Final Landing Site Research Articles

Optimal guidance for lunar soft landing with dynamic low-resolution image sequences

To relieve the burden of obtaining high-precision lunar surface information during lunar soft landing, a new guidance algorithm based on dynamic low-resolution image sequences is proposed. The integrated procedure of dynamic descent stage considering obstacle-avoiding requirement without the hover measurement is designed, which enables the spacecraft to optimize the final landing site by rolling in orbit. A small number of points are used to obtain the variance, slope and roughness of the field of vision subarea, and the shortest distance spiral search algorithm is used to determine the safest landing point. In order to reduce unnecessary control, an acceptance domain algorithm of alternative landing sites is designed to judge whether or not to accept the change of landing sites in dynamic low-precision image sequences. Assuming that the control acceleration is adjustable, the non-singular fast terminal sliding mode (NFTSM) controller based on terminal attractor is adopted and improved, and the stability of the closed-loop system is also proved. The validity, reliability and effectiveness of the proposed guidance algorithm are verified by numerical simulation on the case with disturbance and large sample data. The results demonstrate that the image information required for stable lunar soft landing is far less than that of the traditional methods, which can greatly reduce the burden of the onboard computer.

Mineralogical and Spectral (Near-Infrared) Characterization of Fe-Rich Vermiculite-Bearing Terrestrial Deposits and Constraints for Mineralogy of Oxia Planum, ExoMars 2022 Landing Site.

Oxia Planum is a Noachian plain on Mars. It was chosen as the final landing site for in situ studies by ExoMars 2022 rover. The main scientific objectives of the mission are to understand the mineralogy and aqueous evolution of ancient Mars with relevance to habitability. Oxia is covered by vast deposits of Fe,Mg-phyllosilicates, but the exact nature of these deposits is not yet fully understood. We performed a survey of potential terrestrial analog rocks, and here we show combined mineralogical characterization of these rocks with their near-infrared spectral analysis. Samples from two terrestrial sites were studied: (1) vermiculitized chlorite-schists from Otago, New Zealand, which underwent an alteration process without significant oxidation; and (2) basaltic tuffs from Granby, Massachusetts, USA, with Fe-rich clays filling amygdales of supposedly hydrothermal origin. Both analogues are incorporated into the newly built Planetary Terrestrial Analogue Library (PTAL) collection. Oxia bedrock clay-rich deposits are spectrally matched best by a well-crystallized trioctahedral vermiculite/saponite mixture from the basaltic tuff, although the contribution of saponite must be minor. Otago vermiculite is a good analogue to Oxia vermiculite in terms of overall mineralogy and Fe content. However, spectral inconsistencies related to the Al content in the Otago clays indicate that illitization of vermiculite, which results from postalteration oxidation, did not occur at Oxia. This implies limited water/rock interactions and reducing conditions during deposition of sediments now constituting the bedrock at Oxia. Whereas the spectral match does not conclusively imply the mineralogy, trioctahedral vermiculite should be considered a likely mineral component of the bedrock unit at Oxia Planum. Vermiculite has great potential to store organic matter, and the postdeposition geological context of Oxia Planum derived from understanding of environmental conditions in analog sites is promising for organic matter preservation.

Thermal analysis and constraints for the MASCOT landing site selection on the Asteroid Ryugu

In June 2018, after 4 years of cruise, the Japanese space probe Hayabusa2 [1-Watanabe S. et al.: Hayabusa2 Mission Overview. (2017)] reached the Near-Earth Asteroid (162173) Ryugu. Hayabusa2 carried a small Lander named MASCOT (Mobile Asteroid Surface Scout) [2-Ho T. M. et al.: MASCOT-The Mobile Asteroid Surface Scout on-board the Hayabusa2 mission. (2017)], jointly developed by the German Aerospace Center (DLR) and the French Space Agency (CNES), to investigate Ryugu's surface structure, composition and physical properties including its thermal behaviour and magnetization in-situ. The Microgravity User Support Centre (DLR-MUSC) in Cologne was in charge of providing all thermal conditions and constraints necessary for the selection of the final landing site and for the final operations of the Lander MASCOT on the surface of the asteroid Ryugu.This article provides a comprehensive assessment of these thermal conditions and constraints, based on predictions performed with the Thermal Mathematical Model (TMM) of MASCOT using different asteroid surface thermal models, ephemeris data for approach as well as descent and hopping trajectories, the related operation sequences and scenarios and the possible environmental conditions driven by the Hayabusa2 spacecraft.A comparison with the real telemetry data confirms the analysis and provides further information about the asteroid characteristics.

