On the Isotopic Composition of Primordial Xenon in Terrestrial Planet Atmospheres

Xenon plays a crucial role in models of atmospheric evolution in which noble gases are fractionated from their initial compositions to isotopically heavier distributions by early hydrodynamic escape of primordial planetary atmospheres. With the assumption that nonradiogenic Xe isotope ratios in present-day atmospheres were generated in this way, backward modeling from these ratios through the fractionating process can in principle identify likely parental Xe compositions and thus the probable sources of noble gases in pre-escape atmospheres. Applied to Earth, this approach simultaneously establishes the presence of an atmospheric Xe component due principally to fission of extinct ²⁴⁴Pu and identifies a composition called U-Xe as primordial Xe. Pu-Xe comprises 4.65±0.30% of atmospheric ¹³⁶Xe, and 6.8±0.5% of the present abundance of ¹²⁹Xe derives from decay of extinct ¹²⁹I. U-Xe is identical to the measured composition of solar-wind Xe except for deficits of the two heaviest isotopes – an unexpected difference since the modeling otherwise points to solar wind compositions for the lighter noble gases in the primordial terrestrial atmosphere. Evidence for the presence of U-Xe is not restricted to the early Earth; modeling based on a purely meteoritic data set defines a parental component in chondrites and achondrites with the same isotopic distribution. Results of experimental efforts to measure this composition directly in meteorites are promising but not yet conclusive. U-Xe also appears as a possible base component in interstellar silicon carbide, here with superimposed excesses of ¹³⁴Xe and ¹³⁶Xe six-fold larger than those in the solar wind. These compositional differences imply mixing of U-Xe with a nucleogenetic heavy-isotope component whose relative abundance in the solar accretion disk and in pre-solar environments varied both spatially and temporally. In contrast to Earth, the U-Xe signature on Mars was apparently overwhelmed by local accretion of materials rich in either chondritic Xe or solar-wind Xe. Data currently in hand from SNC meteorites on the composition of the present atmosphere are insufficiently precise to constrain a modeling choice between these two candidates for primordial martian Xe. They likewise do not permit definitive resolution of a ²⁴⁴Pu component in the atmosphere although its presence is allowed within current measurement uncertainties. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar power system options for the Radiation and TechnologyDemonstration spacecraft

The Radiation and Technology Demonstration (RTD) Mission has the primary objective of demonstrating high-power (10 kilowatts) electric thruster technologies in Earth orbit. This paper discusses the conceptual design of the RTD spacecraft photovoltaic (PV) power system and mission performance analyses. These power system studies assessed multiple options for PV arrays, battery technologies and bus voltage levels. To quantify performance attributes of these power system options, a dedicated Fortran code was developed to predict power system performance and estimate system mass. The low-thrust mission trajectory was analyzed and important Earth orbital environments were modeled. Baseline power system design options are recommended on the basis of performance, mass and risk/complexity. Important findings from parametric studies are discussed and the resulting impacts to the spacecraft design and cost     

Optimization of project-ballistic parameters for low earth orbit spacecraft with propulsion systems with energy storage

Optimization of project-ballistic parameters of the spacecraft with propulsion systems with energy accumulation during the orbital transfer between low Earth orbits in the non-central gravitational field of the Earth is considered. The mass model of the spacecraft with an uncontrollable power-limited engine with the energy accumulator is used. The method of the project-ballistic optimization is obtained as an iterative procedure. Energy storage usage efficiency during the orbital transfer between low Earth orbits is estimated.     

The STEREO Observatory

The Solar Terrestrial Relations Observatory (STEREO) is the third mission in NASA’s Solar Terrestrial Probes program. The mission is managed by the Goddard Space Flight Center (GSFC) and implemented by The Johns Hopkins University Applied Physics Laboratory (JHU/APL). This two-year mission provides a unique and revolutionary view of the Sun–Earth system. Consisting of two nearly identical observatories, one ahead of Earth in its orbit around the Sun and the other trailing behind the Earth, the spacecraft trace the flow of energy and matter from the Sun to Earth and reveal the three-dimensional structure of coronal mass ejections (CMEs) to help explain their genesis and propagation. From its unique side-viewing vantage point, STEREO also provides alerts for Earth-directed solar ejections. These alerts are broadcast at all times and received either by NASA’s Deep Space Network (DSN) or by various space-weather partners.     

