Spectral markers are materials with a characteristic emission spectrum, which are added to a spacecraft structure, e.g., a satellite. During an atmospheric re-entry, the marker is released and its radiation can be detected by appropriate instruments. This allows to identify breakup events and to assess the spacecraft’s demise behavior during an observation campaign. In this paper, we study various compounds to select suitable materials for future integration as spectral markers. Six material candidates were tested in the plasma wind tunnel PWK4 at the Institute of Space Systems at the University of Stuttgart. Powdered material was encased in an epoxy resin matrix and mounted to a water-cooled sample holder. Spectra, temperature, and visual data were recorded throughout testing. Lanthanum acetate, lanthanum oxide, and lanthanum oxalate are considered non-viable marker materials. Lanthanum hexaboride features strong atomic lines of lanthanum as well as molecular bands of LaO and BO in a wide wavelength range within the visible and near-infrared (370–820 nm). Rubidium chloride features very strong atomic lines of Rb in the NIR (780–790 nm), well separated from the atomic radiation of air species. Strontium features moderately strong atomic lines. In conclusion, rubidium chloride and lanthanum hexaboride are considered promising marker materials for re-entry event identification through spectroscopic observation.

Established optical techniques for measuring plasma flow speeds, for example Fabry–Pérot interferometry (FPI), are challenging due to the need for accurate alignment and complex data acquisition and evaluation. As a novel alternative, we propose using an event-based camera to determine velocities in a transparent, particle-free plasma jet by tracing the flow of radiating flow features across the field of view. The potential of this new technique is assessed by studying a fast hydrogen-helium plasma flow in an arcjet-driven facility. Data analysis yields temporally and spatially resolved axial velocities. The determined axial velocity profile qualitatively matches the observed supersonic flow structure and ranges between 8 and 20 km/s. For a given position, flow speeds vary by about ±3 km/s, which is either attributed to measurement uncertainties or to actual variations. The mean values compare well to FPI measurements.

This paper reports the approach to determine the luminous efficiency of spacecraft materials experimentally. The idea is that knowing the luminous efficiency of spacecraft materials allows for deriving mass estimates from observation data linking meteor science methods to man-made artificial meteors. This way, observation data from airborne missions and known ground-based meteor observation networks become a versatile tool to analyze destructive spacecraft entry. Material samples were scaled to a typical re-entry condition. The authors recently developed a method to determine the radiant flux in the passbands U, B, and V and the corresponding color indices. The measured mass loss during the experiment is used to determine the luminous efficiency s for the different passbands. These values are reported for three different materials under flow conditions corresponding to two trajectory points at altitudes of 70 km and 65 km in atypical decaying re-entry orbit. The resulting luminous efficiency values are of the order 10 5 , which is about two orders of magnitude lower than meteoroid efficiencies. The found data is applied to fragments observed during the CYGNUS OA-6 observation campaign. The mass loss was extrapolated for the entire duration of the re-entry resulting in amass of around 68-80% of literature values for the mass at the entry interface. (c) 2024 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

This work assesses how experimentally generated plasma flows can contribute to the simulation of the aerothermal loads during entry flight into the atmosphere of the giant planets. Planetary probe missions entering the giant planets' atmospheres require ground test facilities for the qualification of appropriate heat shield materials. Flight path trajectories are computed with the three degree of freedom ballistic solver of TRAJ6D to determine the expected flow conditions during atmospheric entry of future probes to Uranus. Engineering correlations are used to calculate the expected stagnation point convective heat flux for a set of Uranus entry and aerocapture trajectories. Additionally, CFD computations are performed for the estimated peak heating free stream conditions. The best match with the CFD results is obtained for the Carroll-Brandis correlation. For the trajectories considered within this work, the cold wall heat flux calculated with this correlation ranges between 5.1 MW m(-2) and 53.5 MW m(-2). The arc-jet generator driven plasma wind tunnel facilities PWK1 and PWK4 at the Institute of Space Systems of the University of Stuttgart are successfully operated with hydrogen-helium mixtures to match the atmospheric composition of the giant planets. Stagnation point cold wall heat fluxes of up to 9.93 MW m(-2) and Pitot pressures of up to 807 Pa are measured for operation with 90% hydrogen and 10% helium by volume. First estimations indicate that the local mass-specific enthalpy is as high as 472 MJ kg(-1). In the facility PWK4, the local mass-specific enthalpy and cold wall stagnation point heat flux is simultaneously matched to the predictions for a Uranus aerocapture peak heating scenario.

