Photometric Observations of Asteroid 4 Vesta by the OSIRIS Cameras Onboard the Rosetta Spacecraft
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A&A 533, L9 (2011) Astronomy DOI: 10.1051/0004-6361/201117600 & c ESO 2011 Astrophysics Letter to the Editor Photometric observations of asteroid 4 Vesta by the OSIRIS cameras onboard the Rosetta spacecraft S. Fornasier1,2, S. Mottola3, M. A. Barucci1, H. Sierks4, and S. Hviid4 1 LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Pricipal Cedex, France e-mail: [email protected] 2 Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France 3 Institute of Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany 4 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany Received 30 June 2011 / Accepted 13 August 2011 ABSTRACT Aims. We report on new observations of asteroid 4 Vesta obtained on 1 May 2010 with the optical system OSIRIS onboard the ESA Rosetta mission. One lightcurve was taken at a phase angle (52◦) larger than achievable from ground-based observations together with a spectrophotometric sequence covering the 260 to 990 nm wavelength range. Methods. Aperture photometry was used to derive the Vesta flux at several wavelengths. A Fourier analysis and the HG system formalism were applied to derive the Vesta rotational period and characterize its phase function. Results. We find a G parameter value of 0.27 ± 0.01 and an absolute magnitude H(R) = 2.80 ± 0.01. The lightcurve has the largest amplitude ever reported for Vesta (0.19 ± 0.01 mag), and we derive a synodic rotational period of 5.355 ± 0.025 h. The Rosetta spectrophotometry, covering the Vesta western hemisphere, is in perfect agreement with visible spectra from the literature and close to the IUE observations related to the same hemisphere. The new spectrophotometric data reveal that there is no global ultraviolet/visible reversal on Vesta. The Vesta spectrophotometry is well reproduced by spectra of howardite meteorite powders (grain size <25 μm). From the Rosetta absolute spectrophotometry and from the phase function behaviour, we estimate a geometric albedo of 0.36 ± 0.02 at 649 nm and 0.34 ± 0.02 at 535 nm. Key words. minor planets, asteroids: individual: 4 Vesta – techniques: photometric 1. Introduction Vesta is the first target of the NASA Dawn mission, that just entered orbit around the asteroid in July 2011 and will remain in Vesta is the most massive asteroid in the main belt (consider- orbit for one complete year, undertaking a detailed study of its ing that Ceres has been classified as a dwarf planet) orbiting geophysics, mineralogy, and geochemistry (Russell et al. 2007; the Sun at a distance of about 2.36 AU. From HST images, Sierks et al. 2011). Thomas et al. (1997a) derived a Vesta shape fit by an ellipsoid ± In this paper we report on observations of Vesta obtained with semi-axes of 289, 280, 229 ( 5) km, and J2000 pole co- with the scientific optical camera system OSIRIS onboard ordinates RA = 308 ± 10◦,Dec= 48 ± 10◦, which was more = ± ◦ = ± ◦ Rosetta. One new lightcurve taken at an unprecedented large recently updated to RA 305.8 3.1 ,Dec 41.4 1.5 (Li phase angle (52◦) has been obtained together with spectropho- et al. 2011a). These HST data were also analysed to uncover a tometry from 260 to 990 nm. These data, taken at a nearly equa- large (460 km wide) basin near the south pole with a pronounced torial aspect, allow us to more tightly constrain the Vesta phase central peak of 13 km, together with other depressions and large function, to provide the absolute reflectance from the UV to the craters (Thomas et al. 1997b). The discovery of substantial im- NIR range at large phase angle, and to estimate the geometric pact excavation on Vesta is consistent with the idea that this albedo. asteroid is the source of HED meteorites. Several “Vestoids”, i.e. asteroids with spectral properties and orbital parameters sim- ilar to Vesta, have also been discovered. They have therefore 2. Observations and data reduction probably been caused by impact events on Vesta. Vesta is large enough to have experienced a differentiation OSIRIS is the imaging system onboard Rosetta mission. It con- ◦ phase during its accretion and is the only large asteroid known sists of a narrow-angle camera (NAC, field of view of 2 × 2 )and ◦ to have a basaltic surface that retains a record of ancient volcanic a wide-angle camera (WAC, field of view of 12 × 12 ). They are activity (McCord et al. 1970;Gaffey et al. 1997). Geological unobstructed mirror systems with focal lengths of 72 cm and diversity