Publ. Astron. Soc. Japan (2014) 00(0), 1–31 1 doi: 10.1093/pasj/xxx000 Physical properties of near-Earth asteroids with a low delta-v: Survey of target candidates for the Hayabusa2 mission Sunao HASEGAWA,1,* Daisuke KURODA,2 Kohei KITAZATO,3 Toshihiro KASUGA,4,5 Tomohiko SEKIGUCHI,6 Naruhisa TAKATO,7 Kentaro AOKI,7 Akira ARAI,8 Young-Jun CHOI,9 Tetsuharu FUSE,10 Hidekazu HANAYAMA,11 Takashi HATTORI,7 Hsiang-Yao HSIAO,12 Nobunari KASHIKAWA,13 Nobuyuki KAWAI,14 Kyoko KAWAKAMI,1,15 Daisuke KINOSHITA,12 Steve LARSON,16 Chi-Sheng LIN,12 Seidai MIYASAKA,17 Naoya MIURA,18 Shogo NAGAYAMA,4 Yu NAGUMO,5 Setsuko NISHIHARA,1,15 Yohei OHBA,1,15 Kouji OHTA,19 Youichi OHYAMA,20 Shin-ichiro OKUMURA,21 Yuki SARUGAKU,8 Yasuhiro SHIMIZU,22 Yuhei TAKAGI,7 Jun TAKAHASHI,23 Hiroyuki TODA,2 Seitaro URAKAWA,21 Fumihiko USUI,24 Makoto WATANABE,25 Paul WEISSMAN,26 Kenshi YANAGISAWA,27 Hongu YANG,9 Michitoshi YOSHIDA,7 Makoto YOSHIKAWA,1 Masateru ISHIGURO,28 and Masanao ABE1 1Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan 2Okayama Astronomical Observatory, Kyoto University, 3037-5 Honjo, Kamogata-cho, Asakuchi, Okayama 719-0232, Japan 3Research Center for Advanced Information Science and Technology, The University of Aizu, Tsuruga, Ikki-machi, Aizu-Wakamatsu, Fukushima 965-8580, Japan 4Public Relations Center, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka-shi, Tokyo 181-8588, Japan 5Department of Physics, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan 6Department of Education, Hokkaido University of Education, 9 Hokumon, Asahikawa-shi, Hokkaido 070-8621, Japan 7Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 9672, USA arXiv:1810.03706v2 [astro-ph.EP] 10 Oct 2018 8Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan 9Space Science Division, Korea Astronomy and Space Science Institute, 776, Daedeokdae-ro, Yuseong-gu, Daejeon, Korea 10Strategic Planning Department, National Institute of Information and Communications Technology, 4-2-1 Nukuikitama-chi, Koganei, Tokyo 184-8795, Japan 11Ishigakijima Astronomical Observatory, National Astronomical Observatory of Japan, 1024-1 Arakawa, Ishigaki, Okinawa 907-0024, Japan 12Institute of Astronomy, National Central University, 300 Zhongda Rd., Zhongli, Taoyuan City 320, Taiwan, R.O.C. 13Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan c 2014. Astronomical Society of Japan. 2 Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 14Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan 15Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 16Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, Arizona 85721-0092, USA 17Tokyo Metropolitan Government, 2-8-1 Nishishinjyuku, Shinjyuku-ku, Tokyo 163-8001, Japan 18Department of Multi-Disciplinary Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan 19Department of Astronomy, Kyoto University, Kitashirakawaoiwake-cho, Sakyo, Kyoto 606-8502, Japan 20Academia Sinica, Institute of Astronomy and Astrophysics, 11F of Astronomy-Mathematics Building, AS/NTU, No.1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan, R.O.C. 21Japan Spaceguard Association, Bisei Spaceguard Center 1716-3 Okura, Bisei, Ibara, Okayama 714-1411, Japan 22Subaru Telescope Okayama Branch Office, National Astronomical Observatory of Japan, 3037-5 Honjo, Kamogata-cho, Asakuchi, Okayama 719-0232, Japan 23Nishi-Harima Astronomical Observatory, University of Hyogo, 407-2, Nishigaichi, Sayo-cho, Sayo, Hyogo 679-5313, Japan 24Center for Planetary Science, Kobe University, 7-1-48, Minatojima-minamimachi, Chuo-Ku, Kobe 650-0047, Japan 25Department of Applied Physics, Okayama University of Science, 1-1 Ridai, Kita-ku, Okayama, Okayama 700-0005, Japan 26Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, Arizona 85719, USA 27Division of Optical and Infrared Astronomy, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka-shi, Tokyo 181-8588, Japan 28Department of Physical and Astronomy, Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ∗E-mail: [email protected] Received ; Accepted Abstract Sample return from the near-Earth asteroid known as 25143 Itokawa was conducted as part of the Hayabusa mission, with a large number of scientific findings being derived from the re- turned samples. Following the Hayabusa mission, Hayabusa2 was planned, targeting sample return from a primitive asteroid. The primary target body of Hayabusa2 was asteroid 162173 Ryugu; however, it was also necessary to gather physical information for backup target se- lection. Therefore, we examined five asteroids spectroscopically, 43 asteroids