We provide an overview of the MetSpec project, which aims to connect meteorite ablation laboratory experiments with meteor spectral observations in the atmosphere aiming at the development of a methodology to identify incoming planetary material distribution into the Earth’s atmosphere. We have selected 28 meteorites of different types to represent known planetary material compositions coming from asteroids, Vesta, Mars and the Moon. Some samples have been tested twice which resulted in overall 31 experiments. Three distinct test campaigns were realized in 2020, 2021 and 2022 with the High Enthalpy Flow Diagnostics Group in the Plasma Wind Tunnel PWK1 where they have developed a unique testing scenario. During the last and most elaborated campaign, 16 cameras observed the artificial meteors in the laboratory. Besides videos and online live streaming, instruments included several spectrometers, and optical and imaging instruments covering UV, visible and IR spectral range. This special collection in Icarus collects the resulting output from the different instruments and results. This overview article provides an introduction and summarizes the main findings of the experimental campaigns.

Emission spectra and diagnostic spectral features of a diverse range of ablated meteorite samples with a known composition are presented. We aim to provide a reference spectral dataset to improve our abilities to classify meteoroid composition types from meteor spectra observations. The data were obtained by ablating meteorite samples in high-enthalpy plasma wind tunnel facilities recreating conditions characteristic of low-speed meteors. Near-UV to visible-range (320-800 nm) emission spectra of 22 diverse meteorites captured by a high-resolution Echelle spectrometer were analyzed to identify the characteristic spectral features of individual meteorite groups. The same dataset captured by a lower-resolution meteor spectrograph was applied to compare the meteorite data with meteor spectra observations. Spectral modeling revealed that the emitting meteorite plasma was characterized by temperatures of 3700-4800 K, similar to the main temperature component of meteors. The studied line intensity variations were found to trace the differences in the original meteorite composition and thus can be used to constrain the individual meteorite classes. We demonstrate that meteorite composition types, including ordinary chondrites, carbonaceous chondrites, various achondrites, stony-iron and iron meteorites, can be spectrally distinguished by measuring relative line intensities of Mg I, Fe I, Na I, Cr I, Mn I, Si I, H I, CN, Ni I, and Li I. Additionally, we confirm the effect of the incomplete evaporation of refractory elements Al, Ti, and Ca, and the presence of minor species Co I, Cu I, and V I.

Optical emission spectra between 522–580 nm of ablating meteorites have been recorded at frame rates as high as 1 kHz for the first time during ground testing with simultaneous spatial and temporal resolution. A novel high frame rate emission spectroscopy arrangement has been developed and employed to diagnose the ablating meteorites in several experimental campaigns. In addition to the identification of species from emission lines detected, the resulting high-speed spectral data were used to study the temporal and spatial evolution of melting droplets and the associated spectral signatures. The time history of radiance from the atomic species emission was used to interpret the fragmentation behavior of various meteorites. Chelyabinsk meteorite exhibit almost constant radiance over time indicating steady droplet detachment whereas Ragland meteorite shows infrequent radiance peaks corresponding to random fragmentation/droplet detachment of varying sizes. A gradual rise in radiance history from iron meteorite Mount Joy shows that it takes finite time for melting and accumulation of droplets.

This paper introduces a novel approach for transient plasma wind tunnel testing in an arcjet facility, enabling the experimental simulation of the aerothermal loads along reentry trajectory segments. Such experiments newly allow for the investigation of the causes of break-up events. The trajectory segment is simulated by duplicating the time-resolved profiles of the characteristic flow parameters, namely, heat flux and stagnation pressure as well as the mechanical load, which represents the aerodynamic forces. In arcjet facilities, the parameters that govern the plasma condition are split into variable and constant parameters, which define the available altitude range that can be replicated. The variable conditions arc current and ambient pressure were determined with steady-state trajectory points and traces determined between these conditions resulting in a full trajectory segment. The test time then relates directly to an altitude on the trajectory segment, which allows for an experimental determination of the main break-up altitude. The methodology is applied to a trajectory segment of the transport vehicle CYGNUS OA6, covering an altitude range from 85 to 70 km. A transient break-up experiment on an aluminum bar yielded a failure at an altitude corresponding to 77.6 km, showing a thermomechanical characteristic. These results align well with the observed break-up altitude of CYGNUS OA6, showing the potential of the proposed method.

