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Publications of the Astronomical Society of the Pacific Vol. 106 1994

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Publications of the Astronomical Society of the Pacific Vol. 106 1994 Publications of the Astronomical Society of the Pacific Vol. 106 1994 May No. 699 Publications of the Astronomical Society of the Pacific 106: 425-435, 1994 May Invited Review Paper Comets Disguised as Asteroids Jane Luu1 Physics Department, Stanford University, Stanford, California 94305-4060 Electronic mail: [email protected] Received 1993 October 12; accepted 1994 January 24 ABSTRACT. Comets and asteroids were previously thought to be two completely distinct groups of solar- system objects, with marked contrast in both physical and dynamical characteristics. A comet is operationally defined by the presence of a coma, while an asteroid has no coma. However, recent observations have shown that comets can sometimes take on asteroidal appearances and even asteroidal photometric behavior. Thus the observational distinction between comets and asteroids is not as clear cut as it once seemed. The possible presence of comets hidden among known asteroids forces us to reconsider the criterion by which we distinguish comets from asteroids and possibly our inventory of both comet and asteroid populations. 1. INTRODUCTION ets hidden among asteroids. Finally, the Conclusions sum- marize the main ideas presented in the review. This invited review paper is based on a talk given at the 180th American Astronomical Society meeting held in 1992 2. ASTEROIDS June at Columbus, Ohio. Comets and asteroids form two major groups of solar- 2.1 The Main Belt system objects, previously thought to be completely distinct As viewed from the Earth, even the largest asteroid (1 from each other. We identify an object as a comet by its Ceres, diameter 950 km) barely attains an angular diameter coma, an expanding cloud of dust and gas surrounding the of 1 arcsec, and appears marginally resolved in typical ob- nucleus. The coma is created by the sublimation of volatiles serving conditions. Our first close-up glimpse of an asteroid and ejection of entrained dust particles. On the other hand, came in 1991, when the spacecraft Galileo made a close asteroids are ostensibly inert objects; they possess no coma, approach with asteroid 951 Gaspra en route to Jupiter. A and except for the very largest ones, appear unresolved as picture of Gaspra obtained during this encounter is shown in seen from the Earth. Until recently, comets and asteroids Fig. 1. Gaspra is a member of the main asteroid belt with a were thought to share no common characteristic besides the semimajor axis of 2.21 AU. The figure shows an irregularly fact that they both orbit around the Sun. But the last decade shaped body (19X12X11 km) with a rocky surface pock- has seen the emergence of new evidence suggesting that the marked by impact craters (Belton et al. 1992). boundary between comets and asteroids may be much more The majority of asteroids are found in a beltlike distribu- tenuous than previously believed. tion (the "main belt") located between Mars and Jupiter and In this review paper, I will focus on how comets can be effectively marking the boundary between the terrestrial "disguised as asteroids," i.e., how they adopt an asteroidal planets and the gas giants. There is very little mass in the asteroids, compared with the masses of the adjacent planets. appearance and photometric behavior. The paper is intended 21 for the general (nonspecialist) audience. The main goal of the All together, the main belt contains ~3X10 kg, or about 2% of the mass of the Moon and 0.05% that of the Earth. A paper is to show that the observational distinction between third of that mass is in 1 Ceres alone. A sense of the location comets and asteroids is no longer clear, and that the classifi- of asteroids with respect to the planets can be gained from cation of an object as a comet or asteroid, once a trivial task, Fig. 2, which shows a histogram of the asteroid semimajor is no longer a simple matter. The paper begins with a brief axes. introduction to asteroids and comets and their origins. This is The typical orbital periods of main belt asteroids are 3-6 followed by a discussion of how comets can develop aster- yr, and the orbits generally have low eccentricities (^0.1). oidal appearances and the methods by which to identify com- Most asteroid orbits are stable and are subject to only weak planetary perturbations, whose general effect is to make as- 1 Hubble Fellow. teroids oscillate about their mean orbits. As can be seen from 425 © 1994. