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Where Have All the Comets Gone?
Mark E. Bailey

SOURCE: MARK E. BAILEY, ILLUSTRATION; PRESTON MORRIGHAN/SCIENCE

This picture for the home of comets eyond the orbit of the most distant has stood the test of time remarkab ly planets, the Oor t cloud has long well. However, the inability to predict acbeen known as the source of the curatel y the obser v ed flux of nearl y comets that reach the inner solar system. isotropic inter mediate and shor t-period But the distribution of the dynamically comets shows that much in the cometary evolved, "captured" comets from the Oort world remains obscure. It is timely that cloud has been diff icult to explain. In parthe fading problem (613) should be reticular, there is a severe def icit in the assessed in light of new observations and number of comets observed in short-peritheoretical understanding. od orbits like that of comet Halley. This is not the f irst time that our unOn page 2212 of this issue, Levison et derstanding of comets has proved incomal. (1) investigate this so-called "fading plete. For years, it was known that the disproblem" by combining two models: a detribution of perihelion directions of longtailed model of the dynamical evolution of period comets was nonunifor m (1417) comets from the Oort cloud to intermediand that the Oort cloud must be affected ate and short-period orbits, and a statistiby the differential tidal gravitational force cal model that describes the likelihood that cur rent astronomical sur veys would discover these comets or their inert "asteroidal" remnants. This ref ined model still predicts two orders of magnitude more comets in short-period "Halley-type" orEarth Positions in June 2002 Saturn Jupiter bits (periods of revolution P < 200 years) than are observed. The moder n picture for the origin of comets was f irst 180 developed in detail by Oor t 80 ( 2 ). He proposed that the 160 planetary system is surround70 ed by an enormous swarm of comets, with randomly dis140 tributed orbits that extend al60 Hale-Bopp most halfway to the nearest 120 star. Oort's model was spheri50 cally symmetrical, with the 100 Sun at the center, and con11 comets 40 tained about 2 в 10 Halley 80 with diameters greater than a few kilometers. In this model, 30 60 the cometar y space density 3/2, where varied roughly as r 20 40 r is the distance from the Sun. More recent models 10 20 (35) indicate a g reater degree of central condensation, 0 with most of the mass in a 0 0.000 0.001 0.002 0.01 0.02 0.00 hypothetical dense inner core 1/a (AU 1) 1/a (AU 1) at r 103 to 104 astronomical units (AU), where 1 AU is the The fading problem. The frequency distribution of the semimajor axes, 1/a,for 330 long-period and new comets mean distance of Earth from (38), superposed on a plot of the inner solar system and the orbits of comets Halley and Hale-Bopp provided by D. the Sun. J. Asher. The 1/a distribution is shown on two scales (left, bin width 50 в 106 AU1;right, bin width 103 AU1). The

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of the Galaxy (18). Yet it was not until the early 1980s that Oort's picture, in which random stellar per turbations feed new long-period comets into the inner solar system, was over tur ned. The old order was replaced by one in which per turbations by the Galactic disc were dominant ( 1921 ), with stellar and occasional molecular cloud perturbations producing occasional enhancements in the comet flux--so-called comet showers--on time scales of 100 to 1000 million years. Similarly, Jupiter-family comets (those with low-inclination "direct" orbits and P < 20 years) were long believed to originate primarily as a result of the action of Jupiter on long-period comets. This belief persisted e v en though f ar too man y Jupiter-family comets were observed relative to predictions (22, 23). This problem has been resolv ed by introducing into models new sources for Jupiter- f amily comets: either a dense inner core of the Oort cloud (3, 4, 24, 25), necessary in any case to explain the outer Oort cloud's dynamical survival for the age of the solar

N Number of comets

The author is at the Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK. E-mail: meb@arm.ac .uk

whole of the first histogram to the right of 1/a = 0 is contained in the first two positive bins of the second The similar shape of the two plots shows that the fading problem is not solely an issue for dynamically comets. Without fading, apart from the Oort cloud spike, the 1/a distribution would be nearly flat. Note the tence of a few puzzling cases of comets with originally slightly hyperbolic orbits; these are usually explained result of outgassing or poor orbit determination.

