... 20 Meteors and Meteor Streams... 26 The Natural Space Debris System: A Summary...

29 Notes... 31 CHAPTER 3: A Brief Overview of Impact Theory... 33 Mass Extinctions and the Geologic Record... 35 Evidence of Periodic or Recurring Extinctions... 36 Comet Showers...

37 Problems Determining Periodicity-Potential Mechanisms... 39 Nemesis... 40 Passage Through the Galactic Plane... 41. vii Page A Tenth Planet... 42 Problems Determining Periodicity-Accuracy of Dating... 43 Impact Signatures and the K / T Event...

43 Summary... 47 Notes... 48 CHAPTER 4: Natural Space Debris Effects... 50 Impact Energy... 51 Impact Effects... 56 Tunguska...

56 Direct Impact Effects... 59 Indirect Impact Effects... 60 Blast... 60 Tsunamis... 64 Earthquakes... 66 Global Impact Winter...

67 Fires... 69 Hyper canes... 70 Electromagnetic Energy Generation and Electro phonics... 71 The Effect of Meteor Storms and Meteoroid Impact on Space Vehicles... 75 Dust in the Stream... 75 Debris Greater than 1 mm...

76 Summary of Effects... 79 Notes... 80 CHAPTER 5: Natural Space Debris: Clarifying Risk, Hazard and Threat... 82 Terms Defined... 82 Hazard... 82 Risk...

83 Threat... 83 Conveying the Threat Message... 84 Historical Representations of the NSD Threat... 84 The Reasonableness Test...

85 Variations in Historical Risk Assessments... 87 Defining an Acceptable Level of Risk... 89 Action Thresholds... 90 Summary... 91 Notes... 92 CHAPTER 6: The Natural Space Debris Threat...

93 Estimating the Amount of Debris in Earth-crossing Orbits... 94 Risk of Terrestrial Impact... 98 In Search of an NSD Threat Model... 99. vs. Page The Threat to Civilization... 102 Threat to Life... 103 Threat to Economy...

104 An NSD Threat Model Summary... 105 Satellites and the Meteor Storm Threat... 108 Meteor Storms... 109 Meteor Streams...

112 Summary-The NSD Threat... 117 Notes... 120 CHAPTER 7: The NSD Detection and Discrimination Problem... 123 Asteroid Detection and Discrimination... 124 Asteroid Detection Factors-Optical Telescopes... 125 Magnitude...

126 Orbit Geometry and Phase Angle... 127 Distances... 129 Diameter... 130 Albedo... 130 Reflection Phase Law... 131 Apparent Visual Magnitude...

134 Asteroid Detection Factors-IR Telescopes... 135 Optical and IR System Comparison... 136 RADAR Systems (Detection & Discrimination)... 137 Distinguishing Asteroids and Comets from Background Stars (Discrimination)...

139 Proper Motion... 139 Parallax... 142 Effects of Atmosphere on Ground-based Systems... 143 Atmospheric Effect on Apparent Visual Magnitude... 144 Atmospheric Effect on Resolving Power... 145 Atmospheric Effect on IR and Radar Systems...

146 Key Instrument Design / Performance Factors. (Optical and IR Systems)... 146 Detection... 147 Discrimination... 148 Summary... 151 Notes...

152 CHAPTER 8: Search Systems... 154 General System Objectives and Requirements... 155 General System Requirements... 160 Minimum Object Diameter of Interest...

160 Survey Duration... 162 Optical System Requirements... 165 Warning Time Indicator... 165. ix Page Simplified Warning Time Model... 168 Coverage... 173 Impact Trip Wire-Implications for Coverage...

177 Minimum Detectable Proper Motion and Parallax... 178 IR System Requirements... 180 IR Limiting Magnitude... 181 IR Resolution...

185 IR Coverage... 185 IR Coverage Rate... 186 Radar System Requirements... 186 Wide Area Search... 186 Tracking and Orbit Characterization... 186 System Architecture...