Successive Cyclicity in DPs: Evidence from Mongolian Nominalized Clauses

Successive Cyclicity in DPs: Evidence from Mongolian Nominalized Clauses

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

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.

Thermal inertia and roughness of the nucleus of comet 67P/Churyumov–Gerasimenko from MIRO and VIRTIS observations

Aims.Using data from the Rosetta mission to comet 67P/Churyumov–Gerasimenko, we evaluate the physical properties of the surface and subsurface of the nucleus and derive estimates for the thermal inertia (TI) and roughness in several regions on the largest lobe of the nucleus.Methods.We have developed a thermal model to compute the temperature on the surface and in the uppermost subsurface layers of the nucleus. The model takes heat conduction, self-heating, and shadowing effects into account. To reproduce the brightness temperatures measured by the MIRO instrument, the thermal model is coupled to a radiative transfer model to derive the TI. To reproduce the spatially resolved infrared measurements of the VIRTIS instrument, the thermal model is coupled to a radiance model to derive the TI and surface roughness. These methods are applied to Rosetta data from September 2014.Results.The resulting TI values from both instruments are broadly consistent with each other. From the millimetre channel on MIRO, we determine the TI in the subsurface to be <80 JK−1m−2s−0.5for the Seth, Ash, and Aten regions. The submillimetre channel implies similar results but also suggests that higher values could be possible. A low TI is consistent with other MIRO measurements and in situ data from the MUPUS instrument at the final landing site of Philae. The VIRTIS results give a best-fitting value of 80 JK−1m−2s−0.5and values in the range 40–160 JK−1m−2s−0.5in the same areas. These observations also allow the subpixel scale surface roughness to be estimated and compared to images from the OSIRIS camera. The VIRTIS data imply that there is significant roughness on the infrared scale below the resolution of the available shape model and that, counter-intuitively, visually smooth terrain (centimetre scale) can be rough at small (micrometre–millimetre) scales, and visually rough terrain can be smooth at small scales.

The science process for selecting the landing site for the 2020 Mars rover

The process of identifying the landing site for NASA's Mars 2020 rover began in 2013 by defining threshold mission science criteria related to seeking signs of ancient habitable conditions, searching for biosignatures of past microbial life, assembling a returnable cache of samples for possible future return to Earth, and collecting data for planning eventual human missions to the surface of Mars. Mission engineering constraints on elevation and latitude were used to identify candidate landing sites that addressed the scientific objectives of the mission. However, for the first time these constraints did not have a major influence on the viability of candidate sites and, with the new entry, descent, and landing capabilities included in the baseline mission, the vast majority of sites were evaluated and down-selected on the basis of science merit. More than 30 candidate sites with likely acceptable surface and atmospheric conditions were considered at a series of open workshops in the years leading up to the launch. During that period, iteration between engineering constraints and the evolving relative science potential of candidate sites led to the identification of three final candidate sites: Jezero crater (18.4386°N, 77.5031°E), northeast (NE) Syrtis (17.8899°N,77.1599°E) and Columbia Hills (14.5478°S, 175.6255°E). The final landing site will be selected by NASA's Associate Administrator for the Science Mission Directorate. This paper serves as a record of landing site selection activities related primarily to science, an inventory of the number and variety of sites proposed, and a summary of the science potential of the highest-ranking sites.

