Context.
Asteroids with a diameter of up to a few dozen meters may spin very fast and complete an entire rotation within a few minutes. These small and fast-rotating bodies are thought to be ...monolithic objects because the gravitational force due to their small size is not strong enough to counteract the strong centripetal force caused by the fast rotation. This argument means that the rubble-pile structure is not feasible for these objects. Additionally, it is not clear whether the fast spin prevents dust and small particles (regolith) from being kept on their surface.
Aims.
We develop a model for constraining the thermal conductivity of the surface of the small, fast-rotating near-Earth asteroids. This model may suggest whether regolith is likely present on these objects.
Methods.
Our approach is based on the comparison of the measured Yarkovsky drift and a predicted value using a theoretical model that depends on the orbital, physical and thermal parameters of the object. The necessary parameters are either deduced from statistical distribution derived for near-Earth asteroids population or determined from observations with associated uncertainty. With this information, we performed Monte Carlo simulations and produced a probability density distribution for the thermal conductivity.
Results.
Applying our model to the superfast rotator asteroid (499998) 2011 PT, we find that the measured Yarkovsky drift can only be achieved when the thermal conductivity
K
of the surface is low. The resulting probability density function for the conductivity is bimodal, with two most likely values being around 0.0001 and 0.005 W m
−1
K
−1
. Based on this, we find that the probability that
K
is lower than 0.1 W m
−1
K
−1
is at least 95%. This low thermal conductivity might indicate that the surface of 2011 PT is covered with a thermal insulating layer, composed of a regolith-like material similar to lunar dust.
To better estimate which luminous efficiency (τ) value is compatible with contemporary values of the ionization coefficient (β), we report a series of simultaneous optical and specular echo radar ...measurements of low speed (v ¡ 20 km/s) meteors. We focus on the low speed population as secondary ionization is not relevant and the initial trail radii are small, minimizing model assumptions required to estimate electron line density. By using the large decrease in expected ionization coefficient at such low speeds, we attempt to better define the likely ratio of photon to electron production. This provides an estimate of the probable luminous efficiency, given that recent lab measurements of ionization efficiency agree with established theory (Jones, 1997; DeLuca et al., 2018) suggesting β is more constrained than τ.
Optical measurements were performed with two pairs of autonomously operated electron-multiplied charge coupled device cameras (EMCCDs) co-located with the multi-frequency Canadian Meteor Orbit Radar (CMOR) (Brown et al., 2008). Using the timing and geometry of individual meteors measured by both the radar and multi-station EMCCD systems, the portion of the optical lightcurve corresponding to each specular radar echo is measured and the received echo power used to estimate an electron line density. A total of 1249 simultaneous EMCCD and radar meteors were identified from observations between 2017–2019 with 55 having in atmosphere speeds below 20 km/s. A subset of 36 events were analysed in detail, with 29 having speed < 20 km/s. These meteors had G-band magnitudes at the specular radar point between +4 and +7.7, with an average radiant power of 5 W (assuming a 945 W power for a zero magnitude meteor). These correspond to a typical magnitude of +6. Following the procedure in Weryk and Brown (2013), the ratio of electron line density (q) to radiant power (I) provides a direct estimate of the ionization coefficient (β) to luminous efficiency (τ) ratio for each event. We find that β/τ strongly correlates with radiant power. All our simultaneous meteors had asteroidal-like orbits and six were found to be probable iron meteoroids, representing 20% of our slow <20 km/s sample. Luminous efficiency values averaged 0.6% at low speed, ranging from < 0.1% to almost 30%. No trend of luminous efficiency with speed was apparent, though a weak correlation between higher values of τ and radiant power may be present.
•36 optical meteors simultaneously detected by radar were analysed.•The ratio of ionization to luminous efficiency strongly correlates with luminosity.•Luminous efficiency (τ) for most events was in the range 0.1% ¡ τ ¡ 15%.•No correlation was found between speed and τ.•Significant scatter found in τ may be due to fragmentation or spectral variations.
