Future Mission Concepts Research Articles

Applicability of NGGM near-real time simulations in flood detection

Abstract The GRACE mission has demonstrated a tremendous potential for observing mass changes in the Earth system from space for climate research and the observation of climate change. Future mission should on the one hand extend the already existing time series and also provide higher spatial and temporal resolution that is required to fulfil all needs placed on a future mission. To analyse the applicability of such a Next Generation Gravity Mission (NGGM) concept regarding hydrological applications, two GRACE-FO-type pairs in Bender formation are analysed. The numerical closed loop simulations with a realistic noise assumption are based on the short arc approach and make use of the Wiese approach, enabling a self-de-aliasing of high-frequency atmospheric and oceanic signals, and a NRT approach for a short latency. Numerical simulations for future gravity mission concepts are based on geophysical models, representing the time-variable gravity field. First tests regarding the usability of the hydrology component contained in the Earth System Model (ESM) by the European Space Agency (ESA) for the analysis regarding a possible flood monitoring and detection showed a clear signal in a third of the analysed flood cases. Our analysis of selected cases found that detection of floods was clearly possible with the reconstructed AOHIS/HIS signal in 20% of the tested examples, while in 40% of the cases a peak was visible but not clearly recognisable.

Color Classification of Extrasolar Giant Planets: Prospects and Cautions

Abstract Atmospheric characterization of directly imaged planets has thus far been limited to ground-based observations of young, self-luminous, Jovian planets. Near-term space- and ground- based facilities like WFIRST and ELTs will be able to directly image mature Jovian planets in reflected light, a critical step in support of future facilities that aim to directly image terrestrial planets in reflected light (e.g., HabEx, LUVOIR). These future facilities are considering the use of photometry to classify planets. Here, we investigate the intricacies of using colors to classify gas-giant planets by analyzing a grid of 9120 theoretical reflected light spectra spread across different metallicities, pressure–temperature profiles, cloud properties, and phase angles. We determine how correlated these planet parameters are with the colors in the WFIRST photometric bins and other photometric bins proposed in the literature. Then we outline under what conditions giant planet populations can be classified using several supervised multivariate classification algorithms. We find that giant planets imaged in reflected light can be classified by metallicity with an accuracy of >90% if they are a prior known to not have significant cloud coverage in the visible part of the atmosphere, and at least three filter observations are available. If the presence of clouds is not known a priori, directly imaged planets can be more accurately classified by their cloud properties, as oppposed to metallicity or temperature. Furthermore, we are able to distinguish between cloudy and cloud-free populations with >90% accuracy with three filter observations. Our statistical pipeline is available on GitHub and can be extended to optimize science yield of future mission concepts.

Baseline requirements for detecting biosignatures with the HabEx and LUVOIR mission concepts

A milestone in understanding life in the universe is the detection of biosignature gases in the atmospheres of habitable exoplanets. Future mission concepts under study by the 2020 decadal survey, e.g., Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR), have the potential of achieving this goal. We investigate the baseline requirements for detecting four molecular species, H2O, O2, CH4, and CO2, assuming concentrations of these species equal to that of modern Earth. These molecules are highly relevant to habitability and life on Earth and other planets. Through numerical simulations, we find the minimum requirements of spectral resolution, starlight suppression, and exposure time for detecting biosignature and habitability marker gases. The results are highly dependent on cloud conditions. A low-cloud case is more favorable because of deeper and denser lines whereas a no-cloud case is the pessimistic case for its low albedo. The minimum exposure time for detecting a certain molecule species can vary by a large factor (∼10) between the low-cloud case and the no-cloud case. For all cases, we provide baseline requirements for HabEx and LUVOIR. The impact of exozodiacal contamination and thermal background is also discussed and will be included in future studies.

