吉川 真

<p> 2018年11月，「はやぶさ２」のミッションは，その前半が終了した．2018年6月27日に目的地である小惑星リュウグウに到着し，リモートセンシングによる観測は一通り終了した．また，小型ローバMINERVA-II1と小型着陸機MASCOTをリュウグウ表面に降ろすことにも成功し，これらからのデータの取得もできた．ただし，サンプルを採取するためのタッチダウンは，2019年に延期となった．これは，リュウグウ表面の至る所に岩塊が存在しており，安全に着陸できる十分に広い場所が見つからなかったため，探査機のより高精度なナビゲーションが必要になったためである．ここでは，「はやぶさ２」の誕生の経緯からミッション前半までをまとめてみることにする．</p>

The MASCOT Camera (MasCam) is part of the Mobile Asteroid Surface Scout (MASCOT) lander’s science payload. MASCOT has been launched to asteroid (162173) Ryugu onboard JAXA’s Hayabusa 2 asteroid sample return mission on Dec 3rd, 2014. It is scheduled to arrive at Ryugu in 2018, and return samples to Earth by 2020. MasCam was designed and built by DLR’s Institute of Planetary Research, together with Airbus-DS Germany. The scientific goals of the MasCam investigation are to provide ground truth for the orbiter’s remote sensing observations, provide context for measurements by the other lander instruments (radiometer, spectrometer and magnetometer), the orbiter sampling experiment, and characterize the geological context, compositional variations and physical properties of the surface (e.g. rock and regolith particle size distributions). During daytime, clear filter images will be acquired. During night, illumination of the dark surface is performed by an LED array, equipped with 4 × 36 monochromatic light-emitting diodes (LEDs) working in four spectral bands. Color imaging will allow the identification of spectrally distinct surface units. Continued imaging during the surface mission phase and the acquisition of image series at different sun angles over the course of an asteroid day will contribute to the physical characterization of the surface and also allow the investigation of time-dependent processes and to determine the photometric properties of the regolith. The MasCam observations, combined with the MASCOT hyperspectral microscope (MMEGA) and radiometer (MARA) thermal observations, will cover a wide range of observational scales and serve as a strong tie point between Hayabusa 2’s remote-sensing scales (10 –10 m) and sample scales (10 –10 m). The descent sequence and the close-up images will reveal the surface features over a broad range of scales, allowing an assessment of the surface’s diversity and close the gap between the orbital observations and those made by the in-situ measurements. The MasCam is mounted inside the lander slightly tilted, such that the center of its 54.8° square field-of-view is directed towards the surface at an angle of 22° with respect to the surface plane. This is to ensure that both the surface close to the lander and the horizon are observable. The camera optics is designed according to the Scheimpflug principle, thus that the entire scene along the camera’s depth of field (150 mm to infinity) is in focus. The camera utilizes a 1024 × 1024 pixel CMOS sensor sensitive in the 400–1000 nm wavelength range, peaking at 600–700 nm. Together with the f-16 optics, this yields a nominal ground resolution of 150 micron/px at 150 mm distance (diffraction limited). The camera flight model has undergone standard radiometric and geometric calibration both at the component and system (lander) level. MasCam relies on the use of wavelet compression to maximize data return within stringent mission downlink limits. All calibration and flight data products will be generated and archived in the Planetary Data System in PDS image format. 3 − 3 − 3 − 6

<p>1801年に最初の小惑星であるケレス（Ceres）が発見されてから200年余り経った現在，発見されている小惑星の数は70万個を超えた．探査機が接近した小惑星も10個ほどになり，地上観測では分からなかった小惑星のいろいろな特徴が見えてきた．また，日本の探査機「はやぶさ」によって，世界初の小惑星サンプルリターンも成功し，小惑星の表面物質の分析も進んでいる．このように急速に理解が進む小惑星について，ここでは探査機による探査の視点から紹介する．</p>