Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~nolan/astdyn/ADWabs.pdf Äàòà èçìåíåíèÿ: Wed Jan 28 22:44:06 2004 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:47:08 2012 Êîäèðîâêà: ISO8859-5 Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ð ï ð ï ð ï ð ï ð ð ï ï ð ï ï ð ï ï ð ï ð ï ï ð ï

Deduction of the possible excited spin states of 2P/Encke
Michael J.S. Belton (michaelbelton@beltonspace.com) Belton Space Exploration Initiatives, LLC 430 S. Randolph Way, Tucson AZ 85716 The determination of the spin state of an active cometary nucleus presents a somewhat different problem than the determination of the spin state of an asteroid -- although there is also much in common in the two cases. To date the complete spin state has only been determined for two cometary nuclei -- and at least one of them - 1P/Halley - remains controversial (Samarasinha et al. 2004). Partial information is available for about two-dozen more comets, but this information is mainly limited to the primary periodicity in the lightcurve and pole solutions are generally not available. This state of affairs is regrettable for those who wish to explore the implications of observable coma structures and global gas production rates for the properties of specific active regions on the surface. It is knowledge of the spin state that allows such phenomena to be traced back to particular localities on the cometary surface (e.g. Belton et al. 1991). 2P/Encke is a well-observed comet with a short orbital period that displays variable coma structure and also peculiarities in the production rates around perihelion. Its spin state is therefore of considerable interest. Several photometric studies in the late 80's seemed to indicate a fully relaxed spin state with a period near 15 hr, however not all workers agreed pointing out problems with the observed amplitudes of the lightcurves. More recently, observations in 2000 and 2001 have been reported with a period near 11 hr and all signs of a 15 hr period seem to have disappeared. In this paper I revisit these many data sets and show that the many periods latent in the data (not just the two mentioned above) can be consistently explained in terms of an excited spin state. I also show how the pole determinations of Sekanina (1988) and Festou and Barale (2000) can be generalized to give an estimate of the direction of the total angular momentum vector. More difficult is a choice between spin states in the short (SAM) or long (LAM) axis mode (both are allowed by the periodicities). A low-excitation SAM state is the preferred solution, although a highly excited LAM state with rather special properties is also a possibility. While not yet fully specified, four (possibly five) of the parameters describing the spin state are deduced. The rest could, in principle, be determined from radar observations and/or future high quality light curves in the thermal infrared.
Acknowledgements: Much of this work has b een done in consultation with N.H. Samarasinha, Y. Fernandez, and K.J. Meech Belton, M.J.S., Mueller, B.E.A., Julian, W.H., and Anderson, A.J. 1991. The Spin State and Homogeneity of Comet Halley's Nucleus. Icarus 93, 183-193. Festou, M.C., and Barale, O. 2000. The Asymmetric Coma of Comets. I. Asymmetric Outgassing from the Nucleus of Comet 2P/Encke. Astron. J. 119, 3119-3132. Samarasinha, N.H., Mueller, B.E.A., Belton, M.J.S., and Jorda, L. 2004. Rotation of Cometary Nuclei. In Comets I I. Ed. M.C. Festou. In press. Sekanina, Z. 1988b. Outgassing Asymmetry of Perio dic Comet Encke. I I. Apparitions 1868-1918 and a Study of the Nucleus Evolution. Astron. J. 95, 911-924