(Invited) ALD Metal Fluorides for Ultraviolet Filter and Reflective Coating Applications

We report on the development of atomic layer deposition (ALD) processes for integrating optical filter coatings directly onto back-illuminated silicon sensors for ultraviolet applications. These devices, including CCD imaging arrays and large-area avalanche photodiodes, are modified with a two-dimensional doping method utilizing molecular beam epitaxy to provide high internal quantum efficiency in the UV. Detector integrated coatings are then required to recover additional external losses due to natural silicon reflectance at these short wavelengths. This includes the development of broadband AR coatings, multilayer narrowband coatings, and metal-dielectric filters for solar-blind devices. The use of ALD is attractive for this purpose because the low deposition temperature and thermal surface reactions do not degrade the 2D doping layers, and therefore do not result in reduced detector quantum efficiency. Applications implementing these coated detectors related to terrestrial and space astronomy, astrophysics missions, and particle physics experiments will be discussed. Cosmology is experiencing a very exciting time in exploration. Communities of scientists and technology innovators are studying four concepts for flagship future missions selected and supported by NASA. Two of these four missions are Earth-orbiting large aperture telescope observatories that will include UV capabilities. Currently, these ongoing studies have identified high efficiency detectors and high reflectivity mirror coatings as high priority technologies, particularly those that can extend and enhance system performance for UV wavelengths shorter than those previously explored by missions like the Hubble Space Telescope. At wavelengths below 200 nm, typical ALD dielectric oxide materials like TiO2, HfO2, SiO2, and Al2O3 exhibit large optical losses; we have pursued the development of new processes for ALD metal fluorides to extend coating performance into this far UV wavelength range. Thin films of MgF2, AlF3, and LiF are demonstrated using anhydrous hydrogen fluoride (HF) as the fluorine-containing precursor. This is in contrast to alternate ALD methods for fluoride materials which often utilize reactive metal fluorides as the fluorine precursors, and can result in thin films with components of residual metal oxide contamination. The use of HF has allowed for films with good UV optical properties at deposition temperatures as low 100 °C. This enables compatibility with a variety of 2D-doped silicon sensor formats as well as optical components like polymeric diffraction gratings. The deposition rate of these fluoride materials has a more significant substrate temperature dependence than other common ALD chemistries that often display a ‘window’ of uniform film growth per ALD cycle. Characterization of these coatings is performed with measurements of far UV reflectance and transmittance, x-ray photoelectron spectroscopy, x-ray diffraction, ellipsometry, and atomic force microscopy. These ALD fluoride materials are also effective protective coatings for reflective aluminum optics. Protected UV Al mirrors with ALD AlF3 overcoats are shown to be competitive with state-of-the-art commercial mirrors made with conventional physical vapor deposition (PVD) methods. The use of ALD in these components holds promise in reducing the thickness of the overcoat relative to PVD approaches. This has significant performance implications for far UV mirrors and the ultimate short wavelength cutoff of Al mirrors for potential future NASA astrophysics missions. The large scale coating uniformity, and corresponding reflectance uniformity of ALD overcoats is also attractive for the same applications. Additionally, the use of HF-based ALD chemistries can also allow for the thermal atomic layer etching (ALE) of a variety of materials including aluminum oxide. By combining these ALE methods with ALD encapsulation, it is possible to chemically strip the oxidation that occurs on metallic aluminum and effectively replace it with thin fluoride layers. Stable Al mirrors with protective fluoride coatings approximately 3 nm thick are demonstrated with this combined ALE/ALD approach. The environmental stability of these coatings will be described in the context of resistance to high-humidity conditions. Figure 1

From GOCE to NGGM: Automatic Control Breakthroughs for European future Gravity Missions

