岩田 隆浩

第30回大気圏シンポジウム（2016年12月5日-6日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県 30th Atmospheric Science Symposium (December 5-6, 2016. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan 著者人数: 12名ほか 資料番号: SA6000062035 レポート番号: VII-1

第48回月・惑星シンポジウム (2015年7月29-31日. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS)), 相模原市, 神奈川県 48th ISAS Lunar and Planetary Symposium (July 29-31, 2015. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan 著者人数: 14名ほか 資料番号: SA6000045020

木星のラグランジュ点付近に存在するトロヤ群小惑星の起源は、よくわかっていない。我々は（１）種々の氷・含水鉱物・有機物が特徴的な吸収を持つ赤外域（とくに2500$\sim$5000 nmの波長域）でトロヤ群小惑星のイメージング分光を行う母機（２）小惑星表面に降り立ち揮発性の高い炭素・水素・窒素・酸素などの同位体比をその場測定する子機、から成るソーラー電力セイル探査機の検討をすすめている。2020年代に打ち上げるために進行中の、ミッション設計・探査候補天体のサーベイ・搭載機器開発について報告する。

We are developing a small telescope for observations of the Lunar rotation with a target accuracy of 1 milli-second of arc, and it has a potential to observe deflection of the vertical on the ground with high accuracy. As the results of laboratory experiments, it is possible that the vibration of the mercury surface caused by the ground vibrations lead to fluctuations of star positions on CCD as large as 0.1 seconds of arc. The amplitude of the fluctuations depend on the amplitude and the frequency of the ground vibrations and the depth of the mercury pool. The vessel for mercury made by copper with the diameter of 84mm and the depth of 0.5mm is the best judging from our experience. It is important to keep the proper period of the mercury pool away from the period of ground vibrations in order to avoid the resonance. It is also effective to lengthen the integration time not only for the effect of ground vibrations but for the effects of atmospheric fluctuations, and it can improve the reliability of the mean value of the center of a star image by statistical procedure. We investigate the cause of fluctuations which can affect the observations on the ground, and we explore the possibility for a new effective observations with the telescope like PZT.

Japanese lunar explorer SELENE (KAGUYA) was equipped with 14 instruments for various measurements of the Moon. Three of these instruments took geodetic measurements of the Moon. These were two sub-satellites and a laser altimeter. They achieved all the missions expected before launch and obtained excellent results. The main results obtained by the instruments are 1) precise orbit determination with an accuracy of ten meters by Doppler and same-beam VLBI, 2) the first precise gravity fields on the lunar far side by 4-way Doppler measurements, 3) the first topography in latitudes higher than 86 degrees, 4) a global map of the gravity anomaly by using the global topography and the global gravity fields, 5) a global map of the lunar crustal thickness and 6) an illumination rate map in the north and south polar regions. These results open the new stage of lunar geodesy now.

[1] Based on the latest SELENE lunar gravity and topography model obtained by Kaguya mission, we compute the lunar crustal thickness map to investigate differences between farside basin structures. The thickest crust is located in the southern rim of the Dirichlet-Jackson basin and the thinnest crust at the Moscoviense basin. The thickest crust corresponds to the highest topography and is consistent with Airy isostasy. The thinnest crust is due to an abnormally large mantle plug. The crustal thicknesses at Apollo 12/14 sites of our crustal thickness model are 45.1 and 49.9 km. The crustal thickness map indicates that the differences between recently proposed type I and type II basins are probably controlled by the ratio between pre-impact crustal thickness and impact scale. Copyright 2009 by the American Geophysical Union.

The Japanese lunar explorer SELENE (KAGUYA), which has been launched on Sep. 14th, 2007, utilizes VLBI observations in lunar gravimetry investigations. This can particularly improve the accuracy of the low degree gravitational harmonics. Combination of ground based VLBI observations and Doppler measurements of the spacecrafts enable three dimensional orbit determinations and it can improve the knowledge of the gravity field near the limb. Differential VLBI Radio sources called VRAD experiment involves two on-board sub-satellites, Rstar (Okina) and Vstar (Ouna). These will be observed using differential VLBI to measure the trajectories of the satellites with the Japanese network named VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network.<BR>Two new techniques, a multi-frequency VLBI method and the same-beam VLBI method, are used to precisely measure the angular distance between the two sub-satellite radio sources Okina and Ouna. The observations are at three frequencies in S-band, 2212, 2218 and 2287 MHz, and one in X-band, 8456 MHz. We have succeeded in making VLBI observations of Okina/Ouna with VERA and the international network, and have also succeeded in correlating of signals from Okina/Ouna, and obtained phase delays with an accuracy of several pico-seconds in S-band.