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Searching for the Long Lost Precursors of the Primordial Earth
William Bottke (Southwest Research Institute) David Nesvorny (Southwest Research Institute) It is frequently assumed that planetesimals formed at 1 AU, presumably the precursors of our heavily-evolved Earth, could not have survived to the present day. Dynamical models indicate that most ob jects on planetary crossing orbits have a very short dynamical lifetimes (<10 Myr) compared to the age of the solar system (Gladman et al. 1997; Icarus), while cosmochemical models suggest the Earth's bulk chemistry was unlikely to come from any combination of chondritic meteorites in our current collection (e.g., Burbine et al. 2003; LPSC). This is unfortunate, given that samples taken from planetesimals formed at 1 AU could help answer numerous unsolved questions about the formation and evolution of our Earth (e.g., did Earth's water come from 1 AU or was it delivered by primitive asteroids/comets). Results from recent dynamical models, however, are somewhat less pessimistic; they hint at the possibility that a few Earth precursors may still be found, provided we know where to look. For example, Morbidelli et al. (2001; MAPS) showed that during the late stages of planet formation, perturbations with planetary embryos chased a few percent of the initial planetesimal population in the 1-2 AU zone onto highly-inclined orbits in the inner solar system. According to their dynamical model, these bodies could have been long-lived enough to cause an extended bombardment of all of the terrestrial planets for hundreds of Myr. We hypothesize that a very small fraction of this population may have survived all the way to today, provided that: (i) the initial population was large, (ii) the dynamical decay rate of this material was slow (i.e., some of this precursor material found its way into long-lived orbits within the inner solar system), and (iii) these survivors turn out to be protected from extensive comminution. To test our hypothesis, we numerically integrated the known high-inclination NEOs (and their clones) using the code SWIFT-RMVS3 (Levison and Duncan 1994). In our simulation, whenever our initial population decays to less than 10% of the starting population, we clone the remnants and continue the integration. Our goal is to push out the integrations to several Gyr while searching for a long-lived tail to the original population. We believe our method has certain advantages over that used by Morbidelli et al. (2001) because we use the present-day orbital parameters of the planets; Morbidelli et al. (2001) integrated their particles using model-derived planetary parameters that do not match our current solar system. Preliminary results suggest that some clones of our initial population reach orbits near the 2 periphery of the modern-day Hungarias and Phocaeas populations, where their dynamical lifetimes can reach many hundreds of Myr, far longer than typical NEOs (Bottke et al. 2002; Icarus). Even so, it is not yet clear whether these lifetimes are long enough to be consistent with our hypothesis. Our latest results on this topic will be presented at the workshop.

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Using Multi-Polarization Radar to Search for Regolith on Asteroids
Lynn M. Carter, Donald B. Campbell and Michael C. Nolan The presence of regolith material will alter an asteroid's thermal properties and can affect its dynamical history via Yarkovsky forces [1]. It is not clear what sized asteroids have regolith; small asteroids have lower escape velocities, but internal strength and the presence of previously ejected material also determine how much volume is lost to space during an impact [2]. High resolution spacecraft images, such as the NEAR images of Eros, have confirmed that some asteroids possess regoliths [3]. Although many asteroids likely have a regolith, it is hard to detect such surface coverings at the resolutions typically achieved by ground-based optical imaging. At infrared wavelengths, thermal models can be used to extract a value for the thermal inertia, which is related to the density of the surface. However, shape, surface roughness, rotation state and thermal inertia are strongly connected in thermal models, and it is difficult to derive the thermal inertia from observed fluxes without some prior knowledge of the asteroid's physical properties. Multi-polarization radar imaging provides a unique method to search for regolith. In radar experiments, an analysis of the degree of linear polarization in the received echo can be used to investigate whether there is a surface covering on an ob ject. A circularly polarized wave, such as that transmitted by the Arecibo radar, can be thought of as a combination of two linearly polarized waves of equal magnitude, one perpendicular to the plane of incidence and one parallel to the plane of incidence. If a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales and is reflected by embedded scatterers or by an underlying structure, the returned radar echo will have a net linearly polarized component (it will be elliptically polarized). The linear polarization is produced because the horizontally and vertically polarized components of the incident wave have different transmission coefficients into and out of the surface layer. This technique has been used by Stacy [4] to study the lunar regolith and by Carter et al. [5] to study surface deposits on Venus. Benner et al. [6] used the Arecibo and Goldstone radars to observe the asteroid 1999 JM8 during its close approach to Earth. The asteroid is about 7 km in diameter, and delay-Doppler images from these radar runs have resolutions as small as 15 m/pixel, which gives thousands of pixels on the asteroid [6]. The Stokes parameter analysis reveals a significant linearly polarized echo component, which demonstrates that there is some penetration of the radar wave into a surface layer. JM8 has an irregular shape, and the resulting radar ambiguity makes it difficult to model physical properties, such as the dielectric constant and depth, of the regolith. However, the values of the degree of linear polarization also follow the incidence angle trend expected from the transmission coefficients: at near 3 normal incidence angles, no linear polarized echo power is produced, and the values are higher elsewhere on the asteroid. This linear polarization technique can be used with other asteroids, provided that the radar data have sufficient signal-to-noise. We hope to compare the linear polarization properties of different size, shape and perhaps different composition asteroids.
References: [1] Bottke et al. 2002. in Asteroids I I I, p. 395. [2] Scheeres et al. 2002, in Asteroids I I I, p 527. [3] Veverka et al. 2000, Science, 292, 485. [4] Stacy 1993, Ph.D. Thesis, Cornell University [5] Carter et al. 2003, LPSC XXXIV, 1809 [6] Benner et al. 2002, Meteoritics and Planetary Science, 37, 779.

