WPCu& 2BV_Z3| ,Scal) (MF)r\@N:,i|4L  Py?~YYLighettt:rrrrrfssssY?Y?Y?Y?hr~hrrrffff~ssssqqqqqqYYY?Yfy?????YYYYYYYLLL坲heee~?YYLhh~//ttLrvvttV_tM`忿|ʗt$tttuMoeleeeeeeee2G' m"m ^:ENtڨ:ZZt:M:@tttttttttt??j堘UY̭͟b@bt:wvh{cEc}=j;qdnlUVDo[_YWddd2nfffffWYYYYM;M;M;M;qddddoooo~Yfndd~YfffWWWWnYYYYaaaaaaqqMMM;Mbj;;;;;qqqqddUUUtVtVtVtV~D~D~Doooooo~YWWWn;qUtV~D~Y~Yndo//ddAaiiddLKdBUĤjd$dddxxxxdB}wdYuuf}Y}}}}}}}}}}}}}}}}}}}YYYYYYY2A&yS\" R"m ^!+-fB}m!33Bo!,!$BBBBBBBBBB$$oooAf[^k]Rkm3:bU~fkPkfIWmfff_+$+B!AG:HB,AI$#E$lIIIH33+IfVfM:MMMMMMMMMMMMMMMMMMM:::::::f"|^BV\BffBWBGIIgrGr BtwUNSNrrZy~vt BtwwwwgNgNgNgNvvttttwwwwgggNgNNNNNrrrrrrrZZZvtttNrrZvv/                                                /VfdWq  $   Wwwwwwwwww\&7C2,X4L  P1 km diameter) were to strike the Earth, its physical properties would make essentially no difference. To such an impactor, the Earth's atmosphere would be thin, and the object would do damage to the Earth in proportion to the kinetic energy it carries when it explodes at the Earth's surface.  The numerous objects smaller than several hundred meters in diameter may break up in the atmosphere, if they are made of loosely aggregated, weak, low density materials (i.e. "cometary"). Even rocky objects break up in low altitude airbursts if they are smaller than ~70 m, like the ~50 m object that caused the Tunguska explosion. Strong iron meteorites of all sizes can penetrate the Earth's atmosphere practically unscathed. For more details on) -?,?,HH how the physical properties of the impactor affect its passage through the  X atmosphere, see Chyba et al. (1993), Hills et al. (1993), and OTHER CHAPTERS IN THIS BOOK. The implications of this chapter are simply that there is a wide variety of materials among the NEA's, ranging from loose, weak aggregates to solid objects made of metal.  The possibility that a significant proportion of NEA's may be compound bodies raises the possibility of multiple impacts. As we have learned from the example of Comet ShoemakerLevy 9, it is possible for such bodies to be tidally disrupted by passage through a Roche zone and then have the disrupted pieces collide with the planet. This scenario has been considered for the Earth by Melosh and Stansberry (1991) and is under further development. Passage within the Earth's Roche limit is several times more likely than direct impact, and the question then concerns the ultimate fate of such objects. There has been controversy over the existence of "asteroid streams" (Drummond, 1991), and tidal disruption of NEA's may be one explanation for such observations. IMPLICATIONS FOR MITIGATION  One of the most dangerous possible outcomes of an attempt to deflect or destroy an approaching asteroid is the unintended production of several large pieces, which might then do more damage than the single, intact body. For example, a swarm of objects distributed around the Earth could create) -?,?,HH widespread firestorms (both from the direct bolide and from hot ejecta) that would be less likely from a single impact. If a threatening asteroid is to be destroyed, the method must be sufficient to pulverize an object (or compound object) made of the strongest possible material, which is strong metal. If it is to be deflected, it must be guaranteed that the forces applied actually move the object intact, without inducing even small stresses that would cause the body to split (unless the mitigation scenario additionally considers dealing with all such pieces that might result).  