spectrophoto- metrically, and 41 asteroids through periodic analysis. Hence, the physical properties of 74 near-Earth asteroids were obtained, which helped the Hayabusa2 backup target search and, also, furthered understanding of the physical properties of individual asteroids and their origins. Key words: methods: observational — minor planets, asteroids: general — techniques: photometric — techniques:spectroscopic 1 Introduction teorites, and interplanetary dust particles have supplied impor- tant clues on their chemical composition, the temperature and Cosmochemical, mineralogical, and petrological analyses of pressure conditions they have encountered, and on solar sys- extraterrestrial materials such as Apollo and Luna samples, me- Publications of the Astronomical Society of Japan, (2014), Vol. 00, No. 0 3 tem formation, composition, and evolution. Therefore, sample addition, while it is necessary to know the physical charac- return missions from major or minor planets can provide con- teristics of the primary target, it is also important to obtain siderable scientific information compared to flyby and/or ren- those of backup candidates with consideration of potential mis- dezvous missions. sion problems. Therefore, we conducted observations to ac- In the 1990s, the Mu Space Engineering Spacecraft quire the physical properties of backup target candidates for the (MUSES)-C mission was implemented with the aim of success- Hayabusa2 mission. ful sample return from an asteroid (Fujiwara et al. 2000; 2004). In this paper, we report on the physical properties of the The MUSES-C spacecraft was launched in 2003, but renamed NEAs with low delta-v examined for the Hayabusa2 mission. “Hayabusa” after launch. In 2005, Hayabusa rendezvoused with The subsequent sections of this paper describe the observations the S-complex asteroid known as 25143 Itokawa and performed and data reduction procedures (section 2), present the results sample collection of material on the asteroid surface (Fujiwara on the asteroid physical properties (section 3), and discuss the et al. 2006). Samples taken from Itokawa were brought to implications of our findings (section 4). Earth in 2010 and analysis was performed. Those samples provided important planetary science information regarding the meteorite source, the thermal and impact history of Itokawa, 2 Observations and data reduction the space weathering process on asteroids (Ebihara et al. 2011; procedures Nagao et al 2011; Nakamura et al. 2011; Noguchi et al. 2011; The asteroid observations were performed from December 2000 Tsuchiyama et al. 2011; Yurimoto et al. 2011). to January 2015, using 10 different telescopes at eight sites in Following the success of the Itokawa rendezvous performed Japan and the USA, with photometry and spectroscopy obser- by the Hayabusa spacecraft, a subsequent asteroid exploration vations. Five NEA spectra were recorded using the Subaru project, “Hayabusa2”, was planned (Tsuda et al. 2013). The ob- Telescope on Mt. Mauna Kea, Hawaii, USA (Minor Planet jective of the Hayabusa mission was demonstration of sample Center (MPC) code: 568) and at the Okayama Astrophysical return technology; thus, the target body was an object reachable Observatory in Okayama, Japan (MPC code: 371). Colors by the Mu-5 launch vehicle. The purpose of Hayabusa2, which were recorded for 38 NEAs, along with five other asteroids, launched in 2014 and is expected to return to Earth in 2020, at the Kiso Observatory in Nagano, Japan (MPC code: 381); is sample return from a chondritic asteroid such as a C- or D- the Lulin Observatory in Nantou, Taiwan (MPC code: D35); complex body. S-complex asteroids, i.e., of the same spectral the UH88 Telescope on Mt. Mauna Kea (MPC code: 568); type as Itokawa, are excluded. Thus, the C-complex asteroid the Steward Observatory on Mt. Bigelow, Arizona, USA (MPC 162173 Ryugu was selected as the primary target of Hayabusa2. code: 698); and the Nayoro Observatory in Hokkaido, Japan Ryugu was chosen because it is a C-complex asteroid hav- (MPC code: Q33). The lightcurves of 39 NEAs and two ing one of the lowest delta-v among the near-Earth asteroids other asteroids were obtained at the Kiso Observatory; the (NEAs) known at the time (delta-v is the velocity adjustment Lulin Observatory; the Nayoro Observatory; the Ishigakijima needed to transfer a spacecraft from low-Earth orbit to connect Astronomical Observatory at Okinawa, Japan (MPC code: with the asteroid (Shoemaker & Helin 1978)). Hayabusa2 ren- D44); the Nishiharima Astronomical Observatory