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We introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation and apply it to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey. The debiased LPC size-frequency distribution is in agreement with previous estimates for large comets with nuclear diameter ≳ 1 km but we measure a significant drop in the SFD slope for small objects with diameters <1 km and approaching only 100 m diameter. Large objects have a slope α big = 0.72 ± 0.09(stat.) ± 0.15(sys.) while small objects behave as α small = 0.07 ± 0.03(stat.) ± 0.09(sys.) where the SFD is ∝ 1 0 α H N and H N represents the cometary nuclear absolute magnitude. The total number of LPCs that are >1 km diameter and have perihelia q < 10 au is 0.46 ± 0.15 × 109 while there are only 2.4 ± 0.5(stat.) ± 2(sys.) × 109 objects with diameters >100 m due to the shallow slope of the SFD for diameters <1 km. We estimate that the total number of 'potentially active' objects with diameters ≥1 km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to <10 au, is (1.5 ± 1) × 1012 with a combined mass of 1.3 ± 0.9 M ⊕. The debiased LPC orbit distribution is broadly in agreement with expectations from contemporary dynamical models but there are discrepancies that could point towards a future ability to disentangle the relative importance of stellar perturbations and galactic tides in producing the LPC population. • we introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation • we applied the technique to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey • we find a relatively steep slope for the SFD of large LPCs with diameters >∼3 km and a shallow slope for smaller objects • we estimate that there are (0.46±0.15)×109 active LPCs that are >1 km diameter and have perihelia q<10 au • the total number of 'active' objects with diameters ≥1 km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to <10 au, is (1.5±1)×1012 with a combined mass of 1.3±0.9 M ⊕. [ABSTRACT FROM AUTHOR]

Reports on the use of a detector as big as the Earth's protective blanketing atmosphere which acts a scintillator to detect the powerful but rare impacts of small asteroids or comets. Process of creating meteors; Consequences of the collision of big objects with the Earth; Link between comet/asteroid planetary astronomy.

The debiased absolute-magnitude and orbit distributions as well as source regions for near-Earth objects (NEOs) provide a fundamental frame of reference for studies of individual NEOs and more complex population-level questions. We present a new four-dimensional model of the NEO population that describes debiased steady-state distributions of semimajor axis, eccentricity, inclination, and absolute magnitude H in the range 17 <  H  < 25. The modeling approach improves upon the methodology originally developed by Bottke et al. (2000, Science 288, 2190–2194) in that it is, for example, based on more realistic orbit distributions and uses source-specific absolute-magnitude distributions that allow for a power-law slope that varies with H . We divide the main asteroid belt into six different entrance routes or regions (ER) to the NEO region: the ν 6 , 3:1J, 5:2J and 2:1J resonance complexes as well as Hungarias and Phocaeas. In addition we include the Jupiter-family comets as the primary cometary source of NEOs. We calibrate the model against NEO detections by Catalina Sky Surveys’ stations 703 and G96 during 2005–2012, and utilize the complementary nature of these two systems to quantify the systematic uncertainties associated to the resulting model. We find that the (fitted) H distributions have significant differences, although most of them show a minimum power-law slope at H  ∼ 20. As a consequence of the differences between the ER-specific H distributions we find significant variations in, for example, the NEO orbit distribution, average lifetime, and the relative contribution of different ERs as a function of H . The most important ERs are the ν 6 and 3:1J resonance complexes with JFCs contributing a few percent of NEOs on average. A significant contribution from the Hungaria group leads to notable changes compared to the predictions by Bottke et al. in, for example, the orbit distribution and average lifetime of NEOs. We predict that there are 962 − 56 + 52 ( 802 − 42 + 48 × 10 3 ) NEOs with H  < 17.75 ( H  < 25) and these numbers are in agreement with the most recent estimates found in the literature (the uncertainty estimates only account for the random component). Based on our model we find that relative shares between different NEO groups (Amor, Apollo, Aten, Atira, Vatira) are (39.4,54.4,3.5,1.2,0.3)%, respectively, for the considered H range and that these ratios have a negligible dependence on H . Finally, we find an agreement between our estimate for the rate of Earth impacts by NEOs and recent estimates in the literature, but there remains a potentially significant discrepancy in the frequency of Tunguska-sized and Chelyabinsk-sized impacts. [ABSTRACT FROM AUTHOR]

