scholarly journals Tensile strength of 67P/Churyumov–Gerasimenko nucleus material from overhangs

2018 ◽  
Vol 611 ◽  
pp. A33 ◽  
Author(s):  
N. Attree ◽  
O. Groussin ◽  
L. Jorda ◽  
D. Nébouy ◽  
N. Thomas ◽  
...  
Keyword(s):  
Tensile Strength ◽  
Dynamic Stress ◽  
Small Body ◽  
Shape Model ◽  
Lower Limits ◽  
Comet 67P ◽  
Rubble Pile ◽  
Cometary Activity

We directly measured twenty overhanging cliffs on the surface of comet 67P/Churyumov–Gerasimenko extracted from the latest shape model and estimated the minimum tensile strengths needed to support them against collapse under the comet’s gravity. We find extremely low strengths of around 1 Pa or less (1 to 5 Pa, when scaled to a metre length). The presence of eroded material at the base of most overhangs, as well as the observed collapse of two features andthe implied previous collapse of another, suggests that they are prone to failure and that the true material strengths are close to these lower limits (although we only consider static stresses and not dynamic stress from, for example, cometary activity). Thus, a tensile strength of a few pascals is a good approximation for the tensile strength of the 67P nucleus material, which is in agreement with previous work. We find no particular trends in overhang properties either with size over the ~10–100 m range studied here or location on the nucleus. There are no obvious differences, in terms of strength, height or evidence of collapse, between the populations of overhangs on the two cometary lobes, suggesting that 67P is relatively homogenous in terms of tensile strength. Low material strengths are supportive of cometary formation as a primordial rubble pile or by collisional fragmentation of a small body (tens of km).

scholarly journals Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

2020 ◽  
Vol 6 (41) ◽  
pp. eabc3350 ◽  
Author(s):  
D. J. Scheeres ◽  
A. S. French ◽  
P. Tricarico ◽  
S. R. Chesley ◽  
Y. Takahashi ◽  
...  
Keyword(s):  
Gravity Field ◽  
Mass Distribution ◽  
Small Body ◽  
Parent Body ◽  
Constant Density ◽  
Internal Mass ◽  
Shape Model ◽  
Rapid Rotation ◽  
Insight Into ◽  
Rubble Pile

The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu’s surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu’s equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O’Keefe-Radzievskii-Paddack cycle or arising during Bennu’s accretion following the disruption of its parent body.

scholarly journals Plasma properties of suprathermal electrons near comet 67P/Churyumov-Gerasimenko with Rosetta

2019 ◽  
Vol 630 ◽  
pp. A42 ◽  
Author(s):  
M. Myllys ◽  
P. Henri ◽  
M. Galand ◽  
K. L. Heritier ◽  
N. Gilet ◽  
...  
Keyword(s):  
Solar Wind ◽  
Heliocentric Distance ◽  
Radial Distance ◽  
Hot Electron ◽  
Plasma Properties ◽  
Measured Velocity ◽  
Electron Population ◽  
Suprathermal Electrons ◽  
Comet 67P ◽  
Cometary Activity

Context. The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko from 2014 to September 2016. The mission provided in situ observations of the cometary plasma during different phases of the cometary activity, which enabled us to better understand its evolution as a function of heliocentric distance. Aims. In this study, different electron populations, called warm and hot, observed by the Ion and Electron Sensor (IES) of the Rosetta Plasma Consortium (RPC) are investigated near the comet during the escorting phase of the Rosetta mission. Methods. The estimates for the suprathermal electron densities and temperatures were extracted using IES electron data by fitting a double-kappa function to the measured velocity distributions. The fitting results were validated using observations from other RPC instruments. We give upgraded estimates for the warm and hot population densities compared to values previously shown in literature. Results. The fitted density and temperature estimates for both electron populations seen by IES are expressed as a function of heliocentric distance to study their evolution with the cometary activity. In addition, we studied the dependence between the electron properties and cometocentric distance. Conclusions. We observed that when the neutral outgassing rate of the nucleus is high (i.e., near perihelion) the suprathermal electrons are well characterized by a double-kappa distribution. In addition, warm and hot populations show a significant dependence with the heliocentric distance. The populations become clearly denser near perihelion while their temperatures are observed to remain almost constant. Moreover, the warm electron population density is shown to be strongly dependent on the radial distance from the comet. Finally, based on our results we reject the hypothesis that hot electron population seen by IES consists of solely suprathermal (halo) solar wind electrons, while we suggest that the hot electron population mainly consists of solar wind thermal electrons that have undergone acceleration near the comet.