After the successful European gravity mission GOCE (Gravity field and steady-state Ocean Circulation Explorer), which provided an unprecedented high resolution static global map of the Earth’s gravity field, the European Space Agency has proposed several preparatory studies for a Next Generation Gravity Mission (NGGM). The NGGM mission objective aims at measuring the temporal variations of the Earth gravity field over a long time span with an unprecedented level of accuracy, both in spatial and temporal resolution. The GOCE technological heritage is leveraged as starting point while defining the NGGM future mission concept. Nonetheless, to accomplish its challenging scientific objective, the NGGM mission concept envisages a wide range of innovations, with respect to the past or flying missions, both on technological and automatic control side. Thus, this paper focuses on the guidance, navigation and control design evolution for the European gravity missions, from GOCE to NGGM. After recalling the GOCE GNC main design concepts, the paper will describe the most important innovation required by NGGM. Indeed, such a future concept will consist of a two-satellite long-distance loose formation, where each satellite is controlled independently to be drag-free, GOCE-like. The satellite-to-satellite distance variations, encoding gravity anomalies, will be then measured by laser heterodyne interferometry for inter-satellite ranging at 20 nm resolution, or better. Hence, an orbit and formation control is now required to counteract bias and drift of the residual drag-free accelerations, in order to reach a bounded orbit/formation long-term stability. Finally, GOCE control flight results as well as NGGM simulated results, via a high-fidelity simulator, will be provided. These results highlight the GOCE GNC in-flight achievements as well as the NGGM concept validity, showing that the expected control performances are in agreement with the consolidated mission requirements, all over the 10-year mission.

Failure analysis of satellite subsystems to define suitable de-orbit devices

Space missions in Low Earth Orbit (LEO) are severely affected by the build-up of orbital debris. A key practice, to be compliant with IADC (Inter-Agency Space Debris Coordination Committee) mitigation guidelines, is the removal of space systems that interfere with the LEO region not later than 25 years after the End of Mission. It is important to note that the current guidelines are not generally legally binding, even if different Space Agencies are now looking at the compliance for their missions. If the guidelines will change in law, it will be mandatory to have a postmission disposal strategy for all satellites, including micro and smaller classes.A potential increased number of these satellites is confirmed by different projections, in particular in the commercial sector. Micro and smaller spacecraft are, in general, not provided with propulsion capabilities to achieve a controlled re-entry, so they need different de-orbit disposal methods.When considering the utility of different debris mitigation methods, it is useful to understand which spacecraft subsystems are most likely to fail and how this may affect the operation of a de-orbit system. This also helps the consideration of which components are the most relevant or should be redundant depending on the satellite mass class.This work is based on a sample of LEO and MEO satellites launched between January 2000 and December 2014 with mass lower than 1000kg. Failure analysis of satellite subsystems is performed by means of the Kaplan–Meier survival analysis; the parametric fits are conducted with Weibull distributions. The study is carried out by using the satellite database SpaceTrak™ which provides anomalies, failures, and trends information for spacecraft subsystems and launch vehicles. The database identifies five states for each satellite subsystem: three degraded states, one fully operational state, and one failed state (complete failure).The results obtained can guide the identification of the activation procedure for a de-orbit strategy and the level of integration it should have with the host satellite in order to be activated before a total failure.At Cranfield Space Research Centre two different solutions have already been developed as de-orbit sail payloads for microsatellites (Icarus-1 on TechDemoSat-1 and Icarus-3 on Carbonite-1 currently on-orbit, DOM for future ESA ESEO mission). This study will provide a useful input to improve and refine the current de-orbit concepts for future satellite missions.

Review of Thermal Infrared Applications and Requirements for Future High-Resolution Sensors

High-resolution thermal infrared (TIR) remote sensing has a wide range of applications. In this paper, we describe the different applications and requirements identified in a literature review and during a consultation meeting with researcher experts in different fields. As a result, more than 30 applications were identified within three different fields: 1) land and solid Earth; 2) health and hazards; and 3) security and surveillance. A complete set of requirements (spatial, temporal, and radiometric resolution, algorithms used, and supporting data, among others) for each application is also provided. The results presented in this paper provide useful information to enhance the importance of high-resolution TIR data for civil applications and may serve as a reference document for future TIR mission concepts.