Landing Area Selection Based on Closed Environment Avoidance from a Single Image During Optical Coarse Hazard Detection

The success of a landed space exploration depends largely on the final landing site, and the most important factor of landing site selection is the safety of lander, so, hazard detection and avoidance are crucial during asteroid landing. Many approaches have been proposed at present, most of them just detect hazard and select an area that is free of hazard threaten, however, in some cases, the selected site should not be the places that located in closed environment, such as the inner of crater. To tackle the issue, an approach for selecting landing site with closed environment avoidance based on a single image during optical coarse hazard detection was proposed in this paper, the approach was designed under the scheme of Chang’e-3’s landing process. The approach begins with hazard detection based on a proposed binary method. And then, for searching the candidate circular landing areas, the skeletons of areas with no hazard are taken into account, and then, control constraints are considered to select the landing areas that are accessible by the lander. Finally, the final selected circular landing area are chosen by a proposed scoring method, this method combines the factors of circular areas, including radius, the connection among circular areas, circular area’s texture and the cluster relation between circular area’s center and all the hazard. At last, serious of experiments would be conducted to test the performance of the proposed approach.

The pebbles/boulders size distributions on Sais: Rosetta’s final landing site on comet 67P/Churyumov–Gerasimenko

By using the imagery acquired by the Optical, Spectroscopic, and Infrared Remote Imaging System Wide-Angle Camera (OSIRIS WAC), we prepare a high-resolution morphological map of the Rosetta Sais final landing site, characterized by an outcropping consolidated terrain unit, a coarse boulder deposit and a fine particle deposit. Thanks to the 0.014 m resolution images, we derive the pebbles/boulders size-frequency distribution (SFD) of the area in the size range of 0.07–0.70 m. Sais’ SFD is best fitted with a two-segment differential power law: the first segment is in the range 0.07–0.26 m, with an index of −1.7 ± 0.1, while the second is in the range 0.26–0.50 m, with an index of −4.2 +0.4/−0.8. The ‘knee’ of the SFD, located at 0.26 m, is evident both in the coarse and fine deposits. When compared to the Agilkia Rosetta Lander Imaging System images, Sais surface is almost entirely free of the ubiquitous, cm-sized debris blanket observed by Philae. None the less, a similar SFD behaviour of Agilkia, with a steeper distribution above ∼0.3 m, and a flatter trend below that, is observed. The activity evolution of 67P along its orbit provides a coherent scenario of how these deposits were formed. Indeed, different lift pressure values occurring on the two locations and at different heliocentric distances explain the presence of the cm-sized debris blanket on Agilkia observed at 3.0 au inbound. Contrarily, Sais activity after 2.1 au outbound has almost completely eroded the fine deposits fallen during perihelion, resulting in an almost dust-free surface observed at 3.8 au.

Rosetta Lander - Philae: Operations on comet 67P/Churyumov-Gerasimenko, analysis of wake-up activities and final state

The Lander Philae, part of the ESA Rosetta mission successfully landed on comet 67P/Churyumov- Gerasimenko on November 12th, 2014. After several (unplanned) bounces it performed a First Scientific Sequence (FSS), based on the energy stored in its on board batteries. All ten instruments of the payload aboard Philae have been operated at least once. Due to the fact that the final landing site was poorly illuminated, Philae went into hibernation on November 15th. Signals from the Lander were received again in June and July 2015, which indicated multiple awakening episodes of the lander. However, various attempts to re-establish reliable and stable communications links failed.Based on the analysis of the data gained during FSS, and during the contacts in June and July 2015 we draw conclusions on the state of Philae. In addition, images from the OSIRIS camera aboard the Rosetta Orbiter have allowed the identification of the exact position of Philae and its attitude, relative to the local surface terrain. This paper also gives an overview of the implications of Philae results for future engineering comet models, required particularly for the design of in-situ (landing) or sample return missions.Rosetta is an ESA mission with contributions from its member states and NASA.Rosetta's Philae Lander is provided by a consortium led by DLR, MPS, CNES and ASI with additional contributions from Hungary, UK, Finland, Ireland and Austria.