Abstract: The first of a new class of objects now known as main belt comets (MBCs) or “activated asteroids” was identified in 1996. The seven known members of this class have orbital characteristics of main belt asteroids yet exhibit dust ejection like comets. In order to constrain their physical and orbital properties we searched the Thousand Asteroid Light Curve Survey (TALCS; Masiero, J.R., Jedicke, R., Durech, J., Gwyn, S., Denneau, L., Larsen, J. [2009]. Icarus 204, 145–171) for additional candidates using two diagnostics: tail and coma detection. This was the most sensitive MBC survey effort to date, extending the search from MBCs with H ∼18 (D ∼1km) to MBCs as small as H ∼21 (D ∼150m). We fit each of the 924 objects detected by TALCS to a PSF model incorporating both a coma and nuclear component to measure the fractional contribution of the coma to the total surface brightness. We determined the significance of the coma detection using the same algorithm on a sample of null detections of comparable magnitude and rate of motion. We did not identify any MBC candidates with this technique to a sensitivity limit on the order of cometary mass loss rate of about 0.1kg/s. Our tail detection algorithm relied on identifying statistically significant flux in a segmented annulus around the candidate object. We show that the technique can detect tail activity throughout the asteroid belt to the level of the currently known MBCs. Although we did not identify any MBC candidates with this technique, we find a statistically significant detection of faint activity in the entire ensemble of TALCS asteroids. This suggests that many main belt asteroids are active at very low levels. Our null detection of MBCs allows us to set 90% upper confidence limits on the number distribution of MBCs as a function of absolute magnitude, semi-major axis, eccentricity, and inclination. There are ≲400,000 MBCs in the main belt brighter than H V =21 (∼150-m in diameter) and the MBC:MBA ratio is ≲1:400. We further comment on the ability of observations to meaningfully constrain the snow line’s location. Under some reasonable and simple assumptions we claim 85% confidence that the contemporary snow line lies beyond 2.5AU. [Copyright &y& Elsevier]

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 ∼75 dormant JFCs with in the NEO region and that the slope of their cumulative SFD is . Our slope for the SFD of dormant JFCs is very close to that of active JFCs as determined by Weissman and Lowry [2003. Lunar Planet. Sci. 34. Abstract 2003]. Thus, we argue that when JFCs fade they are likely to become dormant rather than to disrupt and that the fate of faded comets is size-independent. Our results imply that the size distribution of the JFC progenitors—the scattered disk trans-neptunian population—either (i) has a similar and shallow SFD or () is slightly steeper and physical processes acting on the comets in a size-dependent manner creates the shallower active comet SFD. Our measured slope, typical of collisionally evolved populations with a size-dependent impact strength [Benz, W., Asphaug, E., 1999. Icarus 142, 5–20], suggests that scattered disk bodies reached collisional equilibrium inside the protoplanetary disk prior to their removal from the planetary region. [Copyright &y& Elsevier]

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≥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±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±1%, 62±1%, and 6±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 (2.52.8 AU), and ∼6% comes from the Jupiter-family comet region (2

