CURRENT STATUS OF DESTINY+ AND UPDATED UNDERSTANDING OF ITS TARGET ASTEROID(3200) PHAETHON. T. Arai1, F. Yoshida2,1, M. Kobayashi1, K. Ishibashi1, H. Kimura1, T. Hirai1, P. Hong1, K.
Wada1, H. Senshu1, M. Yamada1, R. Srama3, H. Krüger4, M. Ishiguro5, H. Yabuta6, T. Nakamura7, S. Kobayashi7, J.
Watanabe8, T. Ito8, T. Ootsubo8, K. Ohtsuka9,, S. Tachibana10, T. Mikouchi10, T. Morota10, M. Komatsu11, K. Nakamura-Messenger12, S. Sasaki13, T. Hiroi14, S. Abe15, S. Urakawa16, N. Hirata17, H. Demura17, G. Komatsu1, 18, T. Noguchi19, T. Sekiguchi20, D. Kinoshita21, H. Kaneda22, S. Kameda23, S. Matsuura24, M. Ito25, A. Yamaguchi26, T. Yanagisawa27, H. Kurosaki27 T. Okamoto28 A. Nakato28, H. Yano28, M. Yoshikawa28, D. W. Dunham29,30, M. W. Buie31,P. A. Taylor32, S. Marshall33, N. Ozaki28, T. Yamamoto28, H. Imamura28, H. Toyota28, K. Nishiyama28, and T. Takashima28, 1Planetary Exploration Research Center (PERC), Chiba Institute of Technology, Chiba, Japan (tomoko.arai@it-chiba.ac.jp), 2 University of Occupational & Environmental Health, Fukuoka, Japan, 3Institut fürRaumfahrtsysteme, Stuttgart University, 4Max Planck Institute for Solar System Research, 5Seoul National University, South Korea, 6Hiroshima University, Japan, 7Tohoku University, Japan, 8National Astronomical Observatory ofJapan, 9Tokyo Meteor Network, Japan, 10The University of Tokyo, Japan, 11The Graduate University for AdvancedStudies, Japan, 12NASA Johnson Space Center, TX, U.S.A, 13Osaka University, Japan, 14Brown University, RI,U.S.A, 15Nihon University, Japan, 16Japan Spaceguard Association, Japan, 17Aizu University, Japan, 18Universitàd'Annunzio, Italy, 19Kyushu University, Japan, 20Hokkaido University of Education, Japan, 21NCU, Taiwan,22Nagoya University, Japan, 23Rikkyo University, Japan, 24Kwansei Gakuin University, Japan, 25JAMSTEC, Japan,26
NIPR, Japan, 27Chohu aerospace center, JAXA, Japan, 28ISAS, JAXA, Japan, 29International Occultation Timing
Association (IOTA), USA, 30KinetX Aerospace, USA, 31Southwest Research Institute, Boulder, CO, USA 32Lunar &Planetary Institute, Houston, TX, USA, 33Arecibo Observatory & University of Central Florida, USA.
Introduction: Asteroid (3200) Phaethon is the parent of Geminid meteor shower [1,2]. It is an activeasteroid, recurrently ejecting dust during the perihelion
passage at 0.14 au [3-5]. Phaethon is the flyby target ofDESTINY+ (Demonstration and Experiment of SpaceTechnology for INterplanetary voYage with Phaethon
fLyby and dUst Science) mission [6]. Detailed understanding of the target asteroid is crucial for the mission
planning and science payload design. Here, we present
the current status of DESTINY+ and review the updated results of extensive astronomical observations ofPhaethon during the close encounter in December,2017 [7] and observation campaigns for stellar occultation by Phaethon in 2019 [8].
Current status of DESTINY+: DESTINY+ is a
joint mission of technology demonstration and scieceobservation, which was selected for JAXA/ISAS
small-class space program in 2017 [8]. The spacecraft
will be launched in 2024 with the enhanced solid-fuelEpsilon rocket into a highly elliptical orbit (270×30000km) and gradually raise its orbit by electric propulsion
for 2 years. After multilple lunar swing-by and subsequent interplanetary cruising, the spacecraft will fly byPhaethon with a distance of 500±50 km at its closest
approach and a relative speed of ~35 km/s in January
2028. The flyby point is around the descending node ofPhaethon with a geocentric distance of 0.33 au and aheliocentric distance of 0.91 au. After Phaethon flyby,
the spacecraft may head to another target asteroid foran extended mission, such as asteroid 2005 UD, which
is a possible breakup body of Phaethon [9].
