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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.017 Byline: Mark Willman (a), Robert Jedicke (a), Nicholas Moskovitz (b), David NesvornA1/2 (c), David VokrouhlickA1/2 (d), Thais Mothe-Diniz (e) Keywords: Asteroids, Surfaces; Spectrophotometry; Spectroscopy Abstract: We have extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages Author Affiliation: (a) Institute for Astronomy, University of Hawai'i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20008, USA (c) Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA (d) Institute of Astronomy, Charles University, V HoleA oviAkach 2, CZ-18000 Prague 8, Czech Republic (e) UFRJ/Observatorio do Valongo, Ladeira Pedro AntA[acute accent]nio, 43, 20080-090 Rio de Janeiro/RJ, Brazil Article History: Received 7 December 2009; Revised 25 February 2010; Accepted 26 February 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.012 Byline: Joseph Masiero (a), Robert Jedicke (a), Josef Aurech (b), Stephen Gwyn (c), Larry Denneau (a), Jeff Larsen (d) Keywords: Asteroids Abstract: We present the results of our Thousand Asteroid Light Curve Survey (TALCS) conducted with the Canada-France-Hawaii Telescope in September 2006. Our untargeted survey detected 828 Main Belt asteroids to a limiting magnitude of g.sup.'[approximately equal to]22.5 corresponding to a diameter range of 0.4km a[c]1/2Da[c]1/210km. Of these, 278 objects had photometry of sufficient quality to perform rotation period fits. We debiased the observations and light curve fitting process to determine the true distribution of rotation periods and light curve amplitudes of Main Belt asteroids. We confirm a previously reported excess in the fraction of fast rotators but find a much larger excess of slow rotating asteroids ([approximately equal to]15% of our sample). A few percent of objects in the TALCS size range have large light curve amplitudes of [approximately equal to]1 mag. Fits to the debiased distribution of light curve amplitudes indicate that the distribution of triaxial ellipsoid asteroid shapes is proportional to the square of the axis ratio, (b/a).sup.2, and may be bi-modal. Finally, we find six objects with rotation periods that may be less than 2h with diameters between 400m and 1.5km, well above the break-up limit for a gravitationally-bound aggregate. Our debiased data indicate that this population represents Author Affiliation: (a) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr, Honolulu, HI 96822, United States (b) Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Holesovickach 2, 18000 Prague, Czech Republic (c) Canadian Astronomical Data Centre, Herzberg Institute of Astrophysics, Victoria, BC, Canada V9E 2E7 (d) Physics Department, United States Naval Academy, Annapolis, MD 21402, United States Article History: Received 4 February 2009; Revised 3 June 2009; Accepted 12 June 2009 Article Note: (footnote) [star] Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.05.010 Byline: Peter VereA (a), Robert Jedicke (b), Richard Wainscoat (b), Mikael Granvik (b), Steve Chesley (c), Shinsuke Abe (d), Larry Denneau (b), Tommy Grav (e) Keywords: Asteroids; Near-Earth objects; Meteors; Impact processes Abstract: We have performed a simulation of a next generation sky survey's (Pan-STARRS 1) efficiency for detecting Earth-impacting asteroids. The steady-state sky-plane distribution of the impactors long before impact is concentrated towards small solar elongations (Chesley, S.R., Spahr T.B., 2004. In: Belton, M.J.S., Morgan, T.H., Samarashinha, N.H., Yeomans, D.K. (Eds.), Mitigation of Hazardous Comets and Asteroids. Cambridge University Press, Cambridge, pp. 22-37) but we find that there is interesting and potentially exploitable behavior in the sky-plane distribution in the months leading up to impact. The next generation surveys will find most of the dangerous impactors (>140m diameter) during their decade-long survey missions though there is the potential to miss difficult objects with long synodic periods appearing in the direction of the Sun, as well as objects with long orbital periods that spend much of their time far from the Sun and Earth. A space-based platform that can observe close to the Sun may be needed to identify many of the potential impactors that spend much of their time interior to the Earth's orbit. The next generation surveys have a good chance of imaging a bolide like 2008 TC.sub.3 before it enters the atmosphere but the difficulty will lie in obtaining enough images in advance of impact to allow an accurate pre-impact orbit to be computed. Author Affiliation: (a) Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska Dolina, 842 48 Bratislava, Slovakia (b) University of