DESTINY+ TARGET ASTEROID (3200) PHAETHON: RECENT UNDERSTANDINGS FROM 2017
OBSERVATION CAMPAIGN AND THE MISSION SCIENCE OVERVIEW.
T. Arai1, M. Kobayashi1, K. Ishibashi1, F. Yoshida1, H. Kimura1, T. Hirai1, P. Hong1, K. Wada1, H. Senshu1, M.
Yamada1, O. Okudaira1, S. Kameda2, R. Srama3, H. Krüger4, M. Ishiguro5, H. Yabuta6, T. Nakamura7, J. Watanabe8,T. Ito8, K. Ohtsuka8,21, S. Tachibana9, T. Mikouchi9, M. Komatsu10, K. Nakamura-Messenger11, S. Messenger11, S.
Sasaki12, T. Hiroi13, S. Abe14, S. Urakawa15, N. Hirata16, H. Demura16, G. Komatsu1, 17, T. Noguchi18, T. Sekiguchi19,
T. Inamori20, T. Yanagisawa22, T. Okamoto22, H. Yano22, M. Yoshikawa22, T. Ohtsubo22, T. Okada22, T. Iwata22, H.
Toyota21, K. Nishiyama22, , Y. Kawakatsu21 and T. Takashima21, 1Planetary Exploration Research Center (PERC),
Chiba Institute of Technology, Chiba 275-0016, Japan (tomoko.arai@it-chiba.ac.jp), 2Rikkyo University, Japan,
3
Institut für Raumfahrtsysteme, Stuttgart University. 4Max Planck Institute for Solar System Research, 5Seoul National University, South Korea, 6Hiroshima University, Japan, 7Tohoku University, Japan, 8National AstronomicalObservatory of Japan, 9The University of Tokyo, Japan, 10The Graduate University for Advanced Studies, Japan,11
NASA Johnson Space Center, TX, U.S.A, 12Osaka University, Japan, 13Brown University, RI, U.S.A, 14Nihon University, Japan, 15Japan Spaceguard Association, Japan, 16Aizu University, Japan, 17Università d'Annunzio, Italy,
18Kyushu University, Japan, 19Hokkaido University of Education, Japan, 20Nagoya University, Japan, 21Tokyo Meteor Network, Japan, 22ISAS, JAXA, Japan.
Introduction: DESTINY+ (Demonstration and Experiment of Space Technology for INterplanetary voYage, Phaethon fLyby and dUst Science) was selected in2017 as a mission for JAXA/ISAS small-class program[1]. It is a joint mission of technology demonstrationand scientific observation. It will test high performanceelectric propelled vehicle technology and high-speed
flyby of asteroid (3200) Phaethon and possibly asteroid
2005UD, which a break-up body from Phaethon [2] asan extended mission. Engineering challenges include
an up-close encounter at a distance of 500 km fromPhaethon with radio-optical hybrid navigation guidanceand control, and autonomous imaging based on opticalinformation for target tracking during a high-speed
flyby of 33km/sec. The science goal is to understandthe nature and origin of cosmic dust brought onto theEarth, in the context of exogenous contribution of carbon and organics for possible prebiotic seeds of theterrestrial life. Phaethon is a parent body of Geminid
meteor shower [3, 4], and thus a known source to periodically provide dust to the Earth, via the dust stream.
The science objectives are two folded: (1) in-situ analyses of velocity, arrival direction,
mass and chemical compositionof interplanetary and interstellar
dust particles around 1 au, thedust trail, and nearby Phaethon,and (2) flyby imaging of Phaethon to study its geology, forunderstanding dust ejection
mechanism of active asteroid
and the surface compositionalvariation. Planned science observation is illustrated in Fig. 1.
Recentunderstandings
from global observation campaign: Despite numerous studies,