A strong and unexpected meteor shower outburst was observed by the Southern Argentina Agile MEteor Radar Orbital System (SAAMER-OS) at high southern ecliptic latitude within the South Toroidal ...region. The outburst, which was active throughout solar longitudes 351° and 352°, peaked at 09:30 UT on 2020 March 12, has a mean Sun-centered ecliptic radiant of λ − λ0 ∼ 307 5 and β ∼ −77 2 and a geocentric velocity of 30.7 km s−1. Using the parameter criterion, we find the corresponding orbital elements of the outburst to match well with both the β Tucanid and δ Mensid meteor showers, suggesting these are in fact the same shower. We also find a promising parent candidate in asteroid (248590) 2006 CS, a large (D ∼ 2 km) highly inclined 52° near-Earth object.
In this paper, we describe an improved technique for using the backscattered phase from meteor radar echo measurements just prior to the specular point (t0) to calculate meteor speeds and their ...uncertainty. Our method, which builds on earlier work of Cervera et al. (1997, https://doi.org/10.1029/96RS03638), scans possible speeds in the Fresnel distance‐time domain with a dynamic, sliding window and derives a best‐speed estimate from the resultant speed distribution. We test the performance of our method, called pre‐t0 speeds by sliding‐slopes technique (PSSST), on transverse scattered meteor echoes observed by the Middle Atmosphere Alomar Radar System (MAARSY) and the Canadian Meteor Orbit Radar (CMOR) and compare the results to time‐of‐flight and Fresnel transform speed estimates. Our novel technique is shown to produce good results when compared to both model and speed measurements using other techniques. We show that our speed precision is ±5% at speeds less than 40 km/s, and we find that more than 90% of all CMOR multistation echoes have PSSST solutions. For CMOR data, PSSST is robust against the selection of critical phase value and poor phase unwrapping. Pick errors of up to ±6 pulses for meteor speeds less than about 50 km/s produce errors of less than ±5% of the meteoroid speed. In addition, the width of the PSSST speed Kernel density estimate (KDE) is used as a natural measure of uncertainty that captures both noise and t0 pick uncertainties.
Key Points
Meteor speed determination by novel means
Our algorithm allows to determine the meteor speed with 5% uncertainty for >90% echoes using only a single radar station
We tested our method for two different radar systems with more than 10 million unique events
We examine meteoroid orbits recorded by the Canadian Meteor Orbit Radar (CMOR) from 2012 to 2019, consisting of just over 11 million orbits in a search for potential interstellar meteoroids. Our 7.5 ...year survey consists of an integrated time-area product of ~ 7× 106 km2 hours. Selecting just over 160000 six station meteor echoes having the highest measured velocity accuracy from within our sample, we found five candidate interstellar events. These five potential interstellar meteoroids were found to be hyperbolic at the 2σ-level using only their raw measured speed. Applying a new atmospheric deceleration correction algorithm developed for CMOR, we show that all five candidate events were likely hyperbolic at better than 3σ, the most significant being a 3.7σ detection. Assuming all five detections are true interstellar meteoroids, we estimate the interstellar meteoroid flux at Earth to be at least 6.6 × 10−7 meteoroids/km2/hr appropriate to a mass of 2 × 10−7kg.
Using estimated measurement uncertainties directly extracted from CMOR data, we simulated CMOR’s ability to detect a hypothetical ‘Oumuamua - associated hyperbolic meteoroid stream. Such a stream was found to be significant at the 1.8σ level, suggesting that CMOR would likely detect such a stream of meteoroids as hyperbolic. We also show that CMOR’s sensitivity to interstellar meteoroid detection is directionally dependent.
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 αbig = 0.72 ± 0.09(stat.) ± 0.15(sys.) while small objects behave as αsmall = 0.07 ± 0.03(stat.) ± 0.09(sys.) where the SFD is ∝10αHN and HN 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⊕.
