Elemental Abundances of the Bulk Solar Wind: Analyses from Genesis and ACE

Analysis of the Genesis samples is underway. Preliminary elemental abundances based on Genesis sample analyses are in good agreement with in situ-measured elemental abundances made by ACE/SWICS during the Genesis collection period. Comparison of these abundances with those of earlier solar cycles indicates that the solar wind composition is relatively stable between cycles for a given type of flow. ACE/SWICS measurements for the Genesis collection period also show a continuum in compositional variation as a function of velocity for the quasi-stationary flow that defies the simple binning of samples into their sources of coronal hole (CH) and interstream (IS).     

MESSENGER: Exploring Mercury’s Magnetosphere

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.     

The Genesis Solar Wind Concentrator Target: Mass Fractionation Characterised by Neon Isotopes

The concentrator on Genesis provided samples of increased fluences of solar wind ions for precise determination of the oxygen isotopic composition. The concentration process caused mass fractionation as a function of the radial target position. This fractionation was measured using Ne released by UV laser ablation and compared with modelled Ne data, obtained from ion-trajectory simulations. Measured data show that the concentrator performed as expected and indicate a radially symmetric concentration process. Measured concentration factors are up to ∼30 at the target centre. The total range of isotopic fractionation along the target radius is 3.8%/amu, with monotonically decreasing ²⁰Ne/²²Ne towards the centre, which differs from model predictions. We discuss potential reasons and propose future attempts to overcome these disagreements.     

MESSENGER Mission Overview

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is nearing the halfway point on its voyage to become the first probe to orbit the planet Mercury. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: (1) What planetary formational processes led to Mercury’s high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury’s magnetic field? (4) What are the structure and state of Mercury’s core? (5) What are the radar-reflective materials at Mercury’s poles? (6) What are the important volatile species and their sources and sinks near Mercury? The mission has focused to date on commissioning the spacecraft and science payload as well as planning for flyby and orbital operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mercury flyby operations in January and October 2008 and September 2009. Those flybys will provide opportunities to image the hemisphere of the planet not seen by Mariner 10, obtain high-resolution spectral observations with which to map surface mineralogy and assay the exosphere, and carry out an exploration of the magnetic field and energetic particle distribution in the near-Mercury environment. The orbital phase, beginning on March 18, 2011, is a one-year-long, near-polar-orbital observational campaign that will address all mission goals. The orbital phase will complete global imaging, yield detailed surface compositional and topographic data over the northern hemisphere, determine the geometry of Mercury’s internal magnetic field and magnetosphere, ascertain the radius and physical state of Mercury’s outer core, assess the nature of Mercury’s polar deposits, and inventory exospheric neutrals and magnetospheric charged particle species over a range of dynamic conditions. Answering the questions that have guided the MESSENGER mission will expand our understanding of the formation and evolution of the terrestrial planets as a family.     

Mercury’s Weather-Beaten Surface: Understanding Mercury in the Context of Lunar and Asteroidal Space Weathering Studies

Mercury’s regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury’s exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury’s regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury’s regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury’s regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury’s dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury’s relatively featureless visible–near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury’s surface composition.     

Precise line-of-sight modelling for angles-only relative navigation

This work presents a precise analytical model to reconstruct the line-of-sight vector to a target satellite over time, as required by angles-only relative navigation systems for application to rendezvous missions. The model includes the effects of the geopotential, featuring: the analytical propagation in the mean relative orbital elements (up to second-order expansion), the analytical two-way osculating/mean orbital elements’ conversion (second-order in $J_2$ and up to a given degree and order of the geopotential), and a second-order mapping from the perturbed osculating elements’ set to the local orbital frame. Performances are assessed against the line-of-sight reconstructed out of the precise GPS-based positioning products of the PRISMA mission. The line-of-sight modelled over a far-range one day long scenario can be fitted against the true one presenting residuals of the order of ten arc-seconds, which is below the typical sensor noise at far-range.     

A novel satellite image encryption algorithm based on hyperchaotic systems and Josephus problem

With the rapid growth of the number of Earth observation satellite (EOS) supporting critical applications, it is required to improve the security techniques to protect the sensitive data and images during the transmission between the satellites and the ground stations. This paper introduces a new satellite image encryption algorithm based on the Linear Feedback Shift Register (LFSR) generator, SHA 512 hash function, hyperchaotic systems, and Josephus problem. LFSR generates a matrix that is used to construct the 512-bits value of the hash function. These bits are used to set the initial values and parameters of the proposed encryption algorithm. Firstly, the six dimensions (6-D) hyperchaotic system is divided into three parts, where every two equations are considered as one part. Secondly, the 1-D hyperchaotic logistic-tent system is considered as the controller to select one part. The selected part is used to generate a matrix that is XORed with the original image. Thirdly, the scrambling operation by Josephus sequences is applied to the output of the previous step by scrambling the rows and the columns according to the selected part to produce the pre-encrypted image. Finally, if the number of iterations is less than the required number which is considered as a parameter of the secret key, the previous operations will be repeated in the pre-encrypted image; otherwise, the pre-encrypted image is considered as the final cipher image. Experimental and analyses results show that the proposed algorithm has good performance in terms of high level of security, large enough key-space, tolerance to Single Event Upsets (SEU) as well as low time complexity.     

Solar particle events and the International Space Station

The high inclination orbit for the International Space Station poses a risk to astronauts on EVA during occasional periods of enhanced high energy particle flux from the sun known as Solar Particle Events. We are currently unable to predict these events within the few-hour lead time required for evasive action. Compounding the threat is the fact that station construction occurs during increasing solar activity and through the peak of the solar cycle. In this paper we present an overview of the risk, the current methods to provide forecasts of SPEs, and potential risk mitigation options.     

Production of neutrons from interactions of GCR-like particles

In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations.     

LISA — a study of the ESA cornerstone mission for observing gravitational waves

The primary objective of the Laser Interferometer Space Antenna (LISA) mission is to detect and observe gravitational waves from massive black holes and galactic binaries in the frequency range 10⁻⁴ to 10⁻¹ Hz. This low-frequency range is inaccessible to ground-based interferometers because of the unshieldable background of local gravitational noise and because ground-based interferometers are limited in length to a few km. LISA is an ESA cornerstone mission and recently had a system study (Ref. 1) carried out by a consortium led by Astrium, which confirmed the basic configuration for the payload with only minor changes, and provided detailed concepts for the spacecraft and mission design. The study confirmed the need for a drag-free technology demonstration mission to develop the inertial sensors for LISA, before embarking on the build of the flight sensors. With a technology demonstration flight in 2005, it would be possible to carry out LISA as a joint ESA-NASA mission with a launch by 2010 subject to the funding programmatics. The baseline for LISA is three disc-like spacecraft each of which consist of a science module which carries the laser interferometer payload (two in each science module) and a propulsion module containing an ion drive and the hydrazine thrusters of the AOCS. The propulsion module is used for the transfer from earth escape trajectory provided by the Delta II launch to the operational orbit. Once there the propulsion module is jettisoned to reduce disturbances on the payload. Detailed analysis of thermal and gravitational disturbances, a model of the drag-free control and of the interferometer operation confirm that the strain sensitivity of the interferometer will be achieved.