189 The Centralized Control and Coordination Center ( )... 190 Responsibility for NSD Search... 191 Data Handling and Storage... 193 Meteor Storm Characterization System Requirements... 195 Predicted Zenithal Hourly Rate and Storm Risk Factor... 195 Minimum Meteor Storm Warning Time...

196 Survey of Optical Systems... 196 Spaceguard Survey Network... 197 Space watch... 197 GEODSS... 198 TOS... 199 NASA Liquid-Mirror Telescopes...

202 LLNL Wide-Field-of-View Telescopes System... 203 Infrared (IR) Systems Survey... 204 IRAS... 204 WIRE... 205 Radar Systems Survey... 205 Search System Costs...

206 General System Architecture... 208 GEODSS / GUPS Based System... 208 Combined NASA Liquid Mirror-GEODSS / GUPS System... 209 Meteor Stream Characterization...

213 Conclusions... 214 Notes... 216 CHAPTER 9: The Military's Response to the Natural Space Debris Threat as a Natural Disaster... 219 Military Involvement in Disaster Response... 220 Doctrine Governing Military Support of Civil Authorities... 224 Employment of Military Forces in Domestic Disaster Relief...

228. x Page Strategic Requirements... 228 Operational Requirements... 230 Preparing for Disaster Assistance Support... 231 NSD as a Natural Disaster: A Summary... 236 Notes...

237 CHAPTER 10: Threat Mitigation... 238 Mitigation to Reduce the Effects of the NSD Threat... 238 Mitigation Measures... 243 Planning... 243 National Planning... 244 International Planning...

247 Preparedness... 248 Warning and Alerting... 248 Training... 250 Defending Against the NSD Threat... 251 Use of Existing Military Assets... 251 Threat Detection and Characterization Systems...

252 Protecting Life and Property from the NSD Threat... 253 Mitigating the NSD Threat: A Summary... 258 Notes... 259 CHAPTER 11: Future Considerations... 261 Asteroid Mining... 261 Extraterrestrial Artifacts...

264 Notes... 267 CHAPTER 12: Recommendation Summary... 268 Recognize the Natural Space Debris Threat... 261 Plan for the NSD Threat Now...

269 Institute an Asteroid and Comet Search Program... 269 Recognize the Meteor Stream Threat... 270 Characterize All Active Meteor Streams... 271 Develop a Meteor Storm Warning Capability... 272 Encourage Satellite Programs to Develop Meteor Storm Procedures... 272. xi List of Tables Page Table 2-1.

Some Short-Period Comets and Their Orbital Periods... 8 Table 2-2. Titius-Bode Sequence Predicted Planet at 2.8 AU from Sun... 12 Table 2-3. Major Asteroid Families and Groups...

16 Table 2-4. Orbit Elements of Earth-crossing Asteroids For Which Families Are Named... 24 Table 2-5. Overlapping Definitions of Some Common Space Debris Terms... 26 Table 2-6. Some Well-known Meteor Showers and Their Parent Objects...

28 Table 5-1. Various Authors Have Put Forth Widely Differing Risk Assessments Regarding Natural Space Debris... 87 Table 6-1. Well Documented Meteor Storms Since 1799...

108 Table 6-2. Estimated Probability of Space Network-Meteoroid Collision... 115 Table 8-1. Asteroid and Comet Search System Key Requirements... 159 Table 8-2.

Warning Time Effect on Interception... 167 Table 8-3. Meteor Stream Characterization and Storm Warning Factors... 195 Table 8-4.

Optical Search System Specifications... 201 Table 8-5. Military Radars... 206 Table 8-6. Detection Instrument Cost Estimates... 207 Table 8-7.

Estimated Optical Search System Costs... 210 Table 8-8. IR, Radar and Costs... 211 Table 8-9.

Estimated Cost of Complete Search System... 212 Table 9-1. Disaster Relief Organizations... 221 Table 9-2.

Emergency Support Function Assignment Matrix... 227 Table 10-1. Issues Affecting NSD Defense Systems... 257. xii List of Figures Page Figure 2-1. Artist Rendering of Comet With Tail... 6 Figure 2-2.