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 426 LUU every revolution of Jupiter. Repeated dynamical interactions with Jupiter have cleared asteroids from these resonances, and the gaps are commonly known as "Kirkwood gaps." 2.2 Non-Main Belt Asteroids Outside the main belt, there are only a few localized re- gions where asteroids are found. Inward of the belt lie the near-Earth asteroids (NEAs). As their name suggests, they travel on orbits which approach or cross that of the Earth. Beyond the main belt, the Trojan asteroids populate the L4 and L5 Lagrangian points of Jupiter at 5 AU. Beyond Jupiter, we presently only know of three objects classified as "aster- oids": 944 Hidalgo, 5145 Pholus, and 1993 HA2. (Another object in this region, 2060 Chiron, was previously classified as an asteroid but now displays a coma and so is a comet by the operational definition.) These objects travel on chaotic, comet-like orbits. It is the asteroids outside the confines of the main belt which play the most important role in the present discussion, and they will subsequently be discussed Fig. 1—Image of asteroid 951 Gaspra taken by the Galileo spacecraft during in greater detail. its encounter with the asteroid in 1991. The picture was taken from a range of 5300 km, with a resolution of —50 m. Photo courtesy of M. Belton, National Optical Astronomy Observatories (NOAO), for the Jet Propulsion 2.3 Physical Properties of Asteroids Laboratory. Asteroids are collisionally evolved: collisions at high rela- Fig. 2, asteroids are distributed almost throughout the entire tive velocities (~5 kms-1) are the main factor responsible 2-4 AU region, with the exceptions of a few clear gaps, such for the evolution of the asteroid's physical properties. A few as at 2.5 AU (3:1 resonance) and 3.3 AU (2:1 resonance). clusters of asteroids in the main belt ("Hirayama families") Asteroids located at these orbital resonances have a mean have similar orbital parameters and are believed to have motion which is in an exact integer ratio to Jupiter's, e.g., formed from common catastrophic collisions (Hirayama those at the 3:1 resonance complete three revolutions for 1918, 1919). Indeed, much of what we know about asteroid Fig. 2—Histogram of the semimajor axes of known asteroids. Arrows point to the locations of resonances. Figure reprinted from Binzel (1989), courtesy of the University of Arizona Press. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System COMETS DISGUISED AS ASTEROIDS 427 collisions come from rock-crashing laboratory experiments. Very crudely, the asteroids follow a inverse power-law size distribution described by q N{r)dr^r dr. (1) where r is the radius, N(r)dr is the number density of ob- jects in the radius range r to r + dr, and ^ is a positive power-law index. The power-law index for known asteroids 1:1 ( lies in the range g—2-3, and may depend on the composi- tional types (Gradie et al. 1989). Asteroids are made mainly of refractory material (e.g., 4:3 olivine, silicates, organics, pyroxene, feldspars), although 3:2 iron and nickel also form a large component in the compo- • * ·. · m -, · · sition of some asteroids. Water may also be present, but usu- 0o no -o vf.» r · ally in the form of chemically bound water (e.g., Jones et al. o C) o^· · . · · O ^v··.. _. 1990). Thus asteroids are generally inert bodies, although the largest asteroids may be able to sustain seasonal "atmo- spheres" by means of sublimating polar caps, as has been 0 suggested for asteroid 1 Ceres (A'Hearn and Feldman 1992). Ο The main belt is roughly divided into three compositional zones (see Fig. 1 of Bell et al. 1989). These compositional zones correspond to clusters of different taxonomic types: at the inner belt (heliocentric distance —2-2.3 AU) we find mostly "S"-type asteroids, then "C"-types further out (i?—2.3-2.8 AU). At the edge of the belt and beyond (ft >2.7 AU), the "P"- and "D"-types dominate. The S-types have moderate to high albedos and a composition of metal, oliv- 0.2 0.6 ine, and pyroxene. The C-types have low albedos and show signs of aqueous alterations. P- and D-types are dark and seem to contain no hydrated minerals, as inferred from their Fig. 3—The distinction between cornets and asteroids in semimajor axis- lack of the 3 μτη water absorption feature (e.g., Jones et al. eccentricity space. The open circles are asteroids, while the solid circles are 1990). The lack of hydrated minerals may indicate that the P- comets. The sizes of the circles and dots are proportional to the sizes of the and D-types have never been sufficiently heated for interior objects. Figure adapted from Kresak (1979) and Weissman et al. (1989), ice to melt. The P- and D-types are thus considered "primi- courtesy of the University of Arizona Press. tive" asteroids. There are also minor asteroid classes, the number and definition of which depend on the particular most of the asteroids were fragmented upon impact and some classification scheme (e.g., Tholen 1984; Barucci et al.
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