plot. new exisas a

Number of comets

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65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 system, or the newly discovered Edgeworth-Kuiper belt (2630). The fading problem, however, has been a persistent thorn in the flesh of the Oort theory. The Oort cloud explains why most long-period comets have such extremely long periods, with orbits stretching in some cases more than halfway to the nearest star (see the f igure). But close examination shows that the frequency distribution of 1/ a values (where a is half the length of the elliptical orbit) has too few comets at large 1/ a . Astronomers have therefore introduced an arbitrary "fading" function that links the physical and dynamical evolution of cometar y nuclei. It has been suggested that 95% of comets that are initially favorably perturbed, so as to return to the inner planetary system, are never seen (1113). Comets are expected to decay (if only because of observed mass loss), but there is no direct evidence that new comets from the Oor t cloud disappear in such large numbers. On the contrary, the repeated return of comet Halley suggests that kilometer-sized cometary nuclei may survive for hundreds or perhaps thousands of revolutions before f inally disintegrating. Herein lies the problem: If comets fade, then they must do so largely out of sight and probably at large heliocentric distances. If comets do not decay, however, then there should be many more intermediate-period comets than observed, and far more Halley-type comets. Previous estimates ( 3136 ) have now been independently conf irmed by the new, comprehensive computer-based model of Levison et al., who argue that the majority of comets must physically disrupt. This impor tant result raises the question: Where does the mass go? An alternative possibility, not favored by Levison et al., is that long-period comets become inert (that is, they lose the capacity to undergo outgassing) and hence evolve into lowalbedo objects resembling asteroids, virtually invisib le against the b lackness of space. These potentially hazardous objects--the astronomical equi v alent of black cats in coal cellars--have important implications for programs aiming to identify the next kilometer-size impactor before it identif ies us. Another alter native is that Oor t cloud comets may easily break up into essentially unobservable smaller bodies after just a few perihelion passages. The Sun-grazing Kreutz family may be a prototype. These comets have orbital periods of hundreds of years, yet they appear to have undergone a hierarchy of fragmentation events during at least the past two millennia. There are at least half a dozen Sun-g razing f amilies, each potentially containing tens of thousands of comets and subcometar y fragments with diameters from 10 to 100 m. Astronomers using data from the Solar and Heliospheric Obser vator y (SOHO) satellite have detected nearly 500 such comets in less than 7 years, with new objects reported on an almost daily basis. With the recognition that the Kreutz group is not unique and the observation of other examples of split comets, a few of them disappearing before our eyes, an important cometary end-state may have been identif ied. This, however, also raises the specter of a cometary risk to civilization, as a result of encounters with possibly dense cometar y meteoroid streams containing fragments of a size that could produce catastrophic events on Earth. A third possibility is that Oor t cloud comets decay into nothing more substantial than dust. The rapid disintegration of a kilometer-sized cometar y nucleus into its component dust and ice grains is perhaps a seriously counterintuitive result, especially as most comets run into nothing harder than the solar wind. However, a few comets have been observed to disinteg rate for no apparent reason. If comets decay rapidly to dust, then the dense meteoroid trails should be detectable--for example, as meteor showers, through impacts on spacecraft, or possibly via thermal infrared emission analogous to the dust trails discovered with the Infrared Astronomical Satellite (IRAS) (37). Perhaps, as with previous questions concer ning the origin of comets, astronomers are collectively missing a trick. A fundamental, though not essential, tenet of the Oor t theor y is that the obser ve d cometary flux is in a steady state, with the numbers of short-period and other comets in balance with the observed near-parabolic flux from the Oor t cloud and other reservoirs. Could the steady-state assumption be mistaken? This would put us in the uncomfortable position of living at a special epoch, perhaps within a few million years of the start of a comet shower (6, 9), with all sor ts of attendant repercussions; but while such models may show promise, none completely resolves the fading problem (33, 36). If comets indeed disr upt, whether to unobservable small bodies or to dust, Levison et al. (1) make the interesting point that perhaps this behavior tr uly distinguishes the nearly isotropic Halley-type shor t-period comets from those of the Jupiter family, which in their model originate from the Edge wo r th-K uiper belt rather than the Oort cloud. In this case, the different fading behavior of the two classes of comet would provide the f irst hard
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evidence for a real physical difference between the "inner" and "outer" comets, perhaps the result of different origins and thermal and collisional histories. At present, comets remain a puzzle: They have to be both strong and weak, and there seems to be a substantial missing mass. Does this provide a clue to the origin of cometary material?
References
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