Artist Rendering of a Fragmented Shoemaker-Levy 9 Impacting Jupiter... 10 Figure 2-3. Asteroid Ida (56 Km Long) and its Moon (1 Km Diameter)... 11 Figure 2-4. Relative Locations of Some Families and the Kirkwood Gaps... 13 Figure 2-5.

Artist Rendering of Asteroids Approaching a Planet... 14 Figure 2-6. Location of Trojan Asteroids at L 4 and L 5 Lagrange Points... 17 Figure 2-7.100 of the Largest Earth-crossing Asteroid Orbits Overlaid on Earth's Orbit... 18 Figure 2-8. Basic Elliptical Orbit Geometry...

20 Figure 2-9. Earth-crossing Obit with Inclination... 23 Figure 2-10. Typical Earth-crossing Orbit of Atens Asteroid Family... 24 Figure 2-11. Typical Earth-approaching Orbit of Amor Asteroid Family...

25 Figure 2-12. Typical Earth-approaching Orbit of Apollo Asteroid Family... 25 Figure 3-1. Mass Extinctions in the Geologic Record...

34 Figure 3-2. Dots Show Approximate Position of Known Impact Sites... 35 Figure 3-3. Approximate Time Scale of Life on Earth According to Fossil Record...

36 Figure 3-4. Theoretical Occurrence of Comet Storms as Calculated by Weissman Using Monte-Carlo Analysis of Random Star and Molecular Cloud Passages... 38 Figure 3-5. Photo of a 0.75 mm Shocked Quartz Grain from K / T Boundary Clay at Teapot Dome, Wyoming. Clearly Shows Two Sets of Planar Deformations... 45 Figure 4-1.

Impact Geometry... 51. x Figure 4-2. Aten Asteroid-Earth Impact Example... 52 Figure 4-3. Typical Meteorite Debris Field... 61 Figure 4-4.

Simulation of a Bolide Passing Through the Atmosphere... 63 Figure 4-5. Function of Hyper cane... 71 Figure 4-6. Meteoroid Energy... 76 Figure 4-7.

A Comparison of the Energy Content of Man-Made and Meteor Stream Debris. Bullet Energy Too Low to Show On Graph... 77 Figure 4-8. Spectrum of Natural Space Debris Effects...

82 Figure 5-1. Threat is a Product of Hazard and Risk... 84 Figure 5-2. Variables Affecting Threat Determination... 85 Figure 5-3.

There is a Relationship Between Risk and Efforts People are Willing to Make to Reduce or Control the Risk... 89 Figure 6-1. Crater and Lava Flows on Moon... 95 Figure 6-2. Number of Earth Crossing Asteroids by Size... 97 Figure 6-3.

Typical Intervals Between Impacts... 98 Figure 6-4. Elements of the NSD Threat Model... 100 Figure 6-5. The Threat to Human Life For Various Impactor Sizes... 104 Figure 6-6.

Conceptual NSD Threat Model... 106 Figure 6-7. Probability of Satellite Collision with Meteoroid... 113 Figure 6-8.

Risk of Satellite Being Struck By Meteor... 116 Figure 6-9. Notional NSD Threat Spectrum... 119 Figure 7-1. General Detection and Discrimination Process for an Optical System...

125 Figure 7-2. Magnitude Scale... 126 Figure 7-3. Orbit Geometry and Phase Angles for a Typical Apollo Asteroid... 127 Figure 7-4. Orbit Geometry and Phase Angles for a Typical Aten Asteroid...

128. xiv Figure 7-5. Light Path for Asteroid Observed From Earth... 129 Figure 7-6. Phase Laws of Three Representative Asteroids... 132 Figure 7-7. Lunar Phase Law...

133 Figure 7-8. Collision With a Typical Aten Asteroid, Showing Small Proper Motion... 141 Figure 7-9. Geocentric Parallax of an Asteroid Seen From Earth... 142 Figure 7-10. Change In Asteroid's Apparent Visual Magnitude Due to Atmospheric Extinction...

144 Figure 7-11. Basic Functional Layout of a Typical Newtonian Telescope System... 147 Figure 7-12. Factors Governing Detection... 149 Figure 7-13. Image Placement on CCD...

