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 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. 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, but owing to lack of observations this is uncertain. 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 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×10(12) joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons--almost always used with this technique--overestimate blast damage. This suggests that earlier damage estimates 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. 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.
Abstract
Trojan asteroids are small bodies orbiting around the
L
4
or
L
5
Lagrangian points of a Sun-planet system. Due to their peculiar orbits, they provide key constraints to the Solar System ...evolution models. Despite numerous dedicated observational efforts in the last decade, asteroid 2010 TK
7
has been the only known Earth Trojan thus far. Here we confirm that the recently discovered 2020 XL
5
is the second transient Earth Trojan known. To study its orbit, we used archival data from 2012 to 2019 and observed the object in 2021 from three ground-based observatories. Our study of its orbital stability shows that 2020 XL
5
will remain in
L
4
for at least 4 000 years. With a photometric analysis we estimate its absolute magnitude to be
$${H}_{r}=18.5{8}_{-0.15}^{+0.16}$$
H
r
=
18.5
8
−
0.15
+
0.16
, and color indices suggestive of a C-complex taxonomy. Assuming an albedo of 0.06 ± 0.03, we obtain a diameter of 1.18 ± 0.08 km, larger than the first known Earth Trojan asteroid.
There are significant uncertainties in the calculation of photometric and ionization masses of meteors, particularly those derived from meteor head echoes observed by high power, large aperture ...radars. Simultaneous observations of meteors with the EISCAT UHF tristatic system and narrow field two‐station intensified video were conducted in October 2007; 11 hours of data produced four useful meteors observed on all three radar receivers and both cameras. The positions and speeds calculated on the two systems generally agree to within the observational uncertainty. The photometric and ionization masses for each meteor were calculated using several values of luminous efficiency and ionization probability from literature, and all of these masses were found to agree to within the estimated error in the methods. More observations are required to select among the various values of ionization coefficient and luminous efficiency.
Key Points
There is significant uncertainty in the calculation of meteor masses
Meteor masses calculated with radar and video methods are useful
Reasonable agreement has been found between radar and video meteor masses
The mass distribution index of the Geminid, Quadrantid, Arietid, Eta Aquariid, Orionid and South Delta Aquariid meteor showers have been measured using data from 2007 to 2010 from the Canadian Meteor ...Orbit Radar (CMOR). Single-station data from the 29.85-MHz system were used to find mass index as a function of solar longitude, and to compare separate years of showers. The variation in mass index with solar longitude can provide information on the structure of the stream and how the structure changes with time. The Geminids and Quadrantids were the most prominent showers seen by CMOR, and had peak mass indices of 1.65 and 1.55, respectively. They had clear, strong peaks and little year-to-year variation. The other showers had slightly higher peak mass indices and a greater variability, even though a more sporadic contamination, which might have affected the results.
We report high-resolution multi-station observations of meteors by the Canadian Automated Meteor Observatory recorded from 2009 June to 2010 August. Our survey has a limiting detection magnitude of ...+5 mag in R band, equivalent to a limiting meteoroid mass of ~2 x 10 super(-7) kg. The high metric trajectory accuracy (of the order of 30 m perpendicular to the solution and 200 m along track) allows us to determine velocities with average uncertainty of < 1.5% in speed and ~0degrees.4 in the radiant direction. A total of 1739 meteors had measured orbits. The data have been searched for meteors in hyperbolic orbits, which are potentially of interstellar origin. We found 22 potential hyperbolic meteors among our sample, with only two of them having a speed at least 3sigma above the hyperbolic limit. For our one-year survey we find no clear evidence of interstellar meteoroids at millimeter sizes in a weighted time-area product of ~10 super(4) km super(2) hr. Backward integrations performed for these 22 potentially hyperbolic meteors to check for close encounters with planets show no considerable changes in their orbits. Detailed examination leads us to conclude that our few identified events are most likely the result of measurement error. We find an upper limit of f sub(ISP) < 2 x 10 super(-4) km super(-2) hr super(-1) for the flux of interstellar meteoroids at Earth with a limiting mass of m > 2 x 10 super(-7) kg.
