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Meteor showers occur when streams of meteoroids originating from a common source intersect the Earth. There will be small dissimilarities between the direction of motion of different meteoroids within a stream, and these small differences will act to broaden the radiant, or apparent point of origin, of the shower. This dispersion in meteor radiant can be particularly important when considering the effect of the Earth's gravity on the stream, as it limits the degree of enhancement of the stream's flux due to gravitational focusing. In this paper, we present measurements of the radiant dispersion of twelve showers using observations from the Global Meteor Network. We find that the median offset of individual meteors from the shower radiant ranges from 0.32$^\circ$ for the eta Aquariids to 1.41$^\circ$ for the Southern Taurids. We also find that there is a small but statistically significant drift in Sun-centered ecliptic radiant and/or geocentric speed over time for most showers. Finally, we compare radiant dispersion with shower duration and find that, in contrast with previous results, the two quantities are not correlated in our data., Accepted for publication in MNRAS, 14 pages, 9 figures

Orbital debris is known to pose a substantial threat to Earth-orbiting spacecraft at certain altitudes. For instance, the orbital debris flux near Sun-synchronous altitudes of 600–800 km is particularly high due in part to the 2007 Fengyun-1C anti-satellite test and the 2009 Iridium-Kosmos collision. At other altitudes, however, the orbital debris population is minimal and the primary impactor population is not man-made debris particles but naturally occurring meteoroids. While the spacecraft community has some awareness of the risk posed by debris, there is a common misconception that orbital debris impacts dominate the risk at all locations. In this paper, we present a damage-limited comparison between meteoroids and orbital debris near the Earth for a range of orbital altitude and inclination, using NASA’s latest models for each environment. Overall, orbital debris dominates the impact risk between altitudes of 600 and 1300 km, while meteoroids dominate below 270 km and above 4800 km.

Meteoroids pose one of the largest risks to spacecraft outside of low Earth orbit. To correctly predict the rate at which meteoroids impact and damage spacecraft, environment models must describe t...

We report results of a four-year survey using Electron Multiplied Charged Coupled Device (EMCCD) cameras recording 34761 two-station video meteor events complete to a limiting magnitude of +6. The survey goal was to characterize probable iron meteoroids. Using only physical properties of the meteor trajectories including early peaking light curves, short luminous trajectories, and high energies accumulated per area at beginning, we identified 1068 iron meteors. Our iron candidates are most abundant at slow speeds < 15 km/s, where they make up approximately 20% of the mm-sized meteoroid population. They are overwhelmingly on asteroidal orbits, and have particularly low orbital eccentricities and smaller semi-major axes when compared to non-irons between 10-20 km/s. Our iron population appears to be more numerous at fainter magnitudes, comprising 15% of slow (10-15 km/s) meteors with peak brightness of +3 with the fraction rising to 25% at +6 to +7, our survey limit. The iron orbits are most consistent with an asteroidal source and are in highly evolved orbits, suggesting long collisional lifetimes (107 years). Metal-rich chondrules (nodules) found in abundance in EL Chondrites are one possible source for this population. We also propose a possible technique using R-band colours to more robustly identify fainter iron meteors with very high confidence., Accepted for publication in MNRAS on September 21st, 2021. 13 pages, 21 figures

NASA's Meteoroid Environment Office (MEO) produces an annual meteor shower forecast in order to help spacecraft operators assess the risk posed by meteoroid streams. Previously, this forecast focused on the International Space Station and therefore reported meteoroid fluxes and enhancement factors at an orbital altitude of 400 km. This paper presents an updated forecast algorithm that has an improved calculation of the flux enhancement produced by showers and can calculate fluxes at any selected Earth or lunar orbital altitude. Finally, we discuss and generate forecasted fluxes for the 2018 Draconid meteor shower, which is expected to produce meteoroid flux enhancements near the Sun-Earth L1 and L2 Lagrange points but not at Earth., Comment: Accepted for publication in the Journal of Spacecraft and Rockets

