Characteristics of Icy Surfaces

The surfaces of the Solar System’s icy satellites show an extraordinary variety of morphological features, which bear witness to exchange processes between the surface and subsurface. In this paper we review the characteristics of surface features on the moons of Jupiter, Saturn, Uranus and Neptune. Using data from spacecraft missions, we discuss the detailed morphology, size, and topography of cryovolcanic, tectonic, aeolian, fluvial, and impact features of both large moons and smaller satellites.     

Mission analysis and performance comparison for an Advanced Solar Photon Thruster

The so-called “compound solar sail”, also known as “Solar Photon Thruster” (SPT), is a design concept, for which the two basic functions of the solar sail, namely light collection and thrust direction, are uncoupled. In this paper, we introduce a novel SPT concept, termed the Advanced Solar Photon Thruster (ASPT), which does not suffer from the simplified assumptions that have been made for the analysis of compound solar sails in previous studies. After having presented the equations that describe the force on the ASPT and after having performed a detailed design analysis, the performance of the ASPT with respect to the conventional flat solar sail (FSS) is investigated for three interplanetary mission scenarios: an Earth–Venus rendezvous, where the solar sail has to spiral towards the Sun, an Earth–Mars rendezvous, where the solar sail has to spiral away from the Sun, and an Earth-NEA rendezvous (to near-Earth asteroid 1996FG3), where a large change in orbital eccentricity is required. The investigated solar sails have realistic near-term characteristic accelerations between 0.1 and 0.2 mm/s². Our results show that an SPT is not superior to the flat solar sail unless very idealistic assumptions are made.     

10Be Production in the Atmosphere by Galactic Cosmic Rays

Galactic cosmic ray nuclei and energetic protons produced in solar flares and accelerated by coronal mass ejections are the main sources of high-energy particles of extraterrestrial origin in near-Earth space and inside the Earth’s atmosphere. The intensity of galactic cosmic rays inside the heliosphere is strongly influenced by the modulation of the interstellar source particles on their way through interplanetary space. Among others, this modulation depends on the activity of the Sun, and the resulting intensity of the energetic particles in the atmosphere is an indicator of the solar activity. Therefore, rare isotopes found in historical archives and produced by spallation reactions of primary and secondary hadrons of cosmic origin in the atmosphere, so-called cosmogenic nuclides, can be used to reconstruct the solar activity in the past. The production rate of ¹⁰Be, one of the cosmogenic nuclides most adequate to study the solar activity, is presented showing its variations with geographic latitude and altitude and the dependence on different production cross-sections present in literature. In addition, estimates for altitude integrated production rates of ¹⁰Be at different locations since the early nineteen sixties are shown.     

Understanding microbialite morphology using a comprehensive suite of three-dimensional analysis tools

Microbialites can have complex morphologies that preserve clues to ancient microbial ecology. However, extracting and interpreting these clues is challenging due to both the complexity of microbial structures and the difficulties of connecting morphology to microbial processes. Fenestrate microbialites from the 2521±3 Ma Gamohaan Formation, South Africa, have intricate structures composed of three distinct microbial structures: steeply dipping supports (surfaces defined by organic inclusions), more shallowly dipping supports with diffuse organic inclusions below them, and draping laminae. In polished slabs, shallowly dipping supports with diffuse organic inclusions show apparent dips from 27° to 60°, and supports without associated zones of diffuse inclusions dip 75° to 88°, which suggests a distinction between support types based on orientation. However, dips exposed in polished slabs are apparent dips, and three-dimensional analysis is required for analysis of true dips. Through the Keck Center for Active Visualization in Earth Sciences (KeckCAVES), we used locally developed software that controls a three-dimensional environment with head and hand tracking (an "immersive environment") to visualize and interpret virtual microbialite data sets. Immersive environments have not penetrated into standard scientific work processes ("workflows") due to their high costs, steep learning curves, and low productivity for users. By contrast, our suite of software tools allowed us to develop a personalized scientific workflow that provides a complete path from initial ideas to characterization of fenestrate microbialites' features. Results of three-dimensional analysis of fenestrate microbialites show that supports with inclusions dip 65° to 75°, whereas supports without inclusions dip 85° to 90°. These results demonstrate that all supports have very steep dips, and a 10° dip gap exists between supports with and without inclusions, which suggests they grew in fundamentally different ways. Results also emphasize how valuable three-dimensional analysis is when combined with a comprehensive workflow for understanding intricate structures such as fenestrate microbialites.     

Simulation model for the operation of space freighters

This paper makes the attempt to illustrate the need for a detailed operational analysis of future space transportation systems with the help of computer-based simulation models. The basic approach deemed suitable for such a systems simulation is explained in some detail. A reference program (100,000 Mg payload per year during 25 years) for a reference mission (heavy cargo transport to GEO for SPS construction) has been selected. A base-line launch vehicle (fully reusable ballistic all chemical three stage vehicle) has been defined, which is considered a serious applicant for such a mission. It was found that the take-off mass of this type of vehicle should be as large as practical from the viewpoint of cost-effectiveness. The example chosen has a GLOW of 10,000 Mg and lifts more than 100 Mg to GEO.With consideration of the operational parameters the simulation model evaluates the annual production rates, inventory of stages, utilization of facilities and operational cost, which amount within this frame of reference to about 96 $/kg net payload delivered to GEO in terms of 1980 dollars and contribute the main share to the total transportation cost.     

Acceleration Profile of the Slow Solar Wind as Inferred from Gradual Mass Ejections Observed by LASCO

The slow solar wind (< 400 km s^-1) appears to initiate from the regions in the corona where magnetic fields are closed, or from the interface between streamers and other coronal regions. The nature of the acceleration of slow solar wind is not yet well known. LASCO observations of gradually evolving mass ejections offer us a good opportunity to study the speed and acceleration profiles of the slow solar wind from a distance of 1.1 up to 30 R_⊙. We present speed and acceleration profiles of slow solar wind, derived on the basis of measurements of mass flows in several cases of gradual mass ejections and present them in perspective of earlier work. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rosetta Radio Science Investigations (RSI)

The Rosetta spacecraft has been successfully launched on 2nd March 2004 to its new target comet 67 P/Churyumov-Gerasimenko. The science objectives of the Rosetta Radio Science Investigations (RSI) experiment address fundamental aspects of cometary physics such as the mass and bulk density of the nucleus, its gravity field, its interplanetary orbit perturbed by nongravitational forces, its size and shape, its internal structure, the composition and roughness of the nucleus surface, the abundance of large dust grains, the plasma content in the coma and the combined dust and gas mass flux. The masses of two asteroids, Steins and Lutetia, shall be determined during flybys in 2008 and 2010, respectively. Secondary objectives are the radio sounding of the solar corona during the superior conjunctions of the spacecraft with the Sun during the cruise phase. The radio carrier links of the spacecraft Telemetry, Tracking and Command (TT&C) subsystem between the orbiter and the Earth will be used for these investigations. An Ultrastable oscillator (USO) connected to both transponders of the radio subsystem serves as a stable frequency reference source for both radio downlinks at X-band (8.4 GHz) and S-band (2.3 GHz) in the one-way mode. The simultaneous and coherent dual-frequency downlinks via the High Gain Antenna (HGA) permit separation of contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively. The investigation relies on the observation of the phase, amplitude, polarization and propagation times of radio signals transmitted from the spacecraft and received with ground station antennas on Earth. The radio signals are affected by the medium through which the signals propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground.