CHARLES UNIVERSITY FACULTY OF SOCIAL SCIENCES Institute of Political Studies Department of International Security Studies Contemporary Challenges of Space Debris Removal: Overview and Outlook Abstract in English Author: Eliška Vojáková Study programme: Security Studies Supervisor: Mgr. Bohumil Doboš, Ph.D. Year of the defence: 2021 Abstract The sustainability of the outer space environment is necessary for all actors to execute all existing and future human space operations safely. While the severe negative consequences of the uncontrolled space debris population are not new, government agencies and intergovernmental organizations' initiatives to lessen the predicament continue to be insufficient. Scientific research and simulation models show that mere mitigation measures cannot stop the ongoing degradation of the outer space environment polluted from the past space missions. Instead, research supports the development of space projects designed with a primary objective to remove debris from space. National administrations attempt to cooperate at the international level to formulate uniform debris mitigation standards and hold each other mutually accountable for worsening the space debris situation. However, joint public international missions to actively remove debris remain unthinkable. The privatization...

Approved for public release; distribution in unlimited. / The International Space Station (ISS) must be able to withstand the hypervelocity impacts of micrometeoroids and orbital debris that strike its many surfaces. In order to design and implement shielding which will prevent hull penetration or other operational losses, NASA must first model the orbital debris and micrometeoroid environment. Based upon this environment, special multi-stage shields called Whipple and Enhanced Stuffed Whipple Shields are developed and implemented to protect ISS surfaces. Ballistic limit curves that establish shield failure criteria are determined via ground testing. These curves are functions of material strength, shield spacing, projectile size, shape and density, as well as a number of other variables. The combination of debris model and ballistic limit equations allows NASA to model risk to ISS using a hydro-code called BUMPER. This thesis modifies and refines existing ballistic limit equations for U.S. Laboratory Module shields to account for the effects of projectile (debris/ micro-meteoroid) densities. Using these refined ballistic limit equations this thesis also examines alternative shielding materials and configurations to optimize shield design for minimum mass and maximum stopping potential, proposing alternate shield designs for future NASA ground testing. A final goal of this thesis is to provide the Department of Defense a background in satellite shield theory and design in order to improve protection against micrometeoroid and orbital debris impacts on future spacebased national systems. / Lieutenant, United States Navy

A instrumentação do canal radicular possui como conseqüência a formação de debris dentinários, os quais acumulam-se especialmente nas áreas mais complexas do sistema de canais radiculares. Por este motivo, atenção especial tem sido concedida aos métodos de agitação da substâ ncia química auxiliar, com o intuito de melhorar a sua eficácia química e mecânica. No entanto, não está claro se debris dentinários removidos do interior do canal radicular possuem efeito na qualidade da obturação. Portanto, os objetivos deste estudo foram, com o auxílio da microtomografia computadorizada (micro-CT): avaliar e quantificar a presença de debris dentinários em canais curvos, após o preparo químico-cirúrgico; comparar a remoção desses debris, após uso de Irrigação Ultrassônica Passiva (IUP) ou do sistema EndoVac; e avaliar a qualidade da obturação dos mesmos canais, mensurando o volume de material obturador e de espaços vazios. Vinte e quatro raízes mesiais de molares inferiores humanos extraídos foram divididos aleatoriamente em 2 grupos (n=12), de acordo com o método de irrigação final a ser avaliado. Cada espécime foi submetido a quatro escaneamentos: exame préoperatório, exame pós-preparo, exame pós-irrigação final e exame pós-obturação, com um microtomógrafo de raios-X SkyScan 1176, a uma resolução de 17,42 m. Após preparo químico-cirúrgico os grupos receberam irrigação final com IUP ou com o sistema EndoVac e em seguida obturados com a técnica termoplástica de ondas contínuas de condensação. Após a reconstrução das imagens resultantes dos quatro escaneamentos, o corregistro das mesmas foi realizado com o programa DataViewer. Os programas CTAn e CTvol foram utilizados para binarização dos objetos de interesse, análise volumétrica e reconstrução de modelos 3D do canal radicular, dos debris dentinários e do material obturador. Os dados foram analisados estatisticamente com o programa GraphPad Prism por meio do teste t de Student (p < 0,05). As análises de micro-CT revelaram debris dentinários acumulados no interior dos canais radiculares, ocupando uma porcentagem média em relação ao volume do canal de 2,71% para o grupo IUP e 4,10% para o grupo EndoVac. Não houve diferença entre os grupos quanto à remoção de debris dentinários, nem em relação ao volume de material obturador e de espaços vazios (p > 0,05). Concluiu-se que a IUP e o sistema EndoVac foram igualmente eficientes na remoção de debris dentinários, reduzindo em média 50% do seu volume. Também, a ativação dos irrigantes pelos dois métodos não interferiu na qualidade da obturação do canal radicular. / Root canal preparation has as a consequence the formation of hard-tissue debris, which accumulates specially in the complex areas of the root canal system. For this reason, special attention has been given to methods of agitation of the irrigants, aiming to improve its chemical and mechanical efficiency. However, it is not clear if dentin debris removed from the root canal have impact on the quality of root canal filling. Therefore, the objectives of this study were, with the aid of microcomputed tomography (micro-CT): evaluate and quantify the presence of hard-tissue debris in