48 research outputs found
Measuring the Terminal Heights of Bolides to Understand the Atmospheric Flight of Large Asteroidal Fragments
The extent of penetration into the Earth's atmosphere of a meteoroid is defined by the point where its kinetic energy is no longer sufficient to produce luminosity. For most of the cases this is the point where the meteoroid disintegrates in the atmosphere due to ablation process and dynamic pressure during flight. However, some of these bodies have particular physical properties (bigger size, higher bulk strength, etc.) or favorable flight conditions (lower entry velocity or/and a convenient trajectory slope, etc.) that allow them to become a meteorite-dropper and reach the ground. In both cases, we define the end of the luminous path of the trajectory as the terminal height or end height. Thus, the end point shows the amount of deceleration till the final braking. We thus assume that the ability of a fireball to produce meteorites is directly related to its terminal height. Previous studies have discussed the likely relationship between fireball atmospheric flight properties and the terminal height. Most of these studies require the knowledge of a set of properties and physical variables which cannot be determined with sufficient accuracy from ground-based observations. The recently validated dimensionless methodology offers a new approach to this problem. All the unknowns can be reduced to only two parameters which are easily derived from observations. Despite the calculation of the analytic solution of the equations of motion is not trivial, some simplifications are admitted. Here, we describe the best performance range and the errors associated with these simplifications. We discuss how terminal heights depend on two or three variables that are easily retrieved from the recordings, provided at least three trajectory (h, v) points. Additionally, we review the importance of terminal heights, and the way they have been estimated in previous studies. Finally we discuss a new approach for calculating terminal heights.Peer reviewe
Physically based alternative to the PE criterion for meteoroids
Meteoroids impacting the Earth atmosphere are commonly classified using the PE criterion. This criterion was introduced to support the identification of the fireball type by empirically linking its orbital origin and composition characteristics. Additionally, it is used as an indicator of the meteoroid tensile strength and its ability to penetrate the atmosphere. However, the level of classification accuracy of the PE criterion depends on the ability to constrain the value of the input data, retrieved from the fireball observation, required to derive the PE value. To overcome these uncertainties and achieve a greater classification detail, we propose a new formulation using scaling laws and dimensionless variables that groups all the input variables into two parameters that are directly obtained from the fireball observations. These two parameters, alpha and beta, represent the drag and the mass-loss rates along the luminous part of the trajectory, respectively, and are linked to the shape, strength, ablation efficiency, mineralogical nature of the projectile, and duration of the fireball. Thus, the new formulation relies on a physical basis. This work shows the mathematical equivalence between the PE criterion and the logarithm of 2 alpha beta under the same PE criterion assumptions. We demonstrate that log(2 alpha beta) offers a more general formulation that does not require any preliminary constraint on the meteor flight scenario and discuss the suitability of the new formulation for expanding the classification beyond fully disintegrating fireballs to larger impactors including meteorite-dropping fireballs. The reliability of the new formulation is validated using the Prairie Network meteor observations.Peer reviewe
Comparing the reflectivity of ungrouped carbonaceous chondrites with that of short period comets like 2P/Encke
Aims. The existence of asteroid complexes produced by the disruption of these
comets suggests that evolved comets could also produce high-strength materials
able to survive as meteorites. We chose as an example comet 2P/Encke, one of
the largest object of the so-called Taurid complex. We compare the reflectance
spectrum of this comet with the laboratory spectra of some Antarctic ungrouped
carbonaceous chondrites to investigate whether some of these meteorites could
be associated with evolved comets. Methods. We compared the spectral behaviour
of 2P/Encke with laboratory spectra of carbonaceous chondrites. Different
specimens of the common carbonaceous chondrite groups do not match the overall
features and slope of comet 2P/Encke. Trying anomalous carbonaceous chondrites,
we found two meteorites, Meteorite Hills 01017 and Grosvenor Mountains 95551,
which could be good proxies for the dark materials forming this short-period
comet. We hypothesise that these two meteorites could be rare surviving
samples, either from the Taurid complex or another compositionally similar
body. In any case, it is difficult to get rid of the effects of terrestrial
weathering in these Antarctic finds, and further studies are needed. Future
sample return from the so-called dormant comets could be also useful to
establish a ground truth on the materials forming evolved short-period comets.
