衝突破片 1999 JU3 のスペクトル特性
S. Sugita1 , D. Kuroda2, S. Kameda3, S. Hasegawa4, S. Kamata1, T. Hiroi4, M. Abe5, M. Ishiguro6, N. Takato2, andM. Yoshikawa5, 1Univ. of Tokyo, Kashiwa, Chiba 277-8561, JAPAN (sugita@k.u-tokyo.ac.jp), 2Natnl. Astron. Obs.
of Japan, 3Rikkyo Univ., 4Japan Aerospace Exploration Agency, 5Brown Univ., 6Seoul Natnl Univ.
Introduction: Among several hundred asteroids
whose visible reflectance spectra have been measured,1999 JU3 is among the few C-type asteroids that can bereached with small !V from the Earth [1]. This makes
this asteroid a very attractive target for a near futuresample-return mission. In fact, 1999 JU3 is currently
chosen for the primary exploration target of JAXA’s
Hayabusa 2 and the secondary target for both OSIRISREx mission by NASA and Marco-Polo mission byESA. Thus, detailed investigation of this NEA is very
important for near future planetary mission projects.
In particular, both spatial inhomogeneity and temporal variation in the spectral properties of the mission
target NEA would give a large impact on remotesensing strategy and sampling site selection processes.
In this study, we observe visible spectrum of 1999 JU3
over multiple spin phases to examine how much spatial
inhomogeneity is present and analyze newly obtained
and literature spectrum of 1999 JU3 closely to infermaterial properties.
Observations and Data Reduction: A series ofvisible spectroscopic observations of 1999 JU3 wereconducted with the 8.1-m-aperture Gemini-South telescope in Chili using GMOS instrument on June 24, 26,and July 5. The apparent visible magnitude of 1999
JU3 was between 19.13 and 19.66 during our observations. The phase angle was relatively small between
22.7 and 30.3°. Two standard stars (HD142801,SA107-998) were used for calibration. Although thethree observations cover a wide rotational phases, theobservation on July 5th suffered from background starcontamination.
Observation Results: Very flat spectra similar tothe Sept. 2007 spectrum by [2] were observed in our
three observations (Fig. 1). There is no obvious 0.7 µm
feature in our spectra. The fact that three flat spectra
without clear 0.7 µm absorption were observed at different rotational phases strongly suggest that most1999 JU3 surface is covered by material with flat spectrum at least the latitude range seen from the Earthduring our observation period.
Data Analyses: We conducted three different
analyses: principal-component analysis (PCA) [3],modified Gaussian modeling (MGM) [4], and comparison with spectra of heated meteorite samples [5].
The PCA indicates that the three of our spectra

Figure 1. Comparison between reflectance spectra ofasteroid 1999 JU3 and those of heated Murchison meteorite samples at different temperatures [4]. The top
and bottom spectra are from [6] and [2], respectively.
The middle three spectra are observed in this study.
are clustered together near the spectra obtained in Sept.
2007 by [2] (Fig. 2). The MGM analysis of the July
2007 spectrum reveals that the central wavelength !ctr
and breadth "! of the best-fit Gaussian for the July
2007 spectrum follow the same !ctr–"! trend for theCh-type MBA’s. This coincidence strongly suggeststhat the possible 0.7 µm absorption feature in the July
2007 spectrum may be real. Thus, the presence of
0.7µm absorption on 1999 JU3 should not be ruled out
at this point. Nevertheless, the fact that most 1999 JU3spectra obtained so far do not exhibit 0.7µm absorption
band signatures indicates that material(s) with 0.7µm
absorption would cover only a small surface area of1999 JU3. Furthermore, the comparison between meteorites and the great variety of 1999 JU3 spectra revealthat the spectrum observed by [3] in May 1999, that by[2] in Sept. 2007, and those in this study in June toJuly 2012 can be fit well by Murchison chondrite samples heated at different temperatures (Fig. 1).