curved root canals, after chemical-surgical preparation; compare the removal of such debris after final irrigation with Passive Ultrasonic Irrigation (PUI) or with the EndoVac system; and evaluate the quality of filling of the same root canals, measuring the volume of filling material and voids. Twenty-four mesial roots of extracted mandibular human molars were randomly divided into 2 groups (n = 12) according to the method of final irrigation being evaluated. Each specimen was subjected to four scans: preoperative, post-operative, post-final irrigation and post-filling, with a microcomputed tomography SkyScan 1176, at a resolution of 17,42 m. After chemosurgical preparation groups received final irrigation with Passive Ultrasonic Irrigation (PUI) or with EndoVac system and were then filled with the continuous wave condensation thermoplastic technique. After the reconstruction of the images resulted from the four scans, co-registration was performed with the DataViewer software. CTAn and CTvol softwares were used for binarization of the objects of interest, volumetric analysis and reconstruction of 3D models of the root canal, hardtissue debris and the filling material. Data were statistically analyzed with the GraphPad Prism program through t students test (p < 0,05). The analysis of micro- CT showed hard-tissue debris accumulated inside the root canal, occupying an average percentage of 2.71 % for the PUI group and 4.10 % for EndoVac group. There was no difference between the groups regarding the removal of hard-tissue debris, or in relation to the volume of filling material and voids (p > 0.05). It was concluded that the EndoVac and PUI were equally efficient in the removal of hardtissue debris, reducing an average of 50% of its volume. Also, irrigants activation with the two methods did not affect the quality of root canal filling.

An increase in the number of debris-covered glaciers and expansion of debris cover across many glaciers has been documented in many of the world’s major glacierised mountain ranges over the last 100 years. Debris cover has a profound impact on glacier mass balance with thick layers insulating the underlying ice and dramatically reducing ablation, while thin or patchy cover accelerates ablation through albedo reduction. Few debris-covered glaciers have been studied in comparison with ‘clean’ glaciers and their response to climatic change is uncertain. Remote sensing, integrated with field data, offers a powerful but as yet unrealised tool for studying and monitoring changes in debris-covered glaciers. Hence, this thesis focuses on two key aims: i) to test the utility of visible/near infrared satellite sensors, such as TERRA ASTER, for studying debris-covered glaciers; ii) to develop techniques to fully exploit the capability of these satellite sensors to extract useful information, and monitor changes over time. Research was focused on four interrelated studies at the Miage Glacier, in the Italian Alps. First, a new method of extracting debris-thickness patterns from ASTER thermal-band imagery was developed, based on a physical energy-balance model for a debris surface. The method was found to be more accurate than previous empirical approaches, when compared with field thickness measurements, and has the potential advantage of transferability to other sites. The high spatial variability of 2 m air temperature, which does not conform to a standard lapse rate, presents a difficulty for this approach and was identified as an important area for future research. Secondly, ASTER and Landsat TM data are used to map debris-cover extent and its change over time using several different methods. A number of problems were encountered in mapping debris extent including cloud cover and snow confusion, spatial resolution, and identifying the boundary between continuous and sporadic debris. Analysis of two images in late summer 1990 and 2004 revealed only a small up glacier increase in debris cover has occurred, confirming other work’s conclusions that the debris cover on Miage Glacier increased to its present extent prior to the 1990s. A third area of research used ASTER DEMs to monitor surface elevation changes of the Miage Glacier over time to update previous studies. Surface velocities on the glacier tongue were also calculated between 2004-2005 using feature-tracking of ASTER orthorectified visible band imagery and ASTER DEMs. However, ASTER DEMs were found to be rather poor for both applications due to large elevation errors in topographically rough parts of the glacier, which prevented a full analysis and comparison of results to previous surface elevation and velocity studies. Finally, the lithological units of the debris cover were mapped, based on the spectral differences of different rock types in the debris layer, providing information both on the location and concentration of different rock types on the surface. Therefore, the identification in the variation in emissivity throughout the glacier surface can be identified, which in turn has an impact upon calculated surface temperatures and ablation respectively. Overall, this research presents a significant contribution to understanding the impact of a debris layer on an alpine glacier, which is an area of key interest and current focus of many present glaciological studies. Since future glacial monitoring will increasingly have to consider supraglacial debris cover as a common occurrence, due to climate warming impacts of glacial retreat and permafrost melting. This contribution is achieved through the successful application of methods which utilise ASTER data to estimate debris thickness and debris extent, and the lithological mapping of debris cover. Therefore, the potential for incorporating these remote sensing techniques for debris-covered glaciers into current global glacier monitoring programs has been highlighted. However the utility of ASTER derived DEMs for surface elevation change analysis and surface velocity estimations in a study site of steep and varied terrain has been identified as questionable, due to issues of ASTER DEM accuracy in these regions.