Decay of COSAC and Ptolemy mass spectra at comet 67P/Churyumov-Gerasimenko

The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae is equipped with two gas analyzers: The Cometary Sampling and Composition experiment (COSAC) and the gas chromatograph and mass spectrometer Ptolemy. On 12 to 14 November 2014 both instruments measured the organic composition of the comet nucleus material through seven measurements in sniffing mode during Philae's hopping and at its final landing site Abydos. We compare the temporal evolution of intensities of several ion species identified by both mass spectrometers. For COSAC this is the first analysis of the temporal behaviour of the measured ion species. All ion species showed the highest intensities in the first spectra measured by both instruments about 20 to 30 minutes after Philae's first touchdown at Agilkia, and a decay during the six consecutive measurements at Abydos. Both instruments measured a nearly identical decay of the water peak (m/z 18), and also CO (m/z 28) behaved similarly. In the COSAC measurements the peak at m/z 44 decays much slower than all the other ion species, including the water peak. In particular, the m/z 44 peak decays much slower in the COSAC measurements than in the Ptolemy data. This supports our earlier interpretation that COSAC for the first time analyzed a regolith sample from a cometary nucleus in situ, while Ptolemy measured cometary gas from the ambient coma. The m/z 44 peak measured by COSAC was likely dominated by organic species, whereas the peak measured by Ptolemy was interpreted to be mostly due to $CO_2$. Ion species heavier than m/z 30 tend to decay somewhat slower in the COSAC measurements than in the Ptolemy data, which may be related to differences in the exhaust designs between both instruments.

A Delimiter Phrase Analysis on the Genitive Subject Construction in Korean

This study aims to propose a multi-layered Functional Category analysis for the genitive subject construction (GSC) of Korean, generally adopting the Delimiter Phrase (DelP) analysis for noun phrases suggested in Yoon (1990). Within the TP-CP-NP-DP-DelP analysis for Korean GSCs, it has been claimed that Korean GSCs are licensed by the syntactic complex of V-T(aspectual morpheme(s))-C and a nominal head and the delimiters such as mace, chocha, kkaji, and so on. It has also been proposed that the inherent [+specificity] feature of the genitive subject in a GSC is checked in the specifier position of DelP. Specifically, the genitive subject which is base-generated in TP-CP first moves to the SPEC of DP for case and EPP. It further moves to the final landing site, the SPEC of DelP, for the [+specificity] feature checking. On the basis of the analysis of considerable data, this study could possibly demonstrate a probability that the DelP analysis could provide a reasonable solution to the difficulty explaining Korean GSCs. This study was concluded with some suggestions for future studies on the DelP analysis.

Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.

Dust Impact Monitor (SESAME-DIM) on board Rosetta/Philae: Millimetric particle flux at comet 67P/Churyumov-Gerasimenko

The Philae lander of the Rosetta mission, aimed at the in situ investigation of comet 67P/C-G, was deployed to the surface of the comet nucleus on 12 Nov 2014 at 2.99 AU heliocentric distance. The Dust Impact Monitor (DIM) as part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) on the lander employed piezoelectric detectors to detect the submillimetre- and millimetre-sized dust and ice particles emitted from the nucleus. We determine the upper limit of the ambient flux of particles in the measurement range of DIM based on the measurements performed with the instrument during Philae's descent to its nominal landing site Agilkia at distances of about 22 km, 18 km, and 5 km from the nucleus barycentre and at the final landing site Abydos. The geometric factor of the DIM sensor is calculated assuming an isotropic ambient flux of the submillimetre- and millimetre-sized particles. For the measurement intervals when no particles were detected the maximum true impact rate was calculated by assuming Poisson distribution of the impacts, and it was given as the detection limit at a 95% confidence level. Based on measurements performed with DIM, the upper limit of the flux of particles in the measurement range of the instrument was of the order of $10^{-8}-10^{-7}m^{-2}s^{-1}sr^{-1}$ during descent and $1.6\cdot10^{-9}m^{-2}s^{-1}sr^{-1}$ at Abydos on 13 and 14 Nov 2014. Considering particle speeds below escape velocity, the upper limit for the volume density of particles in the measurement range of DIM was constrained to $10^{-11}m^{-3}-10^{-12}m^{-3}$. Results of the calculations performed with the GIPSI tool on the expected particle fluxes during the descent of Philae were compatible with the non-detection of compact particles by the DIM instrument.