Implications for 1999 JU3: If the rotation pole is
upright as Müller et al. [7] obtained as the optimal solution, then it would be difficult to understand how astrong 0.7!m absorption feature would be missed bymany observers during this observation opportunity in2012 [8-10]. However, the pole vector determination is
not unique yet. There are multiple local minimum solutions to reproduce the light curve observations results
because 1999 JU3 has a nearly spherical shape andvery small light curve amplitude [e.g., 7].
This apparent contradiction could be resolved if
1999 JU3 has a tilted spin axis and the azimuth of thespin axis is directed to the Earth in summer of 2012 asshown in Fig. 3. In this geometric configuration, wewould have seen only a pole of 1999 JU3 in May-July
2012. Then asteroid spin does not change the area visible from the Earth. In contrast, we would have seen theequatorial regions in summer of 2007 in this configuration. Then more variety of spectra could appear as theasteroid rotates. Higher quality light-curve data would
be able to answer this question.
A possible Parent body for 1999 JU3: Recent dynamical calculations indicate that 1999 JU3 very likely
came from the #6 resonance in the main belt [11].
There are only three asteroid families with low albedo(<0.1) near the #6 resonance: Sulamitis, Polana, andErigone. Because the Sulamitis family members arevery tightly distributed around the main asteroid 752
Sulamitis, however, kilometer-size fragments from thisfamily are unlikely to have reached the #6 resonance[11]. This makes Polana and Erigone families the most
likely source families for 1999 JU3. Although both 142
Polana and 163 Erigone possess low albedo and relatively flat visible spectra, they have distinctly different
visible spectra. The former exhibits clear blue slope
between 0.45 and 1 µm and is classified as B type [12].
None of the 1999 JU3 spectra observed so far is similar
to this 163 Polana spectrum.
In contrast, the flat visible spectra observed in thisstudy as well as that in September 2007 are similar tothat of 163 Erigone as shown in Fig. 4. Also, the potential 0.7 µm absorption feature suggested from theabove MGM analysis of the July 2007 spectrum is also
consistent with the 163 Erigone spectrum. Thesepieces of evidence support the possibility that theparent body of 1999JU3 may be 163 Erigone or its
own larger parent body.
Here it has to be noted, however, that the above
discussion is based on the premise that 1999 JU3 is amember of a collision family. It may have been abackground asteroid near #6. Nevertheless, such distinction between a family asteroid near the #6 resonan-ce and a background asteroid could be made if its
fragmentation age is measured from returned samples.

References: [1] Binzel, R.P. et al., 2004, MAPS, 39,351–366. [2] Vilas, 2008, Astron. J., 135, 1101-1105. [3]Bus, S. and R. Binzel, 2002, Icarus, 158, 146-177. [4] Hiroi,T. et al., 1996, MAPS, 31, 321-327. [5] Hiroi, T. and F. Vilas,
1995, LPSC, 26, 611-612. [6] Binzel, R. P. et al., 2001, Icarus, 151, 139-149. [7] Müller, T. et al., A&A, 525, 1–6. [8]Sugita, S. et al., 2012, DPS, 44, 102.02. [9] Moskovitz, N. etal., DPS 44, 102.04. [10] Lazzaro, D., et al., 2013, A&A, inpress. [11] Campins, H. et al., ACM Mtg., #6452. [12] Clark,B. E., 2011, Icarus 216, 462–475. [13] Abe, M. et al. 2008,LPSC, 39, #1594.

Figure 2. Comparison of principal components, PC2’and PC3’, between SMASSII asteroids [3] and 1999
JU3. The blue asterisk and diamond are for [6] and [2],respectively. The red symbols are for our new spectrain June to July 2012 observed with GEMINI-S.

Figure 3. A possible azimuthal direction of the spinpole of asteroid 1999 JU3.

Figure 4. Comparison of reflectance spectra among
142 Polana [3], 1999JU3 [3, 13], and 163 Erigone [3].