Results of numerical simulations are presented to examine the global gravity field recovery capability of the Japanese lunar exploration project SELENE (SELenological and ENgineering Explorer) which will be launched in 2007. New characteristics of the SELENE lunar gravimetry include 4-way satellite-to-satellite Doppler tracking of main orbiter and differential VLBI tracking of two small free-flier satellites. It is shown that the proposed satellite constellation will provide the first truly global satellite tracking data coverage. The expected results from these data are; (1) drastic reduction in far-side gravity error, (2) estimation of many gravity coefficients by the observation, not by a priori information, and (3) one order of magnitude improvement over existing gravity models for low-degree field. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

The Japanese lunar explorer SELENE (SElenological and Engineering Explorer), to be launched in 2007, will for the first time utilize VLBI observations in lunar gravimetry investigations. This will particularly improve the accuracy to which the low degree gravitational harmonics and the gravity field near the limb can be measured, and when combined with Doppler measurements will enable three-dimensional information to be extracted. Differential VLBI Radio sources called VRAD experiment involves two on-board sub-satellites, Rstar and Vstar. These will be observed using differential VLBI to measure the trajectories of the satellites with the Japanese network named VERA (VLBI Exploration of Radio Astrometry) and an international VLBI network. We will use a multi-frequency VLBI method to measure the angular distance between the two sub-satellite radio sources Rstar and Vstar. The observations will be at three frequencies in S-band, 2212, 2218 and 2287 MHz, and one in X-band, 8456 MHz. This method uses low power consumption carrier waves, and is suitable for the positioning of spacecraft. The Japanese domestic VLBI network, VERA, will conduct VLBI observations for the whole mission period of one year. In addition, we will conduct two periods of intensive observations, each one month in duration, which will also include the international stations, Shanghai, Urumqi (China), Hobart (Australia) and Wettzell (Germany). These observations will measure the phase delay to an accuracy of better than 0.17 rad (10 deg) in X-band. For a baseline length of 2000 km this is equivalent to a positioning accuracy of about 20 cm, on the Moon. Combining the data from the tracking of the 3 SELENE satellites (main orbiter, Rstar and Vstar) at different :Attitudes and from four-way Doppler measurements, and by making long term observations of the sub-satellites (in excess of one year without any maneuvering), we will be able to improve the accuracy to which the lunar gravitational field is known by an order of magnitude. We have completed the development of on-board instruments, and are carrying out pre-flight tests under a variety of conditions. We have also undertaken test VLBI observations of orbiters such as Geotail, Nozomi and Smart-1 with the international network. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

かぐや（SELENE）による月の統合サイエンスのストラテジーとして、１）表面の２次元地質図の作成、２）３次元構造の定量化、３）地殻の形成など個別の科学目標の研究、４）月の熱史などより上位のサイエンスへ、との４ステップを構築した。これにより、特に海のテクトニクス、地殻の形成、極域探査、多重リング盆地の地下構造、月周囲の空間に注目して、解明しようとしており、その内容について論じる。

かぐや（SELENE）には、月の裏側の重力を調べるためのリレー衛星、衛星位置決定精度を高めて重力場精度を向上させる目的のVLBI衛星、さらに月の全面の詳細地形を調べるためのレーザー高度計が搭載されている。この月重力・測地探査の現状・運用計画と、期待できる科学成果について詳説する。

小惑星探査機「はやぶさ」による小惑星イトカワの観測で、500m程度の小さなＳ型小惑星についての理解が深まったが、我々は、さらに次の小天体探査ミッションについての検討を行っている。次のミッションとしては、Ｓ型と同様に小惑星帯で主要なタイプとなっているＣ型小惑星の探査を行いたい。このタイプでは、有機物や水をより多く含んでいると思われており、生命前駆物質の科学としても重要である。ここでは、これまでのミッション検討結果をまとめて報告する。また、是非、多くの研究者に小天体探査に参加してもらうことを呼びかけたい。