The science observation includes high-speed flyby
imaging of Phaethon, and direct measurement of physical and chemical properties of dust particles in theinterplanetary space, dust trail and nearby Phaethon.

The flyby imaging is performed with a panchromatic
telescopic camera (TCAP) with a tracking mirror and aVIS-NIR multiband camera (MCAP) with four bands(425, 550, 700, 850 nm) [10]. In-situ dust analyses
with a dust analyser (DDA) which is a combination of
impact-ionization dust detector and time-of-flight mass
spectrometer enables to analyze mass, arrival direction
speed and element composition for dust particle [11].
Updated understanding of Phaethon: Updated
results of astronomical observation are reviewed below.
Rotation period: 3.6 hours is verified with refined optical light curves [12,13]. The both studies consistently
report 3.603957±0.000001 (hr) [12,13].
Pole orientation: Pole orientation is determined withshape models generated from light curves which wereobserved with variable phase angles. Results amongstudies are consistent within uncertainty [12,13,14].
The latest value [14] obtained with light curves and theshape model generated from the Arecibo radar observation [15] shows pole orientation is found to be λ1=316.0°, β1 = -48.7°, indicating a retrograde rotation.
Color: Phaethon is plotted near B, F, C, G-type on thecolor-color diagrams with slight variation amongst
studies [16-19]. Note that they are all slightly shiftedfrom the typical values of the above spectral types.
Absolute magnitude: Due to the lack of observation at
small phase angle (20 deg) the absolution magnitude ofPhaethon (13.6 - 14.5) is determined with relatively
large uncertainty [17,18].
Polarization: Polarimetric observation made during
the 2016 apparition [20] and during the 2017 apparition [21-26] all reveal large linear polarization. The
large polarization may be attributed to largegrain size
(mm size or greater) or large porosity of the surface[e.g. 20]. Rotational variation are reported [23].

Size and shape: Large differences in the size estimate
were present in the Arecibo radar images (6 km, dia.)[15] and thermophysical model-ling results of observations from NEOWISE mission (4.6 km, dia.) [29]. The
size determined by observations of 2019 stellar occultation by Phaethon is 5.67×4.72 km [27, 28]. The size
defined with the occultation observation data shows
5.2km [30] and 5.55 km [31]. The latest shape model
generated with a combination of the Arecibo radar data,multiple light curves and the occultation outcome
shows that the maximum extent along each axis is
6.4×6.2×5.2km and the volume-equivalent diameter is
5.3km (S. Marshall 2021, personal comm.). A prominent radar dark feature is present near the north poleand km-sized depression-like features are shown near
the equator and low latitude regions [15].
Albedo: The variable size estimates with a range ofabsolute magnitude result in a range of current albedo
estimate of 0.079-0.16 [e.g. 20, 29, 32].
Visible reflectance spectra: Visible spectral observation revealed that blue slopes lacking absorption features, with little rotational variation [16, 32], supporintg previous studies. Some observation shows rotational variation [33], in line with observation duringthat 2007 apparition [34].
NIR reflectance spectra: The NIR observation data ofIRTF shows the lack of a 3 µm feature, suggesting thepaucity of hydrated silicates on Phaethon [35].
Dust ejection: Neither coma nor m-size dust around
Phaethon are detected by optical observation withHubble Space Telescope [36-37] and thermal infraredobservation with VLT [38]. Visibe camera onboard
Parker Solar Probe successfully observed dust trail ofPhaethon [39].
Composition of Geminids dust: Spectroscopic observation of 2017 and 2018 Geminids show Na depletionand variation [40], supporting the previous studies.
Unanswered questions for Phaethon:
Uncertainty in albedo: Uncertainty in absolute magnitude because of the lack of optical observation at thesmall phase angles leads to albedo of Phaethon withrelatively uncertainty. Light-curve observation during
the approach phase and global surface imaging of
TCAP with a range of solar phase angle (0-90deg) will
resolve this question.
Visible spectral variation: Variation in spectral slope
in shorter visible wavelength [33] seems to be related
to viewing geometry and may link with semiglobalfeature and/or depend on observed latitude. Spetiallyresolved imaging by TCAP and MCAP of DESTINY+
will uncover the cause of the visible spectral variation.