Electrical properties and porosity of the first meter of the nucleus of 67P/Churyumov-Gerasimenko

Context. Comets are primitive objects, remnants of the volatile-rich planetesimals from which the solar system condensed. Knowing their structure and composition is thus crucial for the understanding of our origins. After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko in November 2014, for the first time, the Rosetta mission provided the opportunity to measure the low frequency electrical properties of a cometary mantle with the permittivity probe SESAME-PP (Surface Electric Sounding and Acoustic Monitoring Experiment - Permittivity Probe). Aims. In this paper, we conduct an in-depth analysis of the data from active measurements collected by SESAME-PP at Abydos, which is the final landing site of Philae, to constrain the porosity and, to a lesser extent, the composition of the surface material down to a depth of about 1 meter. Methods. SESAME-PP observations on the surface are then analyzed by comparison with data acquired during the descent toward the nucleus and with numerical simulations that explore different possible attitudes and environments of Philae at Abydos using a method called the Capacity-Influence Matrix Method. Results. Reasonably assuming that the two receiving electrode channels have not drifted with respect to each other during the ten-year journey of the Rosetta probe to the comet, we constrain the dielectric constant of the first meter below the surface at Abydos to be > 2.45 p/m 0.20, which is consistent with a porosity < 50% if the dust phase is analogous to carbonaceous chondrites and < 75% in the case of less primitive ordinary chondrites. This indicates that the near surface of the nucleus of 67P/Churyumov-Gerasimenko nucleus is more compacted than its interior and suggests that it could consist of a sintered dust-ice layer.

Philae locating and science support by robotic vision techniques

The ROLIS, CIVA-P and OSIRIS instruments on-board the Philae lander and the Rosetta orbiter acquired high-resolution images during the lander׳s descent towards the targeted landing site Agilkia, during its unexpected rebounds and at the final landing site Abydos on comet 67P/Churyumov–Gerasimenko. We, exploited these images, using robotic vision techniques, to locate the first touchdown on the surface of the comet nucleus, to reconstruct the lander׳s 3D trajectory during the descent and at the beginning of the first rebound, and to create local digital terrain models and depth maps of Agilkia and Abydos sites. Using the ROLIS close-up images we could also determine the actual movements of the lander between the beginning and the end of the First Science Sequence and we propose a new lander׳s bubble movement command meant to increase the probability for a successful drilling during a hypothetical future Long Term Science phase.

Rosetta Lander – Landing and operations on comet 67P/Churyumov–Gerasimenko

The Rosetta Lander Philae is part of the ESA Rosetta Mission which reached comet 67P/Churyumov–Gerasimenko after a 10 year cruise in August 2014. Since then, Rosetta has been studying both its nucleus and coma with instruments aboard the Orbiter. On November 12th, 2014 the Lander, Philae, was successfully delivered to the surface of the comet and operated for approximately 64h after separation from the mother spacecraft. Since the active cold gas system aboard the Lander as well as the anchoring harpoons did not work, Philae bounced after the first touch-down at the planned landing site “Agilkia”. At the final landing site, “Abydos”, a modified First Scientific Sequence was performed. Due to the unexpectedly low illumination conditions and a lack of anchoring the sequence had to be adapted in order to minimize risk and maximize the scientific output. All ten instruments could be activated at least once, before Philae went into hibernation. In June 2015, the Lander contacted Rosetta again having survived successfully a long hibernation phase.This paper describes the Lander operations around separation, during descent and on the surface of the comet. We also address the partly successful attempts to re-establish contact with the Lander in June/July, when the internal temperature & power received were sufficient for Philae to become active again.

