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REASONS FOR NOT HAVING AN EARLY ASTEROID MISSION
Let me first emphasize the area of agreement with Professor Alfvén. I, too, believe that the asteroids are of very great scientific interest; great enough to justify space missions some day. What we differ on is the timing and target selection for these missions (Alfvén and Arrhenius, 1970, and in this volume"). I look upon space missions as a tremendously expensive way of obtaining scientific data, which should not be attempted until all available alternatives are exhausted. Ground-based research on asteroids and meteorites is nowhere near exhaustion; on the contrary, it is moving at an impressive pace. If we maintain this pace for another decade or two, we will not only have answered most of the questions posed for an early mission, but will be able to come up with a more worthwhile, more informative mission.
Harold Urey once said that meteorites are the only samples of extraterrestrial matter delivered to our doorstep free of charge. Although some people will disagree, I think there is more than a slight chance that most meteorites come from the asteroid belt. It would be tremendously embarrassing to our entire profession if it turned out after a mission to Eros that pieces of Eros (erotic meteorites?) have been reposing in our museums all along. I say “embarrassing” because I think it is well within our powers to trace each group of meteorites to its parent body in the sky. What makes the problem tractable is the small number of objects to be matched up: 6 to 11 meteorite parent bodies and about 7 asteroid families. Each successful match reduces the number of combinations remaining. Let me outline some possible approaches.
Arnold's (1965) Monte Carlo method makes it possible to trace meteorites to their parent bodies, by comparing observed meteorite orbits with computergenerated sets for various possible parent bodies. Wetherill (1968, 1969) has made major improvements in the model, and others are undoubtedly feasible. Once a way has been found to treat distant interactions with Jupiter, the model will have reached a degree of realism at which meaningful comparisons with observed meteorite orbits can be made.
Observational material is still scarce; the Prairie Network and the Czechoslovak All Sky Camera Network have thus far yielded orbits for only two meteorites. But similar networks are being built in Canada and Germany, and there is hope that progress in this area will quicken. At least several dozen of the older visual orbits appear to be salvageable (Levin and Simonenko, 1969; Millman, 1969) and additional criteria are available for eliminating the remaining 15 to 20 percent doubtful ones in Millman's selection.
It would be premature to claim any identifications on the basis of the present data. But figures 1 to 3 show that different meteorites and asteroids are readily distinguishable from one another on appropriate plots. The Monte
12 I-T-I-T- 1.2 | H-Chondrites 2 Pollos |OH- - |O - - -—— — — 08H l 08 H l –––– =#| || 308|- eP — 06 H- – 1 – – . H #04– - O4 t #"I / ) — —so - -> 02H- 2= — = 0.2 - H 3 o H # l Eucrites + £ | OH- - # | 0 | - - - - - $ - o :* - 0.8 ------ – # 06H- - 0.6 H – ---- - - # H § 04H- - 04 –1 O2H- — 02 ot-i-- - | - *swo | l I I l l i l 05 || 0 15 20 25 30 35 0.5 !.0 |.5 20 2.5 3.0 3.5 Semimojo quis (AU) Semimajor qxis (AU)
Figure 1.-Observed meteorite orbits. Figure 2.-Monte Carlo orbits of meteorites Photographic orbits for Příbram Pand (Mellick and Anders, unpublished). Both Lost City L are indicated by black were calculated on the assumption that the
circles; visual orbits (Millman, 1969, inclination drops to a small value whenever and personal communication) are the meteorite reaches a near 2.50 AU (1/3 indicated by curves representing loci commensurability with Jupiter), and that of plausible U.-a combinations. the mean life for collisional destruction is Eucrites appear to have systematically 1 x 107 yr. Meteorites from Renzia have smaller orbits. systematically smaller a and lower U. than
those from Pallas.
9 KO H. I I T+ o H-Chondrites © L-Chondnies 8 || D LL-Chondnies 10 F o E-chondrites – • O —orld - pe c. —O- L Ca ió - — - o” I — 2 § - 7 |O # Figure 3.—Radiation age versus perihelion 5 ...6 q. Perihelia of chondrites and Pallas3 |O - derived meteorites tend to be close to 5 2 P o 1 AU, decreasing slightly with increasar allos == ing radiation age. This is true of all Mars-crossing asteroid families, as long 7 - . as older meteorites are eliminated by 10 +– collisional destruction, with mean life - of 107 yr. For Betulia and other | 6 ; initially Earth-crossing parent bodies, O smaller perihelia appear from the very - beginning. Open symbols represent 16 1580 Betulid l visual orbits; black symbols, photo
Carlo orbits were calculated on the “optimistic” assumption that geocentric velocity U is occasionally reoriented by Jupiter perturbations when the semimajor axis a reaches a major commensurability (Anders”). Williams” newly discovered resonances suggest that such an assumption is not grossly unrealistic. As the model is further refined, definite identifications ought to be possible. Some of the parameters most useful in such comparisons can be obtained from sources other than photographic orbits. Perihelia often can be estimated with surprisingly good accuracy from visual observations. Small perihelia (less than 0.5 to 0.7 AU) can be inferred from loss of cosmogenic gases. Some meteorite classes (e.g., H-chondrites) show such gas losses more frequently than others; and because they also show a preference for a.m. falls, they probably have orbits with small a and small q (Wänke, 1966). Geocentric velocities can also be estimated from the ablation loss, which in turn can be determined from measurement of charged particle tracks or cosmogenic radioactivities (Bhandari, 1969).