Dust ejection and abundance: No dust ejection fromPhaethon are not observed by Ground-based andspace-based telescope around 1 au heliocentric distance, where DESTINY+ will flyby Phaethon. However, as is the case of Bennu that OSIRIS-REx navigation

camera unexpectedly observed ejection of cm-sized
dust particles from Bennu during its approach phase[41], DESTINY+ may observe dust ejection from Phaethon with TCAP and/or DDA during the closest flyby.
Mineralogy: CK4 or heated CI/CM chondrites aresuggested for analogues of surface materials of Phaethon [42-44]. In such thermally altered carbonaceouschondrites, phyllosilicates are dehydrated and convered to olivine and thus the absorption of olivine centered at 1.05 micron is typically shown [45,46].
Strangely, 1 micron absorption have not been observed
among any NIR observation [e.g. 32], while MIR spectra of Spitzer suggest the presence of olivine [J. Hanuš,2020 submitted]. This enigmatic question of olivinefeatures between NIR and MIR spectra can not be addressed by DESTINY+ observation and only be resolved by sample return. Returned sample by Bennuand Ryugu may provide the clue for it.
References:[1] Whipple F.L. (1983) IAU Circ., 3881. [2] Williams I. P.
and Wu Z. (1993) MNRAS 262, 231. [3] Jewitt D. and Li J.
(2010) AJ. 140, 1519. [4] Jewitt D.et al. (2013) ApJL, 771,
L36. [5] Hui M.-T. & Li J. (2017) AJ 153, 23. [6] Arai et al.
(2018) LPSC 49th, abstract#2570. [7] Arai et al. (2019) LPSC
50th, abstract#3223. [8] Arai et al. (2020) LPSC 51th, abstract# 2924. [9] Ohtsuka K. et al. (2006) A&A 450, L25.
[10] Ishibashi K. et al. (2021) LPSC 52th, abstract#1405. [11]Kobayashi M. et al. (2018) LPSC 49th, abstract#2050. [12]Kim M. -J. et al. (2018) A&A 619, A123. [13] Hanuš J. et al.
(2018) A&A 620, L8. [14] Marshall S. (2020) In PERC Int'lSymposium on Dust & Parent Bodies 2020. [15] Taylor P. A.
et al. (2019) PSS 167,1. [16] Lee H.-J. et al. (2019) PSS 165,296. [17] Tabeshian M. et al. (2019) AJ 158, 30. [18] Lin ZYi et al. (2020) PSS 180, 104763. [19] Serebryanskiy A. et al.
(2018) In PERC Int'l Symposium on Dust & Parent Bodies
2018.[20] Ito T. et al. (2018) Nature Comm. 9, 2486. [21]Shinnnaka Y. et al. (2018) ApJL 864, L33. [22] Okazaki R.
et al. (2020) PSS 180, 104774. [23] Borisov G. et al. (2018)
MNRAS 480, L131-135. [24] Devogèle M. et al. (2018)
MNRAS 479, 3498. [25] Zheltobryukhov M. et al. (2018)
A&A, A179. [26] Cellino A et al. (2018) MNRAS 481, L49.
[27] Dunham et al. (2019) abstract for Asteroid Science inthe Age of Hayabusa2 and OSIRIS-REx. [28] Buie, M. W.
(2020) In PERC Int'l Symposium on Dust & Parent Bodies
2020. [29] Masiero J. R. et al. (2019) AJ 158, 7. [30] Ye Q.
et al. (2019) Res. Note for AAS, 3, 188. [31] Devogèle M. etal. (2020) PSS 1, 15. [32] Kareta T. et al. (2018) AJ 156, 287.
[33] Lazzarin M. et al. (2019) PSS 165, 115. [34] Ohtsuka K.
et al. (2020) PSS 180, 104940. [35] Takir D. et al. (2020)
Nature Comm, 11:2050. [36] Jewitt D. et al. (2018) AJ 156,238. [37] Ye Q. et al. (2018) ApJL 864, L9. [38] Jewitt D. etal. (2019) AJ 157, 193. [39] Battam K. et al. (2020) ApJS
246, 64. [40] Abe S. et al. (2020) PSS, 194.1. [41] Lauretta D.
S. et al. (2019) Science 366: 3544. [42] Licandro J. et al.
(2007) A&A 461, 751-757. [43] Clark B. E. et al. (2010)
JGRE 115, E06005. [44] de León J. et al. (2012) ICARUS
218, 196. [45] Cloutis E. (2012) Icarus 221, 911. [46] Cloutis
E. (2011) Icarus 216, 309.