The design features of the GGS wind and polar spacecraft

The Global Geospace Science (GGS) WIND and POLAR spacecraft employ unique configuration and design features driven by the requirements of the science instruments which they host. The WIND and POLAR spacecraft are cylindrically shaped spinners (WIND 20 rpm, POLAR 10 rpm) approximately 2.4 m in diameter and 1.8 m high. Each spacecraft has a pair of lanyard booms, which hold magnetometers, four radial wire antennas and two spin-axis antennas. While satisfying different mission requirements, both share a common basic design. The WIND laboratory contains 8 instruments, designed to optimize measurements of waves, fields and particle distributions. The POLAR laboratory contains 12 instruments, with a similar design emphasis on waves, fields and particle measurements, as well as on auroral imaging. The main difference between the two spacecraft is a despun platform on POLAR which provides a stable environment for the auroral imager instruments. Both laboratories are designed to be launched on Delta II model 7925 launch vehicle and have total masses of approximately 1150 g (WIND) and 1240 kg (POLAR).     

Comparison of solar soft X-ray irradiance from broadband photometers to a high spectral resolution rocket observation

The solar soft X-ray (XUV; 1–30 nm) radiation is highly variable on all time scales and strongly affects the ionosphere and upper atmosphere of Earth, Mars, as well as the atmospheres and surfaces of other planets and moons in the solar system; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. While there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the lack of high spectral resolution measurements in this wavelength range. The conversion of the XUV photometer signal into irradiance requires the use of a solar spectral model, but there has not been direct validation of these spectral models for the XUV range. For example, the irradiance algorithm for the XUV Photometer System (XPS) measurements uses multiple CHIANTI spectral models, but validation has been limited to other solar broadband measurements or with comparisons of the atmospheric response to solar variations. A new rocket observation of the solar XUV irradiance with 0.1 nm resolution above 6 nm was obtained on 14 April 2008, and these new results provide a first direct validation of the spectral models used in the XPS data processing. The rocket observation indicates very large differences for the spectral model for many individual emission features, but the differences are significantly smaller at lower resolution, as expected since the spectral models are scaled to match the broadband measurements. While this rocket measurement can help improve a spectral model for quiet Sun conditions, many additional measurements over a wide range of solar activity are needed to fully address the spectral model variations. Such measurements are planned with a similar instrument included on NASA’s Solar Dynamics Observatory (SDO), whose launch is expected in 2009.     

Orbiting Platform Communication and Data Management Systems

The communications and tracking (C&T) system on board the orbiting platform communicates with the ground facilities through the TDRS satellites. The C&T system operates on Ku and S-band. Using a high gain antenna the Ku-band channel can support a downlink data rate of 300 Mbps through the TDRS single axis channel. The S-band system communicates with the orbiter and with both multiple and single axis TDRS channels. The Data Management System (DMS) provides the following services to the orbiting platform: data distribution within and between core systems and payloads, data processing facilities for core systems, data base management, time and frequency standards, and overall platform management and control. The DMS is a distributed data processing network. The nodes are connected by a local area network (LAN). Each node is autonomous. Since the design is modular, nodes can be added or deleted without disturbing the system. Sensors and effectors communicate with the core system software via the network through multiplexers/demultiplexers.     

A far ultraviolet imager for the International Solar-Terrestrial Physics Mission

The aurorae are the result of collisions with the atmosphere of energetic particles that have their origin in the solar wind, and reach the atmosphere after having undergone varying degrees of acceleration and redistribution within the Earth's magnetosphere. The global scale phenomenon represented by the aurorae therefore contains considerable information concerning the solar-terrestrial connection. For example, by correctly measuring specific auroral emissions, and with the aid of comprehensive models of the region, we can infer the total energy flux entering the atmosphere and the average energy of the particles causing these emissions. Furthermore, from these auroral emissions we can determine the ionospheric conductances that are part of the closing of the magnetospheric currents through the ionosphere, and from these we can in turn obtain the electric potentials and convective patterns that are an essential element to our understanding of the global magnetosphere-ionosphere-thermosphere-mesosphere. Simultaneously acquired images of the auroral oval and polar cap not only yield the temporal and spatial morphology from which we can infer activity indices, but in conjunction with simultaneous measurements made on spacecraft at other locations within the magnetosphere, allow us to map the various parts of the oval back to their source regions in the magnetosphere. This paper describes the Ultraviolet Imager for the Global Geospace Sciences portion of the International Solar-Terrestrial Physics program. The instrument operates in the far ultraviolet (FUV) and is capable of imaging the auroral oval regardless of whether it is sunlit or in darkness. The instrument has an 8° circular field of view and is located on a despun platform which permits simultaneous imaging of the entire oval for at least 9 hours of every 18 hour orbit. The three mirror, unobscured aperture, optical system (f/2.9) provides excellent imaging over this full field of view, yielding a per pixel angular resolution of 0.6 milliradians. Its FUV filters have been designed to allow accurate spectral separation of the features of interest, thus allowing quantitative interpretation of the images to provide the parameters mentioned above. The system has been designed to provide ten orders of magnitude blocking against longer wavelength (primarily visible) scattered sunlight, thus allowing the first imaging of key, spectrally resolved, FUV diagnostic features in the fully sunlit midday aurorae. The intensified-CCD detector has a nominal frame rate of 37 s, and the fast optical system has a noise equivalent signal within one frame of 10R. The instantaneous dynamic range is >1000 and can be positioned within an overall gain range of 10⁴, allowing measurement of both the very weak polar cap emissions and the very bright aurora. The optical surfaces have been designed to be sufficiently smooth to permit this dynamic range to be utilized without the scattering of light from bright features into the weaker features. Finally, the data product can only be as good as the degree to which the instrument performance is characterized and calibrated. In the VUV, calibration of an an imager intended for quantitative studies is a task requiring some pioneering methods, but it is now possible to calibrate such an instrument over its focal plane to an accuracy of ±10%. In summary, very recent advances in optical, filter and detector technology have been exploited to produce an auroral imager to meet the ISTP objectives.     