Bounded relative orbits about asteroids for formation flying and applications

The relative motion about 4179 Toutatis is studied in order to investigate the feasibility of formation flying as an alternative concept for future asteroid exploration missions. In particular, the existence of quasi-frozen orbits about slowly rotating bodies allows us to compute families of periodic orbits in the body-fixed frame of the asteroid. Since these periodic orbits are of the center×center type, quasi-periodic invariant tori are calculated via fully numerical procedures and used to initialize spacecraft formations about the central body. Numerical simulations show that the resulting in-plane and out-of-plane relative trajectories remain bounded over long time spans; i.e., more than 30 days.

Science and User Needs for Observing Global Mass Transport to Understand Global Change and to Benefit Society

Satellite gravimetry is a unique measurement technique for observing mass transport processes in the Earth system on a global scale, providing essential indicators of both subtle and dramatic global change. Although past and current satellite gravity missions have achieved spectacular science results, due to their limited spatial and temporal resolution as well as limited length of the available time series numerous important questions are still unresolved. Therefore, it is important to move from current demonstration capabilities to sustained observation of the Earth’s gravity field. In an international initiative performed under the umbrella of the International Union of Geodesy and Geophysics, consensus on the science and user needs for a future satellite gravity observing system has been derived by an international panel of scientists representing the main fields of application, i.e., continental hydrology, cryosphere, ocean, atmosphere and solid Earth. In this paper the main results and findings of this initiative are summarized. The required target performance in terms of equivalent water height has been identified as 5 cm for monthly fields and 0.5 cm/year for long-term trends at a spatial resolution of 150 km. The benefits to meet the main scientific and societal objectives are investigated, and the added value is demonstrated for selected case studies covering the main fields of application. The resulting consolidated view on the required performance of a future sustained satellite gravity observing system represents a solid basis for the definition of technological and mission requirements, and is a prerequisite for mission design studies of future mission concepts and constellations.

A phasemeter concept for space applications that integrates an autonomous signal acquisition stage based on the discrete wavelet transform

We describe a phasemeter designed to autonomously acquire and track a heterodyne signal with low signal-to-noise ratio in a frequency band that spans from 1 MHz to 25 MHz. The background driving some of the design criterions of the phasemeter comes from studies on future space mission concepts such as orbiting gravitational wave observatories and next generation geodesy missions which all rely on tracking phasemeters in order to meet their mission goal. The phasemeter has been implemented within a field programmable gate array trying to minimize the requirement of computational resources and its performance has been tested using signal generators. Laboratory test has shown that the phasemeter is capable of locking to an input signal in less than half a millisecond, while its phase measurement accuracy is in the micro-radian range for measurement frequencies that span from mHz to Hz.

Heterodyne laser frequency stabilization for long baseline optical interferometry in space-based gravitational wave detectors

The European Space Agency (ESA) selected the gravitational universe as the science theme for L3, a large space mission with a planned launch in 2034. NASA expressed a strong interest in joining ESA as a junior partner. The goal of the mission is the detection of gravitational waves of frequencies between 0.1 mHz and 0.1 Hz, where many long-lived sources are expected to be steady emitters of gravitational waves. Most likely, the mission design will evolve out of the earlier Laser Interferometer Space Antenna (LISA) concept. The interferometric heterodyne phase readout in LISA is performed by phase meters developed specifically to handle the low light powers and Doppler-drift of laser frequencies that appear as complications in the mission baseline. LISA requires the frequency noise of its seed lasers to be below $300\text{ }\text{ }\mathrm{Hz}/\sqrt{\mathrm{Hz}}$ throughout the measurement band due to uncertainties in the absolute interferometer arm lengths. We have developed and successfully demonstrated Heterodyne Stabilization (HS), a novel cavity-laser frequency stabilization method that integrates well into the LISA mission baseline due to similar component demand. The cavities for the test setup were assembled with Clearceram-Z spacers, an ultralow thermal expansion coefficient material with potential applicability in interferometric space missions. Using HS, we were able to suppress the frequency noise of two lasers in a bench-top experiment to a level that meets the LISA requirement, suggesting both HS and Clearceram-Z can be considered in future mission concepts.