Finally, some meteorites have radiation ages less than 1 million yr. They are not likely to have experienced any close encounters with Earth prior to impact and thus probably struck Earth from a relatively “unevolved” orbit. Such an orbit should be easy to trace to that of the parent body. If the parent body was Earth-crossing, the two orbits should be identical; if it was Mars-crossing, they should differ by only a single Mars deflection. Sooner or later such a meteorite will be recovered by one of the camera networks. In fact, Lost City, with a radiation age of only 5 million yr, did not intersect Earth's orbit for much of the past 0.5 million yr, according to calculations by Lowrey (1970). Thus it may be suitable for this kind of analysis.
OPTICAL AND CHEMICAL CLUES
McCord et al. (1970) and Chapman et al.” have shown that reflection spectra of asteroids can provide clues to their composition. Dollfus” and Hapke" have used albedo, polarization data, and color indexes (Gehrels, 1970) for this purpose, and have made comparisons with terrestrial, lunar, and meteoritic samples. This is a most promising development. The five known classes of chondrites can be grouped into compositional sequences, on the basis of oxidation state, iron content, Mg/Si ratio, etc. Presumably these trends reflect conditions in the solar nebula that varied in some systematic way with distance, time, height above median plane, etc. (Anders, 1971). It will be most interesting to see whether these sequences can also be recognized among the asteroids, and whether they are functions primarily of a, i, or radius R. The pyroxene band at 0.9 pum, which is the most distinctive feature in the reflection spectra, will depend both on oxidation state (which determines Fe?"/Fetotal. and hence the Fe?" content of the pyroxene) and Mg/Si ratio, which determines the pyroxene/olivine ratio.
The degree of fragmentation of an asteroid family (Anders, 1965) can also provide a useful clue. Iron meteorites with a narrow spread of cooling rates apparently come from near the center of their parent body, which must therefore be highly fragmented. Among the chondrites, the proportion of highly recrystallized ones (petrologic type 6) is a clue to degree of fragmentation. For example, only 27 percent of the H-chondrites are type 6, compared to 68 percent of the L-chondrites. Either the L-chondrite parent body was larger and generally hotter, or it was more highly fragmented. (The latter interpretation is more in line with the 520 million yr outgassing event for the L-chondrites; Anders".)
*See p. 51. *See p. 95. °See p. 67. 7See p. 431.
APPROPRIATE TARGETS FOR AN ASTEROID MISSION
I have argued in my companion paper” that only high-velocity asteroids, about 10 percent of the total, are potential sources of meteorites. Little or no material reaches us from the remaining 90 percent, including the entire outer half.
If we are successful in matching each meteorite class to its parent body, we will certainly know quite a bit about the high-velocity objects in the inner half of the belt. There will not be much point in sending spacecraft to this relatively well-known part of the asteroid belt.
At this point it is well to consider the nature of the Eros group. There can be no question that Eros and Amor are some of the most accessible asteroids, but they seem to be transient objects. All five have short lifetimes against planetary capture. According to Öpik's (1963) calculations, the lifetimes (in aeons) are as follows:
433 Eros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.84
These are probably overestimates, being based on the initial orbit only. Monte Carlo calculations, which consider the change in collision probability after each orbital change in planetary encounters, sometimes give lifetimes up to an order of magnitude shorter for similar objects. In any case, the lifetimes for orbital change are much shorter than those for planetary collision. Thus it is unlikely that these objects formed where they are now found. We can try to estimate the origin of the three Mars-crossing objects, making use of the fact that their velocity relative to Mars remains approximately invariant in successive encounters. (The velocity is not strictly invariant because the orbit of Mars has nonzero e and is this causes a slight acceleration of the asteroid at each encounter.) Figure 4 compares UM, the velocity relative to a circular orbit at 1.524 AU, for the Mars-crossing asteroids. Eros may have been derived from family 31 after some acceleration by Mars, or from family 5. Amor and Ivar may be members of the Hungaria group at 1.9 AU; but this group itself is probably derived from one of the families in the asteroid belt proper: 5, 29, or even 30. Thus there is not much point in obtaining samples of these stray objects because their original location in the asteroid belt is almost as uncertain as that of meteorites. lf missions with sample return capability are ever sent into the asteroid belt, they should seek to complement the knowledge gained from meteorites. As I