If the asteroid to be deflected is a "rubble pile" of unconsolidated pieces, then the largest single impulsive delta-v which can be applied is of the order of the surface escape velocity of the asteroid, or ~1 m/sec per km diameter of the body (see Ahrens & Harris, this volume). This is because an impulse (e.g. from a surface or stand-off nuclear blast) is unevenly distributed throughout the body, and the various pieces will acquire a dispersion of velocities of the same order as the average delta-v; thus if that impulse exceeds the excape velocity, the pieces will become unbound from one another. For initially solid, rocky objects larger than ~100 m, an impulse great enough to produce a delta-v of ~0.1 m/sec would also lead to widespread fracturing of the body, so the above limitation would apply to that case, as well.  NEA's are inherently heterogeneous bodies of great diversity. Any engineering application will have to involve detailed study of the individual object in question to ascertain its particular physical and compositional) -?,?,HH characteristics. Likewise, experimentation on destroying or deflecting a benign asteroid may be of little value: the specific characteristics of the threatening object are likely to be different. Extrapolation from existing groundbased data and current speculations would be most unwise. To date, there has been no "ground truth" obtained for any NEA, although the Clementine mission to Geographos is underway. Even the Galileo flybys of the mainbelt asteroids Gaspra and Ida were at considerable distances and involved only quicklook pictures and remotesensing data (Chapman, 1994). There is much about our current concepts of the makeup of asteroids and dead comets that could be far from the truth. It would be desirable to build up a data base concerning the properties of diverse NEA's so that, if a threatening object is found, we will have a larger and more reliable context in which to assess its characteristics.  It is beyond the scope of this chapter to develop particular approaches to mitigation that take our poor knowledge of physical properties into account. But we may briefly offer the following generic recommendations: (1) Better characterization of NEA's as a population is highly desirable. In addition, precursor missions to assess the properties of the specific hazardous body are essential, or else such assessment capabilities must be built into the deflection mission itself. Such assessment should concentrate on issues such as body strength (e.g. coherent body or rubble pile), body shape and dynamics, and composition. (2) Mitigation procedures must be designed to be as independent of the specific shapes, compositions, heterogeneity, etc. of the body as possible. (3) A controlled deflection scheme such as standoff explosions (Ahrens and)-?,?,HH Harris, 1992) or lowthrust propulsion would help minimize the stresses that might unintentionally disrupt the body into a more dangerous swarm of unpredictable character instead of moving it. ACKNOWLEDGEMENTS  This work has been supported by NASA grants and contracts. This is Planetary Science Institute Contribution No. 318. PSI is a division of Science Applications International Corporation. FIGURE 1 [There is no figure reference in the text. However, if it does not appear elsewhere in this volume, a radar image of Toutatis would be appropriate.] N-?,?,HH REFERENCES  X Ahrens, T. J. and Harris, A. W. 1992. Nature 360 , 429433.  