Atmosphere Impact Losses

Determining the origin of volatiles on terrestrial planets and quantifying atmospheric loss during planet formation is crucial for understanding the history and evolution of planetary atmospheres. Using geochemical observations of noble gases and major volatiles we determine what the present day inventory of volatiles tells us about the sources, the accretion process and the early differentiation of the Earth. We further quantify the key volatile loss mechanisms and the atmospheric loss history during Earth’s formation. Volatiles were accreted throughout the Earth’s formation, but Earth’s early accretion history was volatile poor. Although nebular Ne and possible H in the deep mantle might be a fingerprint of this early accretion, most of the mantle does not remember this signature implying that volatile loss occurred during accretion. Present day geochemistry of volatiles shows no evidence of hydrodynamic escape as the isotopic compositions of most volatiles are chondritic. This suggests that atmospheric loss generated by impacts played a major role during Earth’s formation. While many of the volatiles have chondritic isotopic ratios, their relative abundances are certainly not chondritic again suggesting volatile loss tied to impacts. Geochemical evidence of atmospheric loss comes from the \({}^{3}\mathrm{He}/{}^{22}\mathrm{Ne}\), halogen ratios (e.g., F/Cl) and low H/N ratios. In addition, the geochemical ratios indicate that most of the water could have been delivered prior to the Moon forming impact and that the Moon forming impact did not drive off the ocean. Given the importance of impacts in determining the volatile budget of the Earth we examine the contributions to atmospheric loss from both small and large impacts. We find that atmospheric mass loss due to impacts can be characterized into three different regimes: 1) Giant Impacts, that create a strong shock transversing the whole planet and that can lead to atmospheric loss globally. 2) Large enough impactors (\(m_{\mathit{cap}} \gtrsim \sqrt{2} \rho_{0} (\pi h R)^{3/2}\), \(r_{\mathit{cap}}\sim25~\mbox{km}\) for the current Earth), that are able to eject all the atmosphere above the tangent plane of the impact site, where \(h\), \(R\) and \(\rho_{0}\) are the atmospheric scale height, radius of the target, and its atmospheric density at the ground. 3) Small impactors (\(m_{\mathit{min}}>4 \pi\rho_{0} h^{3}\), \(r_{\mathit {min}}\sim 1~\mbox{km}\) for the current Earth), that are only able to eject a fraction of the atmospheric mass above the tangent plane. We demonstrate that per unit impactor mass, small impactors with \(r_{\mathit{min}} < r < r_{\mathit{cap}}\) are the most efficient impactors in eroding the atmosphere. In fact for the current atmospheric mass of the Earth, they are more than five orders of magnitude more efficient (per unit impactor mass) than giant impacts, implying that atmospheric mass loss must have been common. The enormous atmospheric mass loss efficiency of small impactors is due to the fact that most of their impact energy and momentum is directly available for local mass loss, where as in the giant impact regime a lot of energy and momentum is ’wasted’ by having to create a strong shock that can transverse the entirety of the planet such that global atmospheric loss can be achieved. In the absence of any volatile delivery and outgassing, we show that the population of late impactors inferred from the lunar cratering record containing 0.1% \(M_{\oplus }\) is able to erode the entire current Earth’s atmosphere implying that an interplay of erosion, outgassing and volatile delivery is likely responsible for determining the atmospheric mass and composition of the early Earth. Combining geochemical observations with impact models suggest an interesting synergy between small and big impacts, where giant impacts create large magma oceans and small and larger impacts drive the atmospheric loss.     

Effect of areal expansion and coronal heating on the solar wind

Empirical studies have shown that the solar wind speed at Earth is inversely correlated with the areal expansion rate of magnetic flux tubes near the Sun. Recent model calculations that include a self-consistent determination of the coronal temperature allow one to understand the physical basis of this relationship; they also suggest why the solar wind mass flux is relatively constant.     

The Formation of Mars: Building Blocks and Accretion Time Scale

In this review paper I address the current knowledge of the formation of Mars, focusing on its primary constituents, its formation time scale and its small mass compared to Earth and Venus. I argue that the small mass of Mars requires the terrestrial planets to have formed from a narrow annulus of material, rather than a disc extending to Jupiter. The truncation of the outer edge of the disc was most likely the result of giant planet migration, which kept Mars’ mass small. From cosmochemical constraints it is argued that Mars formed in a couple of million years and is essentially a planetary embryo that never grew to a full-fledged planet. This is in agreement with the latest dynamical models. Most of Mars’ building blocks consists of material that formed in the 2 AU to 3 AU region, and is thus more water-rich than that accreted by Earth and Venus. The putative Mars could have consisted of 0.1 % to 0.2 % by mass of water.     