A Spaceborne Gravity Gradiometer Concept Based on Cold Atom Interferometers for Measuring Earth’s Gravity Field

We propose a concept for future space gravity missions using cold atom interferometers for measuring the diagonal elements of the gravity gradient tensor and the spacecraft angular velocity. The aim is to achieve better performance than previous space gravity missions due to a very low white noise spectral behavior and a very high common mode rejection, with the ultimate goals of determining the fine structures of the gravity field with higher accuracy than GOCE and detecting time-variable signals in the gravity field better than GRACE.

Cross-Correlation Waveform Analysis for Conventional and Interferometric GNSS-R Approaches

Reflectometry using Global Navigation Satellite System (GNSS-R) signals has become an attractive tool for Earth remote sensing. Back to 1993, it was proposed as an alternative to the conventional spaceborne ocean altimetry to estimate the sea surface height over a large field-of-view. After Doppler compensation, the cross-correlation of the received scattered signal with either a locally generated replica [conventional GNSS-R (cGNSS-R)] or either the direct signal (interferometric GNSS-R (iGNSS-R)] is called the waveform. This paper provides a critical review of the cross-correlation waveform model, addressing issues such as the correlation characteristics, the bandwidth, the observation geometry, and the thermal and speckle noises. It provides a comprehensive and systematic overview of the impact of all these effects on the GNSS-R observables, which is important to properly define future GNSS-R instrument and mission concepts.

Comparing seven candidate mission configurations for temporal gravity field retrieval through full-scale numerical simulation

The goal of this contribution is to focus on improving the quality of gravity field models in the form of spherical harmonic representation via alternative configuration scenarios applied in future gravimetric satellite missions. We performed full-scale simulations of various mission scenarios within the frame work of the German joint research project “Concepts for future gravity field satellite missions” as part of the Geotechnologies Program, funded by the German Federal Ministry of Education and Research and the German Research Foundation. In contrast to most previous simulation studies including our own previous work, we extended the simulated time span from one to three consecutive months to improve the robustness of the assessed performance. New is that we performed simulations for seven dedicated satellite configurations in addition to the GRACE scenario, serving as a reference baseline. These scenarios include a “GRACE Follow-on” mission (with some modifications to the currently implemented GRACE-FO mission), and an in-line “Bender” mission, in addition to five mission scenarios that include additional cross-track and radial information. Our results clearly confirm the benefit of radial and cross-track measurement information compared to the GRACE along-track observable: the gravity fields recovered from the related alternative mission scenarios are superior in terms of error level and error isotropy. In fact, one of our main findings is that although the noise levels achievable with the particular configurations do vary between the simulated months, their order of performance remains the same. Our findings show also that the advanced pendulums provide the best performance of the investigated single formations, however an accuracy reduced by about 2–4 times in the important long-wavelength part of the spectrum (for spherical harmonic degrees $${<}50$$ ), compared to the Bender mission, can be observed. Concerning state-of-the-art mission constraints, in particular the severe restriction of heterodyne lasers on maximum range-rates, only the moderate Pendulum and the Bender-mission are beneficial options, of course in addition to GRACE and GRACE-FO. Furthermore, a Bender-type constellation would result in the most accurate gravity field solution by a factor of about 12 at long wavelengths (up to degree/order 40) and by a factor of about 200 at short wavelengths (up to degree/order 120) compared to the present GRACE solution. Finally, we suggest the Pendulum and the Bender missions as candidate mission configurations depending on the available budget and technological progress.

