The Rosetta fly-by with the asteroid 2867 Steins: results from the Osiris imaging system

Sonia Fornasier LESIA/Observatoire de Paris - université Paris VII & the OSIRIS team

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SUMMARY

•

Description of the OSIRIS imaging system

•

Asteroid Steins: what we know fron ground based and space observations before the Rosetta encounter

•

Rosetta Steins fly-by:Results from the Osiris Imaging system

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Mission to 67P/Churyumov-Gerasimenko

After the launch postponement, the targets changed: the new primary target is comet 67P/Churyumov-Gerasimenko, a short period comet of Jupiter's family, larger and dustier than Wirtanen. Secondary scientific targets are two asteroids, Steins and Lutetia. The present scenario is similar to the old one in terms of conditions of the closest approach.

Launch: 2004/03/2 Earth1 2005/03 Mars 2007/03 Earth2 2007/11 Steins 2008/09 Earth3 2009/11 Lutetia 2010/07 Comet RDV 2014/05 Lander deliv. 2014/11 Perihelion: 2015/08 End of mission: 2015/12

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OSIRIS (Optical Spectroscopic and Infrared Remote Imaging System) main scientific imaging system of Rosetta, built by a consortium led by the Max-Planck-Institute for Solar System Research (PI Keller/Sierks, MPS).

It has 2 camera:

•

Narrow Angle Camera (NAC), FoV 2.35x2.35° , resolution of 20



rad/px (PI Lamy, LAM) a Wide Angle Camera (WAC), FoV 12x12°, resolution 100



rad/px (PI Barbieri, UPD).

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Narrow Angle Camera (NAC)

•

Fov: 2.35 X 2.35

 •

Reflective optics, unobstructed and anastigmatic system

•

3 mirrors

•

brightness ratio ≥ 1/1000

•

F/ratio = F/8

Resolution =20



rad/px (about 4 arcsec/px) 10 scientific filters in the 250-1000 nm range +2 refocussing lens for near nucleus imaging CCD: 2048 x 2048 pixel ; pixel: squared, size=13.5



m

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 Wide Fov: 12 X 12   Reflective optics, unobstructed and unvignetted system  Resolution =100  rad/px or 21”/px  Complex baffling system  Contrast capabilities: better than 10

–4

 F/ratio = F/5.6  Entrance pupil=4.9 cm

2



14 filters in the

230--750 nm range 

M1

: off-axis section of an

oblate convex ellipsoid

of squared shape (53  53 mm2) 

M2

:

oblate concave ellipsoidal shape

, circular shape (D= 64 mm) 

CCD:

2048 x 2048 pixel 

pixel:

squared, size=13.5  m

Shutter:

electromechanical, works from

10ms-

100s, high uniformity guaranted. 6

• • • • • • • •

Targets observed with OSIRIS so far

3 comets

– LINEAR 2002 T7 – Machholz 2004 Q2 – 9P/Tempel 1

2 asteroids (Lightcurve)

– 2867 Steins ( 11-12 March 2006) – 21 Lutetia (2-3 Jan. 2007)

Several stars

– Eps. Aqr, 58 Aql, Vega, 16 Cyg (twice), alpha Gru

Starfield for geometric calibration

(area 98, twice)

M42 Several “random” starfields

– Serendipitious observations of Neptune, asteroids, standard star,…

Planets

– Venus, Earth & Moon, Saturn & Titan

2867 Steins fly-by on 5 sept. 2008

About 3000 images already obtained successfully

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What we knew on 2867 Steins before the Rosetta fly-by: results from several observing campaigns

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Polarimetric results on 2867 Steins

Very small polarization values, from -0.26% (



=10.3

º

) to +0.35% (



=28.3

º

) !!!

Inversion angles: V: 17.3

º ±1.5º R: 18.4

º ±1.0º Slope at inv. Angle: V = 0.037 ± 0.003

R = 0.032 ± 0.003

Polarimetric properties are consistent with high albedo E-type asteroids

Padova 30-31 Jan. 06 OSIRIS Flyby and Asteroid Working Group

Polarimetric results on 2867 Steins

Steins albedo (p v ) has been evaluated thanks to the relationship with the polarimetric slope (h) :

log (p v ) = C1 * log (h) + C2

Where C1 and C2 have sligthly different values depending on the dataset used for their determination. Here we used the constants from Bowell & Gradie 1974 (C1=-1, C2=-1.78) derived from laboratory data on meteorites and terrestrial samples.