Interplanetary studies: Propagation of disturbances between the Sun and the magnetosphere

This review is concerned with the interplanetary ‘transmission line’ between the Sun and the Earth's magnetosphere. It starts with comments about coronal mass ejections (CMEs) that are associated with various forms of solar activities. It then continues with some of the current views about their continuation through the heliosphere to Earth and elsewhere. The evolution of energy, mass, and momentum transfer is of prime interest since the temporal/spatial/magnitude behavior of the interplanetary electric field and transient solar wind dynamic pressure is relevant to the magnetospheric response (the presence or absence of geomagnetic storms and substorms) at Earth. Energetec particle flux predictions are discussed in the context of solar activity (flares, prominence eruptions) at various positions on the solar disk relative to Earth's central meridian. A number of multi-dimensional magnetohydrodynamic (MHD) models, applied to the solar, near-Sun, and interplanetary portions of the ‘transmission line’, are discussed. These model simulations, necessary to advancing our understanding beyond the phenomenological or morphological stages, are directed to deceptively simple questions such as the following: can one-to-one associations be made between specific forms of solar activity and magnetosphere response?     

Communication from Mars: Requirements and Limitations

The exploration of our nearest planets will require relaying large amounts of data to Earth for study and evaluation. However, our ability to communicate at interplanetary distances is limited. In this paper, an evaluation is made of our capability to communicate from the vicinity of Mars using the present S-band deep-space network, prospects for enhancing that performance, and limitations beyond which no additional improvement seems feasible. In addition, requirements for real-time television are evaluated and prospects for improving the communication rates by operating at higher microwave frequencies considered.     

Capacity as a consideration for providing aeronautical mobilesatellite air traffic services in the US domestic airspace

A preliminary analysis of the capacity (number of aircraft) that could be handled by the first generation American Mobile Satellite Corporation (AMSC) system in the early part of the 21st century is reported. The analysis is based on assumptions for the service demand, the Aeronautical Telecommunication Network (ATN), the communications scheme, the satellite channel and the aircraft Earth station. Capacity is examined in terms of spectral bandwidth required and satellite power limitations. The sensitivity of the results is examined with regard to variations in service demand; spectral efficiencies of different modulation techniques; aircraft antenna equipage, high-gain or low-gain; and the amount of overhead associated with the ATN. With regard to the ATN, the analysis only illustrates the impact on capacity if some of the ATN overhead were eliminated. The feasibility of eliminating this overhead and the possible resulting loss of functionality are not addressed     

The atmospheric Čerenkov technique in the search for PBH

Searches for primordial black holes are reviewed.     

Satellite Skin-Force Modelling for Atmospheric Drag Calculations

Satellites in low Earth orbits are influenced by the Earth’s atmosphere. The interactions between the molecules and the spacecraft cause the highest non-gravitational force, which in magnitude is comparable to planetary disturbances. Therefore the modelling of atmospheric drag effects is important for many missions with a scientific background like STEP (Satellite Test of Equivalence Principle). With the STEP mission variations between gravitational and inertial mass shall be measured with an accuracy of 10^?18. The results are of great interest for cosmological and gravitational theories. To achieve the aimed accuracy, a precise model of external disturbances is necessary. In this article the method of Ray-Tracing is used to quantify the atmospheric drag forces and torques for spacecrafts of arbitrary shape.     

Influence of the Interplanetary Magnetic Field on the Solar Wind Flow about Planetary Obstacles

The main effects caused by the interplanetary magnetic field (IMF) are analyzed in cases of supersonic solar wind flow around magnetized planets (like Earth) and nonmagnetized (like Venus) planets. The IMF has a relatively weak strength in the solar wind but it is enhanced considerably in the so-called plasma depletion layer or magnetic barrier in the vicinity of the streamlined obstacle (magnetopause of a magnetized planet, or ionopause of a nonmagnetized planet). For magnetized planets, the magnetic barrier is a source of free magnetic energy for magnetic reconnection in cases of large magnetic shear at the magnetopause. For nonmagnetized planets, mass loading of the ionospheric particles is very important. The new created ions are accelerated by the electric field related to the IMF, and thus they gain energy from the solar wind plasma. These ions form the boundary layer within the magnetic barrier. This mass loading process affects considerably the profiles of the magnetic field and plasma parameters in the flow region.     

Big Bang Nucleosynthesis and the Density of Baryons in the Universe

Now that extragalactic deuterium observations are being made, Big Bang Nucleosynthesis (BBN) is on the verge of undergoing a transformation. Previously, the emphasis was on demonstrating the concordance of the Big Bang Nucleosynthesis model with the abundances of the light isotopes extrapolated back to their primordial values using stellar and Galactic evolution theories. Once the primordial deuterium abundance is converged upon, the nature of the field will shift to using the much more precise primordial D/H to constrain the more flexible stellar and Galactic evolution models (although the question of potential systematic error in 4He abundance determinations remains open). The remarkable success of the theory to date in establishing the concordance has led to the very robust conclusion of BBN regarding the baryon density. The BBN constraints on the cosmological baryon density are reviewed and demonstrate that the bulk of the baryons are dark and also that the bulk of the matter in the universe is non-baryonic. Comparison of baryonic density arguments from Lyman- clouds, x-ray gas in clusters, and the microwave anisotropy are made and shown to be consistent with the BBN value.     