Results. As a natural outcome, we think that identifying good proxies of
2P/Encke-forming materials might have interesting implications for future
sample-return missions to evolved, potentially dormant or extinct comets. To
understand the compositional nature of evolved comets is particularly relevant
in the context of the future mitigation of impact hazard from these dark and
dangerous projectiles.Comment: Accepted for publication in A&A on July 6, 202
The effect of aqueous alteration and metamorphism in the survival of presolar silicate grains in chondrites
Relatively small amounts (typically between 2-200 parts per million) of
presolar grains have been preserved in the matrices of chondritic meteorites.
The measured abundances of the different types of grains are highly variable
from one chondrite to another, but are higher in unequilibrated chondrites that
have experienced little or no aqueous alteration and/or metamorphic heating
than in processed meteorites. A general overview of the abundances measured in
presolar grains (particularly the recently identified presolar silicates)
contained in primitive chondrites is presented. Here we will focus on the most
primitive chondrite groups, as typically the highest measured abundances of
presolar grains occur in primitive chondrites that have experienced little
thermal metamorphism. Looking at the most aqueously altered chondrite groups,
we find a clear pattern of decreasing abundance of presolar silicate grains
with increasing level of aqueous alteration. We conclude that the measured
abundances of presolar grains in altered chondrites are strongly biased by
their peculiar histories. Scales quantifying the intensity of aqueous
alteration and shock metamorphism in chondrites could correlate with the
content in presolar silicates. To do this it would be required to infer the
degree of destruction or homogenization of presolar grains in the matrices of
primitive meteorites. To get an unbiased picture of the relative abundance of
presolar grains in the different regions of the protoplanetary disk where first
meteorites consolidated, future dedicated studies of primitive meteorites,
IDPs, and collected materials from sample-return missions (like e.g. the
planned Marco Polo) are urgently required.Comment: 15 pages, 3 figures, published in PASA as part of the Proceedings of
the 2008 Torino Conference "The Origin of the Elements Heavier than Iron
Multi-instrumental observations of the 2014 Ursid meteor outburst
The Ursid meteor shower is an annual shower that usually shows little activity. However, its Zenith hourly rate sometimes increases, usually either when its parent comet, 8P/Tuttle, is close to its perihelion or its aphelion. Outbursts when the comet is away from perihelion are not common and outbursts when the comet is close to aphelion are extremely rare. The most likely explanation offered to date is based on the orbital mean motion resonances. The study of the aphelion outburst of 2000 December provided a means of testing that hypothesis. A new aphelion outburst was predicted for 2014 December. The SPanish Meteor Network, in collaboration with the French Fireball Recovery and InterPlanetary Observation Network, set up a campaign to monitor this outburst and eventually retrieve orbital data that expand and confirm previous preliminary results and predictions. Despite unfavourable weather conditions over the south of Europe over the relevant time period, precise trajectories from multistation meteor data recorded over Spain were obtained, as well as orbital and radiant information for four Ursid meteors. The membership of these four meteors to the expected dust trails that were to provoke the outburst is discussed, and we characterize the origin of the outburst in the dust trail produced by the comet in the year AD 1392.Peer reviewe
ALMA Observations of the DART Impact: Characterizing the Ejecta at Sub-Millimeter Wavelengths
We report observations of the Didymos-Dimorphos binary asteroid system using
the Atacama Large Millimeter/Submillimeter Array (ALMA) and the Atacama Compact
Array (ACA) in support of the Double Asteroid Redirection Test (DART) mission.
Our observations on UT 2022 September 15 provided a pre-impact baseline and the
first measure of Didymos-Dimorphos' spectral emissivity at mm,
which was consistent with the handful of siliceous and carbonaceous asteroids
measured at millimeter wavelengths. Our post-impact observations were conducted
using four consecutive executions each of ALMA and the ACA spanning from
T3.52 to T8.60 hours post-impact, sampling thermal emission from the
asteroids and the impact ejecta. We scaled our pre-impact baseline measurement
and subtracted it from the post-impact observations to isolate the flux density
of mm-sized grains in the ejecta. Ejecta dust masses were calculated for a
range of materials that may be representative of Dimorphos' S-type asteroid
material. The average ejecta mass over our observations is consistent with
1.3--6.4 kg, with the lower and higher values calculated for
amorphous silicates and for crystalline silicates, respectively. Owing to the
likely crystalline nature of S-type asteroid material, the higher value is
favored. These ejecta masses represent 0.3--1.5\% of Dimorphos' total mass and
are in agreement with lower limits on the ejecta mass based on measurements at
optical wavelengths. Our results provide the most sensitive measure of mm-sized
material in the ejecta and demonstrate the power of ALMA for providing
supporting observations to spaceflight missions