Orbit evolution, maintenance and disposal of SpaceChip swarms through electro-chromic control

The combined effect of solar radiation pressure, Earth oblateness and atmospheric drag on the orbital dynamics of satellites-on-a-chip (SpaceChips) is investigated for future swarm mission concepts. The natural evolution of the swarm is exploited to perform spatially distributed measurements of the upper layers of the atmosphere. The energy gain from asymmetric solar radiation pressure can be used to balance the energy dissipation from atmospheric drag. An algorithm for long-term orbit control is then designed, based on changing the reflectivity coefficient of the SpaceChips. The subsequent modulation of the solar radiation pressure allows stabilisation of the swarm in the orbital element phase space. It is shown that the orbit lifetime for such devices can be extended through the interaction of solar radiation pressure and atmospheric drag and indeed selected and the end-of-life re-entry of the swarm can be ensured, by exploiting atmospheric drag.

GeoLab—A habitat-based laboratory for preliminary examination of geological samples

GeoLab is a prototype geological laboratory designed for deployment and testing during NASA's analog demonstrations. Scientists at NASA's Johnson Space Center (JSC) built GeoLab as part of a technology project to support the development of science operational concepts for future planetary surface missions. It was integrated into NASA's Habitat Demonstration Unit 1 – Pressurized Excursion Module (HDU1-PEM), a first generation exploration habitat test bed.As a test bed, GeoLab provides a high fidelity working space for crewmembers to perform preliminary examination and characterization of geologic samples. The GeoLab concept builds from the hardware and cleanroom protocols used in JSC's Astromaterials Sample Curation laboratories. The main hardware component of the GeoLab is a custom-built glovebox, constructed from stainless steel and polycarbonate, and built to provide a positive pressure nitrogen environment. The glovebox is mounted onto the habitat's structural ribs; the unique shape (trapezoidal prism) fits within a pie-shaped section of the cylindrical habitat. A key innovation of GeoLab is the mechanism for transferring samples into the glovebox: three antechambers (airlocks) that pass through the shell of the habitat. These antechambers allow geologic samples to enter and exit the main glovebox chamber directly from (and to) the outside, thereby controlling contamination from inside the habitat. The glovebox is configured to include imaging systems, instrumentation, and computer controls. The first field trials tested a simple configuration including a microscope, a commercially available handheld X-ray Fluorescence instrument, network cameras, and simple sample handling tools inside the glovebox.We present results from the initial field trials of GeoLab during the 2010 Desert Research and Technology Studies (Desert RATS) planetary analog test near Flagstaff, AZ. These field tests examined the general operations of the GeoLab hardware and the crew interfaces for operating the laboratory. GeoLab operations also resulted in the preliminary examination of selected samples for Desert RATS science team decisions regarding prioritization of samples.

Future dark energy constraints from measurements of quasar parallax:Gaia, SIMand beyond

(Abridged) A consequence of the Earth's motion with respect to the CMB is that over a 10 year period it will travel a distance of ~800 AU. As first noted by Kardashev in 1986, this baseline can be used to carry out astrometric measurements of quasar parallaxes, so that only microarcsecond precision is necessary to detect parallax shifts of objects at gigaparsec distances. Such precision will soon be approached with the launch of the astrometric satellites Gaia and SIM. We use a Fisher matrix formalism to investigate the constraints that these and future missions may be able to place on the cosmological distance scale and dark energy. We find that by observing around a million quasars as planned, an extended 10 year Gaia mission could detect quasar parallax shifts at the 2.8 sigma level and so measure the Hubble constant to within 25 km/s. For the interferometer SIMLite, a Key Project using 2.4 % of the total mission time to observe 750 quasars could detect the effect at the 2 sigma level. Gaia and a dedicated SIMLite only weakly constrain the presence of a cosmological constant at the ~1 sigma levels. We also investigate future mission concepts, such as an interferometer similar in scope and design to NASA's Terrestrial Planet Finder. This could in principle measure the dark energy parameters w_0 and w_a with high precision, yielding a Figure of Merit larger than the stage IV experiments considered by the the Dark Energy Task Force. Unlike perhaps all other probes of dark energy there appear to be no obvious astrophysical sources of systematic error. There is however uncertainty regarding the statistical errors. As well as measurement error, there will be small additional contributions from image centroiding of variable sources, quasar peculiar motions and weak microlensing by stars along the line of sight.