The Magnetic Field of the Earth’s Lithosphere

The lithospheric contribution to the Earth’s magnetic field is concealed in magnetic field data that have now been measured over several decades from ground to satellite altitudes. The lithospheric field results from the superposition of induced and remanent magnetisations. It therefore brings an essential constraint on the magnetic properties of rocks of the Earth’s sub-surface that would otherwise be difficult to characterize. Measuring, extracting, interpreting and even defining the magnetic field of the Earth’s lithosphere is however challenging. In this paper, we review the difficulties encountered. We briefly summarize the various contributions to the Earth’s magnetic field that hamper the correct identification of the lithospheric component. Such difficulties could be partially alleviated with the joint analysis of multi-level magnetic field observations, even though one cannot avoid making compromises in building models and maps of the magnetic field of the Earth’s lithosphere at various altitudes. Keeping in mind these compromises is crucial when lithospheric field models are interpreted and correlated with other geophysical information. We illustrate this discussion with recent advances and results that were exploited to infer statistical properties of the Earth’s lithosphere. The lessons learned in measuring and processing Earth’s magnetic field data may prove fruitful in planetary exploration, where magnetism is one of the few remotely accessible internal properties.     

Mass Profiles of Galaxy Clusters from X-ray Analysis

We review the methods adopted to reconstruct the mass profiles in X-ray luminous galaxy clusters. We discuss the limitations and the biases affecting these measurements and how these mass profiles can be used as cosmological proxies.     

连续地月转移系统动力学研究与能量分析

为了研究新型连续地月转移系统的动力学及能量需求,采用Lagrange方法,在系绳为刚性杆假设的前提下,同时忽略第三体引力、地球扁率和系绳轴向变形等扰动因素的影响,建立了驱动型动量交换绳系卫星(MMET)系统的三维刚性动力学模型。对所建立的动力学模型进行了数值仿真及对比分析,仿真结果验证了所建模型的正确性。研究表明,外力矩对系统轨道运动参数影响甚小,对姿态运动参数影响明显。采用MMET方式进行载荷转移,推导出了实现载荷地月轨道转移所需的入口速度条件以及时间周期条件,并求解出了载荷在2次任务之间的时间间隔。给定初始条件下,当MMET系统以0.231 6 rad/s的旋转角速度绕其质心旋转1 448.5圈,其绕地心刚好运行5圈时,载荷可顺利进入地月转移轨道。最后,对连续地月转移系统实现载荷的地月转移进行了能量对比分析,结果表明,相同条件下,MMET载荷转移方式相比于传统脉冲变轨方式在载荷转移过程中消耗更少的能量。     

核聚变空间推进器的初步需求分析

参考一种简化的、解析近似的计算模型，以地球到火星的对接任务和往返任务为例，对核聚变等离子体推进器性能关键参数如推进系统质量、比冲、任务时间、有效载荷份额及比功率等进行分析，得出了任务时间和有效载荷质量份额与聚变堆芯输出功率、推进器结构质量和比冲的依赖关系。在此基础上，结合核聚变地面商业堆的相关进展，对现有的技术做合理适当外推，理论计算表明:飞行任务能够在1~2个月内达到目的星球同时可携带超过10%的有效载荷份额，并提出了未来核聚变空间推进器的初步参数方案和设计构想，总体上能够为未来核聚变空间推进技术的发展提供一定的参考。      

Deuterium Observations in the Galaxy

An accurate measurement of the primordial value of D/H would provide one of the best tests of nucleosynthesis models for the early Universe and the baryon density. Such evaluations have been traditionally made using present estimations of the deuterium abundance in the interstellar medium, extrapolated backwards in time with the use of galactic evolution models. Direct estimations of the primordial deuterium abundance have been carried out only recently in QSOs absorbers at high redshift.We will summarize galactic observations of deuterium and suggest that, perhaps, a single D/H value for the interstellar medium is not representative. These evaluations mainly came from observations completed in the far UV with first the Copernicus satellite over the Lyman lines series followed then by H and D Lyman-alpha lines observations with both the IUE and the GHRS on the Hubble Space Telescope. We discuss different known systematics and show that the situation is not yet clear. It is not possible today to claim that we know "the" D/H value in the interstellar medium, if any.Overall and in the context of additional D observations made in the solar system, we conclude that the actual evolution of deuterium from Big-Bang nucleosynthesis to now is not yet understood. More observations, recently made with IMAPS (the Interstellar Medium Absorption Profile Spectrograph) and hopefully to be made with FUSE (the Far Ultraviolet Spectroscopic Explorer to be launched in the fall of 1998), at higher spectral resolution or in many different galactic sites are certainly needed to help us reach a better global view of the evolution of that key element, and thus better constrain any evaluation of its primordial abundance.