一种新型多普勒噪声抑制技术对BepiColombo任务无线电科学实验的性能提升

在欧洲空间局和日本宇宙开发机构联合开展的BepiColombo水星任务中，将开展水星轨道器无线电科学实验，包括估计水星的引力场及其旋转状态，并对广义相对论进行验证。目前地面系统和星上设备的主流配置可以在无线电科学实验中建立X/X、X/Ka和Ka/Ka多个频段的链路，测速精度可达3 um/s（1 000 s积分），测距精度为20 cm。提出了基于时延机械噪声对消技术提高无线电科学实验性能的方案。时延机械噪声对消技术需要处理在两个测站不同时刻的测量数据，一个测站实施双向多普勒测距，对另一个单收测站的要求较为严格，该测站需要具有较好的对流层条件。这种方法能够显著降低Ka频段双向链路的主要测量噪声，包括由对流层和天线机械系统震动引起的噪声。我们给出了端到端的仿真性能，并估计了在使用时延机械噪声对消技术前提下的水星引力场和旋转状态。考虑使用NASA位于美国本土戈尔德斯敦的DSS-25天线或欧空局位于阿根廷马拉圭的DSA-3天线作为双程测量站，并考虑使用位于智力的APEX天文观测天线作为单收站。分析结果表明在最好的噪声条件下，使用DSA-3天线作为双程测量站时，时延机械噪声对消技术可将待估计的全局和局部参数的估计精度提升一倍。对于无线电科学实验的目标，这一可能的性能提升对行星地质物理学很有意义，它将有益于研究水星内部的结构。     

Mercury Ion Analyzer (MIA) onboard Mercury Magnetospheric Orbiter: MMO

The Mercury Magnetopsheric Orbiter (MMO) is one of the spacecraft of the BepiColombo mission; the mission is scheduled for launch in 2014 and plans to revisit Mercury with modern instrumentation. MMO is to elucidate the detailed plasma structure and dynamics around Mercury, one of the least-explored planets in our solar system. The Mercury Plasma Particle Experiment (MPPE) on board MMO is a comprehensive instrument package for plasma, high-energy particle, and energetic neutral particle atom measurements. The Mercury Ion Analyzer (MIA) is one of the plasma instruments of MPPE, and measures the three dimensional velocity distribution of low-energy ions (from 5 eV to 30 keV) by using a top-hat electrostatic analyzer for half a spin period (2 s). By combining both the mechanical and electrical sensitivity controls, MIA has a wide dynamic range of count rates for the proton flux expected around Mercury, which ranges from 10⁶ to 10¹² cm⁻² s⁻¹ str⁻¹ keV⁻¹, in the solar wind between 0.3 and 0.47 AU from the sun, and in both the hot and cold plasma sheet of Mercury’s magnetosphere. The geometrical factor of MIA is variable, ranging from 1.0 × 10⁻⁷ cm² str keV/keV for large fluxes of solar wind ions to 4.7 × 10⁻⁴ cm² str keV/keV for small fluxes of magnetospheric ions. The entrance grid used for the mechanical sensitivity control of incident ions also work to significantly reduce the contamination of solar UV radiation, whose intensity is about 10 times larger than that around Earth’s orbit.     

Missions to Mercury

Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun’s gravitational well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner 10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late 1990s, two missions—MESSENGER and BepiColombo—received the go-ahead; MESSENGER is on its way to its first encounter with Mercury in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in detail.     

Laboratory calibration measurements of a piezoelectric lead zirconate titanate cosmic dust detector at low velocities

A cosmic dust monitor for use onboard a spacecraft is currently being developed using a piezoelectric lead zirconate titanate element (PZT). Its characteristics of the PZT sensor is studied by ground-based laboratory impact experiments using hypervelocity particles supplied by a Van de Graaff accelerator. The output signals obtained from the sensor just after the impact appeared to have a waveform that was explicitly related to the particle’s impact velocity. For velocities less than ∼6 km/s, the signal showed an oscillation pattern and the amplitude was proportional to the momentum of the impacting particle. For higher velocities, the signal gradually changed to a single waveform. The rise time of this single waveform was proportional to the particle’s velocity for velocities above ∼6 km/s. The present paper reports on results for the low velocity case and especially discusses the effect of an outer coating of the sensor with a paint, which is used to reduce heating by solar radiation.     