Dust devil track survey at Elysium Planitia, Mars: Implications for the InSight landing sites

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) robotic lander is scheduled to land in Elysium Planitia on Mars in September 2016. InSight will perform the first comprehensive surface-based geophysical investigation including seismic measurements. Knowledge about encounter rates of dust devils with the InSight lander are important for two main reasons: (1) dust devils will affect the scientific measurements, i.e., wind-induced seismic noise, and (2) the power-supply of the InSight lander and instruments is provided by solar arrays and previous landers and rovers on Mars were affected by a steady decline in electrical power output due to atmospheric dust deposition on the solar panels. Long term science operations were only made possible by dust clearing events of the solar arrays caused by wind gusts and dust devils. In this study we analyzed dust devil tracks (DDTs) at the final InSight landing site region in Elysium Planitia. Formation of DDTs is caused by the removal of a layer of dust by passing dust devils, hence in principle the same process as clearing of dust from solar panels. We mapped the number, size (width and length), and orientation of DDTs in repeat observations using High Resolution Imaging Science Experiment (HiRISE) images covering the exact same surface area acquired within a relatively short time span (<90martian days). In total, we analyzed 557 newly formed dust devil tracks in 8 study areas. DDTs are morphologically relatively straight with a low mean sinuosity of 1.03 and only reach maximum widths of 30m. The mean DDT width is 4m, indicating that the dust devil size population is dominated by small dust devils with a diameter <10m. The size–frequency distribution of DDTs follows a −2 power law. The mean lengths of DDTs are 0.62km and 1.23km for complete (tracks which are visible from their start to end point) and incomplete DDTs (tracks running across the HiRISE footprint), respectively. The alignment of DDTs in combination with Mars Climate Database (MCD) predicted wind directions imply that dust devils are moving from SE to NW until early northern autumn with a reversal to NW–SE directions of movement at LS=200° consistent with the seasonal reversal in direction of the Hadley circulation. DDT formation rates vary between 0.002 and 0.08ddtkm−2sol−1. DDT area formation rates using the measured DDT widths, lengths, and formation rates are in the range of 0.0003–0.00006km2km−2sol−1, implying that a given spot on the surface may be cleared of dust only once between ∼3000 and 16,000sols (i.e. every ∼5–24Mars years). Measured DDT formation rates were used to find a scaling factor to the seasonal DDA index, and then integrated over the year to estimate a mean annual DDT formation rate of 0.046ddtkm−2sol−1. This translates into a solar panel clearing recurrence interval estimate of ∼11Mars years using the mean annual DDT formation rate, and the mean DDT width and length from all measured DDTs. Due to several uncertainties this solar panel clearing recurrence interval for the InSight landing should be seen as an upper limit estimate.

CONSERT suggests a change in local properties of 67P/Churyumov-Gerasimenko’s nucleus at depth

After the successful landing of Philae on the nucleus of 67P/Churyumov-Gerasimenko, the Rosetta mission provided the first opportunity of performing measurements with the CONSERT tomographic radar in November 2014. CONSERT data were acquired during this first science sequence. They unambiguously showed that propagation through the smaller lobe of the nucleus was achieved. Aims. While the ultimate objective of the CONSERT radar is to perform the tomography of the nucleus, this paper focuses on the local characterization of the shallow subsurface in the area of Philae’s final landing site, specifically determining the possible presence of a permittivity gradient below the nucleus surface. Methods. A number of electromagnetic simulations were made with a ray-tracing code to parametrically study how the gradient of the dielectric constant in the near-subsurface affects the ability of CONSERT to receive signals. Results. At the 90 MHz frequency of CONSERT, the dielectric constant is a function of porosity, composition, and temperature. The dielectric constant values considered for the study are based on observations made by the other instruments of the Rosetta mission, which indicate a possible near-surface gradient in physical properties and on laboratory measurements made on analog samples. Conclusions. The obtained simulated data clearly show that if the dielectric constant were increasing with depth, it would have prevented the reception of signal at the CONSERT location during the first science sequence. We conclude from our simulations that the dielectric constant most probably decreases with depth.

Dust Impact Monitor (SESAME-DIM) measurements at comet 67P/Churyumov-Gerasimenko

The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by sub-millimeter and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. The DIM sensor measures dynamical data like flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. DIM was operated during three mission phases of Philae at the comet: (1) Before Philae's separation from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected. (2) During Philae's descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus' barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in-situ dust detector. (3) At Philae's final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae, or due to shading by obstacles close to Philae, or both. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately $250\, \mathrm{kg\,m^{-3}}$. The particle could have been lifted off from the comet's surface by sublimating water ice.