Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage 1 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects 2 . Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave 3 , but owing to lack of observations this is uncertain 4,5 . Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (6100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT 54.185310 12 joules). We show that a widely referenced technique 4-6 of estimating airburst damage does not reproduce the observations, and that the mathematical relations 7 based on the effects of nuclear weapons-almost always used with this technique-overestimate blast damage. This suggests that earlier damage estimates 5,6 near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques 8,9 . This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes. for the Chelyabinsk airburst, based on indirect illumination measured from video records. The brightness is an average derived from indirect scattered sky brightness from six videos proximal to the airburst, corrected for the sensor gamma setting, autogain, range and airmass extinction, following the procedure used for other airburst light curves generated from video 24,25 . The light curve has been normalized using the US government sensor data peak brightness value of 2.7 3 10 13 W sr 21, corresponding to an absolute astronomical magnitude of 228 in the silicon bandpass. The individual video light curves deviate by less than one magnitude between times 22 and 11.5 with larger deviations outside this interval. Time zero corresponds to 03:20:32.2 UTC on 15 February 2013. b, The energy deposition per unit height for the Chelyabinsk airburst, based on video data. The conversion to absolute energy deposition per unit path length assumes a blackbody emission of 6,000 K and bolometric efficiency of 17%, the same as the assumptions used to convert earlier US government sensor information to energy 26 . The heights are computed us...

Meteoroids pose one of the largest risks to spacecraft outside of low Earth orbit. In order to correctly predict the rate at which meteoroids impact and damage spacecraft, environment models must describe the mass, directionality, velocity, and density distributions of meteoroids. NASA's Meteoroid Engineering Model (MEM) is one such model; MEM 3 is an updated version of the code that better captures the correlation between directionality and velocity and incorporates a bulk density distribution. This paper describes MEM 3 and compares its predictions with the rate of large particle impacts seen on the Long DurationExposure Facility (LDEF) and the Pegasus II and III satellites. Nomenclature a = semimajor axis BH = Brinell hardness b = unitless parameter that relates ∆, y, x, and t t c = speed of sound in meteoroid c 0,t = speed of sound in unstressed target material c t = speed of sound in target d = meteoroid diameter d 0 = crater diameter without supralinearity correction d c = crater diameter E = Young's modulus E t = Young's modulus of target e = orbital eccentricity F = flux F c = crater-or damage-limited flux F m = mass-limited flux F G = Grün et al. flux f = supralinearity correction G = gravitational constant h = altitude h 1 = altitude of 100 km h 2 = altitude of 100,000 km i = orbital inclination M = mass of the Sun M ⊕ = mass of the Earth N c,i = number of craters on side i m = meteoroid mass P = probability p c = crater depth Q = aphelion distance q = perihelion distance R ⊕ = radius of the Earth r = heliocentric distance s t = stress factor of target t t = target thickness v = meteoroid velocity v ⊥ = normal velocity v 0 = minimum speed required to produce a crater v 1 = speed at 100 km v 2 = speed at 100,000 km v esc = local escape velocity v f = meteoroid speed with gravitational focusing v i = meteoroid speed without gravitational focusing x = ratio of uncorrected crater diameter d 0 to meteoroid diameter d Y t = yield strength of target y = unitless parameter that relates t t , f , and d arXiv:1909.05947v2 [astro-ph.EP] 27 Sep 2019 z = unitless parameter that relates y, t t , and d α i, j = angle between surface normal vector i and meteoroid radiant j ∆ = grain size parameter η g = average gravitational focusing factor θ = azimuthal angle µ = mean of a normal distribution ρ = meteoroid density ρ t = target density σ = standard deviation of a normal distribution σ t = ultimate strength of target ψ = angle between the velocity vector and the radius vector φ = elevation angle ξ = depth-to-diameter ratio

