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Photometry of Faint Asteroids and Satellites

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Photometry of Faint Asteroids and Satellites PHOTOMETRY OF FAINT ASTEROIDS AND SATELLITES proefschrift ter verkrijging van de graad van Doctor in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit te Leiden, op gezag van de Rector Magnificus Dr. O.J. Kuenen, hoogleraar in de Faculteit der Wiskunde en Natuurwetenschappen, volgens besluit van het College van Dekanen te verdedigen op vrijdag 1 december 1978 te klokke 14.15 uur door Johan Degewij geboren te Amsterdam in Sterrewacht Leiden Promotores: Prof. T. Gehrels en Prof. H.C. van de Hulst The research described in this dissertation was supported by Leiden University and the National Aeronautics and Space Administration. Voor mijn ouders Voor Leonne CONTENTS page SUMMARY CHAPTER I INTRODUCTION 1.1 Background and Purpose 1 1.2 Detection of Faint Objects in the Solar System 6 1.3 Photometry of Asteroids 8 1.3.1 Definitions 8 1.3.2 Photographic Photometry 11 1.3*3 The Precision and Usefulness of Photographic Photometry 18 \.k Spectroscopy 19 1.5 Polarimetry 19 1.6 Infrared thermal Radiometry 20 CHAPTER II ASTEROIDS IN THE MAIN BELT 2.1 Studies of Photographic Lightcurves 25 2.1.1 Photographic Lightcurves obtained in 1973 27 2.1.2 Photographic Lightcurves obtained in 197^ 3*» 2.1.3 Rotation Periods and Body Shapes 42 2.2 Photoelectric Photometry 2.2.1 UBV Photometry of Faint Asteroids *»7 2.2.2 Compositional Types and Orbital Parameters 57 2.3 Size Distributions 2.3.1 Absolute Magnitude Distributions 60 2.3.2 True Size Distributions 63 2.3.3 Revised Size Distributions 65 2.4 Variations of the Composition over the Surface 2.4.1 Observations 71 2.4.2 Uniformity of Asteroid Surfaces 74 CHAPTER IN DISTANT ASTEROIDS 3.1 Asteroids with Hilda-type Orbits 3.1.1 VBV Colors 80 3.1.2 Radiometric Albedos and Diameters 80 3.2 Trojan Asteroids 3.2.1 UBVRI Reflectances 82 3.2.2 Variability and Body Shapes 88 3.2.3 The Phase Function 90 3.2.4 Radiometric Albedos and Diameters 90 page CHAPTER IV FAINT SATELLITES ' 4.1 Outer Jovian Satellites 3k U.I Saturns' Satellite S9 Phoebe 98 < CHAPTER V HISTORY OF SMALL BODIES IN THE i SOLAR SYSTEM I 5-1 Collisional Processes 101 \ 5.2 Similarities between Distant Asteroids ' and Faint Satellites 107 T 5.3 Where are the Dead Comets? 109 SAHENVATTING 115 STUD IE OVERZICHT 117 ACKNOWLEDGEMENT 118 SUMMARY I began this work in Leiden in 1974 by measuring on photo- graphic plates the 1ight variations of small asteroids as they rotate about their axes. The years 1976-1978 I spent at the University of Arizona making photoelectric observations of faint asteroids, the satellites of Jupiter and faint cometary nuclei. The following general insights were obtained into physical parameters, interrelations, and origin. The smaller asteroids, having diameters of about 1 km, appear to rotate faster than do the larger asteroids (-v-200 km diameter). Most of the bodies may be nearly spherical, proba- bly due to a collisional erosion process in the Main Belt of asteroids. The distributions of diameter versus number were studied for low albedo (C, for carbonaceous) and high albedo (S, for silicaceous) type asteroids in the main belt, down to diameters of 25 km. Among the smaller bodies the S type aster- oids are relatively more abundant, probably due to greater crushing strength for S type asteroids. This indicates that both optical types have also different properties in the in- terior of the body. Areas with slightly different reflectivity over the sur- face of an asteroid were detected; the rotational light varia^- tron of asteroid 4 (Vesta) was found to be caused by spots on Its surface. The spots may be due to differences in composi- tion, or to partial excavation to deeper layers by meteoritic impact. In general, however, the surfaces of asteroids are quite uniform. Impact events result in the excavation of much more than the impacting mass and each asteroid blankets it- self by a well-mixed layer of its own debris. Colorimetry and infrared radiometry of some Hilda aster- oids, Trojans and the fainter satellites of Jupiter and Saturn, all having diameters between 100 and 200 km, show that a mix- ture of types exist. However, the resemblances of reflectance spectrum, albedo, phase function, rotation period and body shape among the majority of Trojans and J6 strongly suggest the possibility of dynamical interrelation during the history of the solar system. If some asteroids are nearly expended nuclei of comets that have lost most of their volatile gaseous material, then their cometary activity is expected to be extinct or at least weak. A dozen asteroids y<ith orbital elements