An Unusual E-Type Asteroid
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Meteoritics & Planetary Science 43, Nr 5, 905–914 (2008) AUTHOR’S PROOF Abstract available online at http://meteoritics.org Rosetta target asteroid 2867 Steins: An unusual E-type asteroid Paul R. WEISSMAN1*, Michael D. HICKS1, Paul A. ABELL2, 3, Young-Jun CHOI1§, and Stephen C. LOWRY4† 1Jet Propulsion Laboratory, 4800 Oak Grove Drive, Mail Stop 183-301, Pasadena, California, USA 2Planetary Science Institute, 1700 East Fort Lowell Rd., Suite 106, Tucson, Arizona 85719, USA 3NASA Johnson Space Center, Mail Code KR, 2101 NASA Parkway, Houston, Texas 77058, USA 4Queen’s University, Belfast, Astrophysics Research Centre, Department of Physics and Astronomy, Belfast BT7 1NN, UK §Current address: Korea Astronomy and Planetary Science Institute, 61-1 Hwaam dong, Yuseong gu, Daejeon 305-348, South Korea †Current address: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Mail Stop 183-301, Pasadena, California 91109, USA *Corresponding author. E-mail: [email protected] (Received 21 June 2007; revision accepted 20 December 2007) Abstract–ESA’s Rosetta spacecraft will fly by main-belt asteroid 2867 Steins on September 5, 2008. We obtained new visible wavelength spectra of 2867 Steins on December 19, 2006 (UT), using the Palomar 5 m telescope and the facility Double Spectrograph. Two sets of spectra, taken ~3 h apart, one half of the rotation period for 2867 Steins, show it to be an E-type asteroid. The asteroid displays a 0.50 µm feature that is considered diagnostic of the E(II) sub-class, but is deeper than any previously observed E-type. This feature is most likely due to the presence of oldhamite (CaS) on the asteroid’s surface. Also, the observed Steins spectra are far redder than any other known E-types. There is potential evidence for heterogeneity on hemispheric scales, one side of the asteroid appearing to be significantly redder than the other. No known recovered meteorite sample matches the unusual spectra of 2867 Steins, but the closest analog would be similar to an enstatite achondrite (aubrite). INTRODUCTION Asteroid 2867 Steins is located at 2.36 AU in the inner third of the main belt in a fairly typical, low eccentricity and ESA’s Rosetta spacecraft was launched on March 2, low inclination orbit. Orbital elements are shown in Table 1. 2004. The goal of the mission is to rendezvous with periodic Our CCD photometry in 2004 (Weissman et al. 2005, 2007) comet 67P/Churyumov-Gerasimenko (67P/C-G) and spend showed that 2867 Steins rotates with a synodic period of approximately two years studying the comet’s nucleus and 6.048 ± 0.007 h (assuming a double-peaked light curve), in coma in detail. En route to 67P/C-G, Rosetta will fly by two good agreement with earlier determinations of 6.06 ± 0.05 h main belt asteroids, 2867 Steins, on September 5, 2008, and by Hicks et al. (2004) and 6.05 ± 0.01 h by Warner (2004). 21 Lutetia on July 10, 2010. Although Lutetia has been The Rosetta OSIRIS imaging team later found a synodic intensively studied in the past, relatively little was known period of 6.052 ± 0.007 h (Küppers et al. 2007), again in good about 2867 Steins at the time of its selection by the Rosetta agreement with our value. mission in 2004. We also showed that the Steins light curve provides a lower We, along with other Rosetta investigators, are limit to the axial ratio, a/b, of 1.30 ± 0.04. After correcting for conducting a ground-based observational program to fully phase effects, this ratio dropped to 1.19. We fitted the available characterize 2867 Steins prior to the spacecraft flyby. R-filter data to find a phase function of H = 12.92 ± 0.22 and Knowledge of the asteroid’s size, shape, rotation period, G = 0.46 ± 0.20. For a typical S-type albedo of 0.20, this pole orientation, phase function, and taxonomic type are corresponds to a mean radius of 3.08 ± 0.30 km, while for a valuable to the spacecraft science and engineering teams in typical E-type albedo of 0.40, the derived H value corresponds planning the encounter geometry and observations. to a mean radius of 2.18 ± 0.20 km. Our VRI photometry Additionally, comparison between the ground-based and showed that Steins was unusually red in color, again in good Rosetta observations provides important “ground-truth” for agreement with broad-band color photometry by Hicks et al. calibrating remote sensing techniques. We are using both (2004). Hicks et al. suggested that Steins was an S-type asteroid, CCD photometry and CCD spectroscopy to accomplish similar to ordinary chondrite meteorites, but could not rule out these goals. a D-type, comparable to more primitive meteorite types. 