This work presents numerical simulations of meteoroid streams released by comet 21P/Giacobini-Zinner over the period 1850-2030. The initial methodology, based on Vaubaillon et al. (2005), has been updated and modified to account for the evolution of the comet's dust production along its orbit. The peak time, intensity, and duration of the shower were assessed using simulated activity profiles that are calibrated to match observations of historic Draconid outbursts. The characteristics of all the main apparitions of the shower are reproduced, with a peak time accuracy of half an hour and an intensity estimate correct to within a factor of 2 (visual showers) or 3 (radio outbursts). Our model also revealed the existence of a previously unreported strong radio outburst on October 9 1999, that has since been confirmed by archival radar measurements. The first results of the model, presented in Egal et al. (2018), provided one of the best predictions of the recent 2018 outburst. Three future radio outbursts are predicted in the next decade, in 2019, 2025 and 2029. The strongest activity is expected in 2025 when the Earth encounters the young 2012 trail. Because of the dynamical uncertainties associated with comet 21P's orbital evolution between the 1959 and 1965 apparitions, observations of the 2019 radio outburst would be particularly helpful to improve the confidence of subsequent forecasts., Comment: Icarus - Accepted

The number density and flux of a meteoroid stream is enhanced near a massive body due to the phenomenon known as gravitational focusing. The greatest enhancement occurs directly opposite the massive body from the stream radiant: as an observer approaches this location, the degree of focusing is unbound for a perfectly collimated stream. However, real meteoroid streams exhibit some dispersion in radiant and speed that will act to eliminate this singularity. In this paper, we derive an analytic approximation for this smoothing that can be used in meteoroid environment models and is based on real measurements of meteor shower radiant dispersion., Comment: Accepted for publication by Monthly Notices of the Royal Astronomical Society

Burns et al. (1979) use the parameter beta to describe the ratio of radiation pressure to gravity experienced by a small particle in the solar system. The central potential that these particles experience is effectively reduced by a factor of (1 - beta), lowering the escape velocity. Burns et al. (1979) also provide a simple expression for the value of beta at which particles ejected from a comet follow parabolic orbits and thus leave the solar system; this paper expands on this to calculate critical beta values that take ejection velocity into account. We first derive an expression for the speed at which particles are ejected from a comet that is valid at all particle sizes. We then calculate beta values for both idealized and real materials to determine which particles will be lost from meteor streams. We perform these calculations for cometary ejecta leading, trailing, and following the parent comet's nucleus for 10 major meteor showers. These values bound mass regimes within which meteoroids are ejected from the solar system and therefore cannot contribute to meteor showers., Comment: Accepted for publication by Icarus

An appearance of the Phoenicids meteor shower was predicted in 2014 by using a dust trail simulation of an outburst of 1956. We detected Phoenicids meteors on December 2 through multiple observation methods. The NASA All Sky Fireball Network and the Southern Ontario Meteor Network detected five meteors of Phoenicids via video observation. The Canadian Meteor Orbit Radar (CMOR) found fourteen candidate meteors, eight of which were confirmed as Phoenicids. The observed radiant point is consistent with that of our model predictions. In addition to the above observations, a visual observation was carried out by the Japanese team near the Observatorio del Roque de los Muchachos (ORM) of Instituto de Astrofisica de Canarias (IAC) in La Palma Island. The obtained zenithal hourly rate (ZHR) was 16.4±4.9. The maximum ZHR was roughly estimated to be between 20 and 30, which indicates that the cometary activity of parent object 289P/Blanpain in the early 20th century was only about one fifth or one eighth as high as its activity in the late 18th and early 19th century. Accordingly, it seems to be the case that 289P/Blanpain is gradually transforming from a comet to a dormant object.

Meteor showers are recognized from the concentration of their radiant distribution on the celestial sphere, and their similar speeds. It has been pointed out that the dispersion of the radiants depends on the velocity of and the spread in velocities between individual stream meteoroids. For meteor showers with low speeds at the Earth, the dispersion can become so large that the shower becomes hard to recognize (Sato and Watanabe, 2014). In this paper, we apply a technique described in Sato and Watanabe (2014) which transforms radiants from a geocentric to a heliocentric frame. The technique is applied to detect two low speed meteor showers, the Phoenicids and the June Bootids, which otherwise would be very difficult to recognize as true meteor showers.

Meteor radars such as the Canadian Meteor Orbit Radar (CMOR) have the ability to detect millions of meteors, making it possible to study the meteoroid environment in great detail. However, meteor radars also suffer from a number of detection biases; these biases must be fully corrected for in order to derive an accurate description of the meteoroid population. We present a bias correction method for patrol radars that accounts for the full form of ionization efficiency and mass distribution. This is an improvement over previous methods such as that of Taylor (1995) , which requires power-law distributions for ionization efficiency and a single mass index. We apply this method to the meteor speed distribution observed by CMOR and find a significant enhancement of slow meteors compared to earlier treatments. However, when the data set is severely restricted to include only meteors with very small uncertainties in speed, the fraction of slow meteors is substantially reduced, indicating that speed uncertainties must be carefully handled.

