A TARGET OF THE ROSETTA MISSION VISIBLE AND INFRARED OBSERVATIONS OF ASTEROID STEINS, A TARGET OF THE ROSETTA MISSION P. LAMY, L. JORDA, S. FORNASSIER, M. KAASALAINEN, S. LOWRY, M.A. BARUCCI, J. CARVANO, CHOI, F. COLAS, D. CRUIKSHANK, E. DOTTO, G. FAURY, M. FULCHIGNONI, O. GROUSSIN, M. HICKS, KRYSZCZYNSKA, M. KUPPERS, I. TOTH, B. WARNER, P. WEISSMAN Laboratoire d’Astrophysique de Marseille

OBSERVATIONS OF STEINS : OVERVIEW VISIBLE (CCD imaging) - OSIRIS: 1 LC - Ground-based : Many telescopes (ESO NTT...): 25 LCs ►Several light curves at different aspect angles > Rotational period ►Shape from inversion, pole solution, rotational period ►Phase function THERMAL INFRARED - Spitzer Space Telescope with IRS ►Scale the size with minimal assumptions ►Thermal properties (interior) ►Albedo when combined with visible - Classification ►Mineralogy – Classification

OBSERVATIONS 1. Weissman, Lowry and Choi (April 2004, 3 nights) Table Mountain Obs. California. 2. Weissman, Choi, and Lowry (August 2005, 5 nights) CTIO/Chile. 3. Warner B. (March 2004, 5 nights) Palmer Divide Observatory 4. Barucci et al. (November & December 2005, 2 nights) Small NTT/ESO/Chile 5. Colas F. and Kryszczynska A. (August 2005, 2 nights) ITAJUBA Brazil 6. Hicks and Bauer (March 2004) Table Mountain Obs. California. 7. OSIRIS (March 2006, 24 hr) 8. Colas F. and Vachier (Sep, Oct, Dec 2006) Pic-du-Midi observatory 9. Weissman et al. (Jan 2007) Table Mountain + Steward observatories

2867 Steins – Phase function

SHAPE OF ASTEROID STEINS Simultaneous Inversion of 16 Light Curves Direction of rotationnal axis : λ = 265° ± 10° β = + 8° ± 10° Mirror solution λ = 82° ± 10° β = +38° ± 10° Rotational period : 6.0481 ± 0.0004 hr

CALCULATED FROM SHAPE MODEL LIGHT CURVES CALCULATED FROM SHAPE MODEL

Update: modeling Rosetta targets Steins and Lutetia Mikko Kaasalainen Dept. of Mathematics, U. Helsinki Finnish CoE in Inverse Problems Research

Steins 24 LCs from 04-07 Best pole: (-89,250)+-5 Almost exactly perpendicular to the ecliptic: lambda error in 70/250 direction P=6.04681+-0.00002h Oct and Dec 06 LCs were crucial in favouring this pole Scale-free solution

Steins

Asteroid 2867 Steins: SST observations

SST OBSERVATIONS : OVERVIEW INSTRUMENTS IRS Low resolution mode - SL : 5.2 - 14.5 μm - LL : 14 - 38 μm Blue peak-up camera CIRCUMSTANCES - 14 visits equally spaced by ~ 30 mn - Total temporal coverage = 6.5 hr (a full period) - Date : 22 November 2005 - rh = 2.13 AU Δ = 1.60 AU α = 27.2°

2867 Steins – SST spectra

Thermal model fit to SED gives constraints on radius, thermal inertia and albedo. Other parameters (beaming factor,...) assumed. Weakly sensitive on radius.

IR lightcurve from SST spectra allows scaling the 3D model Radius = 2.53 ± 0.08 km Ellipsoid: a = 5.74 ± 0.18 km b= 4.90 ± 0.16 km c = 4.60 ± 0.14 km Thermal inertia = 100 – 300 MKS (for a beaming factor between 0.8 and 1)

Albedo comes from visible obervations since size is known Using a linear phase function which excludes the opposition effect p(V) = 0.23-0.27 p(R) = 0.34-0.40 Somewhat unusual type intermediate between S and E

21 Lutetia: SST observations

21 Lutetia 8 hours of full coverage on 10 December 2006, for a total of 14 spectra covering the wavelength range 5-38 micron. Single exposure time of 6.3 s for each channel Observing conditions: DSpitzer = 2.65AU Phase =21.1° DSun= 2.81 AU One partial Light curve obtained on 11 December 2005 telescope was used for the absolute magnitude computation

21 Lutetia – SST spectra

21 Lutetia – SST spectra IR lightcurve

21 Lutetia: the STM gives a geom. albedo = 0.18, beam. Factor=1.49

PRELIMINARY CONCLUSIONS ASTEROID 2867 STEINS : PRELIMINARY CONCLUSIONS Effective radius = 2.3 to 2.8 km Shape: slightly elongated Rotational period = 6.048 hr Direction of rotational axis Albedo 0.3 to 0.4 => E-type asteroid ► Differentiated bodies which experienced significant heating episodes

Determination of the Rotational Period Discrete Fourier Transform OSIRIS NAC OBSERVATIONS : RESULTS Determination of the Rotational Period T = 6.046 ± 0.013 Method Prot Information entropy Discrete Fourier Transform Lomb-Scargle Method Phase Dispersion Min Analysis of Variance WindowClean Fourier Analysis (2 harmonics) (hour) 6.068 6.027 6.046 6.038 6.057 6.045 6.042 ± 0.009 Mean 6.046 ± 0.013

SST OBSERVATIONS : RESULTS Spectral energy distribution 5 – 36 μm STM fit with η = 1.512 Thermal light curves: integrated flux from 5 – 25 μm 5 – 33 μm

DATA ANALYSIS The Standard Thermal Model was applied for a best fit to the observed IR flux. The radiometric method was used to derive the asteroid ‘ albedo and size.