A Note on Integral Equations and Wolf's Theorem

The operation of a large class of physical systems ems can be described through application of singular integral equations written in the time domain. This correspondence shows that Wolf's theorem and a general form of Parseval's theorem can be used to transform the integral equation to the S-plane, the result being an integral equation which readily yields the complex frequency response of the system.     

Lunar astrobiology: a review and suggested laboratory equipment

In October of 2005, the European Space Agency (ESA) and Alcatel Alenia Spazio released a "call to academia for innovative concepts and technologies for lunar exploration." In recent years, interest in lunar exploration has increased in numerous space programs around the globe, and the purpose of our study, in response to the ESA call, was to draw on the expertise of researchers and university students to examine science questions and technologies that could support human astrobiology activity on the Moon. In this mini review, we discuss astrobiology science questions of importance for a human presence on the surface of the Moon and we provide a summary of key instrumentation requirements to support a lunar astrobiology laboratory.     

The IBEX-Lo Sensor

The IBEX-Lo sensor covers the low-energy heliospheric neutral atom spectrum from 0.01 to 2 keV. It shares significant energy overlap and an overall design philosophy with the IBEX-Hi sensor. Both sensors are large geometric factor, single pixel cameras that maximize the relatively weak heliospheric neutral signal while effectively eliminating ion, electron, and UV background sources. The IBEX-Lo sensor is divided into four major subsystems. The entrance subsystem includes an annular collimator that collimates neutrals to approximately 7°×7° in three 90° sectors and approximately 3.5°×3.5° in the fourth 90° sector (called the high angular resolution sector). A fraction of the interstellar neutrals and heliospheric neutrals that pass through the collimator are converted to negative ions in the ENA to ion conversion subsystem. The neutrals are converted on a high yield, inert, diamond-like carbon conversion surface. Negative ions from the conversion surface are accelerated into an electrostatic analyzer (ESA), which sets the energy passband for the sensor. Finally, negative ions exit the ESA, are post-accelerated to 16 kV, and then are analyzed in a time-of-flight (TOF) mass spectrometer. This triple-coincidence, TOF subsystem effectively rejects random background while maintaining high detection efficiency for negative ions. Mass analysis distinguishes heliospheric hydrogen from interstellar helium and oxygen. In normal sensor operations, eight energy steps are sampled on a 2-spin per energy step cadence so that the full energy range is covered in 16 spacecraft spins. Each year in the spring and fall, the sensor is operated in a special interstellar oxygen and helium mode during part of the spacecraft spin. In the spring, this mode includes electrostatic shutoff of the low resolution (7°×7°) quadrants of the collimator so that the interstellar neutrals are detected with 3.5°×3.5° angular resolution. These high angular resolution data are combined with star positions determined from a dedicated star sensor to measure the relative flow difference between filtered and unfiltered interstellar oxygen. At the end of 6 months of operation, full sky maps of heliospheric neutral hydrogen from 0.01 to 2 keV in 8 energy steps are accumulated. These data, similar sky maps from IBEX-Hi, and the first observations of interstellar neutral oxygen will answer the four key science questions of the IBEX mission.     

Development status of AEOLDOS – A deorbit module for small satellites

A prototype CubeSat module to deploy a gossamer aerobrake, using strain stored in tape-springs, at end-of-life is described. A novel hub geometry to reduce bending shock at end-of-deployment while simultaneously permitting radial, as opposed to tangential, deployment is proposed. The rpm of the hub is measured under various deployment conditions to verify that the system offers highly-repeatable performance, while high-speed photography is used to characterise the behaviour of the tape-spring during unspooling and contrast it to the behaviour of a traditional tangential-deployment system. Secondly the folding pattern of the membrane, which takes advantage of the symmetrical deployment offered by the petal hub, is developed and the unfolding mechanism is verified by numerical and experimental analysis. Finally, the release of the stored strain is considered and a novel burn-though device is designed and prototyped to meet this requirement.