We derived for Steins an albedo = 0.45



0.10

Assuming an absolute visual magnitude = 13.18 mag (Hicks et al., 2004)

From polarimetric properties Steins estimated diameter is 4.6 km

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2867 Steins

sharp 0.5



m band (sulfides, troilite or oldhamite) faint 0.9



m band (iron bearing pyroxene, orthopyr. or forsterite)

-- EL6 enstatite chondrite Atlanta .… entatite achondrite (aubrite)

Type E EII type, Angelina like partial melts derived from enstatite chondrite like parent bodies.

( Barucci et al. 2005) 11

E-type asteroids

•

bodies with high albedo (0.4-0.6), are thought to be mostly differentiated and to have experienced high heating episodes (T>1500K).

•

surface composition seems to be dominated by iron-free or iron-poor silicates as enstatite, forsterite or feldspar

•

supposed to be parent bodies of enstatite meteorites

•

small population (albedo needed!) ~ 25 bodies, located in the inner main belt and in the Hungaria region, but

3 different mineralogies have been identified

(Gaffey & Kelley, 2004; Clark et al., 2004):

Subtype I

Featureless slightly reddish spectra

Subtype II

strong band at 0.49



m (calcium sulfide oldhamite) and occasionally at 0.96

Subtype III

strong band at 0.88- 0.9



m characteristic of enstatite pyroxene containing Fe2+

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(forsterite) & 1.8



m

H h

SPITZER (Nov. 2005)

r

= 2.46 ± 0.20 km thermal inertia

I

= 150 ±60 JK−1 m−2 s−1/2

P

(R) = 0.40 ± 0.07

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2867 Steins: observed by OSIRIS on 11-12 March 2006

24 h of imaging with the NAC (clear filter) D Rosetta = 1.06 AU Phase =42 ° D Sun = 2.30 AU Images of 300 s Psyn = 6.052

±0.035 h, amplitude=0.23

This LC, together with those available from the Earth, allows the first evaluation of Steins rotational properties: λ 1 λ 2 = 8° ± 10°; β =38° ± 10°; β 1 2 = 265° ± 10° =82° ± 10° a/b=1.2 ; b/c=1.4

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Steins Fly-by Overview

4 August 2008 to 3 October 2008 Closest approach:

5 Sept. 2008 18:38

r H = 2.14 AU, Δ = 2.41 AU Relative velocity: 8.62 km/s Targeted minimum flyby distance: 803 km 15

ROSETTA - Steins Fly-by Closest approach took place at 18:38:20 UTC on 5 Sept. 2008 at a minimum distance of 803 km. Minimum phase angle of 0.27° (opposition) 2 minutes before CA. It increased again to 51° at CA, and finally to 141° at the end of observations. Approximately 60 % of the surface of Steins was resolved during the fly-by

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17

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Keller et al. 2010, Science 327, 190

Copyright: ESA ©2008 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

NAC best res. Image (100m/px) 5Sept UT: 18:28, dist:5200 km, phase=30° WAC best res. Image (80m/px) 5Sept UT: 18:38:15 dist:806km,phase=50 °

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STEINS SHAPE

Steins shape modelled using limb positions from one NAC and 61 WAC images and the simultaneous inversion of a set of 28 light curves taken from Earth and during approach. Its shape is best approximated by an oblate spheroid rotating about its short axis

Keller et al. 2010, Science 327, 190

Equatorial view

Pole direction :

RA = 91.6° and DEC = –68.2°, close to perpendicular to the ecliptic plane

Polar view

rotation retrograde sidereal period

:

6.04679 ± 0.00002 h

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STEINS SHAPE

•

Mean radius: 2.69 km

–

projected dimension at phase zero: 5.3 x 3.9 km 2

•

Surface: 94 km 2 ± 9

•

Volume: 76 km 3 ± 11

•

Principal axes: a = 6.7, b = 5.9, c = 4.3 km

•

North pole orientation: RA = + 91.6° and DEC = -68.2°

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The 2km-sized crater has well defined rim: relatively young?

•The 2km-sized crater: possibly two big craters on top?

•Other large craters are shallow: old and/or regolith jolting?

•The large depression in the NAC image seems to be connected to the 'chain' in the WAC images: a possible large fracture, maybe triggered by the 2km-sized crater?

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Crater counting and age estimation

34 features have been counted on the whole visible area of 34.5

km 2 Reliable detection threshold of D=4pix (~0.3 km): 29 craters.