905 © The Meteoritical Society, 2008. Printed in USA. 906 P. R. Weissman et al. Table 1. Orbital elements for asteroid 2867 Steins.* The Double Spectrograph is a low-to-medium resolution Semi-major axis 2.36342 AU (R ~ 1,000 to 10,000) grating instrument that uses a dichroic Eccentricity 0.14586 to split light into separate red and blue channels observed Inclination 9.9454 degrees simultaneously. Slits are 128′′ in length and are available in a Argument of perihelion 250.5196 degrees variety of widths. The red channel detector is a 1024 × Longitude of the node 55.5353 degrees 1024 CCD with 24 µm pixels, and the blue channel detector is Time of perihelion 2005 June 23.4379 a 2048 × 4096 CCD with 15 µm pixels. Both CCDs are *Epoch: 2007-Apr-10.0. thinned and anti-reflection coated. From JPL Solar System Dynamics Group. Within the Double Spectrograph, the night sky and object are first imaged on the slit before being divided by the Barucci et al. (2005) classified Steins as an E-type asteroid dichroic into red and blue beams, which are then dispersed based on visual and near-infrared spectra, in particular on the and re-imaged with individual grating and camera set ups. For presence of a strong 0.50 µm absorption feature. E-type our observations we used the 5500 Å dichroic with the 300 asteroids may be similar to enstatite achondrite meteorites and lines mm−1 (3990 Å blaze) grating for the blue channel, and generally display only grey to moderately red colors. However, the 158 lines mm−1 (7500 Å blaze) grating for the red channel. the visual spectrum obtained by Barucci et al. displayed the This gave a spectral dispersion of 2.14 Å channel−1 in the blue same strongly red color as seen in the broad-band photometry and 4.91 Å channel−1 in the red. The telescope was tracked at by Hicks et al. (2004) and Weissman et al. (2005, 2007). the predicted rate of motion of the asteroid. We chose a Fornasier et al. (2006) used polarimetric measurements relatively wide, 6 arcsecond spectroscopic slit. The tailpiece of to estimate the albedo of 2867 Steins. They found a relatively the telescope was rotated such that the slit matched the high albedo of 0.45 ± 0.10. High albedo is consistent with an parallactic angle, in order to negate any possibility of differential E-type taxonomic classification. Lamy et al. (2006) used the refraction effects, which have the potential to cause spurious Spitzer Space Telescope to provide a preliminary albedo slopes at the blue end of high air mass spectra. estimate of 0.35 ± 0.05, again relatively high, though also In addition to 2867 Steins we also collected spectral consistent with some high albedo S-type asteroids. exposures of the solar analog stars 97–29, Ru 149B, SAO This paper reports results of CCD spectroscopy of 2867 130415, PG 918 + 29 c, and 103–487 at a variety of air masses Steins using the facility Double Spectrograph on the Palomar that bracketed and matched the air masses observed for 5 m Hale telescope, in order to further clarify the taxonomic Steins. Wavelength calibrations were accomplished with arc- identity of 2867 Steins. Our results show that this is an unusual lamp exposures and flat-fields were taken using the illuminated E(II)-type asteroid with the strongest 0.50 µm absorption dome as well as the twilight sky. We took care to place Steins band ever seen, and unusually red colors. Furthermore, by and the solar analog stars in the identical place within the slit, obtaining spectra spaced approximately three hours apart, re-centering the objects between exposures. The asteroid was half the rotation period of Steins, we show that there are a few weeks short of opposition at a favorable northerly potential differences in the strength of the red color on the two declination of +33 degrees. The estimated visual magnitude opposing hemispheres. of the asteroid was 16.82 according to the JPL Horizons The organization of this paper is as follows. The next two ephemeris service (Giorgini et al. 2007). Weather conditions sections describe the observations and instrumentation, and the were dry and cold, and the sky was not photometric, with thin data reduction, respectively. After that, four sections present clouds and seeing of ~3 arc-seconds. our analysis of the Steins spectra: a principal component analysis, the spectral type identification, a mineralogic analysis, DATA REDUCTION and a comparison with both asteroid and meteorite analogs, respectively. The last section provides a discussion and The two channels of the spectrograph were analyzed summary of our results. separately and recombined into a composite spectrum spanning from approximately 3500 Å to 9500 Å. Our reductions proceeded OBSERVATIONS in the standard manner and our methods are discussed in greater detail in Hicks and Buratti (2004).