Hawai'i, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822-1897, USA (c) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA (d) Institute of Astronomy, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 320, Taiwan (e) Department of Physics and Astronomy, Bloomberg 243, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218-2686, USA Article History: Received 19 January 2009; Revised 5 May 2009; Accepted 7 May 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.05.021 Byline: Maximilian Zavodny (a), Robert Jedicke (a), Edward C. Beshore (b), Fabrizio Bernardi (a), Stephen Larson (b) Keywords: Near-Earth objects; Asteroids; dynamics Abstract: We present the first observational measurement of the orbit and size distribution of small Solar System objects whose orbits are wholly interior to the Earth's (Inner Earth Objects, IEOs, with aphelion Author Affiliation: (a) Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA (b) Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA Article History: Received 27 January 2008; Revised 4 May 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.07.006 Byline: Nicholas A. Moskovitz (a), Robert Jedicke (a), Eric Gaidos (b)(c), Mark Willman (a), David NesvornA1/2 (d), Ronald Fevig (e), A1/2eljko IveziA (f) Keywords: Asteroids; Spectroscopy; Asteroids; composition; Asteroid Vesta Abstract: We present the observational results of a survey designed to target and detect asteroids whose photometric colors are similar to those of Vesta family members and thus may be considered as candidates for having a basaltic composition. Fifty basaltic candidates were selected with orbital elements that lie outside of the Vesta dynamical family. Optical and near-infrared spectra were used to assign a taxonomic type to 11 of the 50 candidates. Ten of these were spectroscopically confirmed as V-type asteroids, suggesting that most of the candidates are basaltic and can be used to constrain the distribution of basaltic material in the Main Belt. Using our catalog of V-type candidates and the success rate of the survey, we calculate unbiased size-frequency and semi-major axis distributions of V-type asteroids. These distributions, in addition to an estimate for the total mass of basaltic material, suggest that Vesta was the predominant contributor to the basaltic asteroid inventory of the Main Belt, however scattered planetesimals from the inner Solar System (a Author Affiliation: (a) Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) Department of Geology and Geophysics, University of Hawaii, POST 701, 1680 East-West Road, Honolulu, HI 96822, USA (c) NASA Astrobiology Institute, University of Hawaii, PSB 213, 2565 McCarthy Mall, Honolulu, HI 96822, USA (d) Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA (e) Lunar & Planetary Laboratory, University of Arizona, 1629 University Blvd., Tucson, AZ 85721-0092, USA (f) Department of Astronomy, University of Washington, PO Box 351580, Seattle, WA 98195-1580, USA Article History: Received 22 November 2007; Revised 27 June 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.02.007 Byline: Mark Willman (a), Robert Jedicke (a), David NesvornA1/2 (b), Nicholas Moskovitz (a), A1/2eljko IveziA (c), Ronald Fevig (d) Keywords: Asteroids; surfaces Abstract: We have obtained moderate S/N ([approximately equal to]85) spectra at a realized resolution of R[approximately equal to]100 for 11 members of the Iannini family, until recently the youngest known family at under 5 million years of age [NesvornA1/2, D., Bottke, W.F., Levison, H.F., Dones, L., 2003. Astrophys. J. 591, 486-497, 720-771]. The spectra were acquired using the Echellette Spectrograph and Imager in its low-resolution prism mode on the Keck II telescope. The family members belong to the S-complex of asteroids with perhaps some K class members. The Iannini family members' average spectral slope, defined as the slope of the best-fit line constrained to pivot about 1 at 550 nm, is (0.30[+ or -]0.04)/[mu]m, matching the (0.26[+ or -]0.03)/[mu]m reported by Jedicke et al. [Jedicke, R., NesvornA1/2, D., Whiteley, R.J., IveziA, A1/2., JuriA, M., 2004. Nature 429, 275-277] using SDSS [IveziA, A1/2., JuriA, M., Lupton, R.H., Tabachnik, S., Quinn, T., 2002. In: Tyson, J.A., Wolff, S. (Eds.), Survey and Other Telescope Technologies and Discoveries. In: Proc. SPIE, vol. 4836. SPIE, Bellingham, pp. 98-103] color photometry. Using our spectra for this family as well as new observations of Karin family members [Vernazza, P., Birlan, M., Rossi, A., Dotto, E., NesvornA1/2, D., Brunetto, R., Fornasier, S., Fulchignoni, M., Renner, S., 2006. Astron. Astrophys. 