Fresnel Interferometric Imager: ground-based prototype

The Fresnel Interferometric Imager is a space-based astronomical telescope project yielding milli-arcsecond angular resolution and high contrast images with loose manufacturing constraints. This optical concept involves diffractive focusing and formation flying: a first "primary optics" space module holds a large binary Fresnel array, and a second "focal module" holds optical elements and focal instruments that allow for chromatic dispersion correction. We have designed a reduced-size Fresnel Interferometric Imager prototype and made optical tests in our laboratory in order to validate the concept for future space missions. The primary module of this prototype consists of a square, 8 cm side, 23 m focal length Fresnel array. The focal module is composed of a diaphragmed small telescope used as "field lens," a small cophased diverging Fresnel zone lens that cancels the dispersion, and a detector. An additional module collimates the artificial targets of various shapes, sizes, and dynamic ranges to be imaged. We describe the experimental setup, different designs of the primary Fresnel array, and the cophased Fresnel zone lens that achieves rigorous chromatic correction. We give quantitative measurements of the diffraction limited performances and dynamic range on double sources. The tests have been performed in the visible domain, lambda = 400-700 nm. In addition, we present computer simulations of the prototype optics based on Fresnel propagation that corroborate the optical tests. This numerical tool has been used to simulate the large aperture Fresnel arrays that could be sent to space with diameters of 3 to 30 m, foreseen to operate from Lyman alpha (121 nm) to mid IR (25 microm).

Optical studies of noctilucent clouds in the extreme ultraviolet

Abstract. In order to better understand noctilucent clouds (NLC) and their sensitivity to the variable environment of the polar mesosphere, more needs to be learned about the actual cloud particle population. Optical measurements are today the only means of obtaining information about the size of mesospheric ice particles. In order to efficiently access particle sizes, scattering experiments need to be performed in the Mie scattering regime, thus requiring wavelengths of the order of the particle size. Previous studies of NLC have been performed at wavelengths down to 355 nm from the ground and down to about 200 nm from rockets and satellites. However, from these measurements it is not possible to access the smaller particles in the mesospheric ice population. This current lack of knowledge is a major limitation when studying important questions about the nucleation and growth processes governing NLC and related particle phenomena in the mesosphere. We show that NLC measurements in the extreme ultraviolet, in particular using solar Lyman-α radiation at 121.57 nm, are an efficient way to further promote our understanding of NLC particle size distributions. This applies both to global measurements from satellites and to detailed in situ studies from sounding rockets. Here, we present examples from recent rocket-borne studies that demonstrate how ambiguities in the size retrieval at longer wavelengths can be removed by invoking Lyman-α. We discuss basic requirements and instrument concepts for future rocket-borne NLC missions. In order for Lyman-α radiation to reach NLC altitudes, high solar elevation and, hence, daytime conditions are needed. Considering the effects of Lyman-α on NLC in general, we argue that the traditional focus of rocket-borne NLC missions on twilight conditions has limited our ability to study the full complexity of the summer mesopause environment.

300 mK and 50 mK cooling systems for long-life space applications

The X-ray Evolving Universe Spectroscopy mission is a candidate for the ESA Cosmic Vision 2015–2025 plan, with two possible instruments at 300 mK and 50 mK. Astrium, under ESA contract, has worked with MSSL, RAL, and CEA-SBT, to propose a suitable payload accommodation scheme meeting ESA’s requirements. Exhaustive trade-offs were performed, according to a described methodology, leading to two designs incorporating closed-cycle active coolers maximizing well proven heritage concepts. These concepts are described, and although they have been prepared for XEUS, there are other future mission concepts which could also benefit from 50 mK cooling with a lifetime greater than 10 years.