scholarly journals Spectrophotometric characterization of the Philae landing site and surroundings with the Rosetta/OSIRIS cameras

2020 ◽  
Vol 498 (1) ◽  
pp. 1221-1238
Author(s):  
Hong Van Hoang ◽  
S Fornasier ◽  
E Quirico ◽  
P H Hasselmann ◽  
M A Barucci ◽  
...  
Keyword(s):  
Total Mass ◽  
Morphological Changes ◽  
Landing Site ◽  
Spectral Slope ◽  
Water Ice ◽  
Total Mass Loss ◽  
Before And After ◽  
Comet 67P ◽  
Cometary Activity

ABSTRACT We investigate Abydos, the final landing site of the Philae lander after its eventful landing from the Rosetta spacecraft on comet 67P/Churyumov–Gerasimenko on 2014 November 12. Over 1000 OSIRIS-level 3B images were analysed, which cover the 2014 August–2016 September timeframe, with spatial resolution ranging from 7.6 m pixel−1 to approximately 0.06 m pixel−1. We found that the Abydos site is as dark as the global 67P nucleus and spectrally red, with an average albedo of 6.5 per cent at 649 nm and a spectral slope value of about 17 per cent/(100 nm) at 50° phase angle. Similar to the whole nucleus, the Abydos site also shows phase reddening but with lower coefficients than other regions of the comet, which may imply a thinner cover of microscopically rough regolith compared to other areas. Seasonal variations, as already noticed for the whole nucleus, were also observed. We identified some potential morphological changes near the landing site implying a total mass-loss of (4.7–7.0) × 105 kg. Small spots ranging from 0.1 to 27 m2 were observed close to Abydos before and after perihelion. Their estimated water ice abundance reaches 30–40 per cent locally, indicating fresh exposures of volatiles. Their lifetime ranges from a few hours up to three months for two pre-perihelion spots. The Abydos surroundings showed a low level of cometary activity compared to other regions of the nucleus. Only a few jets are reported originating nearby Abydos, including a bright outburst that lasted for about 1 h.

scholarly journals Tidal distortion and disruption of rubble-pile bodies revisited

2020 ◽  
Vol 640 ◽  
pp. A102
Author(s):  
Yun Zhang ◽  
Patrick Michel
Keyword(s):  
Small Body ◽  
Influence Factors ◽  
Dynamical Evolution ◽  
Continuum Theory ◽  
Close Approach ◽  
Material Strength ◽  
Discrete Element Modeling ◽  
Small Bodies ◽  
Tidal Forces ◽  
Rubble Pile

Context. In the course of a close approach to planets or stars, the morphological and dynamical properties of rubble-pile small bodies can be dramatically modified, and some may catastrophically break up, as in the case of comet Shoemaker-Levy 9. This phenomenon is of particular interest for the understanding of the evolution and population of small bodies, and for making predictions regarding the outcomes of future encounters. Previous numerical explorations have typically used methods that do not adequately represent the nature of rubble piles. The encounter outcomes and influence factors are still poorly constrained. Aims. Based on recent advances in modeling rubble-pile physics, we aim to provide a better understanding of the tidal encounter processes of rubble piles through soft-sphere discrete element modeling (SSDEM) and to establish a database of encounter outcomes and their dependencies on encounter conditions and rubble-pile properties. Methods. We performed thousands of numerical simulations using the SSDEM implemented in the N-body code pkdgrav to study the dynamical evolution of rubble piles during close encounters with the Earth. The effects of encounter conditions, material strength, arrangement, and resolution of constituent particles are explored. Results. Three typical tidal encounter outcomes are classified, namely: deformation, mass shedding, and disruption, ranging from mild modifications to severe damages of the progenitor. The outcome is highly dependent on the encounter conditions and on the structure and strength of the involved rubble pile. The encounter speed and distance required for causing disruption events are much smaller than those predicted by previous studies, indicating a smaller creation rate of tidally disrupted small body populations. Extremely elongated fragments with axis ratios ~1:6 can be formed by moderate tidal encounters. Our analyses of the spin-shape evolution of the largest remnants reveal reshaping mechanisms of rubble piles in response to tidal forces, which is consistent with stable rubble-pile configurations derived by continuum theory. A case study for Shoemaker-Levy 9 suggests a low bulk density (0.2–0.3 g cc−1) for its progenitor.