460, 945-951] and new classifications of some older families we have revised the space weathering rate of S-complex asteroids originally determined by Jedicke et al. [Jedicke, R., NesvornA1/2, D., Whiteley, R.J., IveziA, A1/2., JuriA, M., 2004. Nature 429, 275-277]. Following Jedicke et al. [Jedicke, R., NesvornA1/2, D., Whiteley, R.J., IveziA, A1/2., JuriA, M., 2004. Nature 429, 275-277] we parameterize the space weathering rate of the principal component color of the spectrum (PC.sub.1), which is correlated with the spectral slope, as PC.sub.1(t)=PC.sub.1(0)+[DELTA]PC.sub.1[1-exp.sup.-(t/I).sup.[alpha]]. Our revised rate suggests that the characteristic time scale for space weathering is I=570[+ or -]220 Myr and that new S-complex clusters will have an initial color of PC.sub.1(0)=0.31[+ or -]0.04. The revised time scale is in better agreement with lab measurements and our measurements support the use of space weathering as a dating method. Under the assumption that all the spectra should be identical, since the members all derive from the same parent body and are presumably covered with similar regolith, we combined them to obtain a high-S/N composite spectrum for the family. The combined spectrum is within the S-complex. Author Affiliation: (a) Institute for Astronomy, University of Hawaii'i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA (c) Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580, USA (d) Lunar & Planetary Laboratory, University of Arizona, 1629 University Blvd., Tucson, AZ 85721-0092, USA Article History: Received 18 July 2007; Revised 2 February 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.02.016 Byline: Kathryn Whitman (a), Alessandro Morbidelli (b), Robert Jedicke (a) Keywords: Comets; Asteroids Abstract: We estimate the total number and the slope of the size-frequency distribution (SFD) of dormant Jupiter family comets (JFCs) by fitting a one-parameter model to the known population. We first select 61 near-Earth objects (NEOs) that are likely to be dormant JFCs because their orbits are dynamically coupled to Jupiter [Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J., Levison, H.F., Michel, P., Metcalfe, T.S., 2002a. Icarus 156, 399-433]. Then, from the numerical simulations of Levison and Duncan [1997. Icarus 127, 13-32], we construct an orbit distribution model for JFCs in the NEO orbital element space. We assume an orbit-independent SFD for all JFCs, the slope of which is our unique free parameter. Finally, we compute observational biases for dormant JFCs using a calibrated NEO survey simulator [Jedicke, R., Morbidelli, A., Spahr, T., Petit, J., Bottke, W.F., 2003. Icarus 161, 17-33]. By fitting the biased model to the data, we estimate that there are [approximately equal to]75 dormant JFCs with H Author Affiliation: (a) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822, USA (b) Observatory of Nice, B.P. 4229, 06304 Nice Cedex 4, France Article History: Received 17 May 2005; Revised 23 December 2005

The main belt is believed to have originally contained an Earth mass or more of material, enough to allow the asteroids to accrete on relatively short timescales. The present-day main belt, however, only contains ~5 x [10.sup.-4] Earth masses. Numerical simulations suggest that this mass loss can be explained by the dynamical depletion of main belt material via gravitational perturbations from planetary embryos and a newly-formed Jupiter. To explore this scenario, we combined dynamical results from Petit et al. [Petit, J. Morbidelli, A., Chambers, J., 2001. The primordial excitation and clearing of the asteroid belt. Icarus 153, 338-347] with a collisional evolution code capable of tracking how the main belt undergoes comminution and dynamical depletion over 4.6 Gyr [Bottke, W.F., Durda, D., Nesvorny, D., Jedicke, R., Morbidelli, A., Vokrouhlicky, D., Levison, H., 2005. The fossilized size distribution of the main asteroid belt. Icarus 175, 111-140]. Our results were constrained by the main belt's size--frequency distribution, the number of asteroid families produced by disruption events from diameter D > 100 km parent bodies over the last 3-4 Gyr, the presence of a single large impact crater on Vesta's intact basaltic crust, and the relatively constant lunar and terrestrial impactor flux over the last 3 Gyr. We used our model to set limits on the initial size of the main belt as well as Jupiter's formation time. We find the most likely formation time for Jupiter was 3.3 [+ or -] 2.6 Myr after the onset of fragmentation in the main belt. These results are consistent with the estimated mean disk lifetime of 3 Myr predicted by Haisch et al. [Haisch, K.E., Lada, E.A., Lada, C.J., 2001. Disk frequencies and lifetimes in young clusters. Astrophys. J. 553, LI53-L156]. The post-accretion main belt population, in the form of diameter D ~ < 1000 km planetesimals, was likely to have been 160 [+ or -] 40 times the current main belt's