Ground testing meteorite samples offer in-situ measurements of known materials in conditions that occur during entry into Earth's atmosphere. 22 meteorite samples with a wide range of origins and classifications were tested in the plasma wind tunnel facility PWK1 at the Institute of Space Systems in Stuttgart. These tests recreate the flow condition of a meteoroid during entry into earth's atmosphere at an altitude of 78.8 km altitude and a velocity of 11.7 km s-1. Four optical diagnostic techniques were used to measure the surface temperature above 1000 K. 2-D methods showed that the surface temperature is evenly distributed over the sample surface, while time-resolved analyses show that the samples reach a steady state temperature within 0.5 s. The steady-state temperature for chondritic samples was consistent but varied significantly for achondrites and iron meteorite samples. The composition data showed a strong dependency of the surface temperature on the silicon content. The surface temperatures were shown to be dependent on the material and a database of temperatures was set up. The Planck fit methodology could be directly adapted to spectral meteor observation systems. A comparison of the method to established methods showed an offset between the methods. This data could be applied to thermal models to better understand the energy transfer processes during meteor flight.

Abstract Space exploration missions rely on ablative heat shields for the thermal protection of spacecraft during atmospheric entry flights. While dedicated research is needed for future missions, the scientific community has limited access to ablative materials typically used in aerospace. In this paper, we report the development of the HEFDiG Ablation Research Laboratory ExperimentMaterial (HARLEM), a carbon–phenolic ablator designed to supply the need for ablative materials in laboratory experiments. HARLEM is manufactured using polyacrylonitrile-based carbon fiber preforms and a simplified processing route for phenolic impregnation. We characterized the thermal protection performance of HARLEM in arcjet experiments conducted in the plasma wind tunnel PWK1 of the Institute of Space Systems at the University of Stuttgart. We assessed the performance of the new material by measuring surface recession rate and temperature using photogrammetry and thermography setups during the experiments, respectively. Our results show that HARLEM’s thermal protection performance is comparable to legacy carbon–phenolic ablators that have been validated in different arcjet facilities or in-flight, as demonstrated by calculations of the effective heat of ablation and scanning electron microscopy of as-produced samples. In-house manufacturing of carbon–phenolic ablators enables the addition of embedded diagnostics to ablators, allowing for the acquisition of data on internal pressure and more sophisticated pyrolysis analysis techniques.

Fragments of small solar system bodies entering Earth’s atmosphere have possibly been important contributors of organic compounds to the early Earth. The cyano radical (CN) emission from meteors is considered as potentially one of the most suitable markers of organic compounds in meteoroids, however, its detection in meteor spectra has been thus far unsuccessful. With the aim to improve our abilities to identify CN emission in meteor observations and use its spectral features to characterize the composition of incoming asteroidal meteoroids, we present a detailed analysis of CN emission from high-resolution spectra of 22 laboratory simulated meteors including ordinary, carbonaceous, and enstatite chondrites, as well as a large diversity of achondrites (i.e., ureilite, aubrite, lunar, martian, howardite, eucrite, and diogenite), mesosiderite, and iron meteorites. We describe the variations of CN emission from different classes of asteroidal meteor analogues, its correlation and time evolution relative to other major meteoroid components. We demonstrate that CN can be used as a diagnostic spectral feature of carbonaceous and carbon-rich meteoroids, while most ordinary chondrites show no signs of CN. Our results point out strong correlation between CN and H emission and suggest both volatile features are suitable to trace contents of organic matter and water molecules present within meteoroids. For the application in lower resolution meteor observations, we demonstrate that CN can be best recognized in the early stages of ablation and for carbon-rich materials by measuring relative intensity ratio of CN band peak to the nearby Fe I-4 lines.