X Binzel, R. et al. 1992. Icarus 99 , 225237. Binzel, R. P., Xu, S., Bus, S. J., and Bowell, E. 1992. Origins for the  X_ Near-Earth Asteroids. Science 257 , 779-782. Binzel, R. P., Xu, S., Bus, S. J., Skrutskie, M. F., Meyer, M., Knezek, P., and Barker, E. S. 1993. The asteroidmeteorite connection: the discovery of a main  X belt ordinary chondrite asteroid (abstract). Meteoritics 28 , 324.  X Chapman, C. R. 1994. The Galileo encounters with Gaspra and Ida. Planetary  X and Spa. Sci., in press.  X Chyba, C. F., Thomas, P. J., and Zahnle, K. J. 1993. Nature 361 , 4044.  Xb Drummond, J. D. 1991. Earthapproaching asteroid streams. Icarus 89 , 1425. Gaffey, M. J., Burbine, T. H., and Binzel, R. P. 1993. Asteroid spectroscopy:  X progress and perspectives. Meteoritics 28 , 161187. Gradie, J. C., Chapman, C. R., and Tedesco, E. F. 1989. Distribution of  X taxonomic classes and compositional structure of the asteroid belt. In Asteroids  X II, ed. R. P. Binzel, T. Gehrels, and M. S. Matthews (Tucson: Univ. of Arizona Press), pp. 316335. Hartmann, W. K., Tholen, D. J., and Cruikshank, D. P. 1987. The relationship  Xe of active comets, "extinct" comets, and dark asteroids. Icarus 69 , 3350.  X7 Hills, J. G. and Goda, M. P. 1993. Astron. J. 105 , 11141144. Lebofsky, L. A., Lebofsky, M. J., and Rieke, G. H. 1979. Radiometry and  X surface properties of Apollo, Amor, and Aten asteroids. Astron. J. 84 , 885888. Luu, J. and Jewitt,D. 1989. On the relative numbers of C types and S types  X! among nearEarth asteroids. Astro. J. 98 , 19051911. McFadden, L.A., Tholen, D. J., and Veeder, G. J. 1989. Physical properties of  Xh$ Aten, Apollo, and Amor asteroids. In Asteroids II, ed. R. P. Binzel, T. Gehrels, and M. S. Matthews (Tucson: Univ. of Arizona Press), pp. 442467. Melosh, H. J. and Stansberry, J. A. 1991. Doublet craters and the tidal  X ( disruption of binary asteroids. Icarus 94 , 171179.  X) Ostro, S. J. et al. 1990. Science 248 , 15231528. *-?,?,HHԌOstro, S. J., Campbell, D. B., Hine, A., and Shapiro, I. I. 1987. Radar echoes  X from asteroid 1986 DA indicate a metallic composition (abstract). Bull. Am.  X Astron. Soc. 19 , 840.  X Wetherill, G. W. 1988. Where do the Apollo objects come from? Icarus 76 , 118. #d6X@8;S@#_-?,?,HH     Appendix ???. Table 1. Physical Properties of NEA's Asteroid a e i H Alb Dia Cl Per Amp Rad 433 Eros 1.46 .223 10.8 11.16 0.12 22. S 5.270 0.05-1.5 2 887 Alinda 2.49 .559 9.3 13.76 0.31 4.2 S 73.97 0.35 944 Hidalgo 5.84 .658 42.4 10.77 d 38. D 10.064 0.6 1036 Ganymed 2.67 .537 26.5 9.45 0.19 39. S 10.31 0.12-0.45 1 1221 Amor 1.92 .435 11.9 17.70 m 1.0 0.1* 1566 Icarus 1.08 .827 22.9 16.40 0.51 1.0 S 2.273 0.05-0.22 1 1580 Betulia 2.19 .490 52.1 14.52 0.08 5.8 C 6.130 0.21-0.50 2 1620 Geographos 1.24 .335 13.3 15.60 0.25 2.0 S 5.223 1.10-2.03 1 1627 Ivar 1.86 .397 8.4 13.20 0.14 8.1 S 4.797 0.25-0.60 1 1685 Toro 1.37 .436 9.4 14.23 0.31 3.4 S 10.196 0.6 -0.8 3 1862 Apollo 1.47 .560 6.3 16.23 0.25 1.5 Q 3.065 0.15-0.60 1 1863 Antinous 2.26 .606 18.4 15.54 0.24 2.1 S 4.02 0.12 1864 Daedalus 1.46 .615 22.2 14.85 m 3.7 SQ 8.57 0.85 1865 Cerberus 1.08 .467 16.1 16.84 0.22 1.2 S 6.800 1.48-2.0 1866 Sisyphus 