150 Figure 8-1. Contributions of Optical, IR and Radar Systems to Search Mission in Terms of General Search Areas... 157 Figure 8-2. Objects Outside of Our Detection Sphere Will Not Be Found Until Their Orbits Carry Them Within Range of the Instruments...

163 Figure 8-3. Discovery Completeness For 1 km and Larger Earth-crossing Objects (Hypothetical Whole Sky Survey)... 164 Figure 8-4. Geometry of Orbits Used to Determine Warning Time Indicator, Asteroid Orbit Intersects Earth Orbit at Aphelion...

168 Figure 8-5. Geometry of Orbits Used to Determine Maximum Warning Time Indicator, Asteroid Orbits Intersects Earth Orbit at Perihelion... 169 Figure 8-6. The Maximum Warning Time an Instrument Can Provide Depends On Its Limiting Magnitude and Object Diameter...

171 Figure 8-7. Realistic Maximum Warning Times... 172 Figure 8-8. Percent of Objects Found in a 25 Year, Monthly Survey Using Dark Sky and Standard Search Areas... 173 Figure 8-9. Orbit Geometry Used to Bound IR Limiting Magnitude Requirements...

181 Figure 8-10. IR Magnitude of Object in Position #1... 182. xv Figure 8-11. IR Magnitude of Object in Position #2... 182 Figure 8-12. Smallest Visible Object for a Given Instrument Limiting Magnitude...

183 Figure 8-13. Efficient Search System Architecture With Centralized Control & Coordination Center... 189 Figure 8-14. GEODSS Site... 199 Figure 9-1. Strategic Decision Sequence...

230 Figure 9-2. Operational Command Relationships... 232 Figure 9-3. Disaster Stages and Levels of Effort...

234 Figure 9-4. Effort vs. Time for an NSD Disaster... 235 Figure 10-1. NSD Effect for Debris Less Than 56 Meters in Size...

241 Figure 10-2. Synergistic Effect of Multiple Mitigation Measures... 243 Figure 10-3. Types of Joint Operation Planning...

246. xvi List of Equations Formula Page (1)... 21 (2)... 21 (3)... 21 (4)... 21 (5)... 52 (6)...

53 (7)... 53 (8)... 54 (9)... 54 (10)... 54 (11)... 55 (12)...

55 (13)... 62 (14)... 62 (15)... 64 (16)... 67 (17)... 77 (18)...

77 (19)... 130 (20)... 132 (21)... 133 (22)... 134 (23)... 135 (24)...

136 (25)... 136 (26)... 137 (27)... 144 (28)...

147 (29)... 148 (30)... 148 (31)... 170 (32)... 170 (33)...

170 (34)... 170 (35)... 170 (36)... 176. xvii Abstract The threat posed to Earth and Earth-orbiting spacecraft by natural space debris (asteroids, comets and meteor streams) is examined in an effort to quantify the threat and identify available, low cost mitigation measures. Our study found that the Earth resides in a swarm of natural debris that consists of at least three families of asteroids (the Apollo, Aten and Amor asteroids), several short-period comets and at least 11 active meteor streams. The results of recent studies regarding the risk of a significant asteroid or comet impact on Earth are presented.

Best estimates indicate the probability of a large impact within the next century is about 1 chance in 10,000. Further, there is a much higher probability of a smaller (Tunguska sized) impact sometime in the next century. The myriad of potential impact effects are discussed in detail for various impactor sizes. The threat that meteor storms pose to space-borne assets is also discussed.

There has not been a major meteor storm since 1965, hence our modern space systems have never been subjected to a severe storm. There is a very high probability that we will see an extremely active storm from the Leonid stream around 17 November 1999. We discuss the meteor stream threat to our space systems (as an integrated network), and what we should do to lessen the possibility of losing satellites in future meteor storms. The natural space debris threat is real and mitigation measures should be implemented.

Before this can happen, the threat must be communicated. Problems communicating the natural space debris threat are discussed using historical examples. With these problems in mind, we offer suggestions to more clearly quantify and. xv communicate the threat to decision-makers in the future. The need for a better threat model is discussed and the framework for an improved model is provided.