mass. This corresponds to 0.06--0.1 Earth masses, only a small fraction of the total mass thought to have existed in the main belt zone during planet formation. The remaining mass was most likely taken up by planetary embryos formed in the same region. Our results suggest that numerous D > 200 km planetesimals disrupted early in Solar System history, but only a small fraction of their fragments survived the dynamical depletion event described above. We believe this may explain the limited presence of iron-rich M-type, olivine-rich A-type, and non-Vesta V-type asteroids in the main belt today. The collisional lifetimes determined for main belt asteroids agree with the cosmic ray exposure ages of stony meteorites and are consistent with the limited collisional evolution detected among large Koronis family members. Using the same model, we investigated the near-Earth object (NEO) population. We show the shape of the NEO size distribution is a reflection of the main belt population, with main belt asteroids driven to resonances by Yarkovsky thermal forces. We used our model of the NEO population over the last 3 Gyr, which is consistent with the current population determined by telescopic and satellite data, to explore whether the majority of small craters (D < 0.1-1 km) formed on Mercury, the Moon, and Mars were produced by primary impacts or by secondary impacts generated by ejecta from large craters. Our results suggest that most small craters formed on these worlds were a by-product of secondary rather than primary impacts. Keywords: Asteroids; Asteroids, dynamics; Collisional physics; Impact processes; Origin, Solar System

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.05.004 Byline: Hai Fu (a), Robert Jedicke (a), Daniel D. Durda (b), Ronald Fevig (c), James V. Scotti (c) Keywords: Asteroids; Dynamics Abstract: The study of asteroid families has provided tremendous insight into the forces that sculpted the main belt and continue to drive the collisional and dynamical evolution of asteroids. The identification of asteroid families within the NEO population could provide a similar boon to studies of their formation and interiors. In this study we examine the purported identification of NEO families by Drummond [Drummond, J.D., 2000. Icarus 146, 453-475] and conclude that it is unlikely that they are anything more than random fluctuations in the distribution of NEO osculating orbital elements. We arrive at this conclusion after examining the expected formation rate of NEO families, the identification of NEO groups in synthetic populations that contain no genetically related NEOs, the orbital evolution of the largest association identified by Drummond [Drummond, J.D., 2000. Icarus 146, 453-475], and the decoherence of synthetic NEO families intended to reproduce the observed members of the same association. These studies allowed us to identify a new criterion that can be used to select real NEO families for further study in future analyses, based on the ratio of the number of pairs and the size of strings to the number of objects in an identified association. Author Affiliation: (a) Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA (b) Southwest Research Institute, 1050 Walnut Street Suite 400, Boulder, CO 80302, USA (c) Lunar and Planetary Lab, University of Arizona, P.O. Box 210092, Tucson, AZ 85721-0092, USA Article History: Received 14 February 2005; Revised 3 May 2005

Planet formation models suggest the primordial main belt experienced a short but intense period of collisional evolution shortly after the formation of planetary embryos. This period is believed to have lasted until Jupiter reached its full size, when dynamical processes (e.g., sweeping resonances, excitation via planetary embryos) ejected most planetesimals from the main belt zone. The few planetesimals left behind continued to undergo comminution at a reduced rate until the present day. We investigated how this scenario affects the main belt size distribution over Solar System history using a collisional evolution model (CoEM) that accounts for these events. CoEM does not explicitly include results from dynamical models, but instead treats the unknown size of the primordial main belt and the nature/timing of its dynamical depletion using innovative but approximate methods. Model constraints were provided by the observed size frequency distribution of the asteroid belt, the observed population of asteroid families, the cratered surface of differentiated Asteroid (4) Vesta, and the relatively constant crater production rate of the Earth and Moon over the last 3 Gyr. Using CoEM, we solved for both the shape of the initial main belt size distribution after accretion and the asteroid disruption scaling law [Q.sup.