The first detected exoplanets found were "hot Jupiters"; these are large Jupiter-like planets in close orbits with their host star. The stars in these so-called "hot Jupiter systems" can have significant X-ray emission and the X-ray flux likely changes the evolution of the overall star-planetary system in at least two ways: (1) the intense high energy flux alters the structure of the upper atmosphere of the planet - in some cases leading to significant mass loss; (2) the angular momentum and magnetic field of the planet induces even more activity on the star, enhancing its X-rays, which are then subsequently absorbed by the planet. If the alignment of the systems is appropriate, the planet will transit the host star. The resulting drop in flux from the star allows us to measure the distribution of the low-density planetary atmosphere. We describe a science mission concept for a SmallSat Exosphere Explorer of hot Jupiters (SEEJ; pronounced "siege"). SEEJ will monitor the X-ray emission of nearby X-ray bright stars with transiting hot Jupiters in order to measure the lowest density portion of exoplanet atmospheres and the coronae of the exoplanet hosts. SEEJ will use revolutionary Miniature X-ray Optics (MiXO) and CMOS X-ray detectors to obtain sufficient collecting area and high sensitivity in a low mass, small volume and low-cost package. SEEJ will observe scores of transits occurring on select systems to make detailed measurements of the transit depth and shape which can be compared to out-of-transit behavior of the target system. The depth and duration of the flux change will allow us to characterize the exospheres of multiple hot Jupiters in a single year. In addition, the long baselines (covering multiple stellar rotation periods) from the transit data will allow us to characterize the temperature, flux and flare rates of the exoplanet hosts at an unprecedented level. This, in turn, will provide valuable constraints for models of atmospheric loss. In this contribution we outline the science of SEEJ and focus on the enabling technologies Miniature X-ray Optics and CMOS X-ray detectors.

Predictions of the 2018 Draconid activity at the Earth and the Sun-Earth L1 and L2 Lagrange points are presented. Numerical simulations of the meteoroids' ejection and evolution from comet 21P/Giacobini-Zinner are performed with a careful implementation of the results analysis and weighting. Model meteoroid fluxes at Earth are derived using as calibration the main peak date, intensity, and shower profiles of previous Draconid outbursts. Good agreement between the model and measurements is found for the 1933, 1946, 1998 and 2011 showers for a meteoroid size distribution index at ejection of about 2.6. A less accurate estimate of the peak time for the 1985, 2005 and 2012 predominantly radio-observed outbursts was found by considering the contribution of individual ejection epochs, while the model peak flux estimate was found to agree with observations to within a factor 3. Despite the promising geometrical configuration in 2018, our simulations predict low Draconid activity is expected on Earth, with a maximum of less than a few tens of meteors per hour around midnight the 9th of October, confirming previous models. At the L1 and L2 Lagrange points, however, the flux estimates suggest a 'meteoroid storm'. The Gaia spacecraft at the L2 region might be able to detect small (~ {\mu}g) Draconid meteoroid impacts centered in a two-hour window around 18h30 UT on the 8th of October, 2018., Comment: Accepted for publication by APJ Letter the 19/09/2018

We present a new method to detect meteor showers using the Density-Based Spatial Clustering of Applications with Noise algorithm (DBSCAN; Ester et al. 1996). DBSCAN is a modern cluster detection algorithm that is well suited to the problem of extracting meteor showers from all-sky camera data because of its ability to efficiently extract clusters of different shapes and sizes from large datasets. We apply this shower detection algorithm on a dataset that contains 25,885 meteor trajectories and orbits obtained from the NASA All-Sky Fireball Network and the Southern Ontario Meteor Network (SOMN). Using a distance metric based on solar longitude, geocentric velocity, and Sun-centered ecliptic radiant, we find 25 strong cluster detections and 6 weak detections in the data, all of which are good matches to known showers. We include measurement errors in our analysis to quantify the reliability of cluster occurrence and the probability that each meteor belongs to a given cluster. We validate our method through false positive/negative analysis and with a comparison to an established shower detection algorithm., Comment: 8 figures, 2 tables, published in Meteoritics & Planetary Science

