Gaia: an unprecedented observatory for Solar System objects

Paolo TangaObservatoire de la Côte d’Azur (France)

P. Tanga – ELSA school 2007

SummaryIntro: the Solar System

The general picture

Most relevant objects for Gaia

Asteroids properties, main facts and unknowns

Traditional observing techniques

Asteroids as seen by Gaia

Peculiar issues in SSO observations by Gaia

P. Tanga – ELSA school 2007100000 AU

P. Tanga – ELSA school 2007

Gaia will mainly observe…Asteroids

Remnants of Solar System formationAltered/shattered by mutual collisionsMain Belt: source of Earth Crossers

CometsPrimitive material from the outer Solar System

« Small » planetary satellites« regular »« irregular » (retrograde orbits)

Gaia will probably NOT collect observations of « large » bodies (~200 mas ?):

Main PlanetsA few largest asteroidsLarge satellites (Galilean, Titan..)

P. Tanga – ELSA school 2007

How many asteroids will be seen by Gaia?

The large majority of asteroids observed by Gaia (~250.000) will be known

P. Tanga – ELSA school 2007

Asteroids are …small!Astrometry

Orbit determinationMass from mutual perturbations

PhotometryLightcurves shapes

Visible spectroscopyTaxonomy

Thermal IR…+ visiblesize & albedomodel-dependent (thermal conductivity of the surface)

P. Tanga – ELSA school 2007

Other techniquesPolarimetry

Albedo determination

Adaptive optics / HSTSatellite discovery ( mass)

Disk resolution of the largestasteroids

Radar ranging3D shape / size (NEOs mainly)

Occultations of starsShape/size (> 40 km)

P. Tanga – ELSA school 2007

Location, classification

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Asteroid colours: taxonomyBased on:

UBVRI spectrophotometry

8 colors + albedo (Tholen 1984, ~500 objects)

Bus & Binzel (2002) CCD spectra, ~1500 objects

Problems:Poor statistics in some cases (S)Meteorite equivalents

P. Tanga – ELSA school 2007

Asteroid distributionColour gradient primitivecomposition gradient in the disk

Other effects:MixingSpace weathering

semi-major axis (AU)

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Asteroid sizeThe problem of size determination:

unresolved sources albedo

Debiased distributionRed: S typeBlue: C type

T. Quinn, Z. Isevic (SDSS)

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Dynamics, collisions

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Collisional life: dynamical families

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Gravitational re-accumulationCumulative size distribution and dispersion in orbital elements: compatible with theobservations.

Binary systems are frequentlyformed in the process

Michel, Benz, Tanga, Richardson 2001Tanga, Delbò, Richardson 2007

a (AU)e

sin i

P. Tanga – ELSA school 2007

Itokawa by the Hayabusa missionIs this a gravitational aggregate?

540 m

P. Tanga – ELSA school 2007

So, why are we interested in asteroids?

To understand their structure and evolution :

To improve Solar System ephemeris:Current accuracy : ~1 km/ 10 yr Earth, Mars (Standish, Fienga 2001)

~100 km / 10 yr for several NEOs

The great unknowns : density, porosity…Gravitational aggregates or solid bodies?The origin of shapesThe collisional historyImpact risks and mitigation strategy

P. Tanga – ELSA school 2007

Gaia, 5 years of sky scans…

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t1

t3

t4t5

t6 t7

t2

P. Tanga – ELSA school 2007

Gaia and the Solar System - observablesAll objects observed, V<20, size <200 mas

~70 observations / object / 5 years

Astrometry and photometry for:3·105 Main Belt & Near Earth AsteroidsComets and TNOsasteroid / planet satellites

Spectroscopic properties (small dispersion)

P. Tanga – ELSA school 2007

What Gaia can do for asteroids?Uncertainties from Earth from Gaia (each obs.)