Attempt to identify a source mechanism of Mercury’s sodium exosphere by a spectrometer using Fabry–Perot etalon

The Mercury’s Sodium Atmosphere Spectral Imager (MSASI) on BepiColombo (BC) will address a range of fundamental scientific questions pertaining to Mercury’s exosphere. The measurements will provide new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral range around sodium D2 emission (589 nm). A tandem Fabry–Perot etalon is used to achieve a compact design. We presents a design of the spectral analyzer using Fabry–Perot interferometer. We conclude that: (1) The MSASI optical design is practical and can be implemented without new or critical technology developments; (2) The thermally-tuned etalon design is based on concepts, designs and materials that have good space heritage.     

Current status of the BepiColombo/MMO spacecraft design

This paper shows the current baseline of the conceptual design of the BepiColombo/MMO (mercury magnetospheric orbiter) spacecraft, which is conducted by the ISAS Mercury Exploration Working Group. The MMO is a spinning spacecraft of 200 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and subsystems such as thermal control, communication, power, etc. are described. The latest status of the development of critical technologies for the MMO and the outline of the international cooperation between ESA and ISAS are also presented.     

Optical performance of PHEBUS/EUV detector onboard BepiColombo

BepiColombo, a mission of ESA (European Space Agency) in cooperation with JAXA (Japan Aerospace Exploration Agency), will explore Mercury, the planet closest to the Sun. BepiColombo will launch in 2014 on a journey lasting up to six and a half years; the data gathering phase should occupy a one year nominal mission, with a possible extension of another year. The data which will be brought back from the orbiters will tell us about the Hermean surface, atmospheric composition, and magnetospheric dynamics; it will also contribute to understanding the history and formation of terrestrial planets. The PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) instrument will be flown on MPO: Mercury Planetary Orbiter, one of the two BepiColombo orbiters. The main purpose of the instrument is to reveal the composition and the distribution of the exosphere of Mercury through EUV (Extreme Ultraviolet: 55–155 nm) and FUV (Far Ultraviolet: 145–315 nm) measurements. A consortium composed of four main countries has been formed to build it. Japan provides the two detectors (EUV and FUV), Russia implements the scanning system, and France and Italy take charge of the overall design, assembly, test, integration, and also provide two small NUV (Near Ultraviolet) detectors (for the light from calcium and potassium molecules). An optical prototype of the EUV detector which is identical to the flight configuration has been manufactured and evaluated. In this paper, we show the first spectra results observed by the EUV channel optical prototype. We also describe the design of PHEBUS and discuss the possibility of detecting noble gases in Mercury’s exosphere taking the experimental results so far into account.     

Development of the EUV detector for the BepiColombo mission

An ultraviolet spectrometer, PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) that is loaded onto the Mercury Planetary Orbiter in the BepiColombo mission is under development. The instrument, basically consisting of two spectrophotometers (EUV: 50–150 nm, FUV: 145–330 nm) and one scanning mirror, aims at measuring emission lines from molecules, atoms and ions present in the tenuous atmosphere of Mercury. The detectors employ microchannel plates as 2-D photon-counting devices. In order to enhance the quantum detection efficiencies, the surface of the top microchannel plates of EUV detector is covered with photocathode. This method enables us to identify weak atmospheric signatures such as neon (73.5 nm) and argon (104.8 nm), which could not be detected with conventional detector systems. This paper presents measurements of the performance characteristics of potassium bromide and esium iodide photocathodes, which have been evaluated for use in the EUV channel.     

Mercury’s sodium exosphere explored by the BepiColombo mission

The Mercury’s Sodium Atmosphere Spectral Imager (MSASI) on BepiColombo will address fundamental scientific questions pertaining to the Mercury’s sodium exosphere. Together, our measurements on the overall scale will provide ample new information on regolith–exosphere–magnetosphere coupling as well as new understanding of the dynamics governing the surface-bounded exosphere. We will compare the four different source mechanisms in preparation for modeling MSASI data and show the feasibility of identifying a process.     

Navigating BepiColombo during the weak-stability capture at Mercury

BepiColombo is scheduled for launch in August 2013 and to arrive after a nearly six-year long transfer at Mercury in June 2019. The trajectory has a number of challenging elements: a launch with Soyuz/Fregat into a geostationary transfer orbit, followed by a lunar flyby, long low-thrust arcs and five more planetary flybys (one at the Earth, two at Venus and two at Mercury). At arrival the low thrust arcs reduce the approach velocity so much that BepiColombo passes by the Sun–Mercury Lagrange points L₁ and L₂ and gets weakly captured in a highly eccentric orbit around Mercury in case the orbit insertion manoeuvre would fail.This paper describes the navigation strategy during the final phase. Five trajectory correction manouevres during the last 65 days requiring up to 20 m/s (3σ) are proposed. With this strategy it is possible to navigate BepiColombo safely through the weak-stability boundary of Mercury and to reach the target periherm with a precision of 11 km.