Abstract not available

The construction and demolition (C and D) recycling industry creates economic opportunity through business activity; promotes equity through workforce training and partnerships; and helps to conserve natural resources through the reuse, remanufacturing, and recycling of C and D debris. While C and D recycling satisfies traditional local economic development goals, it also addresses broader goals of progressive, and sustainable local economic development. The general planning literature shows an increasing interest in sustainability; however, there have been fewer studies on sustainable local economic development initiatives. This research examines the current state of local economic development agency support for the C and D recycling industry as an economic development strategy. In doing so, the dissertation assembles the array of activities local economic development agencies used to support the industry; identifies distinguishing policy or contextual characteristics of agencies that actively supported the industry from those that did not; and assesses whether the agencies support for C and D recycling fit within the rational planning model. Using data from a national survey of local economic development agencies, the study categorizes the local economic development tools used to support the industry. Results show that a combination of traditional, progressive, and sustainable local economic development tools are adapted and newly created to satisfy the specific needs of this specialized industry. Multiple discriminant analyses identify key characteristics of the agencies that support C and D recycling. These characteristics include previous support for the general recycling industry, the presence and support of environmental enterprise zones and eco-industrial parks, and knowledge of local landfill capacity issues. Overall, the agencies that actively supported C and D recycling engage in activities associated with sustainable local economic development. The descriptive and statistical analyses are combined with the surveys qualitative responses to determine that local economic development agencies do not operate under a strict interpretation of the comprehensive rational planning model in their support of C and D recycling. Instead, local economic development agencies use modified rational and reactive planning strategies in their support of the industry. The study concludes with policy recommendations to increase local economic development agency support for C and D recycling.

Vikram Sarabhai Space Centre,Trivandrum / In the space surrounding the earth there are two major regions where orbital debris causes concern. They are the Low Earth Orbits (LEO) up to about 2000 km, and Geosynchronous Orbits (GEO) at an altitude of around 36000 km. The impact of the debris accumulations are in principle the same in the two regions; nevertheless they require different approaches and solutions, due to the fact that the perturbations in the orbital decay due to atmospheric drag effects predominates in LEO, gravitational forces including earth’s oblateness and luni solar effects dominating in GEO are different in these two regions. In LEO it is generally known that the debris population dominates even the natural meteoroid population for object sizes 1 mm and larger. This thesis focuses the study mainly in the LEO region. Since the first satellite breakup in 1961 up to 01 January 2003 more than 180 spacecraft and rocket bodies have been known to fragment in orbit. The resulting debris fragments constitute nearly 40% of the 9000 or more of the presently tracked and catalogued objects by USSPACECOM. The catalogued fragment count does not include the much more numerous fragments, which are too small to be detected from ground. Hence in order to describe the trackable orbital debris environment, it is important to develop mathematical models to simulate the trackable fragments and later expand it to untrackable objects. Apart from the need to better characterize the orbital debris environment down to sub millimeter particles, there is also a pressing necessity of simulation tools able to model in a realistic way the long term evolution of space debris, to highlight areas, which require further investigations, and to study the actual mitigation effects of space policy measures. The present thesis has provided newer perspectives for five major issues in space debris modeling studies. The issues are (i) breakup modeling, (ii) environment modeling, (iii) evolution of the debris environment, (iv) collision probability analysis and (v) reentry prediction. The Chapter 1 briefly describes an overview of space debris environment and the issues associated with the growing space debris populations. A literature survey of important earlier work carried out regarding the above mentioned five issues are provided in the Chapter 2. The new contributions of the thesis commence from Chapter 3. The Chapter 3 proposes a new breakup model to simulate the