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The First Direct Measurement of Yarkovsky Acceleration on an Asteroid
Steven R. Chesley (JPL), Steven J. Ostro (JPL), David Vokrouhlicky (Charles Univ., Ä ek (Charles Univ., Prague), Jon D. Giorgini (JPL), Michael C. Nolan Prague), David Cap (Arecibo Obs.), Jean-Luc Margot (UCLA), Alice A. Hine (Arecibo Obs.), Lance A. M. Benner (JPL), Alan B. Chamberlin (JPL) Radar ranging at three apparitions over twelve years has unambiguously revealed the action of the Yarkovsky Effect on near-Earth asteroid 6489 Golevka. The most recent measurements, from Arecibo in May 2003, indicate a 15 km displacement that cannot be reconciled with a purely gravitational force model. This kind of detection requires careful consideration of astrometric uncertainties and potential mismodeling of gravitational perturbations. Indeed, uncertainty in the masses of perturbing asteroids has the potential to completely obscure Yarkovsky-induced deflections in some cases. The magnitude of the displacement is consistent with the predictions from simplified Yarkovsky models, and thus many previous theoretical studies that relied on such models are substantially validated by this detection. Moreover, the Yarkovsky measurement permits us to place constraints on the mass and bulk density of the asteroid. However the quality of the mass estimate is degraded by uncertainty in the surface thermal characteristics. A Yarkovsky detection, along with independent measurement of the surface thermal conductivity would allow a much more precise estimate of the asteroid's mass. Conversely, if the mass can be independently determined, such as for a binary asteroid system, then a Yarkovsky detection would allow a precise estimate of the surface thermal properties.

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that is an for Synchronous Asteroidal Satellitesacting on an ideal Ida. Of YORP order of magnitude shorter than that for YORP course, real asteroids have very different shapes; nevertheless this exercise demonstrates Matija Cuk, Cornell University Many of the lesser components in binary asteroid pairs are expected to be in synchronous rotation. Geometrically, such satellites appear to be rigidly attached to their primaries, since they always keep the same orientations relative to their primary's center of mass (assuming the satellite's orbit is circular). Recently, YORP - the non-gravitational force affecting an asteroid's rotation - has received considerable attention (Rubincam 2000, Vokrouhlicky and Capek 2002, Vokrouhlicky et al. 2003). I argue that a similar effect should influence the orbits of at least some synchronous asteroidal satellites, owing to their similarity to promontories of the primary. To illustrate this mechanism, we visualize an irregularly shaped asteroid as a sphere with a wedge-shaped attachment (cf. Rubincam 2000, but we assume only a single attachment). This appendage is shaped like a prism with a right triangular base, which is attached to sphere's equator, with one of the smaller rectangular sides perpendicular to the sphere's rotation axis. This attachment reflects sunlight directly back off one side but out-of-plane off the other; each side has the same vertical cross-section. The infrared radiation re-emitted by the body itself has a similar distribution. Hence a torque, which can change spins, arises. I now consider an identically shaped asteroidal satellite in synchronous rotation, with the prism glued to the satellite's long axis, which is assumed to point toward the primary. In this way one side of the prism is always facing forward relative to the satellite's orbital motion, while the other faces backwards. Depending on the prism's exact placement and its orientation, re-radiated energy from the prism can produce a positive or negative torque on the satellite's orbit. To estimate roughly the significance of this effect, we compare the timescales for YORP acting on the primary and "satellite YORP" acting on a secondary, assuming our "sphere-plus-wedge" shape and taking binary parameters like those of the Ida-Dactyl system. The YORP's magnitude varies with the prism's size, and therefore with radius squared, while the torque is the product of this force times the distance from the center of motion (the primary's radius for the usual YORP, but the binary's separation for the satellite YORP). The timescales for these two effects are estimated by dividing the relevant angular momenta by the respective torques. For YORP, the timescale ty = const 2/5 M R2 Wrot /(R R2 ) = const R2 W
rot