Long-period comet C/2013 A1 (Siding Spring) will experience a close encounter with Mars on 2014 Oct 19. A collision between the comet and the planet has been ruled out, but the comet's coma may envelop Mars and its man-made satellites. By the time of the close encounter, five operational spacecraft will be present near Mars. Characterizing the coma is crucial for assessing the risk posed to these satellites by meteoroid impacts. We present an analytic model of cometary comae that describes the spatial and size distributions of cometary dust and meteoroids. This model correctly reproduces, to within an order of magnitude, the number of impacts recorded by Giotto near 1P/Halley [1] and by Stardust near comet 81P/Wild 2 [2]. Applied to Siding Spring, our model predicts a total particle fluence near Mars of 0.02 particles per square meter. In order to determine the degree to which Siding Spring's coma deviates from a sphere, we perform numerical simulations which take into account both gravitational effects and radiative forces. We take the entire dust component of the coma and tail continuum into account by simulating the ejection and evolution of dust particles from comet Siding Spring. The total number of particles simulated is essentially a free parameter and does not provide a check on the total fluence. Instead, these simulations illustrate the degree to which the coma of Siding Spring deviates from the perfect sphere described by our analytic model (see Figure). We conclude that our analytic model sacrifices less than an order of magnitude in accuracy by neglecting particle dynamics and radiation pressure and is thus adequate for order-of-magnitude fluence estimates. Comet properties may change unpredictably and therefore an analytic coma model that enables quick recalculation of the meteoroid fluence is highly desirable. NASA's Meteoroid Environment Office is monitoring comet Siding Spring and taking measurements of cometary brightness and dust production. We will discuss our coma model and nominal fluence taking the latest observations into account.

We present a method to confirm the planetary nature of objects in systems with multiple transiting exoplanet candidates. This method involves a Fourier-Domain analysis of the deviations in the transit times from a constant period that result from dynamical interactions within the system. The combination of observed anti-correlations in the transit times and mass constraints from dynamical stability allow us to claim the discovery of four planetary systems Kepler-25, Kepler-26, Kepler-27, and Kepler-28, containing eight planets and one additional planet candidate.

Separating meteor showers from the sporadic meteor background is critical for the study of both showers and the sporadic complex. The linkage of meteors to meteor showers, to parent bodies, and to other meteors is done using measures of orbital similarity. These measures often take the form of so-called D-parameters and are generally paired with some cutoff value within which two orbits are considered related. The appropriate cutoff value can depend on the size of the data-set (Southworth & Hawkins 1963), the sporadic contribution within the observed size range (Jopek 1995), or the inclination of the shower (Galligan 2001). If the goal is to minimize sporadic contamination of the extracted shower, the cutoff value should also reflect the strength of the shower compared to the local sporadic background. In this paper, we present a method for determining, on a per-shower basis, the orbital similarity cutoff value that corresponds to a chosen acceptable false-positive rate. This method also assists us in distinguishing which showers are significant within a set of data. We apply these methods to optical meteor observations from the NASA All-Sky and Southern Ontario Meteor Networks., Accepted for publication in MNRAS

This paper presents a parametric study of giant planet migration through the combined action of disk torques and planet-planet scattering. The torques exerted on planets during Type II migration in circumstellar disks readily decrease the semi-major axes, whereas scattering between planets increases the orbital eccentricities. This paper presents a parametric exploration of the possible parameter space for this migration scenario using two (initial) planetary mass distributions and a range of values for the time scale of eccentricity damping (due to the disk). For each class of systems, many realizations of the simulations are performed in order to determine the distributions of the resulting orbital elements of the surviving planets; this paper presents the results of 8500 numerical experiments. Our goal is to study the physics of this particular migration mechanism and to test it against observations of extrasolar planets. The action of disk torques and planet-planet scattering results in a distribution of final orbital elements that fills the a-e plane, in rough agreement with the orbital elements of observed extrasolar planets. In addition to specifying the orbital elements, we characterize this migration mechanism by finding the percentages of ejected and accreted planets, the number of collisions, the dependence of outcomes on planetary masses, the time spent in 2:1 and 3:1 resonances, and the effects of the planetary IMF. We also determine the distribution of inclination angles of surviving planets and the distribution of ejection speeds for exiled planets., 46 pages including 15 figures; accepted to ICARUS

Eighty planetary systems of two or more planets are known to orbit stars other than the Sun. For most, the data can be sufficiently explained by non-interacting Keplerian orbits, so the dynamical interactions of these systems have not been observed. Here we present four sets of light curves from the Kepler spacecraft, each which of shows multiple planets transiting the same star. Departure of the timing of these transits from strict periodicity indicates that the planets are perturbing each other: the observed timing variations match the forcing frequency of the other planet. This confirms that these objects are in the same system. Next we limit their masses to the planetary regime by requiring the system remain stable for astronomical timescales. Finally, we report dynamical fits to the transit times, yielding possible values for the planets' masses and eccentricities. As the timespan of timing data increases, dynamical fits may allow detailed constraints on the systems' architectures, even in cases for which high-precision Doppler follow-up is impractical.