creation of debris objects by explosion in LEO named “A Semi Stochastic Environment Modeling for Breakup in LEO” (ASSEMBLE). This model is based on a study of the characteristics of the fragments from on orbit breakups as provided in the TLE sets for the INDIAN PSLV-TES mission spent upper stage breakup. It turned out that based on the physical mechanisms in the breakup process the apogee, perigee heights (limited by the breakup altitude) closely fit suitable Laplace distributions and the eccentricity follows a lognormal distribution. The location parameters of these depend on the orbit of the parent body at the time of breakup and their scale parameters on the intensity of explosion. The distribution of the ballistic coefficient in the catalogue was also found to follow a lognormal distribution. These observations were used to arrive at the proper physical, aerodynamic, and orbital characteristics of the fragments. Subsequently it has been applied as an inverse problem to simulate and further validate it based on some more typical well known historical on orbit fragmentation events. All the simulated results compare quite well with the observations both at the time of breakup and at a later epoch. This model is called semi stochastic in nature since the size and mass characteristics have to be obtained from empirical relations and is capable of simulating the complete scenario of the breakup. A new stochastic environment model of the debris scenario in LEO that is simple and impressionistic in nature named SIMPLE is proposed in Chapter 4. Firstly among the orbital debris, the distribution of the orbital elements namely altitude, perigee height, eccentricity and the ballistic coefficient values for TLE sets of data in each of the years were analyzed to arrive at their characteristic probability distributions. It is observed that the altitude distribution for the number of fragments exhibits peaks and it turned out that such a feature can be best modeled with a tertiary mixture of Laplace distributions with eight parameters. It was noticed that no statistically significant variations could be observed for the parameters across the years. Hence it is concluded that the probability density function of the altitude distribution of the debris objects has some kind of equilibrium and it follows a three component mixture of Laplace distributions. For the eccentricity ‘e’ and the ballistic parameter ‘B’ values the present analysis showed that they could be acceptably quite well fitted by Lognormal distributions with two parameters. In the case of eccentricity also the describing parameter values do not vary much across the years. But for the parameters of the B distribution there is some trend across the years which perhaps may be attributed to causes such as decay effect, miniaturization of space systems and even the uncertainty in the measurement data of B. However in the absence of definitive cause that can be attributed for the variation across the years, it turns out to be best to have the most recent value as the model value. Lastly the same kind of analysis has also been carried out with respect to the various inclination bands. Here the orbital parameters are analyzed with respect to the inclination bands as is done in ORDEM (Kessler et al 1997, Liou et al 2001) for near circular orbits in LEO. The five inclination bands considered here are 0-36 deg (in ORDEM they consider 19-36 deg, and did not consider 0-19 deg), 36-61 deg, 61-73 deg, 73-91 deg and 91- 180 deg, and corresponding to each band, the altitude, eccentricity and B values were modeled. It is found that the third band shows the models with single Laplace distribution for altitude and Lognormal for eccentricity and B fit quite well. The altitude of other bands is modeled using tertiary mixture of Laplace distributions, with the ‘e’ and ‘B’ following once again a Lognormal distribution. The number of parameter values in SIMPLE is, in general, just 8 for each description of altitude or perigee distributions whereas in ORDEM96 it is more. The present SIMPLE model captures closely all the peak densities without losing the accuracy at other altitudes. The Chapter 5 treats the evolution of the debris objects generated by on orbit breakup. A novel innovative approach based on the propagation of an equivalent fragment in a three dimensional bin of semi major axis, eccentricity, and the ballistic coefficient (a, e, B) together with a constant gain Kalman filter technique is described in this chapter. This new approach propagates the number density in a bin of ‘a’ and ‘e’ rapidly and accurately without propagating each and every of the space debris objects in the above bin. It is able to assimilate the information from other breakups as well with the passage of time. Further this approach expands the scenario to provide suitable equivalent ballistic coefficient values for the conglomeration of the fragments in the various bins. The heart of the technique is to use a constant Kalman gain filter, which