The unexpected 2012 Draconid meteor storm Ye, Quanzhi; Wiegert, Paul A; Brown, Peter G ...
Monthly notices of the Royal Astronomical Society,
02/2014, Letnik:
437, Številka:
4
Journal Article
Recenzirano
Odprti dostop
An unexpected intense outburst of the Draconid meteor shower was detected by the Canadian Meteor Orbit Radar on 2012 October 8. The peak flux occurred at ∼16:40 ut on October 8 with a maximum of 2.4 ...± 0.3 h−1 km−2 (appropriate to meteoroid mass larger than 10−7 kg), equivalent to a ZHRmax 9000 ± 1000 using 5-min intervals, using a mass distribution index of s = 1.88 ± 0.01 as determined from the amplitude distribution of underdense Draconid echoes. This makes the outburst among the strongest Draconid returns since 1946 and the highest flux shower since the 1966 Leonid meteor storm, assuming that a constant power-law distribution holds from radar to visual meteoroid sizes. The weighted mean geocentric radiant in the time interval of 15-19 h ut, 2012 October 8, was αg = 262
4 ± 0
1, δg = 55
7 ± 0
1 (epoch J2000.0). Visual observers also reported increased activity around the peak time, but with a much lower rate (ZHR ∼ 200), suggesting that the magnitude-cumulative number relationship is not a simple power law. Ablation modelling of the observed meteors as a population does not yield a unique solution for the grain size and distribution of Draconid meteoroids, but is consistent with a typical Draconid meteoroid of m
total between 10−6 and 10−4 kg being composed of 10-100 grains. Dynamical simulations indicate that the outburst was caused by dust particles released during the 1966 perihelion passage of the parent comet, 21P/Giacobini-Zinner, although there are discrepancies between the modelled and observed timing of the encounter, presumably caused by approaches of the comet to Jupiter during 1966-1972. Based on the results of our dynamical simulation, we predict possible increased activity of the Draconid meteor shower in 2018, 2019, 2021 and 2025.
A strong outburst of the October Draconid meteor shower was predicted for 2011 October 8. Here we present the observations obtained by the Canadian Meteor Orbit Radar (CMOR) during the 2011 outburst. ...CMOR recorded 61 multistation Draconid echoes and 179 single-station overdense Draconid echoes (covering the magnitude range of +3 ≤ M
V
≤ +7) between 16 and 20 h ut on 2011 October 8. The mean radiant for the outburst was determined to be αg = 261
9 ± 0
3, δg = +55
3 ± 0
3 (J2000) from observations of the underdense multistation echoes. This radiant location agrees with model predictions to ∼1°. The determined geocentric velocity was found to be ∼10-15 per cent lower than the model value (17.0-19.1 km s−1 versus 20.4 km s−1), a discrepancy we attribute to undercorrection for atmospheric deceleration of low-density Draconid meteoroids as well as to poor radar radiant geometry during the outburst peak. The mass index at the time of the outburst was determined to be ∼1.75 using the amplitude distribution of underdense echoes, in general agreement with the value of ∼1.72 found using the diffusion-limited durations of overdense Draconid echoes. The relative flux derived from overdense echo counts showed a similar variation to the meteor rate derived from visual observations. We were unable to measure the peak flux due to the high elevation of the radiant (and hence low elevation of specular Draconid echoes). Using the observed speed and electron line density measured by CMOR for all underdense Draconid echoes as a function of height as a constraint, we have applied the ablation model developed by Campbell-Brown & Koschny. From these model comparisons, we find that Draconid meteoroids at radar sizes are consistent with a fixed grain number n
grain = 100 and a variable grain mass m
grain between 2 × 10−8 and 5 × 10−7 kg, with bulk and grain density of 300 and 3000 kg m−3, respectively. One particular Draconid underdense echo displayed well-defined Fresnel amplitude oscillations at four stations. The internal synchronization allowing us to measure absolute length as a function of time by combining the absolute timing offsets between stations. This event showed clear deceleration and modelling suggests that the number of grains for this meteoroid was of the order of 1000 with grain masses between 10−10 and 10−9 kg, and a total mass of 2 × 10−6 kg.