similar to those of short-period comets were observed with sensitive spectroscopic equipment, but no activity indicating cometary outgassing has been detected and no proof of their cometary origin has as yet been obtained. CHAPTER i INTRODUCTION 1.1 Background and Purpose . Small bodies in the solar system (0.5-500 km) are of three ; types: asteroids, satellites, and comets. A comet differs from an asteroid by the appearance of a coma and/or a tail due to f outgassing, as the comet approaches the sun. KresSk (1972) 1 showed that a plot of semi-major axis a and orbital eccentri- ; city e (Fig. 1.1) distinguishes the asteroids from the comets. ' Only SUU Hidalgo is in a cometary orbit and does not show come- : tary activity. The relation between apparent magnitude, distance and dia- meter of a solid body without an atmosphere, is given in Fig. 1.2. With a 1.5 meter telescope (V,. = 18) bodies that have albedos of 0.05 can be observed phoioelectrically with sizes down to 5 km in the main asteroid belt and down to 30 km at Ju- piter distance. Objects smaller by a factor of 10 can still be observed photographically with large Schmidt telescopes. The location of these bodies (Fig. 1.1) is mainly in the asteroid belt (2 AU<a<3.5 AU). There exist about 105 asteroids with sizes larger than 0.5 km in the asteroid belt. About 103 asteroids are known to occur in the Lagrangian Sun-Jupiter points prece- ding and following Jupiter. In addition, there are fewer than 300 bodies known to move in other orbits. The dominant mass of Jupiter rules the evolution of the orbits of the smaller bodies beyond Mars. Gaps can be seen in the main-belt (Fig. 1.1), where the revolution period is in re- sonance with that of Jupiter. Only objects that librate with Jupiter, like the Hildas (a~3.S AU) and Trojans (a=5.2 AU) can stay there for a long time. The orbits of the short period co- mets, however, evolve strongly with time, because of close en- counters with Jupiter. The population of asteroids crossing the orbit of Mars is poorly surveyed, mainly because cf their large orbital variation in distance to the earth. The lifetime, before a collision with the planets takes place, may be on the order of 107 years (Wetherill ^) i All detailed information about magnitudes and orbital ele- j merits of asteroids was obtained with systematic photographic I surveys. The Yerkes-McDonald Survey (MDS), published in 1958 by ; Kuiper, Fujita, Gehrels, Groeneveld, Kent, van Biesbroeck, and - van Houten, provided magnitudes for asteroids down to blue ap- \ parent magnitude 16. The Palomar-Leiden Survey (?LS), published :: in 1970 by van Houten, van Houten-Groeneveld, Herget, and I Gehrels, sampled only 2% of the belt. It yielded orbital ele- k ments and an absolute magnitude distribution down to blue appa- ll rent magnitude 20. With an estimate for albedo and density, the 2 3 3 2 2 1 3 1 Fig.l.l The distribution of orbital elements a and e for asteroids (•), librating asteroids (A), well observed comets (O), and lost comets (o). Every object to the right side of the curved lines, representing the aphelia of Earth and Mars, crosses the orbit of the planet. Also the orbital resonances with Jupiter are shown. Adapted from KresSk (1972). derived mass distribution provided an idea about the total mass in the main-belt, namely about twice the mass of the largest as- teroid, Ceres, or 2.Jt*102ltg, or O.^IO-3 the mass of Earth (Schubart 1972). The mass distribution at the faintest end gave reasonable agreement with the nieteroid mass distribution, sugges- ting a meteoroid origin from crushed asteroids (Oohnanyi 1969). A series of papers (see Chapter 2.3) was devoted to the in- terpretation of a non-linearity or "hump" at 9.5<g<11.5 in the absolute magnitude distribution of the inner zone of the main- belt between 2.0 and 2.6 AU. This corresponds with asteroids having diameters between 30 and 70 km, if an albedo of 0.05 is assumed. With the assumption that the absolute magnitude distri- bution represents a size distribution, it is thought that this non-linearity is the transition region between an original dis- tribution of less than 100 large aocretional objects and nume- rous smaller aollisional fragments. 25 Fig.1.2 The relation between the apparent visual mag- nitude V, semi-major axis a, and diameter D for an asteroid in opposition with the sun. Circular orbits and an albedo of 0.05 are assumed. Table 1.1 Terminology and Symbols Asteroid A moving object of stellar appearance, without any trace of cometary activity. k Vesta Asteroid moving in the main belt of asteroids between Mars and Jupiter; this one is the fourth discovered, numbered k in the asteroid catalogue, and named Vesta.
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