Abbildungen der Seite
PDF
EPUB
[ocr errors]

Figure 4.—Mars-crossing asteroids. Eros has nearly the same encounter velocity with Mars UM as families 31 or 5, and may be a stray member of one of them. Amor and Ivar appear to be related to the Hungaria group at 1.9 AU, which in turn may be derived from families 5, 29, or even 30.

argued in my companion paper,” perhaps as much as 10 percent of the asteroid belt (mainly the high-velocity objects in the inner half) may contribute meteorites to the Earth. The remaining > 90 percent is out of communication with us. If the optimists are right, we may soon gain a good understanding of the “communicating” 10 percent of the belt. Combining this knowledge with telescopic observations, we can then extrapolate at least the gross trends of the “noncommunicating” 90 percent. Some crucial questions will undoubtedly remain when all ground-based studies have been pushed to the limit, and at that stage, perhaps 10 yr from now, further progress will require space missions. We do not know what sort of target will have highest scientific interest at that time: a Trojan, a Hilda group asteroid, a few nearly spherical asteroids (small or large), in the near or far parts of the belt, a few highly irregular objects, a Hirayama family, etc. Any choice we make now is likely to seem trivial or uninformative a decade hence.

If the pessimists are right, even the “communicating” 10 percent of the asteroid belt will be terra incognita to us by the time ground-based work has reached its limits. In that case, a methodical exploration of the asteroid belt may well be justified. Whatever the GNP at that time, it seems certain that the number of missions will be orders of magnitude smaller than the number of asteroids. Targets therefore will have to be selected with very great care. In any event, because proximity is not a critical mission constraint (Bender and Bourke'9), asteroids in the main belt would seem more worthwhile than stray objects such as Eros.

[ocr errors]

ACKNOWLEDGMENTS

The Monte Carlo results in figures 2 and 3 are unpublished data by P. J. Mellick. This work was supported in part by AEC Contract AT(11-1)-382 and NASA Grant NGL 14-001-010.

REFERENCES

Alfvén, H., and Arrhenius, G. 1970, Mission to an Asteroid. Science 167, 139-141.
Anders, E. 1965, Fragmentation History of Asteroids. Icarus 4,399-408.
Anders, E. 1971, Meteorites and the Early Solar System. Ann. Rev. Astron. Astrophys. 9,
1-34.
Arnold, J. R. 1965, The Origin of Meteorites as Small Bodies. II. The Model. Astrophys. J.
141, 1536-1547.
Bhandari, N. 1969, On Orbital Velocities of Some Stone Meteorites. Paper presented at
the Cosmic Ray Conf. (Delhi).
Gehrels, T. 1970, Photometry of Asteroids. Surfaces and Interiors of Planets and Satellites
(ed., A. Dollfus), p. 319. Academic Press, Inc. New York.
Levin, B. Yu., and Simonenko, A. N. 1969, Meteorite Radiants and Orbits. Meteorite
Research (ed., P. M. Millman), ch. 46, pp. 552-558. D. Reidel. Dordrecht.
Lowrey, B. E. 1970, Orbital Evolution of Lost City Meteorite. GSFC preprint
X-644-70-444.
McCord, T. B., Adams, J. B., and Johnson, T. V. 1970, Asteroid Vesta: Spectral
Reflectivity and Compositional Implications. Science 168, 1445-1447.
Millman, P. M. 1969, Astronomical Information on Meteorite Orbits. Meteorite Research
(ed., P. M. Millman), ch. 45, pp. 541-551. D. Reidel. Dordrecht.
Öpik, E. J. 1963, Survival of Comet Nuclei and the Asteroids. Advances in Astronomy and
Astrophysics 2 (ed., Z. Kopal), pp. 219-262. Academic Press, Inc. New York.
Wänke, H. 1966, Der Mondals Mutterkörper der Bronzit-Chondrite. Z. Naturforsch. A 21,
93-1 10.
Wetherill, G. W. 1968, Dynamical Studies of Asteroidal and Cometary Orbits and Their
Relation to the Origin of Meteorites. Origin and Distribution of the Elements (ed., L.
H. Ahrens), pp. 423-443. Pergamon Press. Oxford.
Wetherill, G. W. 1969, Relationships Between Orbits and Sources of Chondritic
Meteorites. Meteorite Research (ed., P. M. Millman), ch. 48, pp. 573-589. D. Reidel.
Dordrecht.

DISCUSSION

KUIPER: As a ground-based observer most of my life, I would naturally sympathize with the point of view of Anders. I think all of us who are working in the laboratory or as astronomers feel that we can do 10 times better or we would not be in the business in the first place. We see many possibilities ahead. I think at the same time it is not right for us to say that the rest must wait until we have made progress. I am all for an accelerated ground-based program for 5 yr, but let us not stop the space missions if they can be ready before that.