The need for an asteroid and comet search program is discussed, and basic search system requirements are derived. Using these requirements, we evaluate the utility of several existing and proposed systems. Then, the general architecture and approximate cost of a suitable search program is presented. We estimate the program cost to be $56.5 M to $57 M non-recurring, and $12.6 M to $15.4 M / yr for operations. A limited search capability could be had for $19.5 M to $20 M non-recurring, and $10.6 M / yr to $13.4 M / yr for operations.

The need for meteor stream characterization and the development of a storm warning capability is introduced, and a cost estimate presented. To characterize all 11 active streams and develop a basic meteor storm warning capability for our satellite programs will cost approximately $3.2 M over eight years. Given that the threat is real, we examine the roles and responsibilities of the US military regarding the defense of Earth and our space assets from asteroids, comets and meteor storms. Within the last 15 years, the military has responded to natural disasters such as floods, hurricanes, earthquakes and volcanic eruptions. Based on existing policies, and the historical role of the military in disaster response, we believe that the military has a responsibility to address the natural space debris threat. Finally, several threat mitigation measures are presented.

Active measures such as the deflection or destruction of potential impactor are briefly discussed. However, we recommend the search and planning measures be given first priority. A summary of key recommendations is provided in the final chapter... 1 PLANETARY ASTEROID DEFENSE STUDY: ASSESSING AND RESPONDING TO THE NATURAL SPACE DEBRIS THREAT CHAPTER 1 Introduction On 1 February 1994 at 22: 38 Universal Time, a piece of natural space debris entered the Earth's atmosphere just north of Kosrae island, off the coast of New Guinea.

Traveling at ~15 km / sec (33,555 mph), it streaked across the sky toward the northwest and exploded about 20 km above the sea, near the island of Tokelau, with a force of ~11 kilotons of TNT. 1 At its peak, the brightness exceeded magnitude -25 (similar to the Sun). 2 The explosion triggered sensors on several US early warning satellites. 3 Fortunately, the blast occurred at high altitude and over a sparsely populated area; thus, no damage was done. On 23 March 1989, an asteroid about 800 meters (1/2 mile) in diameter missed Earth by about 6 hours. 4 If it had hit, the impact would have released energy equivalent to about 40,000 Megatons of TNT or 2,000 hydrogen bombs.

On 8 December 1992, another asteroid, named Toutatis missed hitting the Earth by about 2 lunar distances. 5 Toutatis is nearly 4 km in diameter (2.5 miles), more than twice the size required to create a global catastrophe. 6 Its impact would release more energy than all the nuclear weapons in existence, about 9 million megatons... 2 These are just a few examples of the risks we face each day from Natural Space Debris (NSD).

While the probability of a large asteroid like Toutatis hitting us is relatively low, it may not be as low as we have traditionally believed. The Threat In 1989 the US Congress commissioned NASA to study the threat posed by Earth-orbit crossing debris and investigate means of mitigating that threat. Christened the "Spaceguard" study, a team of over one hundred of the top US and international scientists participated. Their conclusion was that natural space debris does present a real (though not eminent) threat to Earth and that some reasonable effort should be made to find, catalog and track Earth-orbit crossing objects.

7 Further, they found that, if a large asteroid were on a collision course with Earth, we now have the technology to deflect or destroy it and prevent catastrophe. Since the publication of the Spaceguard Survey report in 1992, much work has been done by scientists around the world to further define the risks presented by NSD. In the following pages we will present our assessment of the risk based on the most current data available and what should be done about it. We intend to assess the results of the Spaceguard Survey and to use additional new information to better understand the threat posed by NSD and investigate tools that the military, particularly the Air Force, may have available to counter the threat.

Roles and Responsibilities of the United States Military Regarding NSD Defense The US military's role in providing domestic disaster relief is not a new one, but it is indeed an ever-changing one. In the past, military assistance was simply welcomed; . 3 today it is expected. Further, there is growing pressure at all levels of government to ensure designated agencies provide the necessary assistance and relief in a timely manner. Our paper will discuss how evolving policy, doctrine and detailed preparedness planning have all contributed to improving the military's response to domestic emergencies. We will also discuss several challenges and concerns that the military must address in order to plan for, and respond to, a disaster resulting from an asteroid impact.

