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Chapter 9 Conclusions and the Future Chapter Conclusions and The Future In this chapter I will sp ecify some conclusions that can b e drawn by combin ing the results presented in this thesis with earlier studies of cometary nuclei Of course my work has not made the sample of nuclei complete and a few dozen more ob jects with welldetermined physical prop erties would b e helpful b efore any anal ysis b ecomes statistically defensible However it is interesting to collate the current information and see what trends may b e app earing and what observational biases are dominating the study of comets Comets and Their Disguised Relations Figure shows a plot of the eective diameters and geometric alb edos for several cometary nuclei including the ones I have discussed in this thesis Also plotted are nuclei studied by others and several NEAs and Centaurs The data are listed in Table with the information from this thesis having an arrow in the Ref column Not all data in Table are plotted on Fig Here are some caveats ab out this table Most of the entries are from rep orts of a nuclear size measurement made using thermal infrared techniques In a few cases radar or optical observations that have spatiallyresolved images of the ob ject were used Observations in those wavelength regimes that just have cross sectionintegrated photometry were not used The vast ma jority of the radii and alb edos were derived using the Standard Thermal Mo del A few used the Rapid Rotator Mo del and one was even derived from the Isothermal Mo del I have not made an attempt to reanalyze these data I simply have quoted the values and errors that the authors themselves state even though there are very clearly cases where the error bars are underestimated Consid ering the uncertainties in some of the parameters that go into the thermal mo dels such as the b eaming factor and the phase b ehavior Chapter the systematic error of the absolute ux calibration ab out Tokunaga Rieke et al and the exp erience of the several comets presented in this thesis it seems that some of the diameters error bars could b e closer to and the alb edos error bars clos er to This is esp ecially true where the thermal data is of low S N and this do es not even include any systematic error with using an idealized mo del Excep tions to this include but are not limited to Comet Halley and Asteroid Eros which have b een optically imaged with subkm spatial resolution Asteroid Toutatis which has b een the sub ject of multiple extensive radar exp eriments and Comet IRASArakiAlco ck which passed so close to Earth and resulted in a mul tiwavelength data cache so large that it was probably only a matter of time b efore someone collated everything into a coherent picture Sekanina c Figure The sizes and alb edos of active cometary nuclei some nearEarth aster oids and Centaurs The cometary region is now starting to ll out thanks to many thermal studies done since the mids One notices that some cometary diameters have no attendant alb edos Iron ically it is often the case that reliable optical cross sections do not exist for the comets that have b een observed in the midIR In the future more co ordination b etween observations in the multiple wavelength regimes is needed In Chapter I discussed some of the problems of nucleus observation that contribute to this lack of optical cross sections The Rotation column shows many more entries with N than with Y ie for most of the listed ob jects the rotational context is unknown This has not b een reected in the error bars of the diameters and alb edos so the true error bars are even higher for many ob jects For some ob jects this is not a problem b ecause the observations to ok so long that the rotational variation has probably b een averaged out and so is incorp orated into the error estimate already For example the multiple midIR exp osures of Hyakutake cover several hours of time and hence a large fraction of the rotation p erio d Note that the Cometary Nuclei in the title is in quotation marks I have included many asteroids in the table some fraction of which are extinct comets I will now discuss this p oint in more detail The Tisserand parameter T is a constant of motion in a restricted threeb o dy J problem Considering the Sun Jupiter and a small b o dy as the three memb ers of a system as long as the b o dy is not having a close encounter with Jupiter at the time of the observation the value of T is constant In practice the value uctuates J by a few p ercent due to p erturbations by other planets The denition is Danby p v u 2 u e a J t T cos i J a a a J where a is the semima jor axis of Jupiter AU a is the ob jects semima jor axis J e is the ob jects eccentricity and i is the ob jects orbital inclination