revealing longitudinal variations in albedo and miner- 14 cm, respectively. Both cameras are equipped with 2048 × alogy was detected from polarimetric and spectroscopic mea- 2048 pixel CCD detectors with a pixel size of 13.5 μm. The im- surements obtained at different rotational phases (Gaffey 1997; age scale is 3.9 arcsec/pixel for the NAC and 20.5 arcsec/pixel Binzel et al. 1997;Lietal.2010). for the WAC. The cameras have a set of broadband and nar- rowband filters covering the wavelength range 240−990 nm. Table 2 is available in electronic form at We refer to Keller et al. (2007) for a detailed description of http://www.aanda.org the instrument. Article published by EDP Sciences L9, page 1 of 5 A&A 533, L9 (2011) During its approach to asteroid 21 Lutetia, which was suc- 4.3 cessfully flown by on 10 July 2010, Rosetta observed the aster- 4 Vesta oid 4 Vesta on 1 May 2010 for a total duration of 10.8 h. These observations included lightcurve coverage with the two “red” fil- 4.4 ) ters of the cameras (the NAC filter F22 centred at 649.2 nm, ° and the WAC filter F12, centred at 629.8 nm), and a spectropho- =52.26 4.5 tometric sequence including 12 NAC filters and 9 WAC filters, α covering the wavelength range 269−989 nm (Table 1). The ob- servations were made at a spacecraft-target distance 0.285879 < R(1, 1, Δ < 0.283682 AU, at a Vesta heliocentric distance of 2.324 AU, 4.6 F12 P = 5.355 hr and at a phase angle 52.1 <α<52.8◦, the largest one covered syn F22 T = 55317.0 MJD up to date, before the DAWN encounter. Vesta was observed at 0 equatorial aspect, at a sub-spacecraft latitude of 3.0−2.7◦,and 4.7 was not resolved (resolution of 800 km/px and 4000 km/px with 0.0 0.2 0.4 0.6 0.8 1.0 1.2 the NAC and WAC cameras, respectively). Rotational Phase The data were reduced using the OSIRIS standard pipeline, Fig. 1. The lightcurve of asteroid 4 Vesta from OSIRIS observations. but the flux calibration was done using revised values of the The arrow show the rotational phase corresponding to the spectropho- conversion factors that had been recently computed. The data tometric data set acquisition. Points beyond rotational phase 1.0 are reduction steps are the same as those described in Küppers repeated for clarity. F12 and F22 are the red filters of the WAC and et al. (2007). The data are converted from digital units NAC cameras, respectively. to W m−2 nm−1 sr−1, using conversion factors derived from observations of Vega taken on 1 May 2010 and 12 July 2010. These factors were reduced to a solar in- put spectrum. The absolute calibration factors were com- puted using the Vega (alpha_lyr_stis_005.ascii)andsun flux (sun_reference_stis_001.ascii) standard spectra from the HST CALSPEC catalogue1. The Vesta flux was calculated from the images using aper- ture photometry with an aperture radius of five (WAC) and six (NAC) pixels. These radii allow us to acquire more than 99% of the target flux and to minimize the background contribution and cosmic ray hits. Aperture correction factors were derived from the high signal-to-noise ratio observations of Vega and the fluxes were divided by 0.992446 for the WAC and 0.992918 for the NAC observations to get the total flux (estimated from Vega growth curves for an aperture of 15 px). The background was evaluated in four rectangular regions around the target and then subtracted from the total flux. Fig. 2. Phase curve of asteroid 4 Vesta. Beside the Rosetta point, data come from the Asteroid Photometric Catalogue (Lagerkvist et al. 1995) and from Hasegawa et al. (2009). The dashed-line represents the linear 3. Lightcurve and phase function fit to the data. We used the 44 WAC and NAC images in the red filters to build the Vesta lightcurve. To derive the absolute magnitude at the ob- served phase angle, we follow the method described by Küppers behaviour. Its amplitude is 0.19 ± 0.01, the largest one observed et al. (2007). We first corrected the Vesta flux by considering its so far for Vesta as a consequence of the well-known amplitude- spectrum, which is redder than that of the sun, using: phase effect (Zappalá et al. 1995). The Vesta time-series were modelled with a ninth order F(λ)T(λ)dλ Fourier polynomial (Harris et al. 1989), whose best-fit relation = × λ Fc Fo , (1) corresponded to a synodic rotation period (P ) of 5.355 ± F (λ)T(λ)dλ syn λ Vesta 0.025 h. The relatively large error is related to the limited time- span of the Rosetta observations. The data folded with the best- where Fc and Fo are the corrected and uncorrected Vesta fit period are shown in Fig.