1. 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

2. 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

3. 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

4. 2867 Steins – Phase function

5. 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 : ± hr

6. CALCULATED FROM SHAPE MODEL
LIGHT CURVES
CALCULATED FROM SHAPE MODEL

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

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

9. Steins

10. Asteroid 2867 Steins: SST observations

11. SST OBSERVATIONS : OVERVIEW
INSTRUMENTS
IRS Low resolution mode
- SL : μm
- LL : 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°

13. 2867 Steins – SST spectra

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

15. IR lightcurve from SST spectra allows scaling the 3D model
Radius = 2.53 ± 0.08 km
Ellipsoid:
a = 5.74 ± 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)

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

17. 21 Lutetia: SST observations

18. 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

19. 21 Lutetia – SST spectra

20. 21 Lutetia – SST spectra IR lightcurve

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

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

24. Determination of the Rotational Period Discrete Fourier Transform
OSIRIS NAC OBSERVATIONS : RESULTS
Determination of the Rotational Period
T = ± 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

25. 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

26. 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.