(where M, R and Wrot are the primary's mass, radius and rotation rate). For satellite YORP, ts = const m a2 Worb /(a r2 ) = const a r Worb (where m, r, a, Worb are the satellite's mass, radius, semima jor axis and mean motion). Assuming identical densities, the ratio of timescales is: (ts )/(ty ) = 2.5 (a/R) (r/R) (W
orb

5

)/(Wrot )

For the Ida-Dactyl pair, (a/R) = 7, (r/R) = 1/22, (Worb /Wrot ) = 1/8, so the timescale ratio should be (ts /ty )= 0.1. Therefore, our ideal Dactyl would change its orbit on a timescale

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the otential importance of the satellite YORP. Since the orbital angular momentum varies p as a, and the satellite YORP torque is proportional to a, a satellite's outward migration is a runaway process (at least until the synchronous rotation is broken), quite unlike tidal evolution. Also, for some close binaries with similar-sized, mutually-synchronous components ("contact binaries"), YORP and satellite YORP should become indistinguishable.

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Does Microporosity or Macroporosity Dominate in Stone Asteroids?: Evidence from Stone Meteorites
George J. Flynn, Dept. of Physics, SUNY-Plattsburgh, 101 Broad St., Plattsburgh, NY 12901 Comparison of the densities of asteroids with the densities of the minerals presumed to dominate their composition (based on reflection spectroscopy) indicates that most asteroids exhibit significant porosity (Flynn, 1994). The type of porosity ranging from microporosity to macroporosity (in the extreme case, a rubble pile) determines the physical behavior of the asteroids, e.g., the response to collisions, the maximum rotation rate, etc. Since the meteorites are generally accepted to be samples of the asteroids, the study of porosity in meteorites can provide constraints on the type and amount of microporosity in the asteroids. Britt and Consolmagno (2003) reviewed the density and porosity measurements on stone meteorites obtained by several groups. The lowest bulk density in their tabulation is 1.5 gm/cc for a large (47.2 gm) sample of the hydrated CI carbonaceous chondrite Orgueil. Four of the hydrated CM carbonaceous chondrites in the tabulation have bulk density values < 2.0 gm/cc (ranging from 1.79 gm/cc for Santa Cruz to 1.96 gm/cc for Nogoya), but other hydrated CM meteorites Cold Bokkeveld, Kivesvaara, and Murchison have bulk densities ranging from 2.3 to 2.4 gm/cc. The lowest bulk density reported for an anhydrous meteorite is 2.38 gm/cc for the LL ordinary chondrite Y-75258. However, we might reasonably expect that the bulk density of the parent asteroid would be significantly lower than that of the meteorite derived from that parent since ob jects tend to break into their strongest subunits, thus minimizing their microporosity. This trend is evident in the Orgueil data (Britt and Consolmagno, 2003), where the sample having the lowest bulk density (1.5 gm/cc) is the largest one measured, while the smaller Orgueil samples have much higher bulk densities (averaging 2.1 gm/cc). Recent asteroid flyby missions have permitted accurate determinations of the bulk densities of several asteroids: Mathilda at 1.3 gm/cc, Ida at 2.7 gm/cc, and Eros at 2.67 gm/cc. Each of these bulk densities is high enough to be explained entirely by microporosity, especially when the measured bulk densities of the meteorites are adjusted down to account for ob jects breaking into their strongest (most likely least porous) subunits. The meteorites exhibit three distinct types of porosity on the microscale - cracks, vugs, and gaps or low density regions separating chondrules from the matrix (Flynn et al., 2000). But the dominant type of porosity is cracks, which, in some cases, may not reflect the condition of their parent body. Meteorites are believed to be ejected from their parent body by collisions, which can induce new cracks through shock or fill pre-existing cracks through 7 injection of melted material into the interior of the rock. In addition, we have performed Computed MicroTomography (CMT) on several small whole stone meteorites. We found cracks that cut the fusion crust on two meteorites. Once the cracks were identified using CMT, we were able to locate these microscopic cracks on the exterior surface of these stones using a low-power magnifying lens. Thus, at least in some cases, the cracks in meteorites were either produced or significantly enlarged after production of the fusion crust.

