Titan’s new pole: Implications for the Huygens entry and descent trajectory and landing coordinates

The European Space Agency’s Huygens probe separated from the NASA Cassini spacecraft on 25 December 2004, after having been attached for a 7-year interplanetary journey and three orbits around Saturn. The probe reached the predefined NASA/ESA interface point on 14 January 2005 at 09:05:52.523 (UTC). It performed a successful entry and descent sequence and softly landed on Titan’s surface on the same day at 11:38:10.77 (UTC) with a speed of about 4.54 m/s. Since the publication of the official project entry and descent trajectory reconstruction effort by the Descent Trajectory Working Group in 2007 (referred to as DTWG#4) various other efforts have been performed and published. This paper presents an overview of the most relevant reconstructions and compares their methodologies and results. Furthermore, the results of a new reconstruction effort (DTWG#5) are presented, which is based on the same methodology as DTWG#4 but takes into account new estimates of Titan’s pole coordinates which were derived from radar images of different Cassini Titan flybys. It can be shown that the primary effect can be observed in the meridional direction which is represented by a stark southward shift of the trajectory by about 0.3 deg. A much smaller effect is seen in the zonal direction (i.e., less than 0.01 deg in the west to east direction). The revised probe landing coordinates are 192.335 deg W and 10.573 deg S. A comparison of these coordinates with results of recent landing site investigations using visual and radar images of the Cassini VIMS instrument shows excellent agreement of the two independently derived landing coordinates, i.e., longitude and latitude residuals of respectively 0.035 deg and 0.007 deg.     

“嫦娥4号”月球背面着陆区月壳及深部结构特征

深部月壳和月幔物质结构是月球科学探测的关键问题之一。“嫦娥4号”初步将月球背面南极—艾肯（South Pole-Aitken,SPA）盆地内的冯·卡门（Von Kármán）撞击坑作为着陆点,具有重要的科学研究价值。结合月球重力、地形、布格重力、月壳厚度等地球物理数据,综合对冯·卡门撞击坑的月壳及其深部结构特征进行了分析。结果显示：冯·卡门撞击坑重复撞击到南部的冯·卡门M撞击坑上,后者的中央峰具有明显的布格正重力异常和线性的布格重力梯度特征,显示出高密度的幔部物质向上涌起;冯·卡门撞击坑极有可能穿透了该区域的整个月壳,并挖掘出了深部月幔的物质;该区域南部月壳厚度较薄小于5 km,北部平均月壳厚度在15～20 km,月壳平均密度为2 630 kg·m^-3,比背面高地月壳密度高,且平均孔隙度为9%,低于月球的平均孔隙度12%。     

冯·卡门撞击坑物质成分研究

“嫦娥4号”预计在月球背面的冯·卡门（Von Kármán）撞击坑着陆,为近距离研究月球最大和最古老的南极—艾肯（South Pole-Aitken）盆地的形成和演化提供了机会。综合利用多种光谱数据,本研究分析了冯·卡门撞击坑及其周边地区TiO₂和FeO的含量变化,以及铁镁质矿物的分布情况,并推断了其主要的岩石类型。研究结果表明：冯·卡门撞击坑内部贫TiO₂（约1.5~2.5 wt%）富FeO（约12~16 wt%）,主要岩石类型为低钛玄武岩。撞击坑以外的地层则以苏长岩质物质（低钙辉石）为主,并含有约1 wt%的TiO₂和10 wt%的FeO。此外,冯·卡门撞击坑外的南部局fFf), and infilled by low-Ti basalts. The materials outside the crater are dominated by noritic materials (Low-Ca pyroxene) with abundance of TiO₂(~1 wt%)and FeO(~10 wt%). In addition, some plagioclase-rich layers are also exposed on the southern region outside the Von Kármán crater.     