Our premise is that the hazard posed by natural space debris is much like that of any other natural hazard. The military, particularly the Air Force, can't afford to ignore natural space debris and its potential for causing serious damage. In the last decade, military units participated in relief operations stemming from volcanic eruptions, earthquakes, hurricanes and floods. It seems only logical that national leaders and the public will continue to look to the military for help in times of disaster. Thus, it follows that the military must assume some degree of responsibility for NSD defense. The necessity of planning for a domestic disaster resulting from natural space debris has apparently never been seriously considered within the Air Force.

The defense of Earth from asteroid impact has, in the past, been considered both expensive and unnecessary. Recent events, such as those presented above, combined with new data and theories regarding the nature of the NSD threat, give reason to re-examine these issues. Notes 1 "Satellites Detect Record Meteor", Sky & Telescope: 11 (June 1994). 2 Ibid. 3 Ibid... 4 4 George E. Brown Jr., Chairman, House of Representatives, Committee on Science, Space and Technology.

"The Threat of Large Earth-Orbit Crossing Asteroids", Hearings before the House Sub-committee on Space on Results of Spaceguard Study. 24 March 1993.5 Corey S. Powell, "Asteroid Hunters", Scientific American: 34-40 (April 1993). 6 Clark Chapman and David Morrison, "Impacts on the Earth by Asteroids and Comets: Assessing the Hazard", Nature 367: 35 (6 January 1994). 7 David Morrison, Chairman of Asteroid Detection Workshop (Spaceguard Study), NASA Ames Research Center.

"Statement Given House of Representatives, Committee on Science, Space and Technology, before the House Sub-committee on Space. 24 March 1993... 5 CHAPTER 2 Natural Space Debris Definition of Natural Space Debris (NSD) Natural space debris, for the purposes of our discussion, consists of all naturally occurring solid matter orbiting the Sun whose orbits intersect or share that of the Earth from time to time, or might do so in the future. Thus, we are specifically excluding man-made debris (i.e. : objects orbiting the Earth or Sun placed in orbit by man). We are also excluding natural debris in permanent orbit around the Earth since most of it is relatively small and the quantity is fairly constant. While the Spaceguard study focused on objects greater than 1 km in diameter, our discussions will include objects of all sizes; from the smallest grain of sand to the 10 km diameter planet-busters.

1 Sources of Natural Space Debris There are two major sources of natural space debris: asteroids and comets. Both are considered to be left-over material from the formation of the planets in our solar system. Occasionally, these objects are perturbed by chaotic interaction with gravitational fields of the Sun and planets into paths that cross Earth's orbit. 2 Comets and asteroids are not always very different. Both can occupy the same types of orbits as illustrated by the fact that some of the Earth-crossing asteroids are actually burnt-out comets. 3 The primary differences have to do with their composition and origins.

As you " ll see in the following discussion these differences have an effect on the detection problem... 6 The Comets. A comet, unlike an asteroid, contains a large quantity of various ices. Common ices would include materials such as water, methane, ammonia, carbon dioxide, hydrogen and nitrogen. 4 The ices act as a glue to hold the comet together; thus comets are often thought of as dirty snowballs containing a mixture of rock and metals all held together in a frozen mass. As such they would be physically more fragile than an asteroid made of solid rock (which is important if you want to deflect one).

As they orbit the Sun and the ices boil away, the core of the comet will be weakened. Over time, it will lose all of its ices leaving only rock or metal. Because of this process, comets are responsible for two magnificent astronomical displays: the comet with its tail as seen in Figure 2-1, and some well-known annual meteor showers. The most visible difference between an asteroid and a comet is the tail.