Astrometry ~0.2-1 asPhotometry ~0.05 magV lim ~23

~0.2-1 mas~ 0.005 ~ 20

Gaia data can be used for:Orbit accuracy improvementDynamical familiesMass determination for largest asteroidsShapes and sizesSatellites of planets and asteroidsRelativity testsDetection of cometary activity…

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Orbit improvement

Accuracy from a pure Gaia data set:102-103 better than current

M. Granvik

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Asteroid massesPerturbed motion of a minor planet

m

22VGm

m D VMsol km ua km/s mas

10-10 ~500 0.1 3 400.05 3 80

10-11 ~200 0.1 3 40.05 3 8

V

P. Tanga – ELSA school 2007

Asteroid masses: today

Asteroid Mass (M ) Reference

10 Hygiea (4.7 ± 2.3) × 10-11 Scholl et al. 1987(5.6 ± 0.7) × 10-11 Michalak 2001

11 Parthenope (2.6 ± 0.10) × 10-12 Viateau Rapaport 199715 Eunomia (4.2 ± 1.1) × 10-12 Hilton 1997

(1.2 ± 0.4) × 10-11 Michalak 2001

limited astrometric precision, long periods of observation perturbations by other unknown masses

Uncertainty > 10-11 M (10-30% Ceres, Pallas, Vesta)~40 asteroids at better than 60% (Mouret et al. 2007)

P. Tanga – ELSA school 2007

Masses as computed with Gaia data:N-body system of « unknown » masses

The global solution (orbits + masses) must take into account the complete system.

Results on simulations of 20.000 asteroids (5 years):

(S. Mouret, 2007)

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Final statistics for mass determinationNumber of masses(over 20.000 simulated!)

But in reality we will observe ~10 times more objects with Gaia.(Mouret et al. 2007)

Supplementary Earth-based observations will help…!

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From photometry to shapes…

Courtesy of Marco D

elbò

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p = 30

p = 60

b/a = 0.7

c/a = 0.5

P = 7h.527

0 = 0.4

Simulated Gaia photometry

(mag) wrt first observation

Orbit of 39 Laetitia

A. Cellino, P. Tanga, M. Delbo

P. Tanga – ELSA school 2007

Ellipsoidal model inversion:

when the problem has a solution?

Inversion limits

A. Cellino, P. Tanga

P. Tanga – ELSA school 2007

Spin properties: important constraint to modern models of the collisional evolution of Main Belt asteroids.

Tests of preferential alignments of family members, or the effectiveness of the Yarkovsky-YORP effects.

Implications

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Rotation periods

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Putting all the results together…Gaia can offer a new, complete overview of asteroid properties:

Dynamical propertiesNew spectral taxonomyShapes (sizes)

Complete picture of family member properties.For a subset: masses & densities

New definition of families« Minor » families discovery

P. Tanga – ELSA school 2007

Problems… (each result has its price)Solar System objects are moving

They are not always point-like sourcesSome of them (>10 mas) are marginally resolved

(>10-25 km in the Main Belt)

Sparse observations of a new object must be linked together (threading)

Photocentre discrepancy: see exercise presentation….

P. Tanga – ELSA school 2007

= 1.7 au = 3.7 au

Dkm

Hmag

N ( < H)mag mas

Vmag mas

Vmag

250 6.1 5 200 9.4 90 11.1100 8.1 200 80 11.4 40 13.050 9.5 2000 40 12.9 20 14.525 11.3 10 000 20 14.4 10 16.110 13.0 5 x 104 8 16.4 4 18.25 14.5 2 x 105 4 17.9 2 19.52 16.5 10 6 1.6 19.8 0.8 21.51 18.0 5 x 106 0.8 21.4 0.4 23.0

5

D : diameter - H : abs. mag. - : angular diameter - V : apparent mag.

Albedo = 0.15

Apparent sizes: some useful numbers

P. Tanga – ELSA school 2007

Focal Plane – TDI modeSM1-2 AF1 - 9 BP

420 mm0.69°

RP

RVS

BAM

BAM

WFS

WFS

0s 10.6 15.5 49.5 56.3 64.130.1

0s 5.8 10.7 44.7 51.5 59.325.3

sec

secFOV1

FOV2

106 CCDs (4.5 x 2 kpix), 1 pixel 60 x 180 mas

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MB

MB NEOs

NEOs

Along and across scan proper speeds

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Windows on moving sourcesWindows are allocated from ASM centroiding

centroiding errors lead to offset in the window

transit velocity errors lead to a drift in the window

A moving object will also drift relative to the windowthe total effect depends on the window size and Val

SM

Signal recorded

AF1 AF2

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Gaia: the global solution for Solar System

CCD level processingof the single observation:

positions & flux

Dynamical model

masses, non-grav. effects…

Shape, spindetermination

Physical ephemeris