is optimal to track the dynamically evolving fragment scenario and further expand the scenario to provide time varying equivalent ballistic coefficients for the various bins. In the next chapter 6 a new approach for the collision probability assessment utilizing the closed form solution of Wiesel (1989) by the way of a three dimensional look up table, which takes only air drag effect and an exponential model of the atmosphere, is presented. This approach can serve as a reference collision probability assessment tool for LEO debris cloud environment. This approach takes into account the dynamical behavior of the debris objects propagation and the model utilizes a simple propagation for quick assessment of collision probability. This chapter also brings out a comparison of presently available collision probability assessment algorithms based on their complexities, application areas and sample space on which they operate. Further the quantitative assessment of the collision probability estimates between different presently available methods is carried out and the obtained collision probabilities are match qualitatively. The Chapter 7 utilizes once again the efficient and robust constant Kalman gain filter approach that is able to handle the many uncertain, variable, and complex features existing in the scenario to predict the reentry time of the risk objects. The constant gain obtained by using only a simple orbit propagator by considering drag alone is capable of handling the other modeling errors in a real life situation. A detailed validation of the approach was carried out based on a few recently reentered objects and comparison of the results with the predictions of other agencies during IADC reentry campaigns are also presented. The final Chapter 8 provides the conclusions based on the present work carried together with suggestions for future efforts needed in the study of space debris. Also the application of the techniques evolved in the present work to other areas such as atmospheric data assimilation and forecasting have also been suggested.

No description available.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 107-117). Also available on the World Wide Web.

Les Hautes Montagnes d’Asie (HMA) abritent la plus grande superficie de glaciers en dehors des régions polaires. Environ 15 % des ~100 000 km² de glaciers des HMA sont couverts de débris d’épaisseur variable. L’influence de cette couverture détritique sur la réponse des glaciers au changement climatique reste méconnue. Au-delà d’une épaisseur critique (quelques cm), les débris protègent les glaciers de la fonte par effet isolant. Mais ces glaciers présentent des structures qui pourraient sensiblement accentuer leur fonte : en surface ce sont les falaises où la glace est à nue et les lacs supra-glaciaires, alors qu’au cœur des glaciers c’est leur réseau hydrologique intra-glaciaire complexe. L’objectif de cette thèse est d’évaluer l’influence de la couverture détritique sur le bilan de masse des glaciers des HMA. Jusqu’à présent, cette influence a été évaluée à partir de changements de longueurs ou sur des échantillons de glaciers restreints, et aucune étude n’a quantifié l’influence de la couverture détritique sur le bilan de masse des glaciers à grande échelle.Nous avons d’abord traité plus de 50 000 couples stéréoscopiques du capteur ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) pour dériver des modèles numériques de terrain (MNTs) sur la quasi-totalité des glaciers des HMA. Nous avons mesuré ainsi le bilan de masse régional entre 2000 et 2016 avec une résolution jamais atteinte auparavant. La perte totale est de 16.3 ± 3.5 Gt a-1 soit un bilan de masse moyen de -0.18 ± 0.04 m équivalent (éq.) eau a-1, très variable spatialement, avec une perte de masse record pour le Nyainqentanglha (-0.62 ± 0.23 m éq. eau a-1) et un léger gain pour le Kunlun (+0.14 ± 0.08 m éq. eau a-1).Cette variabilité spatiale des bilans de masse reflète au premier ordre la variabilité des climats, différents d’un bout à l’autre des HMA. Pour s’en affranchir, nous avons découpé cette région en 12 sous-régions supposées homogènes climatiquement, où nous avons étudié l’influence de la couverture détritique sur le bilan de masse des glaciers de plus de 2 km² (>6 500 glaciers soit 54 % de la surface englacée totale). Statistiquement, la couverture de débris n’est pas un bon prédicteur du bilan de masse. Dans quatre sous-régions, les glaciers couverts ont des bilans de masse plus négatifs que les glaciers blancs, c’est l’opposé dans le Tien Shan alors que pour les sept sous-régions restantes, les bilans ne sont pas différents statistiquement entre glaciers blancs et couverts. Souvent, la couverture détritique a une influence plus faible que la pente de la langue ou l’altitude moyenne du glacier, car les langues couvertes de débris descendent plus bas en altitude, là où l’ablation est la plus forte.Ce type d’étude statistique est intéressant pour se forger une intuition, mais reste peu informatif