We present a new method for confirming transiting planets based on the combination of transit timingn variations (TTVs) and dynamical stability. Correlated TTVs provide evidence that the pair of bodies are in the same physical system. Orbital stability provides upper limits for the masses of the transiting companions that are in the planetary regime. This paper describes a non-parametric technique for quantifying the statistical significance of TTVs based on the correlation of two TTV data sets. We apply this method to an analysis of the transit timing variations of two stars with multiple transiting planet candidates identified by Kepler. We confirm four transiting planets in two multiple planet systems based on their TTVs and the constraints imposed by dynamical stability. An additional three candidates in these same systems are not confirmed as planets, but are likely to be validated as real planets once further observations and analyses are possible. If all were confirmed, these systems would be near 4:6:9 and 2:4:6:9 period commensurabilities. Our results demonstrate that TTVs provide a powerful tool for confirming transiting planets, including low-mass planets and planets around faint stars for which Doppler follow-up is not practical with existing facilities. Continued Kepler observations will dramatically improve the constraints on the planet masses and orbits and provide sensitivity for detecting additional non-transiting planets. If Kepler observations were extended to eight years, then a similar analysis could likely confirm systems with multiple closely spaced, small transiting planets in or near the habitable zone of solar-type stars., Comment: 23 pages, 8 figures, 4 tables, 1 electronic table, accepted to ApJ

We present a Bayesian surrogate model for the analysis of periodic or quasi-periodic time series data. We describe a computationally efficient implementation that enables Bayesian model comparison. We apply this model to simulated and real exoplanet observations. We discuss the results and demonstrate some of the challenges for applying our surrogate model to realistic exoplanet data sets. In particular, we find that analyses of real world data should pay careful attention to the effects of uneven spacing of observations and the choice of prior for the "jitter" parameter., 25 pages, 4 figures, accepted to Bayesian Analysis , special issue for Ninth Valencia International Conference on Bayesian Statistics

We report the distribution of planets as a function of planet radius (R_p), orbital period (P), and stellar effective temperature (Teff) for P < 50 day orbits around GK stars. These results are based on the 1,235 planets (formally "planet candidates") from the Kepler mission that include a nearly complete set of detected planets as small as 2 Earth radii (Re). For each of the 156,000 target stars we assess the detectability of planets as a function of R_p and P. We also correct for the geometric probability of transit, R*/a. We consider first stars within the "solar subset" having Teff = 4100-6100 K, logg = 4.0-4.9, and Kepler magnitude Kp < 15 mag. We include only those stars having noise low enough to permit detection of planets down to 2 Re. We count planets in small domains of R_p and P and divide by the included target stars to calculate planet occurrence in each domain. Occurrence of planets varies by more than three orders of magnitude and increases substantially down to the smallest radius (2 Re) and out to the longest orbital period (50 days, ~0.25 AU) in our study. For P < 50 days, the radius distribution is given by a power law, df/dlogR= k R^��. This rapid increase in planet occurrence with decreasing planet size agrees with core-accretion, but disagrees with population synthesis models. We fit occurrence as a function of P to a power law model with an exponential cutoff below a critical period P_0. For smaller planets, P_0 has larger values, suggesting that the "parking distance" for migrating planets moves outward with decreasing planet size. We also measured planet occurrence over Teff = 3600-7100 K, spanning M0 to F2 dwarfs. The occurrence of 2-4 Re planets in the Kepler field increases with decreasing Teff, making these small planets seven times more abundant around cool stars than the hottest stars in our sample. [abridged], Submitted to ApJ, 22 pages, 10 figures