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Britt, D. T. and G. J. Consolmagno (2003) Stony meteorite porosities and densities: A review of the data through 2001, Meteoritics & Planetary Science, 38, 1161-1180. Flynn, G. J. (1994) Interplanetary dust particles collected from the stratosphere: Physical, chemical, and mineralogical properties and implications for their sources, Planetary and Space Science, 42, 1151-1161. Flynn, G. J.; Rivers, M.; Sutton, S. R.; Eng, P.; Klock, W. (2000) X-Ray Computed Microtomography (CMT): A Non-invasive Screening Tool for Characterization of Returned Rock Cores from Mars and Other Solar System Bodies, 31st Annual Lunar and Planetary Science Conference, March 13-17, 2000, Houston, Texas, Abstract no. 1893.

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Asteroid Orbit Determination and Radar Astrometry: Small Forces and Long-Term Prediction


Asteroid Lightcurve Parameters Data File
Alan W. Harris1, Brian D. Warner2, Petr Pravec3
1. Space Science Institute, La Canada, CA-91011, U.S.A. 2. Palmer Divide Observatory, Colorado Springs, CO 80908 U.S.A. 3. Astronomical Institute AS CR, Ondrejov, CZ-25165, Czech Republic

Beginning about the time of the first Asteroids conference in Tucson in 1979, I (AWH) began compiling data on asteroid rotations from literature reports of lightcurve observations. The first research based on that list was published that year (Harris and Burns, Icarus 40, 115-144, 1979). Along with the statistical analysis of rotations, we published the entire list, which consisted of 304 entries referring to 191 different asteroids. Of these, 182 asteroids had some indication of rotational properties, and 151 had "reliable" rotation periods published. The entire list fit on six partial pages of the old small Icarus page layout. In following years, numerous papers have been published analyzing the data contained on various updates of the data file, and updates of the file itself have been published in the Asteroids II book, an Icarus paper in 1983, and in abridged form in the annual Ephemeredes of Minor Planets (EMP), and is posted on the MPC and CALL web sites. Here we present the latest revision, which has now grown to a file with 6,183 entries referring to 1,813 different asteroids, and including 1,371 reliable rotation periods. The current data file, if published in format similar to the original Icarus publication, would occupy a fair fraction of a single issue of the journal. Fortunately electronic publication has come to the rescue of the data explosion, so it is again possible to publish one's underlying data. This file, and updates thereof, will continue to be available on the CALL and MPC web sites, or available for download, or CD copy on request (to AWH). However, it is desirable from time to time to "freeze" a version of the data file so that analyses based on that set can be examined, checked and compared against other analyses using the same data. With that in mind, we offer the data set as used in the analyses presented at thi