the true nature of Phaethon, such as physical properties(size and geometric albedo), dust ejection mechanism[5], chemical makeup (heated CI/CM chondrites [6] or
CK4 chondrites [7,8] for the meteorite analogues), andthe origin (a Main-belt asteroid (2) Pallas [6,9] or a
Jupiter-family comet [10]) have not been well understood. Phaethon passed 0.07 au from the Earth on December 16, 2017, which is the closest approach since
1974 and until 2093. This apparition is a great opportunity to study this intriguing object for better characterization and for further mission planning and payload
design for DESTINY+. Global astronomical observation campaign of Phaethon was conducted, including
optical, spectroscopic, polarimetric and radar observation with ground and space-based telescopes.
The rotation period of 3.6 hours is verified with refined optical light curves and the shape model has been
updated [11, 12]. A likely larger size (D〜5.7 km) thanpreviously reported values (i.e. 5.1 ± 0.2 km [13] wasreported from the analyses of delay-Doppler observation of the Arecibo observatory [14]. Note that there isa prominent radar dark feature near one of the poles

Fig. 1. Overview of DESTINY+ science observation.

and about 1 km-sized depression-like feature near theequator [14]. With the Arecibo radar data [14] and newlight curves, the bulk density was estimated to be 1.67
± 0.47 gcm-3 [12], which is somewhat smaller compared to that estimated for Geminids meteoroids (2.9g/cm3) [15]. Polarimetric observation made during the2016 apparition [16] and those during the 2017 apparition [17-20] all revealed that Phaethon exhibits extremely large linear polarization. The large polarization
degree with the updated size (5.7 km) led to a significant lower geometric albedo (0.08) than previously
reported values (0.11 ± 0.01, IRAS-derived albedo[21], 0.122 ± 0.008, Spitzer-derived value [13]). The
similar lower albedo is also reported based on thethermal emission [22]. It is suggested that the large
polarization may be attributed to relatively large grain
size on the surface [16]. Visible spectral observation
revealed that blue slopes lacking absorption features,with little rotational variation [22, 23], in line with previous studies [6-9]. Note that visible spectral observation during that 2007 apparition shows rotational color
variation from very blue to neutral [24, 25]. NIR spectral observation at the IRTF revealed concave upwards
from very blue to blue-neutral with increasing wavelength with little rotational variation [22]. The NIR
data also show the lack of a 3 µm feature, suggestingthat the surface is devoid of any hydrated silicates [26].
Recurrent dust ejection has been reported upon theperihelion passage with the STEREO spacecraft [2729]. Optical observation with Hubble Space Telescopeduring the 2017 apparition revealed neither coma nor
dust ejection from Phaethon around 1 au [30-31].
Mission science overview of DESTINY+: Two sets ofcamera (Telescopic Camera for Phaethon (TCAP) andMultiband Camera for Phaethon (MCAP)) are used for
Flyby imaging of Phaethon [32, 33] [Fig. 2]. The surface of Phaethon can be viewed with a spatial resolution of 5m per pixel during the closest flyby around
500 km distant from Phaethon. The high resolution
images are effective to search for geologic features
possibly related to dust ejection. TCAP is equippedwith a one-axis tracking mirror, which can observe
Phaethon with a wide range of solar phase angle (-30deg to 90 deg). MCAP is used to survey the spectralreflectance with multiple bands (400, 480, 550, 700,860, 950 nm). Due to the high-speed flyby of 33km/sec,a filter wheel mechanism for multiband observation is
not feasible. Instead, a compound-eye camera isadapted for simultaneous multiband observation. With
the six bands images, spectral variation in conjunction
with local geologic features obtained by TCAP, i.e.
craters, cracks, and circular pits, will be studied.
In-situ analyses of dust derived from Phaethon isconducted with DESTINY+ Dust Analyzer is a combi-nation of impact-ionization dust detector and time-offlight mass spectrometer, which enables to analyze
mass, arrival direction speed and element composition
for each dust particle [34] (Fig. 2). As implied by the
optical observation with Hubble Space Telescope [3031], dense dust clouds around Phaethon are unlikely.
Yet, there might be dust clouds which are generated as
impact ejecta by bombardment of micrometeoroids on
the surface of Phaethon. Recent numerical simulationsuggests 10 dust particles or more can be detected during flyby at the distance of 500 km from Phaethon [35].
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Fig. 2. DESTINY+ science payloads