-Crater counting -modelling of impactors flux at Steins -cratering scaling laws - derivation of crater cumulative distribution

Estimation of Steins surface age

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STEINS SURFACE AGE ESTIMATION

Keller et al. 2010, Science 327, 190

The cumulative distribution of craters with diameter larger than 3 pixels (~240 m) visible in the WAC images around CA. The 7 pits of the catena are excluded. The pair of solid and dashed lines on the right represent best fit models based on NSL and HSL for craters > 0.5 km. The pair on the left represent fits for craters < 0.4 km.

An age of 154 ± 35 Myr based on NSL (Nolan et al. (1992,1996, 2001) fits the distribution for craters larger than 0.5 km in diameter. Cratering ages based on HSL (Holsapple&Housen scaling law

)

are typically up to a factor of ten larger, and depend on the asteroid tensile strength. We find 0.4 ± 0.2 and 1.6 ± 0.5 Gyr for 10 5 and 10 6 dyne cm –2 24

OSIRIS Spectrophotometry

Keller et al. 2010, Science 327, 190

The UV drop-off shows that the surface of Steins is made by iron poor minerals. The E[II] classification is confirmed.

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Analysis of Steins’ surface composition

G-mode statistical method applied to the OSIRIS disk resolved images reveal no significant variations with 95% of confidence level, confirming the great homogeneity of the surface. This homogeneity seems to be consistent with possible outcome of an impact that may have ejected the first layer of the regolith on the whole surface

NAC (CA-10min) WAC (CA)

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Steins Phase function

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Steins Phase function

Geometric albedo near 0 phase image: 0.40 ± 0.01

Small linear slope value (0.024) and high G value (0.45) are typical of E type asteroids

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Belskaya & Shevchenko 1999

Steins

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Satellite Search

From close up images the limit size that could be detected would be:

•

In the region from 120 to 15 Steins radii: objects with a size from 10 to 20 m.

•

For distances smaller than 15 Steins radii: objects with a size of about 5 m.

NO SATELLITES FOUND AROUND STEINS

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Steins shape (‘conical’) seems to be the result of a reshaping of the YORP effect

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The reshaping of Steins by YORP effect acting in the past + Scarceness of small craters (attributed to YORP reshaping) + Big impact crater (big impact would probably disrupt a monolithic body) Steins is probably a rubble pile asteroid

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Steins: summary

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60 % of Steins are illuminated and imaged by OSIRIS

•

Shape model gives an oblate spheroid with a spherical

•

equivalent radius of 2.65 km, seems modelled by YORP effect

•

Rotation retrograde, close to perpendicular to the ecliptic

•

pole RA = 91.6° and DEC = –68.2°

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sidereal period

:

6.04679 ± 0.00002 h

•

Strong opposition effect, albedo = 0.40 ± 0.01

•

Phase function is typical for high albedo E type asteroids Spectrophotometry confirm the E[II] classsification, the presence of a strong band at 0.5 micron (sulfides) and show a

•

strong UV drop-off of reflectance (iron poor minerals) • No Colour variegation : homogeneou surface • Surface age: very young • No satellites found It is very probably a rubble pile asteroid

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Mineralogy

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E[II] type confirmed

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Characterized by deep absorption (~10%) at 490 nm

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Overall spectrum indicates iron poor silicates such as enstatite, fosterite or feldspar

•

Parent bodies for enstatite achondrites – aubrite meteorites

•

Absorption at 0.49 micron probably due to calcium sulfide oldhamite

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Igneous, highly reduced rocks (not from shock melts)

–

melting T > 1000 C

•

Surface homogeneity => chemical homogeneity of interior

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Enstatite achondrite are composed of large white crystals of the Fe-poor, Mg reach orthopyroxene, or enstatite (Mg

2

Si

2

O

6

).

Other minerals present in aubrites are: feldspates (1 16%), diopside (0.2-8%), olivine (0.3-10%), Fe-Ni reach minerals (0.3-7%), the sulfides troilite and oldhamite (0.1-7%)

Aubrites

ALH84007 38

Steins surface morphology

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Morphology is dominated by a large (2.1 km of diameter) crater near the south pole, a lack of small craters and the presence of linear faults

•

crater shape and depth-to-diameter ratio (~0.12) are consistent with degradation due to ejecta blanketing and regolith disturbance by impact seismic shaking

•

Catena with 7 pits of similar size may be linked to the impact that caused the large crater. It indicates partial drainage of loose surface material into a fracture within stronger deeper material, possibly marking pre existing physical inhomogeneities

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