The mass distribution index of sporadic meteors and the indices of specific sporadic sources have been measured using data from 2007 to 2010 from the Canadian Meteor Orbit Radar. Single-station data ...from the 29.85-MHz system were used and the trends throughout the year are examined. We find that the average values for the sources are approximately equal, but the annual variation of each source is unique. We find that the overall average sporadic mass index is 2.17 ± 0.07, less than the frequently cited 2.34. The variation in the mass index throughout the year can provide information about the meteoroid populations that the Earth is encountering at different parts of its orbit.
– The Grimsby meteorite (H4–6) fell on September 25, 2009. As of mid‐2010, 13 fragments totaling 215 g have been recovered. Records of the accompanying fireball from the Southern Ontario Meteor ...Network, including six all‐sky video cameras, a large format CCD, infrasound and radar records, have been used to characterize the trajectory, speed, orbit, and initial mass of the meteoroid. From the four highest quality all‐sky video records, the initial entry velocity was 20.91 ± 0.19 km s−1 while the derived radiant has a local azimuth of 309.40° ± 0.19° and entry angle of 55.20° ± 0.13°. Three major fragmentation episodes are identified at 39, 33, and 30 km height, with corresponding uncertainties of approximately 2 km. Evidence for early fragmentation at heights of approximately 70 km is found in radar data; dynamic pressure of this earliest fragmentation is near 0.1 MPa while the main flare at 39 km occurred under ram pressures of 1.5 MPa. The fireball was luminous to at least 19.7 km altitude and the dynamic mass estimate of the largest remaining fragment at this height is approximately several kilograms. The initial mass is constrained to be <100 kg from infrasound data and ablation modeling, with a most probable mass of 20–50 kg. The preatmospheric orbit is typical of an Apollo asteroid with a likely immediate origin in either the 3:1 or ν6 resonances.
We present results from a 12 year survey of the Daytime Arietid meteor shower using the Canadian Meteor Orbit Radar, a VHF backscattering orbital meteor radar, covering the interval 2002-2013. This ...survey recorded more than 2 x 10... Daytime Arietid orbits having representative masses of 8 x 10... kg and sizes of ...0.4 mm. The core activity for the Arietids is found in the range 73 ...5 ≤ ... ≤ 84 ...5 and shows a broad 4-d maximum centred near ... = 80 ...5 of 0.04 meteoroids km... h... producing meteors of equivalent radio magnitude of +6.5 from a mean radiant at α... = 44 ...9 ± 1 ...1, ... = 25 ...5 ± 1 ...0. During the plateau of shower peak activity, the mass index of the stream reaches a minimum with s = 1.6-1.7. Contamination from another nearby shower (likely the Daytime Zeta Perseids) and/or sub-streams showing different orbits compared to the core of the stream is evident in the interval 60 ...5 ≤ ... ≤ 71 ...5. Similar contamination beyond ... = 84 ...5 may be due to the Helion sporadic source. We also characterized the deceleration profiles for Daytime Arietid meteor echoes using several independent speed techniques including Fresnel pre-t..., Fresnel amplitude oscillation and time-of-flight speeds which together with modelling produced a best estimate for the stream's out-of-atmosphere speed of v... = 40.5 ± 0.7 km s... The mean radar orbit from our study is noticeably smaller in semi-major axis and eccentricity than is found for larger Arietids measured with optical systems, a difference which if real indicates a particle-size sorting of the stream orbit. The broad activity maximum, long duration of activity and particle-size dependence of the orbital elements suggest the stream is too old to have been solely formed during the breakup of the parent comet of the Marsden sunskirters about a millennium ago as proposed by Sekanina & Chodas. (ProQuest: ... denotes formulae/symbols omitted.)
We report the discovery of an Hr = 3.4 0.1 dwarf planet candidate by the Pan-STARRS Outer Solar System Survey. 2010 JO179 is red with (g − r) = 0.88 0.21, roughly round, and slowly rotating, with a ...period of 30.6 hr. Estimates of its albedo imply a diameter of 600-900 km. Observations sampling the span between 2005 and 2016 provide an exceptionally well determined orbit for 2010 JO179, with a semimajor axis of 78.307 0.009 au; distant orbits known to this precision are rare. We find that 2010 JO179 librates securely within the 21:5 mean-motion resonance with Neptune on 100 Myr timescales, joining the small but growing set of known distant dwarf planets on metastable resonant orbits. These imply a substantial trans-Neptunian population that shifts between stability in high-order resonances, the detached population, and the eroding population of the scattering disk.
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