*.sub.D]. In contrast to previous efforts, we find our derived [Q.sup.*.sub.D] function is very similar to results produced by numerical hydrocode simulations of asteroid impacts. Our best fit results suggest the asteroid belt experienced as much comminution over its early history as it has since it reached its low-mass state approximately 3.9-4.5 Ga. These results suggest the main belt's wavy-shaped size-frequency distribution is a 'fossil' from this violent early epoch. We find that most diameter D [??] 120 km asteroids are primordial, with their physical properties likely determined during the accretion epoch. Conversely, most smaller asteroids are byproducts of fragmentation events. The observed changes in the asteroid spin rate and lightcurve distributions near D [??] 100-120 km are likely to be a byproduct of this difference. Estimates based on our results imply the primordial main belt population (in the form of D < 1000 km bodies) was 150-250 times larger than it is today, in agreement with recent dynamical simulations. Keywords: Asteroids; Asteroids, dynamics; Collisional physics; Impact processes; Origin, Solar System

By studying color variations between young and old asteroid families we find evidence for processes that modify colors of asteroids over time. We show that colors of aging surfaces of S-type asteroids become increasingly 'redder' and measure the rate of these spectral changes. We estimate that the mean spectral slope between 0.35 and 0.9 [micro] increases with time t (given in My) as [approximately equals] 0.01 [[micro]m.sup.-1] x [log.sub.l0.sup.t]. This empirical fit is valid only for 2.5 [??] t [??] 3000 My (the time interval where we have data) and for the mean spectral slope determined from wide-wavelength filter photometry obtained by the Sloan Digital Sky Survey. We also find that Gy-old terrains of S-type asteroids reflect about 15% more light at ~ 1-[micro]m wavelengths than an ~ 5-My-old S-type asteroid surface when the flux is normalized by the reflected light at 0.55 [micro]m. We attribute these effects to space weathering. This result has important implications for asteroid geology and the origin of meteorites that reach the Earth. Our results also suggest that surfaces of C-type asteroids exhibit color alterations opposite to those of the S-type asteroids. Keywords: Asteroids; Composition; Surfaces; Asteroids; Regoliths

We have used an improved model of the orbit and absolute magnitude distribution of Near Earth Objects (NEOs) to simulate the performance of asteroid surveys. Our results support general conclusions of previous studies using preliminary Near Earth Asteroid (NEA) orbit and magnitude distributions and suggest that meeting the Spaceguard Goal of 90% completion for Near Earth Objects (NEOs) greater than 1 km diameter by 2008 is impossible given contemporary surveying capabilities. The NEO model was derived from NEO detections by the Spacewatch Project. For this paper we developed a simulator for the Catalina Sky Survey (CSS) for which we had a complete pointing history and NEO detection efficiency. The good match between the output of the simulator and the actual CSS performance gives confidence that both the NEO model and simulator are correct. Then, in order to determine if existing surveys can meet the Spaceguard Goal, we developed a simulator to mimic the LINEAR survey, for which detailed performance characteristics were unavailable. This simulator serendipitously provided an estimate for the currently undiscovered population of NEOs upon which we base all our estimates of time to 90% completion. We also developed a set of idealized NEO surveys in order to constrain the best possible survey performance in contrast to more realistic systems. A 100% efficient, all-sky, every night survey, subject only to the constraints of detection above a specified air mass and when the Sun is 18[degrees] below the horizon provides a benchmark from which to examine the effect of imposing more restrictions and the efficacy of some simple survey strategies. Such a survey must have a limiting V-magnitude of 20.1 [+ or -] 0.2 to meet the Spaceguard Goal. More realistic surveys, limited by latitude, the galaxy, minimum rates of NEO motion, etc., require fainter limiting magnitudes to reach the same completion. Our most realistic simulations, which have been normalized to the performance of the LINEAR detector system's operation in the period 1999-2000, indicate that it would take them another 33 [+ or -] 5 years to reach 90% completeness for the larger asteroids ([greater than or equal to] 1 km diameter). They would need to immediately increase the limiting magnitude to about 24 in order to meet the Spaceguard Goal. The simulations suggest that there may be little need for distributing survey telescopes in longitude and latitude as long as there is sufficient sky coverage from a telescope or network of telescopes which may be geographically close. An idealized space-based survey, especially from a satellite orbit much interior to Earth, would offer an advantage over their terrestrial counterparts. We do not consider a cost-benefit analysis for any of the simulations but suspect that a local-area network of telescopes capable of covering much of the sky in a month to V ~ 21.5 may be administratively, financially, and scientifically the best compromise tot reaching 90% completion of NEOs larger than 1 km diameter. Keyword: Asteroids