“嫦娥4号”中继星任务分析与系统设计

作为"嫦娥4号"任务的重要组成部分,中继星将为着陆器和巡视器提供中继通信支持。不同于其它月球探测器,中继星首次选择了绕地月L2平动点运行的晕(Halo)轨道以保证对月球背面的着陆器和巡视器提供连续的中继通信服务,面临诸多技术挑战。在对中继星任务特点进行分析的基础上,梳理了研制中的技术难题,包括使命轨道的选择、使命轨道的到达和长期维持、中继通信体制选择等,并提出了解决方案。中继星的总体设计方案概述也在文中给出。     

Improving magnetosphere in situ observations using solar sails

Past and current magnetosphere missions employ conventional spacecraft formations for in situ observations of the geomagnetic tail. Conventional spacecraft flying in inertially fixed Keplerian orbits are only aligned with the geomagnetic tail once per year, since the geomagnetic tail is always aligned with the Earth-Sun line, and therefore, rotates annually. Solar sails are able to artificially create sun-synchronous orbits such that the orbit apse line remains aligned with the geomagnetic tail line throughout the entire year. This continuous presence in the geomagnetic tail can significantly increase the science phase for magnetosphere missions. In this paper, the problem of solar sail formation design is explored using nonlinear programming to design optimal two-craft, triangle, and tetrahedron solar sail formations, in terms of formation quality and formation stability. The designed formations are directly compared to the formations used in NASA’s Magnetospheric Multi-Scale mission.     

Traverses for the ISECG-GER design reference mission for humans on the lunar surface

This study explores the Design Reference Mission (DRM) architecture developed by Hufenbach et al. (2015) as a prelude to the release of the 2018 Global Exploration Roadmap (GER) developed by the International Space Exploration Coordination Group (ISECG). The focus of this study is the exploration of the south polar region of the Moon, a region that has not been visited by any human missions, yet exhibits a multitude of scientifically important locations – the investigation of which will address long standing questions in lunar research. This DRM architecture involves five landing sites (Malapert massif, South Pole/Shackleton crater, Schrödinger basin, Antoniadi crater, and the South Pole-Aitken basin center), to be visited in sequential years by crew, beginning in 2028. Two Lunar Electric Rovers (LER) are proposed to be tele-robotically operated between sites to rendez-vous with crew at the time of the next landing. With engineering parameters in mind we explore the feasibility of tele-robotic operation of these LERs between lunar landing sites, and identify potential high interest sampling locations en-route. Additionally, in-depth sample collection and return traverses are identified for each individual landing site across key geologic terrains that also detail crew Extra-Vehicular Activity (EVA). Exploration at and between landing sites is designed to address a suite of National Research Council (2007) scientific concepts.     

Impact of a novel hybrid accelerometer on satellite gravimetry performance

The state-of-the-art electrostatic accelerometers (EA) used for the retrieval of non-gravitational forces acting on a satellite constitute a core component of every dedicated gravity field mission. However, due to their difficult-to-control thermal drift in the low observation frequencies, they are also one of the most limiting factors of the achievable performance of gravity recovery. Recently, a hybrid accelerometer consisting of a regular EA and a novel cold atom interferometer (CAI) that features a time-invariant observation stability and constantly recalibrates the EA has been developed in order to remedy this major drawback. In this paper we aim to assess the value of the hybrid accelerometer for gravity field retrieval in the context of GRACE-type and Bender-type missions by means of numerical closed-loop simulations where possible noise specifications of the novel instrument are considered and different components of the Earth’s gravity field signal are added subsequently. It is shown that the quality of the gravity field solutions is mainly dependent on the CAI’s measurement accuracy. While a low CAI performance (10^?8 to 10^?9?m/s²/Hz^1/2) does not lead to any gains compared to a stand-alone EA, a sufficiently high one (10^?11?m/s²/Hz^1/2) may improve the retrieval performance by over one order of magnitude. We also show that improvements which are limited to low-frequency observations may even propagate into high spherical harmonic degrees. Further, the accelerometer performance seems to play a less prominent role if the overall observation geometry is improved as it is the case for a Bender-type mission. The impact of the accelerometer measurements diminishes further when temporal variations of the gravity field are introduced, pointing out the need for proper de-aliasing techniques. An additional study reveals that the hybrid accelerometer is – contrary to a stand-alone EA – widely unaffected by scale factor instabilities.     