Space exploration missions rely on ablative heat shields for the thermal protection of spacecraft during atmospheric entry flights. While dedicated research is needed for future missions, the scientific community has limited access to ablative materials typically used in aerospace. In this paper, we report the development of the HEFDiG Ablation-Research Laboratory Experiment Material (HARLEM), a carbon-phenolic ablator designed to supply the need for ablative materials in laboratory experiments. HARLEM is manufactured using polyacrylonitrile-based carbon fiber preforms and a simplified processing route for phenolic impregnation. We characterized the thermal protection performance of HARLEM in arcjet experiments conducted in the plasma wind tunnel PWK1 of the Institute of Space Systems at the University of Stuttgart. We assessed the performance of the new material by measuring surface recession rate and temperature using photogrammetry and thermography setups during the experiments, respectively. Our results show that HARLEM's thermal protection performance is comparable to legacy carbon-phenolic ablators that have been validated in different arcjet facilities or in-flight, as demonstrated by calculations of the effective heat of ablation and scanning electron microscopy of as-produced samples. In-house manufacturing of carbon-phenolic ablators enables the addition of embedded diagnostics to ablators, allowing for the acquisition of data on internal pressure and more sophisticated pyrolysis analysis techniques.

This paper proposes a methodology to scale the stagnation point plasma conditions of an axially symmetric body to a two-dimensional planar body. The method is required to correlate material samples tested under thermochemical loads combined with aeromechanical loads in order to relate the measurements to actual flight scenarios. The equations governing the boundary-layer and heat transfer equations are introduced and analyzed using the commonly known local heat transfer simulation concept. This technique is then adapted to the given constraints and results in a two-step flight-to-ground scaling approach. Flight conditions are first transformed to axisymmetric ground testing equivalents before being scaled to planar bodies. Thereby, the mass-specific enthalpy, total pressure, and Stanton number stay constant; and the velocity gradient doubles when scaling from axisymmetric to planar. Formulations for the velocity gradient are analyzed for both the sub- and supersonic cases. The results are compared between a theoretical approach and plasma wind-tunnel tests. Three heat flux gauges were tested at two conditions. The planar sensors were evaluated with two independent methods, and the results were scaled to a comparable condition. The results compare very well with the theoretically calculated values. The axisymmetric to planar conversion theory detailed in this paper is therefore considered experimentally verified.

This paper shows new findings for the break-up of large spacecraft during reentry into Earth's atmosphere. The break-up scenario at high altitude drives the ground impact area for parts surviving the reentry. Therefore, based on a combined experimental and numerical analysis of the reentry of the International Space Station (ISS), loads at the module connections have been assessed. In this study, experiments were conducted in an impulse facility to determine the aerothermodynamic forces that apply to models of three components from the ISS. This paper marks the first approach to the experimental testing of complex geometric models coupled with a ground-to-flight scaling of the resulting internal stress. A high-speed schlieren setup was used to record the movement of the multibody free-flight models. A 3D positional analysis tracked the bodies separately. The experimental situation was simulated using the eilmer4 computational fluid dynamics code. A comparison of the experimental and simulated flowfield shows a reasonable agreement in shock structure and resulting forces. The forces were then scaled from testing to flight, extrapolated along a trajectory path, and transformed into spacecraft internal stress. While the resulting forces are significant, it is shown that these forces are significantly smaller than the yield strength of structural materials. Although no aeroheating was simulated in this study, the separation of the large segments of the ISS might not be driven by the mechanical forces alone. This study shows that comparatively simple shock tunnel experiments offer a comprehensive high-fidelity analysis of the interbody forces of complex multibody structures.

