Cometary and interstellar dust grains: Analysis by ion microprobe mass spectrometry and other techniques

A survey of microanalytical measurements on interplanetary dust particles (IDPs) and interstellar dust grains from primitive meteorites is presented. Ion microprobe mass spectrometry with its capability to determine isotopic compositions of many elements on a m spatial scale has played a special role. Examples are measurements of H, N, and O isotopes and refrectory trace elements in IDPs, and C, N, Mg, and Si isotopes in interstellar silicon carbide grains, and C and N isotopes as well as H, N, Al and Si concentrations in interstellar graphite grams. Possible future instrumental developments are also outlined.     

Quantitative Assessments of the Martian Hydrosphere

In this paper, we review current estimates of the global water inventory of Mars, potential loss mechanisms, the thermophysical characteristics of the different reservoirs that water may be currently stored in, and assess how the planet’s hydrosphere and cryosphere evolved with time. First, we summarize the water inventory quantified from geological analyses of surface features related to both liquid water erosion, and ice-related landscapes. They indicate that, throughout most of Martian geologic history (and possibly continuing through to the present day), water was present to substantial depths, with a total inventory ranging from several 100 to as much as 1000 m Global Equivalent Layer (GEL). We then review the most recent estimates of water content based on subsurface detection by orbital and landed instruments, including deep penetrating radars such as SHARAD and MARSIS. We show that the total amount of water measured so far is about 30 m GEL, although a far larger amount of water may be stored below the sounding depths of currently operational instruments. Finally, a global picture of the current state of the subsurface water reservoirs and their evolution is discussed.     

The Mawrth Vallis region of Mars: A potential landing site for the Mars Science Laboratory (MSL) mission

The primary objective of NASA's Mars Science Laboratory (MSL) mission, which will launch in 2011, is to characterize the habitability of a site on Mars through detailed analyses of the composition and geological context of surface materials. Within the framework of established mission goals, we have evaluated the value of a possible landing site in the Mawrth Vallis region of Mars that is targeted directly on some of the most geologically and astrobiologically enticing materials in the Solar System. The area around Mawrth Vallis contains a vast (>1?×?10? km2) deposit of phyllosilicate-rich, ancient, layered rocks. A thick (>150?m) stratigraphic section that exhibits spectral evidence for nontronite, montmorillonite, amorphous silica, kaolinite, saponite, other smectite clay minerals, ferrous mica, and sulfate minerals indicates a rich geological history that may have included multiple aqueous environments. Because phyllosilicates are strong indicators of ancient aqueous activity, and the preservation potential of biosignatures within sedimentary clay deposits is high, martian phyllosilicate deposits are desirable astrobiological targets. The proposed MSL landing site at Mawrth Vallis is located directly on the largest and most phyllosilicate-rich deposit on Mars and is therefore an excellent place to explore for evidence of life or habitability.     

Geophysical Constraints on the Evolution of Mars

The evolution of Mars is discussed using results from the recent Mars Global Surveyor (MGS) and Mars Pathfinder missions together with results from mantle convection and thermal history models and the chemistry of Martian meteorites. The new MGS topography and gravity data and the data on the rotation of Mars from Mars Pathfinder constrain models of the present interior structure and allow estimates of present crust thickness and thickness variations. The data also allow estimates of lithosphere thickness variation and heat flow assuming that the base of the lithosphere is an isotherm. Although the interpretation is not unambiguous, it can be concluded that Mars has a substantial crust. It may be about 50 km thick on average with thickness variations of another ±50 km. Alternatively, the crust may be substantially thicker with smaller thickness variations. The former estimate of crust thickness can be shown to be in agreement with estimates of volcanic production rates from geologic mapping using data from the camera on MGS and previous missions. According to these estimates most of the crust was produced in the Noachian, roughly the first Gyr of evolution. A substantial part of the lava generated during this time apparently poured onto the surface to produce the Tharsis bulge, the largest tectonic unit in the solar system and the major volcanic center of Mars. Models of crust growth that couple crust growth to mantle convection and thermal evolution are consistent with an early 1 Gyr long phase of vigorous volcanic activity. The simplest explanation for the remnant magnetization of crustal units of mostly the southern hemisphere calls for an active dynamo in the Noachian, again consistent with thermal history calculations that predict the core to become stably stratified after some hundred Myr of convective cooling and dynamo action. The isotope record of the Martian meteorites suggest that the core formed early and rapidly within a few tens of Myr. These data also suggest that the silicate rock component of the planet was partially molten during that time. The isotope data suggest that heterogeneity resulted from core formation and early differentiation and persisted to the recent past. This is often taken as evidence against vigorous mantle convection and early plate tectonics on Mars although the latter assumption can most easily explain the early magnetic field. The physics of mantle convection suggests that there may be a few hundred km thick stagnant, near surface layer in the mantle that would have formed rapidly and may have provided the reservoirs required to explain the isotope data. The relation between the planform of mantle convection and the tectonic features on the surface is difficult to entangle. Models call for long wavelength forms of flow and possibly a few strong plumes in the very early evolution. These plumes may have dissolved with time as the core cooled and may have died off by the end of the Noachian.     