scholarly journals The Rosetta mission orbiter science overview: the comet phase

2017 ◽  
Vol 375 (2097) ◽  
pp. 20160262 ◽  
Author(s):  
M. G. G. T. Taylor ◽  
N. Altobelli ◽  
B. J. Buratti ◽  
M. Choukroun
Keyword(s):  
Remote Sensing ◽  
Solar System ◽  
Cometary Nucleus ◽  
Rosetta Mission ◽  
Interstellar Material ◽  
The Relationship ◽  
Mission Operations ◽  
Comet 67P ◽  
Cometary Activity

The international Rosetta mission was launched in 2004 and consists of the orbiter spacecraft Rosetta and the lander Philae. The aim of the mission is to map the comet 67P/Churyumov–Gerasimenko by remote sensing, and to examine its environment in situ and its evolution in the inner Solar System. Rosetta was the first spacecraft to rendezvous with and orbit a comet, accompanying it as it passes through the inner Solar System, and to deploy a lander, Philae, and perform in situ science on the comet's surface. The primary goals of the mission were to: characterize the comet's nucleus; examine the chemical, mineralogical and isotopic composition of volatiles and refractories; examine the physical properties and interrelation of volatiles and refractories in a cometary nucleus; study the development of cometary activity and the processes in the surface layer of the nucleus and in the coma; detail the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the Solar System; and characterize asteroids 2867 Steins and 21 Lutetia. This paper presents a summary of mission operations and science, focusing on the Rosetta orbiter component of the mission during its comet phase, from early 2014 up to September 2016. This article is part of the themed issue ‘Cometary science after Rosetta’.

scholarly journals Time evolution of dust deposits in the Hapi region of comet 67P/Churyumov-Gerasimenko

2020 ◽  
Vol 636 ◽  
pp. A91
Author(s):  
P. Cambianica ◽  
M. Fulle ◽  
G. Cremonese ◽  
E. Simioni ◽  
G. Naletto ◽  
...  
Keyword(s):  
Time Evolution ◽  
Southern Hemisphere ◽  
Loss Rate ◽  
Dust Layer ◽  
Water Ice ◽  
Narrow Angle ◽  
Dust Deposit ◽  
Erosion And Accretion ◽  
Comet 67P ◽  
Cometary Activity

Aims. We provide a measurement of the seasonal evolution of the dust deposit erosion and accretion in the Hapi region of comet 67P/Churyumov-Gerasimenko with a vertical accuracy of 0.2–0.9 m. Methods. We used OSIRIS Narrow Angle Camera images with a spatial scale of lower than 1.30 m px−1 and developed a tool to monitor the time evolution of 22 boulder heights with respect to the surrounding dust deposit. The tool is based on the measurement of the shadow length projected by the boulder on the surrounding pebble deposit. Assuming the position of the boulders does not change during the observational period, boulder height variations provide an indication of how the thickness of the surrounding dust layer varies over time through erosion and accretion phenomena. Results. We measured an erosion of the dust deposit of 1.7 ± 0.2 m during the inbound orbit until 12 December, 2014. This value nearly balances the fallout from the southern hemisphere during perihelion cometary activity. During the perihelion phase, the dust deposit then increased by 1.4 ± 0.8 m. This is interpreted as a direct measurement of the fallout thickness. By comparing the erosion rate and dust volume loss rate at the Hapi region measured in the coma, the fallout represents ~96% in volume of the ejecta. The amount of the eroded pristine material from the southern hemisphere, together with its subsequent transport and fallout on the nucleus, led us to discuss the pristine water ice abundance in comet 67P. We determine that the refractory-to-ice mass ratio ranges from 6 to 110 in the perihelion-eroded pristine nucleus, providing a pristine ice mass fraction of (8 ± 7)% in mass.