Simultaneous radar and video measurements of meteors were made using the Canadian Meteor Orbit Radar (CMOR) and several Gen-III image-intensified CCD cameras primarily to relate radar meteor electron ...line density, q, to video meteor photon radiant power, I. We find that log10q=log10I+(12.56±0.49) leading to M=(38.7±1.2)−2.5log10q, where M is the meteor magnitude in the Gen-III video bandpass (470–850nm) corresponding to q at the radar specular point. The ratio of the ionisation coefficient to luminous efficiency, β/τI, was estimated from our observations of q/I to functionally depend on speed and radiant power. For our average meteor photon radiant power of I=64W, we find log10β/τI=(3.00±0.62)log10v−(4.27±1.37). By adopting β computed according to Jones (1997), which we approximate as log10β=5.84−0.09v0.5−9.56/log10v (roughly proportional to v4 between 20 and 40km/s), a corresponding estimate of τI for our intensified spectral bandpass was made using our measurements of q/I. We find a peak bolometric value of τI=5.9% at 41km/s. The main uncertainties associated with our analysis are the unknown spectra of individual meteors which affect our estimate of absolute radiant power, and uncertain values of the initial trail radius which makes estimates of q problematic. Our results suggest that the video meteor mass scale is an order of magnitude smaller than previously thought at these higher speeds, and implies that the total meteoroid mass influx between 10−5 and 10−8kg is lower than previous studies would suggest.
•We made simultaneous radar and video meteor observations.•Electron line density and photon radiant power have the same dependence on speed.•The ratio of ionisation coefficient to luminous efficiency depends on radiant power (and therefore, mass)•We determine the bolometric luminous efficiency for our dataset: a peak of 5.9% at 41km/s.•The total meteoroid mass influx for video sizes (10−5 to 10−8kg) is likely overestimated by others.
The mirror tracking system of the Canadian Automated Meteor Observatory (CAMO) can track meteors in real time, providing an effective angular resolution of 1 arc sec and a temporal resolution of 100 ...frames per second.
We describe the upgraded hardware and give details of the data calibration and reduction pipeline. We investigate the influence of meteor morphology on radiant and velocity measurement precision, and use direct observations of meteoroid fragmentation to constrain their compressive strengths.
On July 21, 2017, CAMO observed a ~4 second meteor on a JFC orbit. It had a shallow entry angle (~8°) and 12 fragments were visible in the narrow-field video. The event was manually reduced and the exact moment of fragmentation was determined. The aerodynamic ram pressure at the moment of fragmentation was used as a proxy for compressive strength, and strengths of an additional 19 fragmenting meteoroids were measured in the same way. The uncertainty in the atmosphere mass density was estimated to be ±25% using NAVGEM-HA data.
We find that meteor trajectory accuracy significantly depends on meteor morphology. The CAMO radiant and initial velocity precision for non-fragmenting meteors with short wakes is ~0. 5′ and 1 m s−1, while that for meteors with fragments or long wakes is similar to non-tracking, moderate field of view optical systems (~5′, 50 m s−1). Measured compressive strengths of 20 fragmenting meteoroids (with less precise radiants due to their morphology) was in the range of 1–4 kPa, which is in excellent accord with Rosetta in-situ measurements of 67P. Fragmentation type and strength do not appear to be dependent on orbit. The mass index of the 12 fragments in the July 21 meteoroid was very high (s = 2.8), indicating possible progressive fragmentation.
•The Canadian Automated Meteor Observatory (CAMO) mirror tracking system is presented.•Data calibration and reduction methods are described in detail using example meteors.•Meteor radiant and velocity measurement accuracy depends on the meteor morphology.•Compressive strenghts of 20 meteors are measured by directly observing fragmentation.•Measured strengths are 1–4 kPa, in agreement with in-situ measurements by Philae.
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