en termes de compréhension des processus glaciologiques. Pour mieux contraindre les contributions des processus responsables de la fonte, nous avons travaillé en parallèle à une échelle plus fine en nous intéressant au glacier du Changri Nup (2.7 km²) situé non loin de l’Everest au Népal. A partir de MNTs haute résolution dérivés d’images des satellites Pléiades ou acquises avec un drone, nous avons montré que les falaises de glace, bien qu’elles n’occupent que 7 à 8 % de la surface de la langue de ce glacier, ont contribué à ~23 ± 5 % de l’ablation nette totale au cours de deux années contrastées. Ces falaises sont donc des zones d’ablation préférentielle mais couvrent des surfaces trop faibles pour compenser la réduction d’ablation induite par la couverture détritique environnante. Si l’on observe des taux d’amincissement similaires sur les langues couvertes ou non de débris, c’est que la vitesse d’émergence est plus faible sur les langues couvertes ce qui compense un bilan de masse de surface moins négatif que sur les glaciers blancs. Il est néanmoins nécessaire de mieux comprendre la dynamique des langues couvertes de débris. / High Mountain Asia (HMA) hosts the largest glacierized area outside the polar regions. Approximately 15 % of the ~100 000 km² of HMA glaciers is covered by a debris layer of various thickness. The influence of this debris on the HMA glacier response to climate change remains debated. In principle, the presence of a thick layer of debris reduces the melt of the ice beneath it, due to the insulating effect. However, other processes such as ablation of bare ice cliff faces, subaqueous melt of supraglacial ponds and internal ablation due to englacial hydrology could substantially contribute to enhance the debris-covered glacier mass losses. The aim of this PhD work is to assess the impact of the debris on glacier mass balance in HMA. Up to now, the influence of the debris cover has been assessed through glacier front position changes or on a restricted sample of glaciers, and no large scale study of the influence of the debris cover on the glacier-wide mass balance is available.As a starting point, we derived glacier mass changes for the period 2000-2016 for the entire HMA, with an unprecedented resolution, using time series of digital elevation models (DEMs) derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) optical satellite imagery. We calculated a total mass loss of -16.3 ± 3.5 Gt yr-1 (-0.18 ± 0.04 m w.e. yr-1) with contrasted rates of regionally-averaged mass changes ranging from -0.62 ± 0.23 m w.e. yr-1 for the eastern Nyainqentanglha to +0.14 ± 0.08 m w.e. yr-1 for the western Kunlun Shan.At the scale of HMA, the pattern of glacier mass changes is not related to the presence of debris, but is linked with the climatology. Consequently, we studied the influence of the debris-cover on mass balance within climatically homogeneous regions. Based on the mass balances of individual glaciers larger than 2 km² (more than 6 500 glaciers, which represent 54% of the total glacierized area), we found that debris-covered glaciers have significantly more negative mass balances for four regions out of twelve, a significantly less negative mass balance for one region and non-significantly different mass balances for the remaining seven regions. The debris-cover is generally a less significant predictor of the mass balance than the slope of the glacier tongue or the glacier mean elevation. The influence of the debris is not completely clear and complicated to untangle from the effect of the other morphological parameters, because heavily debris-covered tongues are situated at lower elevations than debris-free tongues, where ablation is higher.However, such a statistical analysis of the influence of the debris-cover on the glacier-wide mass balance variability is not very informative in terms of glaciological processes. In order to better constrain the contribution of the different ablation processes on debris-covered tongues, work at a finer scale is required. For the debris-covered tongue of Changri Nup Glacier, Everest region, Nepal, we quantified the contribution of ice cliffs to the ablation budget. Using a combination of very high resolution DEMs derived from Pléiades images and an unmanned aerial vehicle, we found that ice cliffs contributed to ~23 ± 5 % of the total net ablation of the tongue, over two contrasted years, although they occupy only 7 to 8 % of its area. Ice cliffs are large contributors to the ablation of a debris-covered tongue, but they cannot alone explain the so-called debris cover anomaly, i.e. the fact that debris free and debris covered tongues have similar thinning rates. This anomaly is probably due to smaller emergence velocity over debris-covered tongues than over debris-free tongues, resulting in similar thinning rates, despite less negative surface mass balance rates. We advocate for more measurements of ice thickness of debris-covered tongues in order to better understand their dynamics.