We present and discuss five candidate exoplanetary systems identified with the Kepler spacecraft. These five systems show transits from multiple exoplanet candidates. Should these objects prove to be planetary in nature, then these five systems open new opportunities for the field of exoplanets and provide new insights into the formation and dynamical evolution of planetary systems. We discuss the methods used to identify multiple transiting objects from the Kepler photometry as well as the false-positive rejection methods that have been applied to these data. One system shows transits from three distinct objects while the remaining four systems show transits from two objects. Three systems have planet candidates that are near mean motion commensurabilities---two near 2:1 and one just outside 5:2. We discuss the implications that multitransiting systems have on the distribution of orbital inclinations in planetary systems, and hence their dynamical histories; as well as their likely masses and chemical compositions. A Monte Carlo study indicates that, with additional data, most of these systems should exhibit detectable transit timing variations (TTV) due to gravitational interactions---though none are apparent in these data. We also discuss new challenges that arise in TTV analyses due to the presence of more than two planets in a system., Comment: Accepted to ApJ

The extrasolar planets discovered to date possess unexpected orbital elements. Most orbit their host stars with larger eccentricities and smaller semi-major axes than similarly sized planets in our own solar system do. It is generally agreed that the interaction between giant planets and circumstellar disks (Type II migration) drives these planets inward to small radii, but the effect of these same disks on orbital eccentricity, e, is controversial. Several recent analytic calculations suggest that disk-planet interactions can excite eccentricity, while numerical studies generally produce eccentricity damping. This paper addresses this controversy using a quasi-analytic approach, drawing on several preceding analytic studies. This work refines the current treatment of eccentricity evolution by removing several approximations from the calculation of disk torques. We encounter neither uniform damping nor uniform excitation of orbital eccentricity, but rather a function de/dt that varies in both sign and magnitude depending on eccentricity and other solar system properties. Most significantly, we find that for every combination of disk and planet properties investigated herein, corotation torques produce negative values of de/dt for some range in e within the interval [0.1, 0.5]. If corotation torques are saturated, this region of eccentricity damping disappears, and excitation occurs on a short timescale of less than 0.08 Myr. Thus, our study does not produce eccentricity excitation on a timescale of a few Myr -- we obtain either eccentricity excitation on a short time scale, or eccentricity damping on a longer time scale. Finally, we discuss the implications of this result for producing the observed range in extrasolar planet eccentricity., 24 pages including 13 figures; accepted to ICARUS

The Kappa Cygnid (KCG) meteor shower exhibited unusually high activity in 2014, producing ten times the typical number of meteors. The shower was detected in both radar and optical systems and meteoroids associated with the outburst spanned at least five decades in mass. In total, the Canadian Meteor Orbit Radar, European Network, and NASA All Sky and Southern Ontario Meteor Network produced thousands of KCG meteor trajectories. Using these data, we have undertaken a new and improved characterization of the dynamics of this little-studied, variable meteor shower. The Cygnids have a di use radiant and a significant spread in orbital characteristics, with multiple resonances appearing to play a role in the shower dynamics. We conducted a new search for parent bodies and found that several known asteroids are orbitally similar to the KCGs. N-body simulations show that the two best parent body candidates readily transfer meteoroids to the Earth in recent centuries, but neither produces an exact match to the KCG radiant, velocity, and solar longitude. We nevertheless identify asteroid 2001 MG1 as a promising parent body candidate.

Doppler planet searches have discovered that giant planets follow orbits with a wide range of orbital eccentricities, revolutionizing theories of planet formation. The discovery of hundreds of exoplanet candidates by NASA's Kepler mission enables astronomers to characterize the eccentricity distribution of small exoplanets. Measuring the eccentricity of individual planets is only practical in favorable cases that are amenable to complementary techniques (e.g., radial velocities, transit timing variations, occultation photometry). Yet even in the absence of individual eccentricities, it is possible to study the distribution of eccentricities based on the distribution of transit durations (relative to the maximum transit duration for a circular orbit). We analyze the transit duration distribution of Kepler planet candidates. We find that for host stars with T_eff > 5100 K we cannot invert this to infer the eccentricity distribution at this time due to uncertainties and possible systematics in the host star densities. With this limitation in mind, we compare the observed transit duration distribution with models to rule out extreme distributions. If we assume a Rayleigh eccentricity distribution for Kepler planet candidates, then we find best-fits with a mean eccentricity of 0.1-0.25 for host stars with T_eff < 5100 K. We compare the transit duration distribution for different subsets of Kepler planet candidates and discuss tentative trends with planetary radius and multiplicity. High-precision spectroscopic follow-up observations for a large sample of host stars will be required to confirm which trends are real and which are the results of systematic errors in stellar radii. Finally, we identify planet candidates that must be eccentric or have a significantly underestimated stellar radius., 30 pages, 19 figures