The Type IIn supernova SN 2010jl was relatively nearby and luminous, allowing detailed studies of the near-infrared (NIR) emission. We present 1 − 2.4 µm spectroscopy over the age range of 36 -565 days from the earliest detection of the supernova. On day 36, the H lines show an unresolved narrow emission component along with a symmetric broad component that can be modeled as the result of electron scattering by a thermal distribution of electrons. Over the next hundreds of days, the broad components of the H lines shift to the blue by 700 km s −1 , as is also observed in optical lines. The narrow lines do not show a shift, indicating they originate in a different region. He I λ10830 and λ20587 lines both show an asymmetric broad emission component, with a shoulder on the blue side that varies in prominence and velocity from −5500 km s −1 on day 108 to −4000 km s −1 on day 219. This component may be associated with the higher velocity flow indicated by X-ray observations of the supernova. The absence of the feature in the H lines suggests that this is from a He rich ejecta flow. The He I λ10830 feature has a narrow P Cygni line, with absorption extending to ∼ 100 km s −1 and strengthening over the first 200 days, and an emission component which weakens with time. At day 403, the continuum emission becomes dominated by a blackbody spectrum with a temperature of ∼ 1900 K, suggestive of dust emission.

The Chelyabinsk superbolide of February 15, 2013, was caused by the atmospheric entry of a ∼19 m asteroid with a kinetic energy of 500 kT TNT just south of the city of Chelyabinsk, Russia. It was a rare event; impacts of similar energy occur on the Earth only a few times per century. Impacts of this energy near such a large urban area are expected only a few times per 10 000 years. A number of video records obtained by casual eyewitnesses, dashboard cameras in cars, security, and traffic cameras were made publicly available by their authors on the Internet. These represent a rich repository for future scientific studies of this unique event. To aid researchers in the archival study of this airburst, we provide and document a catalog of 960 videos showing various aspects of the event. Among the video records are 400 distinct videos showing the bolide itself and 108 videos showing the illumination caused by the bolide. Other videos show the dust trail left in the atmosphere, the arrival of the blast wave on the ground, or the damage caused by the blast wave. As these video recordings have high scientific, historical, and archival value for future studies of this airburst, a systematic documentation and description of records is desirable. Many have already been used for scientific analyses. We give the exact locations where 715 videos were taken as well as details of the visible/audible phenomena in each video recording. An online version of the published catalog has been developed and will be regularly updated to provide a long-term database for investigators.

This paper outlines new methods to measure optical meteor fluxes for showers and sporadic sources. Many past approaches have found the collecting area of a detector at a fixed 100 km altitude, but this approach considers the full volume, finding the area in two km height intervals based on the position of the shower or sporadic source radiant and the population's velocity. Here, the stellar limiting magnitude is found every 10 minutes during clear periods and converted to a limiting meteor magnitude for the shower or sporadic source having fluxes measured, which is then converted to a limiting mass. The final output is a mass limited flux for meteor showers or sporadic sources. Presented are the results of these flux methods as applied to the 2015 Perseid meteor shower as seen by the Meteoroid Environment Office's eight wide-field cameras. The peak Perseid flux on the night of August 13, 2015, was measured to be 0.002989 meteoroids/km 2 /hr down to 0.00051 grams, corresponding to a ZHR of 100.7.

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We present the results of new calibration tests performed by the NASA Meteoroid Environment Office (MEO) designed to help quantify and minimize systematic uncertainties in meteor photometry from video camera observations. These systematic uncertainties can be categorized by two main sources: an imperfect understanding of the linearity correction for the MEO's Watec 902H2 Ultimate video cameras and uncertainties in meteor magnitudes arising from transformations between the Watec camera's Sony EX-View HAD bandpass and the bandpasses used to determine reference star magnitudes. To address the first point, we have measured the linearity response of the MEO's standard meteor video cameras using two independent laboratory tests on eight cameras. Our empirically determined linearity correction is critical for performing accurate photometry at low camera intensity levels. With regards to the second point, we have calculated synthetic magnitudes in the bandpass for reference stars. These synthetic magnitudes enable direct calculations of the meteor's photometric flux within the camera band pass without requiring any assumptions of its spectral energy distribution. Systematic uncertainties in the synthetic magnitudes of individual reference stars are estimated at ∼ 0.20 mag, and are limited by the available spectral information in the reference catalogs. These two improvements allow for zero-points accurate to ∼ 0.05 - 0.10 mag in both filtered and unfiltered camera observations with no evidence for lingering systematics. These improvements are essential to accurately measuring photometric masses of individual meteors and source mass indexes.

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