The hydrogen emission from meteors is assumed to originate mainly from the meteoroid composition, making it a potential tracer of H$_{2}$O molecules and organic compounds. H$\alpha$ line was previously detected in individual fireballs, but its variation in a larger meteor dataset and dependency on the dynamical origin and physical properties have not yet been studied. Here we investigate the relative intensity of H$\alpha$ within 304 meteor spectra observed by the AMOS network. We demonstrate that H$\alpha$ emission is favored in faster meteors ($v_i >>$ 30 km s$^{-1}$) which form the high-temperature spectral component. H$\alpha$ was found to be a characteristic spectral feature of cometary meteoroids with $\sim$ 92% of all meteoroids with detected H$\alpha$ originating from Halley-type and long-period orbits. Our results suggest that hydrogen is being depleted from meteoroids with lower perihelion distances (q $<$ 0.4 au). No asteroidal meteoroids with detected H emission were found. However, using spectral data from simulated ablation of different meteorite types, we show that H emission from asteroidal materials can occur, and apparently correlates with their water and organic matter content. Strongest H emission was detected from carbonaceous chondrites (CM and CV) and achondrites (ureilite and aubrite), while it was lacking in most ordinary chondrites. The detection of H$\alpha$ in asteroidal meteoroids could be used to identify meteoroids of carbonaceous or achondritic composition. Overall, our results suggest that H$\alpha$ emission correlates with the emission of other volatiles (Na and CN) and presents a suitable tracer of water and organic matter in meteoroids.

Carbon–phenolic ablators have been the materials of choice for interplanetary return heat shields, particularly because of their low density and good characteristics for the dissipation of absorbed heat through ablation and re-radiation. During atmospheric entry, their surfaces reach very high temperatures. Part of the radiation penetrates the material and contributes to the heat transfer. In this paper, the rationale and the results of an idea to enhance the performance of ablators by the inclusion of additives are presented. Ceramic additives are homogeneously dispersed into the materials with the goal of reflecting the radiation emitted by the upper layers of the material to the lower ones. Carbon–phenolic ablators were manufactured with and without ceramic platelets as additives and tested in the plasma wind tunnel facility PWK1 at the Institute of Space Systems in Stuttgart. The material performance was assessed in-situ by measuring the recession through photogrammetry and the surface temperature using IR thermography. The collected data is analyzed to clarify the impact of ceramic additives on the ablator performance during atmospheric entry. The addition of additives yields higher surface temperatures, which enhances the heat dispersion by re-radiation. However, the recession rate is also significantly increased. This would require an increased heat shield mass, potentially compromising the payload of spacecraft.

The fragmentation of two aerospace aluminum alloys is investigated in a ground testing facility including mechanical loads as occurring due to aerodynamic forces in a real atmospheric entry event at three trajectory points. The emission spectroscopic analysis shows that these materials fail after distinct alkali metal features are observed in the spectra. The two alloys feature characteristic emissions of the different alkali metals. The presence of lithium lines that have previously been exclusively attributed to battery failure in observation campaigns may be considered as a marker for aluminum breakup. This is particularly interesting for future entry observations because it allows a new insight into the structural failure processes of the demising spacecraft. The lack of emissions of alloying elements points to these spectra being candidates for the determination of spacecraft demise. The identification of such features in ground testing will allow a more certain identification of specific breakup events.

The fragmentation of two aerospace aluminum alloys is investigated in a ground testing facility including mechanical loads as occurring due to aerodynamic forces in a real atmospheric entry event at three trajectory points. The emission spectroscopic analysis shows that these materials fail after distinct alkali metal features are observed in the spectra. The two alloys feature characteristic emissions of the different alkali metals. The presence of lithium lines that have previously been exclusively attributed to battery failure in observation campaigns may be considered as a marker for aluminum breakup. This is particularly interesting for future entry observations because it allows a new insight into the structural failure processes of the demising spacecraft. The lack of emission of alloying elements points to these spectra being a candidate for the determination of spacecraft demise. The identification of such features in ground testing will allow a more certain identification of specific break-up events

This article describes a novel high frame rate emission spectroscopy setup developed for measurements in high enthalpy flow fields. The optical setup and the associated hardware arrangements are described in detail followed by test case data to demonstrate the capability of recording spectral images at 1 kHz frame rate. The new system is based on a classical Czerny-Turner spectrograph but with a particular setup for high frame rate detection using a Generation II intensifier coupled with a high-speed camera. The high frame rate spectral images acquired enable, for the first time, investigation of the spatial distribution and temporal tracking and evolution of molten droplets of an ablating sample. In this paper, an example is shown from ablating meteorite samples tested in a high enthalpy plasma flow field corresponding to a flight scenario at an altitude of 80 km. This new instrumental configuration allows emission spectroscopic analysis of transient phenomena simulated in the high enthalpy ground test facilities with kHz resolution. The particular feature of this system is the ability to measure very faint spectral lines at high temporal and spatial resolution.