E-Type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta

Science ◽  
2010 ◽  
Vol 327 (5962) ◽  
pp. 190-193 ◽  
Author(s):  
H. U. Keller ◽  
C. Barbieri ◽  
D. Koschny ◽  
P. Lamy ◽  
H. Rickman ◽  
...  
Keyword(s):  
Solid Rock ◽  
Direct Evidence ◽  
Imaging System ◽  
Main Belt ◽  
Rosetta Mission ◽  
Oblate Body ◽  
Spherical Diameter ◽  
Yorp Effect ◽  
Comet 67P ◽  
Rubble Pile

The European Space Agency’s Rosetta mission encountered the main-belt asteroid (2867) Steins while on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Images taken with the OSIRIS (optical, spectroscopic, and infrared remoteimaging system) cameras on board Rosetta show that Steins is an oblate body with an effective spherical diameter of 5.3 kilometers. Its surface does not show color variations. The morphology of Steins is dominated by linear faults and a large 2.1-kilometer-diameter crater near its south pole. Crater counts reveal a distinct lack of small craters. Steins is not solid rock but a rubble pile and has a conical appearance that is probably the result of reshaping due to Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) spin-up. The OSIRIS images constitute direct evidence for the YORP effect on a main-belt asteroid.

scholarly journals Rosetta/OSIRIS observations of the 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry

2019 ◽  
Vol 630 ◽  
pp. A9 ◽  
Author(s):  
C. Feller ◽  
S. Fornasier ◽  
S. Ferrari ◽  
P. H. Hasselmann ◽  
A. Barucci ◽  
...  
Keyword(s):  
High Resolution ◽  
Source Region ◽  
Spectral Slope ◽  
Shape Model ◽  
Water Ice ◽  
Narrow Angle ◽  
Average Reflectance ◽  
The One ◽  
Phase Angles ◽  
Comet 67P

Context. From August 2014 to September 2016, the Rosetta spacecraft followed comet 67P/Churyumov–Gerasimenko along its orbit. After the comet passed perihelion, Rosetta performed a flyby manoeuvre over the Imhotep–Khepry transition in April 2016. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 observations with mainly three broadband filters (centered at 480, 649, and 743 nm) at a resolution of up to 0.53 m/px and for phase angles between 0.095° and 62°. Aims. We have investigated the morphological and spectrophotometrical properties of this area using the OSIRIS/NAC high-resolution observations. Methods. We assembled the observations into coregistered color cubes. Using a 3D shape model, we produced the illumination conditions and georeference for each observation. We mapped the observations of the transition to investigate its geomorphology. Observations were photometrically corrected using the Lommel–Seeliger disk law. Spectrophotometric analyses were performed on the coregistered color cubes. These data were used to estimate the local phase reddening. Results. The Imhotep–Khepry transition hosts numerous and varied types of terrains and features. We observe an association between a feature’s nature, its reflectance, and its spectral slopes. Fine material deposits exhibit an average reflectance and spectral slope, while terrains with diamictons, consolidated material, degraded outcrops, or features such as somber boulders present a lower-than-average reflectance and higher-than-average spectral slope. Bright surfaces present here a spectral behavior consistent with terrains enriched in water-ice. We find a phase-reddening slope of 0.064 ± 0.001%/100 nm/° at 2.7 au outbound, similar to the one obtained at 2.3 au inbound during the February 2015 flyby. Conclusions. Identified as the source region of multiple jets and a host of water-ice material, the Imhotep–Khepry transition appeared in April 2016, close to the frost line, to further harbor several potential locations with exposed water-ice material among its numerous different morphological terrain units.

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