As a comet approaches the Sun, solar radiation vaporizes the ices on the surface of the nucleus, forming a luminous cloud around the nucleus called the coma which blends into the tail. Source: David Irizarry. Figure 2-1.7 The tail always points away from the Sun since it is made up of vaporized material being blown away from the nucleus by the solar wind. The length and brightness of the tail can vary considerably since the ablation rate of the comet material varies with the composition of the nucleus, distance from the Sun and orientation of the nucleus. As the comet passes perihelion (closest approach to the Sun) it loses a lot of its ices and gravitational forces will severely stress the nucleus.

Eventually, due to loss of the ices, the comet will break up, or it will lose so much material that it will no longer be capable of generating a bright tail. It's the slow disintegration of a comet that produces many of our annual meteor showers. As the ices in the nucleus warm and subsequently vaporize, they leave behind the rock which is itself eventually ejected from the surface. Therefore, in the path of a comet, clouds of debris begin to form. Over time, debris can become distributed (albeit unevenly) around the orbit. 5 This effect is extremely important since these clouds of debris form the streams that are the source for at least some of our annual meteor showers.

Streams will be discussed in more detail later. The Origin of the Comets. The origin of comets is unknown. No one has ever seen or irrefutably proven the existence of a single source of new cometary debris; however, it seems certain a source does exist. Short-period comets can not survive more than a few tens-of-thousands of years before the Sun boils away their ices. Thus, a supply of comets must exist someplace in deep space where the Sun can not destroy them.

The most accepted theory today proposes the existence of a cloud of icy debris at the very edge of the Sun's gravitational influence. At this great distance, the accretion process that created the planets some 4,800 million years ago did not happen. Material in the outer. 8 reaches of the solar system combined with leftover debris ejected by the planets to form a spherical cloud of debris around the solar system called the Oort cloud, which consists of somewhere between 10 12 and 10 14 potential comets.

Occasionally, for reasons not yet fully understood, debris from this cloud (comets) are sent sunward where they may eventually hit one of the planets. Types of Comets. Comets are classified as either long, intermediate or short period, where period is defined as the time it takes the comet to orbit the Sun. Cometary orbits are often different than those of the asteroids. They are usually highly elliptical and are often inclined at large angles to the orbit plane of the planets. The significance is that, unlike the asteroids, comets could approach the Earth from almost any direction and at very high velocity.

Therefore, to find them you would have to continuously survey the entire sky. Table 2-1. Some Short-Period Comets and Their Orbital Periods Comet Name Period (years) En cke 3.30 Schwassman-Wachmann 3 5.35 Giacobini-Inner 6.59 Halley 76.03 Swift-Tuttle 119.60 Source: Comets and Meteor Streams, Vol 2, Porter. Short-period Comets.

To simplify our discussion, we will combine the traditional short and intermediate period comets into the category of Short-period comets. Though called short-period comets, periods range from only a few years up to 200 hundred years. The number of short-period comets in our solar system is unknown but is. 9 believed to be on the order of 15,000 with a diameter greater than 100 meters. 6 The number of these that are Earth-crossing is estimated to be about 10-20% of all short-period comets or about 3,000.7 Unfortunately, only a small portion of these have been discovered and have known orbits. While the orbits of short-period comets are more stable than many in the long-period class, their orbits are still subject to perturbations by the planets and collisions with other minor solar system objects.

Comet Shoemaker-Levy 9 is a prime example of the drastic orbit changes that can occur when a comet has a close encounter with one of the planets. Some time ago, Shoemaker-Levy 9 was captured by Jupiter where it orbited in a highly elliptical orbit until 8 July 1992 when Jupiter's tidal forces tore it apart. 8 One orbit later, on 16 July 1994, pieces of the fragmented comet began colliding with Jupiter sending fireballs rising out of Jupiter's atmosphere and leaving dark scars that were visible from small Earth-based telescopes. 9 A similar impact on Earth would be disastrous.

Source: David A. Seal, Paul W. Codas and Donald K. Yeomans of JPL. Figure 2-2.10 Long-period Comets. Long-period comets consist of all comets with a period greater than 200 years. As with the short-period comets, the number of long-period comets is not known. Its likely that there are literally trillions in the solar system, waiting in the Oort cloud. Approximately 700 are known to have passed through the inner solar system and about half of them had Earth-crossing orbits.