We analyze a set of 760 475 observations of 333 026 unique main-belt objects obtained by the Pan-STARRS1 (PS1) survey telescope between 2012 May 20 and 2013 November 9, a period during which PS1 discovered two main-belt comets, P/2012 T1 (PANSTARRS) and P/2013 R3 (Catalina-PANSTARRS). PS1 comet detection procedures currently consist of the comparison of the point spread functions (PSFs) of moving objects to those of reference stars, and the flagging of objects that show anomalously large radial PSF widths for human evaluation and possible observational follow-up. Based on the number of missed discovery opportunities among comets discovered by other observers, we estimate an upper limit comet discovery efficiency rate of ∼70% for PS1. Additional analyses that could improve comet discovery yields in future surveys include linear PSF analysis, modeling of trailed stellar PSFs for comparison to trailed moving object PSFs, searches for azimuthally localized activity, comparison of point-source-optimized photometry to extended-source-optimized photometry, searches for photometric excesses in objects with known absolute magnitudes, and crowd-sourcing. Analysis of the discovery statistics of the PS1 survey indicates an expected fraction of 59 MBCs per 10 6 outer main-belt asteroids (corresponding to a total expected population of ∼140 MBCs among the outer main-belt asteroid population with absolute magnitudes of 12 < H V < 19.5 ), and a 95% confidence upper limit of 96 MBCs per 10 6 outer main-belt asteroids (corresponding to a total of ∼230 MBCs), assuming a detection efficiency of 50%. We note however that significantly more sensitive future surveys (particularly those utilizing larger aperture telescopes) could detect many more MBCs than estimated here. Examination of the orbital element distribution of all known MBCs reveals an excess of high eccentricities ( 0.1 < e < 0.3 ) relative to the background asteroid population. Theoretical calculations show that, given these eccentricities, the sublimation rate for a typical MBC is orders of magnitude larger at perihelion than at aphelion, providing a plausible physical explanation for the observed behavior of MBCs peaking in observed activity strength near perihelion. These results indicate that the overall rate of mantle growth should be slow, consistent with observational evidence that MBC activity can be sustained over multiple orbit passages. [ABSTRACT FROM AUTHOR]

Abstract: The discovery of new objects in modern wide-field asteroid and comet surveys can be enhanced by first identifying observations belonging to known Solar System objects. The assignation of new observations to a known object is an attribution problem that occurs when a least squares orbit already exists for the object but a separate fit is not possible to just the set of new observations. In this work we explore the strongly asymmetric attribution problem in which the existing least squares orbit is very well constrained and the new data are sparse. We describe an attribution algorithm that introduces new quality control metrics in the presence of strong biases in the astrometric residuals. The main biases arise from the stellar catalogs used in the reduction of asteroid observations and we show that a simple debiasing with measured regional catalog biases significantly improves the results. We tested the attribution algorithm using data from the PS1 survey that used the 2MASS star catalog for the astrometric reduction. We found small but statistically significant biases in the data of up to 0.1arcsec that are relevant only when the observations reach the level of accuracy made possible by instruments like PS1. The false attribution rate was measured to be <1/1000 with a simple additional condition that can reduce it to zero while the attribution efficiency is consistent with 100%. [Copyright &y& Elsevier]

Abstract: The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) under development at the University of Hawaii''s Institute for Astronomy is creating the first fully automated end-to-end Moving Object Processing System (MOPS) in the world. It will be capable of identifying detections of moving objects in our solar system and linking those detections within and between nights, attributing those detections to known objects, calculating initial and differentially corrected orbits for linked detections, precovering detections when they exist, and orbit identification. Here we describe new kd-tree and variable-tree algorithms that allow fast, efficient, scalable linking of intra and inter-night detections. Using a pseudo-realistic simulation of the Pan-STARRS survey strategy incorporating weather, astrometric accuracy and false detections we have achieved nearly 100% efficiency and accuracy for intra-night linking and nearly 100% efficiency for inter-night linking within a lunation. At realistic sky-plane densities for both real and false detections the intra-night linking of detections into ‘tracks’ currently has an accuracy of 0.3%. Successful tests of the MOPS on real source detections from the Spacewatch asteroid survey indicate that the MOPS is capable of identifying asteroids in real data. [Copyright &y& Elsevier]