KIANG: Past attempts at determining the size and shape have had little success, and this is not due to any lack of effort. Here we seem to have an objective that will always evade us however hard we may try from the ground but that can easily be attained by an asteroid mission. Another objective of this nature is the number density of small bodies in the asteroidal belt. At present this is based on brute extrapolation.

DUBIN: I agree with Anders regarding the importance of the ground-based studies of meteorites relative to the understanding of the asteroids, but it appears that the ground-based work cannot resolve the problem in the sense indicated from the Prairie Network results shown by McCrosky'' earlier. McCrosky showed that most of the bright meteors were of type A and C and of low density. The chondrite, Lost City, was one case in a sample of between 100 and 1000 events. In addition, Anders showed that a significant fraction of the mass of the meteoroid, in fact the interesting outer crust, is always ablated during atmospheric entry. I question, accordingly, how these limitations of the sample of meteorites can give a satisfactory model of the early solar nebula and the asteroids. ALFVEN: Anders feels that so much information about asteroids could be obtained from the study of meteorites that this, for the time being, ought to substitute for actual exploration in space. To investigate the evolutionary history of the solar system, it is necessary to pool information from a number of sciences including chemistry, geology, meteoritics, celestial mechanics, and plasma physics. Specialists in any one of these fields may make important contributions but hardly without collaboration with scientists in other fields. This seems not to be generally understood. There have been numerous attempts from astrophysicists to clarify the evolutionary history of the solar system without taking notice of the results of the chemists. Similarly, some competent chemists still tend to draw their conclusions from chemical considerations alone, neglecting the laws of physics. It is obviously unreasonable to write one evolutionary history for physicists and another, completely different, for chemists. The most important task of the cosmic sciences today is to draw borderlines between speculation and science in its real sense. It is obvious that one of the borders is set by the reach of spacecraft. For example, before the first soft landing on the Moon there were, understandably in view of lacking evidence, many speculations about the chemical composition and physical structure of the lunar surface. We know now that all this had very limited scientific value except as a stimulus toward the actual revealing experiment. For asteroids, a similar situation prevails today, only the uncertainties are wider yet. During this symposium there has been much discussion about the origin of meteorites. There has been rather extensive support for the view that there is an association between meteorites, comets, and Mars-crossing asteroids, although the arguments for this view are still speculative. Anders, however, believes that the meteorites also come from the main belt asteroids and that certain groups of meteorites should be associated with certain asteroid families. His hypothesis is not demonstrably supported by the laws of celestial mechanics. UREY: It is my expectation that Eros would have a composition very similar to some meteorite with only minor differences in composition and mineralogy. Also, I suspect that it is a fragment from some violent collision because of its elongated shape. Because of this origin and its very low gravitational field, I believe that it will have no “soil” on its surface and therefore no scooping of soil or drilling of a core can be made. I strongly suspect that a mission to Eros will be very disappointing to experimenters, the scientific community, NASA, Congress, and the public. I would be delighted to support a mission to Ceres. It may be a fragment of some larger body or it may be a primary object. It may have free rocks and some layer of soil on its surface. Possibly it is a little sister of my lunar-sized bodies or it may be a daughter of them. I would be most interested in seeing a mission to Ceres or the other larger asteroids. WHIPPLE: I do feel that interest by astronomers in the solar system, particularly in asteroids and smaller bodies, has been negative over the years. Thus I certainly am delighted to see this broad interest of so many coming here to discuss this. It looks to me that it is a healthy attitude toward these problems but I feel this should be supported at = higher level than ever. However, I have a point about landing on an asteroid. It seems to me that we would like to find a broken-up body so that its surface would give us a historical record.

[ocr errors]

analogous to geology on the walls of the Grand Canyon. I am sure that the formation processes of asteroids must lead to such historical layers. Perhaps Ceres is not a good choice because its early formations will probably be covered over. On the other hand, I suspect it would be hard to choose an asteroid that would not lead to exciting and unexpected results.

GREENSTADT. Anders' argument against an early asteroid mission and Anders' and Urey's skepticism regarding the value of an Eros mission obtain part of their force from an implicit assumption that such a mission would be created at great expense solely for its own objectives. However, this is not the case. If solar electric and nuclear electric propulsion are to be developed in an orderly way because they are useful for future solar system missions in general, then a proper question is whether Eros, or another Mars-crossing asteroid, would be a suitable target for an early launch with electricpropulsion technology. The choice may be not whether an Eros rendezvous is the best of all asteroid mission objectives, but whether it is best among alternatives for application of early electric-propulsion launches.

« ZurückWeiter »