10 The total population of long-period comets is hard to characterize for two reasons. First, its difficult to find and catalog them. Comets are most visible when they are close to the Sun. The long-period comets will spend most of their time in deep space where very little Sun light will reach them. Thus, most of the population is far enough out in space that we can't see them.

Secondly, the orbits will be greatly affected by the outer planets, especially Jupiter. For example, comet 1910 I has a calculated period of 3,910,000 Source: NASA, Galileo Photo. Figure 2-3.11 years. 11 Little credence should be placed in calculations of such an orbit since before it can return to the inner solar system (assuming that it will return) it is likely that its orbit will be perturbed. In fact, it will probably be difficult to recognize 1910 I if it reappears since its orbit may be changed so much that it would be indistinguishable from a new comet.

There are potentially many thousands of comets within the solar system with such long periods that we will never be able to say with confidence that we know where they all are. If only a fraction of these are Earth-crossing they could pose a significant risk. The Asteroids. There are three very general groups of asteroids that need to be addressed: planet-crossing, main belt and extra-belt asteroids. Of these, the planet-crossing bodies are of greatest concern since they frequently cross Earth's orbit; thus they offer the greatest probability of impact. The main belt and extra-belt bodies do not pose an immanent threat since they stay well beyond Earth's orbit; however, the chaotic gravitational interaction between the planets and the asteroids may perturb them into Earth-crossing orbits sometime in the future.

12 Table 2-2. Titius-Bode Sequence Predicted Planet at 2.8 AU from Sun Planet Series Titius Series Value Distance From Sun in AU Mercury 0 0.4 0.39 Venus 3 0.7 0.72 Earth 6 1.0 1.00 Mars 12 1.6 1.52 24 2.8 Jupiter 48 5.2 5.20 Saturn 96 9.6 9.54 Source: Cosmic Impact, John K. Davies. 12 Main Belt Asteroids. In 1772, a German professor named Johann Daniel Titius found an interesting mathematical relation between the sequence of numbers: 0, 3, 6, 12, 24, 48, 96 and the orbits of the planets.

13 Notice that in this series each number is double the previous one (except for the second). If the number 4 is added to each number in the series the resulting new series gives the ratios of the distances of the planets from the Sun. If you define the distance from the Earth to the Sun as 1 Astronomical Unit (AU) and divide by 10 the series gives the distance of each planet from the Sun in AU. The series is nearly perfect for all planets through Uranus. The significance of this is that Titius, and later a German Astronomer named Johann Bode, noticed that planets existed at each of the predicted locations except 2.8 AU.

The discovery of Uranus in 1781 by William Herschel at almost exactly the orbit predicted by the Titius Series (mean orbital distance from the Sun 19.6 AU) started a search for the missing planet at 2.8 AU. 1 2 4 6 Semi-Major Axis (AU) Earth Jupiter 4: 1 7.2 3: 1 5: 2 7: 3 2: 1 5: 3 3: 2 4: 3 1: 1 Jovian Resonances Atens Apollos Amor Hungarian Phocaea Koronis Trojans Cybele's Cental Main Belt Hilda's Kirkwood Gaps are Areas Between Families at Resonance Points. Not a Complete List. Figure 2-4.13 In 1801 an Italian astronomer Giuseppe Piazza accidentally discovered Ceres at 2.77 AU. The search for the missing planet would probably have ended there; however another astronomer named Heinrich Ollers found another object in the same area. The object was named Pallas; the second body in a region that has come to be known as the asteroid belt.

For reasons that are still not clearly understood the region between Mars and Jupiter contains many planetesimals rather than a single planet. The most accepted explanation is that the gravitational field of Jupiter created a disturbance that prevented the debris from coming together to form a planet. 14 Further study has shown that the belt itself isn't just a random collection of objects. There's a definite structure which was discovered by Daniel Kirkwood in 1857. He showed that, within the belt, there were no asteroids with an orbital period equal to an even fraction of Jupiter's period. 15 He explained this by saying that all objects in the belt receive a gravitational "tug" by Jupiter.

For most of the objects, this tug occurs at different places in their orbits so the effects.