The orbital and absolute magnitude distribution of the near-Earth objects (NEOs) is difficult to compute, partly because only a modest fraction of the entire NEO population has been discovered so far, but also because the known NEOs are biased by complicated observational selection effects. To circumvent these problems, we created a model NEO population which was fit to known NEOs discovered or accidentally rediscovered by Spacewatch. Our method was to numerically integrate thousands of test particles from five source regions that we believe provide most NEOs to the inner Solar System. Four of these source regions are in or adjacent to the main asteroid belt, while the fifth one is associated with the transneptunian disk. The nearly isotropic comets, which include the Halley-type comets and the long-period comets, were not included in our model. Test bodies from our source regions that passed into the NEO region (perihelia q < 1.3 AU and aphelia Q [greater than or equal to] 0.983 AU) were tracked until they were eliminated by striking the Sun or a planet or were ejected out of the inner Solar System. These integrations were used to create five residence time probability distributions in semimajor axis, eccentricity, and inclination space (one for each source). These distributions show where NEOs from a given source are statistically most likely to be located. Combining these five residence time probability distributions with an NEO absolute magnitude distribution computed from previous work and a probability function representing the observational biases associated with the Spacewatch NEO survey, we produced an NEO model population that could be fit to 138 NEOs discovered or accidentally rediscovered by Spacewatch. By testing a range of possible source combinations, a best-fit NEO model was computed which (i) provided the debiased orbital and absolute magnitude distributions for the NEO population and (ii) indicated the relative importance of each NEO source region. Our best-fit model is consistent with 960 [+ or -] 120 NEOs having H < 18 and a < 7.4 AU. Approximately 44% (as of December 2000) have been found so far. The limits on this estimate are conditional, since our model does not include nearly isotropic comets. Nearly isotropic comets are generally restricted to a Tisserand parameter (with respect to Jupiter) of T < 2, such that few are believed to have a < 7.4 AU. Our computed NEO orbital distribution, which is valid for bodies as faint as H < 22, indicates that the Amor, Apollo, and Aten populations contain 32 [+ or -] 1%, 62 [+ or -] 1%, and 6 [+ or -] 1% of the NEO population, respectively. We estimate that the population of objects completely inside Earth's orbit (IEOs) arising from our source regions is 2% the size of the NEO population. This value does not include the putative Vulcanoid population located inside Mercury's orbit. Overall, our model predicts that ~61% of the NEO population comes from the inner main belt (a < 2.5 AU), ~24% comes from the central main belt (2.5 < a < 2.8 AU), ~8% comes from the outer main belt (a > 2.8 AU), and ~6% comes from the Jupiter-family comet region (2 < T >[??] 3). The steady-state population in each NEO source region, as well as the influx rates needed to replenish each region, were calculated as a by-product of our method. The population of extinct comets in the Jupiter-family comet region was also computed. Key Words: asteroids; asteroid dynamics; orbits.

Numerical models of the collisional evolution of the main-belt asteroids lead to a new interpretation of the shape of the observed size distribution. Using recent hydrocode predictions of shattering and disruption criteria as starting points, we adjust the size-strength scaling relation for asteroidal strengths within our collisional model until a best least-squares fit to the observed size distribution is obtained. We show for the first time general agreement between the predictions of hydrocodes, results of numerical collisional models, and the observed size distribution of the main-belt population. Key Words: asteroids; collisional physics; impact processes.