1.89 .539 41.1 13.00 0.16 8.2 S 2.48 0.12 1 1915 Quetzalcoatl 2.54 .574 20.5 18.97 0.21 0.5 S 4.9 0.26 1 1916 Boreas 2.27 .451 12.9 14.93 m 3.5 S 1917 Cuyo 2.15 .505 24.0 13.90 m 5.7 2.697 0.43 1 1943 Anteros 1.43 .256 8.7 15.75 0.17 2.3 S long 0.04 1951 Lick 1.39 .062 39.1 14.70 m 3.9 A 4.417 0.28* 1980 Tezcatlipoca 1.71 .365 26.8 13.92 0.25 4.3 S 7.853 0.74* 1981 Midas 1.78 .650 39.8 15.00 m 3.4 S 5.218 0.85* 1 2061 Anza 2.26 .537 3.7 16.56 d 2.6 TCG: 11.50 0.3 2062 Aten .97 .183 18.9 16.80 0.26 1.1 S 2100 Ra-Shalom .83 .437 15.8 16.05 0.06 3.4 C 19.79 0.34 2 2101 Adonis 1.87 .764 1.4 18.70 m 0.6 1 2201 Oljato 2.18 .711 2.5 15.55 0.54 1.4 S? 24.? >0.1 1 2212 Hephaistos 2.17 .833 11.8 13.87 m 5.7 SG 0.05* 2340 Hathor .84 .450 5.9 20.26 m 0.3 CSU 2368 Beltrovata 2.11 .414 5.2 15.21 0.27 2.3 SQ 5.9 0.84 2608 Seneca 2.49 .581 15.3 17.52 0.21 0.9 S 8. 0.5 3102 1981 QA 2.15 .449 8.4 16.70 m 1.6 QRS 147.8 >1.0 3103 1982 BB 1.41 .355 20.9 15.21 0.64 1.5 E 5.709 0.72-0.9 2 3122 1981 ET3 1.77 .423 22.2 14.20 m 4.9 5 or 10 0.25* 3199 Nefertiti 1.57 .284 33.0 14.84 0.42 2.2 S 2.82 0.12 1 3200 Phaethon 1.27 .890 22.1 14.60 0.09 6.9 F 4.0 0.12* 3288 Seleucus 2.03 .457 5.9 15.00 0.22 2.8 S 75. 1.0 3360 1981 VA 2.47 .743 21.7 16.20 0.17 1.8 3361 Orpheus 1.21 .323 2.7 19.03 l 0.3 V 3.58 0.32* 3362 Khufu .99 .469 9.9 18.10 0.21 0.7 3551 1983 RD 2.09 .488 9.5 16.81 0.37 0.9 V 4.930 0.11-0.15 3552 Don Quixote 4.24 .714 30.8 13.00 0.03 19.0 D 7.7 >0.41 3554 Amun .97 .280 23.4 15.82 0.20 2.0 M 2.53 0.16* 3671 Dionysius 2.19 .543 13.6 16.30 m 1.9 2.4 0.26 3691 1982 FT 1.77 .284 20.4 14.50 m 4.3 days? * 3757 1982 XB 1.84 .446 3.9 18.95 0.18 0.5 S 9.012 0.20 1 3908 1980 PA 1.93 .458 2.2 17.30 0.23 1.0 V 4.426 0.25-0.46 1 3988 1986 LA 1.54 .317 10.8 18.30 m 0.7 8. 0.2' 4015 1979 VA 2.64 .623 2.8 15.99 d 3.4 CF 3.556 0.06 4034 1986 PA 1.06 .444 11.2 18.10 m 0.8 1 4055 Magellan 1.82 .327 23.2 14.50 0.23 3.4 V 7.5 0.5( 4179 Toutatis 2.51 .641 .5 14.00 m 5.4 S 200. 1. 4197 1982 TA 2.30 .773 12.2 15.40 0.37 1.8 4544 Xanthus 1.04 .250 14.1 17.10 m 1.3 1 4688 1980 WF 2.23 .515 6.4 18.60 0.18 0.6 SQ 4769 Castalia 1.06 .483 8.9 16.90 m 1.4 4.088 1.0 1 4954 Eric 2.00 .449 17.5 12.50 m 10.8 S >11. 0.6* 5332 1990 DA 2.16 .456 25.4 14.90 m 3.6 S 5.816 0.36 5370 1986 RA 3.35 .631 19.0 15.90 d 3.6 C 0.02 1977 VA 1.86 .394 3.0 19.40 m 0.4 XC 1978 CA 1.12 .215 26.1 16.90 0.09 1.9 D? 3.756 0.8 1980 AA 1.89 .444 4.2 19.40 m 0.4 S 2.697 0.12 +-?,?,HHԌ 1984 BC 3.44 .457 22.4 16.00 d 3.4 D 1984 KB 2.22 .765 4.9 16.60 0.21 1.4 S 1986 DA 2.82 .582 4.3 15.90 0.15 2.3 M 3.58 0.32 1 1986 JK 2.80 .680 2.1 18.90 d 0.9 C 0.05 1 1988 TA 1.54 .479 2.5 20.90 d 0.4 C 1989 DA 2.16 .544 6.4 17.90 m 0.9 7.867 0.12* 1989 JA 1.77 .484 15.2 16.40 m 1.8 * 1 1989 UP 1.86 .473 3.9 20.60 m 0.3 6.983 1.2* 1989 VA .73 .595 28.8 16.90 m 1.4 >0.15* 1989 VB 1.86 .461 2.1 19.90 m 0.4 >24. >0.3* 1990 HA 2.57 .692 3.9 16.90 m 1.4 8.51 0.06* 1990 KA 2.20 .433 7.6 15.90 m 2.3 6. >0.4 1990 MF 1.75 .456 1.9 18.50 m 0.7 1 1990 OS 1.67 .459 1.1 19.90 m 0.4 1 1990 SA 1.96 .430 37.5 16.90 m 1.4 S * 1990 TR 2.14 .437 7.9 14.50 m 4.3 S 6.25 0.19 1990 UA 1.72 .552 1.0 19.40 m 0.4 long 0.08 1990 UP 1.33 .169 28.1 20.50 m 0.3 >20. >0.07* 1991 AQ 2.16 .769 3.2 17.50 m 1.1 * 1 1991 EE 2.25 .624 9.8 18.50 m 0.7 3.00 0.12* 1 1991 JX 2.53 .599 2.3 18.50 m 0.7 1 1992 AC 2.10 .421 16.1 13.60 m 6.3 <0.02* #Z4L  P