Electric space propulsion systems

Active development of electric thrustors began 10 years ago. Today, several kinds of thrustors have achieved efficiencies above 90 % and lifetimes of several thousand hours. The following article derives the basic theory of electric thrust production at constant exhaust velocity, and at variable exhaust velocity programmed for optimum vehicle performance. Electrothermal or arcjet; electrostatic or ion; and electrodynamic or plasma thrustors are described. At the present time, ion thrustors of the electron bombardment and of the surface ionization types are the most promising systems. Electric power in space may be generated by solar cells or nuclear-electric generators. It is expected that the incore thermionic converter will eventually be the preferred system. A variety of missions with electric propulsion systems appear feasible and highly desirable, among them orbital station keeping, attitude control, planetary probes, solar and out-of-the-ecliptic probes, deep-space probes, and manned Mars and Venus exploration. For each mission, a careful systems-design study must be made, which will provide the optimum selection of thrustor type, thrust level, exhaust velocity, thrust program, power source, trajectory, and flight plan.     

Legal consequences of a SETI detection

If a detection of ETI takes place, this will in all probability be the result of either: (a) detecting and recognising a signal or other emission of ETI; or (b) the finding of an alien artifact (for instance on the Moon or other Celestial Body of our Solar System); or (c) the highly improbable event of an actual encounter. First and foremost, legal consequences regarding any of these contingencies will result from immediate consultations between nations on Earth. Understandings, memoranda and even agreements might be proposed and/or concluded. Such results within the field of terrestrial law will surely be a new branch of International Law, and particularly of International Space Law. At the same time, terrestrial nations will have to realize that any ETI will be self-determined intelligent individualities or organizations who might have their own understanding of “rules of behaviour” and thus, be legal subjects. Whether one calls such rules “law” or not: if two intelligent races—both of which have specific rules of behaviour—come into contact with each other, the basic understanding of such mutual rules will lead to a kind of “code of conduct”. This might be the starting point for a kind of Law—Metalaw—between different races in the Universe.     

Prediction of Differential Time-e-Delay Errors in HF Hyperbolic Position-Fixing Systems

Differential time-delay errors in HF hyperbolic position-fixing systems arising from ionospheric propagation time delays are estimated by employing a propagation prediction model for several path-pairs. A comparison of the prediction model results with experimental observations indicates that the differential time-delay errors can be reduced significantly by the use of the prediction model.     

A liquid propulsion panorama

Liquid-propellant rocket engines are widely used all over the world, thanks to their high performances, in particular high thrust-to-weight ratio. The present paper presents a general panorama of liquid propulsion as a contribution of the IAF Advanced Propulsion Prospective Group.After a brief history of its past development in the different parts of the world, the current status of liquid propulsion, the currently observed trends, the possible areas of future improvement and a summarized road map of future developments are presented. The road map includes a summary of the liquid propulsion status presented in the “Year in review 2007” of Aerospace America.Although liquid propulsion is often seen as a mature technology with few areas of potential improvement, the requirements of an active commercial market and a renewed interest for space exploration has led to the development of a family of new engines, with more design margins, simpler to use and to produce associated with a wide variety of thrust and life requirements.     

Precision orbit determination performance for CryoSat-2

In this paper we discuss our efforts to perform precision orbit determination (POD) of CryoSat-2 which depends on Doppler and satellite laser ranging tracking data. A dynamic orbit model is set-up and the residuals between the model and the tracking data is evaluated. The average r.m.s. of the 10?s averaged Doppler tracking pass residuals is approximately 0.39?mm/s; and the average of the laser tracking pass residuals becomes 1.42?cm. There are a number of other tests to verify the quality of the orbit solution, we compare our computed orbits against three independent external trajectories provided by the CNES. The CNES products are part of the CryoSat-2 products distributed by ESA. The radial differences of our solution relative to the CNES precision orbits shows an average r.m.s. of 1.25?cm between Jun-2010 and Apr-2017. The SIRAL altimeter crossover difference statistics demonstrate that the quality of our orbit solution is comparable to that of the POE solution computed by the CNES. In this paper we will discuss three important changes in our POD activities that have brought the orbit performance to this level. The improvements concern the way we implement temporal gravity accelerations observed by GRACE; the implementation of ITRF2014 coordinates and velocities for the DORIS beacons and the SLR tracking sites. We also discuss an adjustment of the SLR retroreflector position within the satellite reference frame. An unexpected result is that we find a systematic difference between the median of the 10 s Doppler tracking residuals which displays a statistically significant pattern in the South Atlantic Anomaly (SSA) area where the median of the velocity residuals varies in the range of ?0.15 to +0.15?mm/s.     

LDEF: Chemical and isotopic measurements of micrometeoroids by SIMS

Described is a passive experiment for LDEF (Long Duration Exposure Facility) to measure the chemical and isotopic composition of interplanetary dust particles >10⁻¹⁰g for most of the major elements expected to be present. The detector consists of Ge targets covered with a metallized plastic film. During impact micrometeoroid vapor and melt are deposited on the underside of the foil which can be analyzed be secondary ion mass spectroscopy (SIMS) after the return of LDEF. Additional information on projectile mass, velocity and density can be obtained from the study of the penetration hole and the impact crater. Criteria for the choice of materials are given and first results of impact simulation experiments are reported which demonstrate the viability of the basic concept and show that isotopic data can be obtained from the deposits.