Tisserand himself recognized in the late th century that this constant of motion could b e used to identify two comets observed far apart in time as the same ob ject if the comet had had a close encounter with Jupiter in the interval and thus had its orbital elements drastically changed The value of the parameter indicates the strength of the dynamical coupling of the ob jects orbit to Jupiter Most asteroids have T while the shortp erio d J comets mostly have T ie T is the b oundary b etween the coupling J J almost all shortp erio d comets are dynamically coupled to Jupiter while most asteroids are not An indication of this can b e seen in the q column in Table ap for ob jects with T the aphelion is close to Jupiter J Of course the T b order is not p erfect There are asteroids that have T J J these are usually NEAs with a suciently large aphelion distance and there are comets that have T the socalled Encke Family Levison which so J far only has two known memb ers b oth of which are in Table These are comets in more classic NEA orbits ie the aphelion distance is never high enough to bring them close to Jupiter The explanation for the asteroids in cometary orbits follows from the supp osed typical life cycle of a shortp erio d Jupiter Family comet After b eing p erturb ed out of the Kuip er Belt and into the outer planetary region the nucleus is at the mercy of the gas giants Approximately thirty p ercent of these comets that leave the Kuip er Belt b ecome part of the Jupiter Family Levison and Duncan the rest are either ejected or sent farther out in the Solar System Centaurs are thought to b e Kuip er Belt ob jects currently in transition since their dynamical lifetimes is 6 roughly only year Dones et al Once an ob ject is in the Jupiter Family its dynamical lifetime there is ab out 5 years Wetherill afterwhich the comet collides with a planet or the Sun or is sent into a classic NEA orbit decoupled from Jupiter Of course during those 5 years the comet is outgassing since it passes close enough to the Sun but the 4 store of volatile material in the comet will only last ab out years either the comet will disintegrate by then or the mantled surface will b e to o thick choking o the available ice Levison and Duncan Hence on average a comet will b ecome dormant while still coupled to Jupiter Observationally one would discover an asteroid in a cometlike orbit T a few of which are noted in Table J However it is p ossible that a comet will b e quickly sent into an NEA orbit and decoupled from Jupiter b efore all available ice is gone and we will see active comets in NEA T orbits which we do see most famously as comet Encke The J existence of this comet and comet WilsonHarrington guarantee that despite the fact that the Main Belt can p otentially provide a large fraction of the kilometer size and larger NEAs Rabinowitz some fraction of the NEAs must b e dead cometary nuclei The trick which we have not yet solved is to nd some diagnostic that indicates which of the NEAs are cometary and which are asteroidal McFadden Future studies of NEAs and nuclei may shed light on this problem In Table I have made an arbitrary separation at T to mark which J asteroids might dynamically have a higher probability of b eing dead comets How ever there are two intriguing asteroids that have high T and yet could very well J b e cometary Asteroid Phaethon is the parent to the Geminid meteor stream Whipple which is strong evidence for a cometary origin despite the fact that its aphelion distance is almost full AU smaller than the next smallest cometary one Encke Asteroid Oljato was observed to have a transient blue excess by McFadden et al which they argued was caused by a cometary outburst One problem with Oljato is its high alb edo much higher than all of the known com etary nuclei Nevertheless I have separated these ob jects from the other high T J crowd to emphasize that these ob jects have additional extenuating circumstances Immediately one notices that there are some very black asteroids in b oth the lowT and highT sections In my opinion these ob jects are the prime candidates J J for b eing extinct nuclei and further study is needed to nd out if there is any distinguishing characteristic observable from Earth that separates them from the other Main Beltderived NEAs Furthermore there are many asteroids such as Hidalgo Damo cles and BC that have low T and resp ectively but that simply J have not yet had their thermal ux measured Presumably when that happ ens they will take their place alongside the other lowalb edo ob jects Table Sizes and Alb edos of Cometary Nuclei Ob ject Diameter Alb edo T q Rotation Ref In Fig J ap a b c km AU Halley Family Comets PHalley NA Y PTemp elTuttl e N Y N N
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