Takahiro Iwata

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。

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.

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.

We have examined the integrated science of the Moon using KAGUYA (SELENE). The strategy is summarized as: 1) drawing two-dimensional maps to integrate various geologic units into a coherent map, 2) drawing three-dimensional maps of subsurface structures beneath maria and highlands, 3) joint studies of special topics such as mare tectonics and crustal formation, 4) joint studies of advanced topics such as dichotomy and bulk composition. Using the strategy, we will shed light on the tectonics of maria; crustal formation; explorations of polar regions; inner structure of multi-ring crater - Orientale; and lunar environments to figure out the origin and evolution of the Moon.

KAGUYA (SELENE) is a Japanese lunar mission which will be launched using H-IIA vehicle in September 2007 by JAXA. We are planning to improve lunar gravity field model by four-way Doppler measurements and differential VLBI observations using three orbiters of KAGUYA (SELENE) mission. KAGUYA is composed of the main orbiter and two small free-flyer subsatellites, i.e., the Relay Satellite (Rstar) and the VLBI Radio Satellite (Vstar). Each subsatellite weighs approximately 50kg.

The observation of Asteroid Itokawa by Hayabusa spacecraft has brought us a lot of new knowledge for very small (about 500m in length) S-type asteroid. We already have started to consider next space mission to small bodies in the solar system. For the next asteroid mission, we would like to explore C-type asteroid, which is also major type in the asteroid belt. This type of asteroid is supposed to have organic matter or water more than S-type asteroid, and it is important to study the material related to life. In this paper, we present the current plan for the next exploration to asteroids. We also want to encourage many researchers to join our asteroid mission.