Numerical simulation of time delay interferometry for a LISA-like mission with the simplification of having only one interferometer

In order to attain the requisite sensitivity for LISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In a previous paper (Dhurandhar, S.V., Nayak, K.R., Vinet, J.-Y. Time delay interferometry for LISA with one arm dysfunctional. Class. Quantum Grav. 27, 135013, 2010), we have found a large family of second-generation analytic solutions of time delay interferometry with one arm dysfunctional, and we also estimated the laser noise due to residual time-delay semi-analytically from orbit perturbations due to Earth. Since other planets and solar-system bodies also perturb the orbits of LISA spacecraft and affect the time delay interferometry (TDI), we simulate the time delay numerically in this paper for all solutions with the generation number n ? 3. We have worked out a set of 3-year optimized mission orbits of LISA spacecraft starting at January 1, 2021 using the CGC2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second-generation solutions of our previous paper, and compare with the semi-analytic error estimate. The accuracy of this calculation is better than 1 cm (or 30 ps). The maximum path length difference, for all configuration calculated, is below 1 m (3 ns). This is well below the limit under which the laser frequency noise is required to be suppressed. The numerical simulation in this paper can be applied to other space-borne interferometers for gravitational wave detection with the simplification of having only one interferometer.     

Dark energy,co-evolution of massive black holes with galaxies,and ASTROD-GW

The detection of low frequency band (100 nHz–100 mHz) and very low frequency band (300 pHz–100 nHz) gravitational waves (GWs) is important for exploration of the equation of state of dark energy and the co-evolution of massive black holes (MBHs) with galaxies. Most galaxies are believed to have a massive black hole in the galactic core. In the formation of these black holes, merging and accretion are the two main processes. Merging of massive black holes generate GWs which could be detected by space GW detectors and Pulsar Timing Arrays (PTAs) to cosmological distances. LISA (Laser-Interferometric Space Antenna) is most sensitive to the frequency band 1 mHz–100 mHz, ASTROD-GW (ASTROD [Astrodynamical Space Test of Relativity using Optical Devices] optimized for Gravitational Wave detection) is most sensitive to the frequency band 100 nHz–1 mHz and PTAs are most sensitive to the frequency band 300 pHz–100 nHz. In this paper, we discuss the sensitivities and outlooks of detection of GWs from binary massive black holes in these frequency bands with an emphasis on ASTROD-GW. The GWs generated by the inspirals, merging and subsequent ringdowns of binary black holes are standard sirens to the cosmological distance. Using GW observations, we discuss the methods for determining the equation of state of dark energy and for testing the co-evolution models of massive black holes. ASTROD-GW is an optimization of ASTROD to focus on the goal of detection of GWs. The mission orbits of the 3 spacecraft forming a nearly equilateral triangular array are chosen to be near the Sun-Earth Lagrange points L3, L4 and L5. The 3 spacecraft range interferometrically with one another with arm length about 260 million kilometers. With 52 times longer in arm length compared to that of LISA, the strain detection sensitivity is 52 times better toward larger wavelength. The scientific aim is focused for gravitational wave detection at low frequency. The science goals include detection of GWs from MBHs, and Extreme-Mass-Ratio Black Hole Inspirals (EMRI), and using these observations to find the evolution of the equation of state of dark energy and to explore the co-evolution of massive black holes with galaxies.     

深空探测器自主控制技术综述

深空探测是人类考察、勘探和定居地球外其它天体的第1步,而深空探测器的自主控制技术则是确保深空探测任务成功完成的重要关键技术之一。从"自主导航、自主制导与控制、自主任务规划、自主故障诊断与重构"4个方面对深空探测器自主控制技术的研究现状进行综述,分析了已有的深空探测器自主控制技术存在的问题,并根据深空探测技术发展和任务实施的需求,提出了深空探测器自主控制技术未来研究的发展趋势。