Gaia: Solar System observations

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Gaia: Solar System observations

Paolo Tanga

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Gaia will also observe…

Asteroids (~250.000 – most known)

Mainly Main Belt Asteroids (MBA) Several NEOs

Other populations (trojans, Centaurs,..)

Comets

Primitive 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 Planets, large satellites (Galilean, Titan..) A few largest asteroids

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How many asteroids will be seen by Gaia?

Cumulated population 2001 2006 (F. Mignard)

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Present situation

Photometry

Æ shapes, poles

rotation periods

Satellites

Low-res spectroscopy:

surface composition

Astrometry, orbit

determination

Æ masses, σ<60%

Size / albedos

~100

~1000

~20 (MBA)

~1500

~40

~2000

(indirect method)

N

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The scanning law

Sole Spin axis

Spin axis trajectory, 4 months

4 rotations/day

Sun trajectory, 4 months 45°

Rotation axis movement

Scan path 4 days

Spin axis trajectory 4 days

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Discovery potential and follow-up

Observable region on the ecliptic plane

Discovery space:

Low elongations

(~45-60°)

Inner Earth Objects

(unknown

population)

Other NEOs

Need of

ground-based follow-up

(resp. W. Thuillot)

unobservable unobservable Sun Gaia

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Focal plane

SM1-2 _{AF1 - 9} BP 420 mm 0.69° RP RVS BAM BAM WFS WFS 0s _10.6 _15.5 _30.1 _49.5 _56.3 _64.1 0s 5.8 10.7 _25.3 _44.7 _51.5 _59.3 sec sec FOV1 FOV2

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MB

MB _NEOs

NEOs

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Windows on moving sources

Windows 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 window

the total effect depends on the window size and V_al

SM

Signal recorded

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Gaia data for asteroids

Astrometric Field

Astrometry

Accuracy 0.1 - 1 mas /observation

(conservative)

Depending upon centroiding model Better for « slow » objects

Photometry

Very accurate (but linked to centroiding prob.)

Red / Blue Photometer

Equiv. to ~20 filters 330 – 1000 nm

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Asteroid dynamics and physics

by Gaia

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Gaia and asteroid dynamics

Astrometry

ground-based

Gaia single measurement

0.1 - 1 arcsec

0.1 – 1 mas

Larger sensitivity to « small » effects:

Mutual perturbations (<100 mas) …among several bodies!

Masses of ~100 objects

Shape effects (<0.1 x diameter)

Photocenter-barycenter difference

Non-gravitational accelerations

Thermal emission (Yarkovsky, ~0.1 mas) Comet jets

Orbit improvement (> 100)

Uncertainty < d for d > 2 km

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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 1997 15 Eunomia (4.2 ± 1.1) × 10-12 Hilton 1997

(1.2 ± 0.4) × 10-11 Michalak 2001

limited astrometric precision, long periods of observation

Æ perturbations by other unknown masses

uncertainty

>

10

-11

M

_~

(10-30% Ceres, Pallas, Vesta)

~40 asteroids at better than 60% (Mouret et al. 2007)

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Final statistics for mass determination

N-body system of « unknown » masses

The global solution (orbits + masses) must take

into account the complete system.

But:

Reality will be better : ~10 times more objects

observed

Problem (opportunity): several encounters

Mouret 2007 ~100 Larger perturbers better than 15% !!

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λ

_p

= 30

β

_p

= 60

b/a = 0.7

c/a = 0.5

P = 7

h

_.527

φ

₀

= 0.4

Simulated Gaia photometry

Δ(mag) wrt first observation Orbit of 39 Laetitia

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Ellipsoidal model inversion: when the problem has a solution?

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Binary asteroids - today

Gaia window -AL Gaia pixel -A L

Centroiding

acc

. radar/

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Gaia and the asteroids: a new global picture

1.000 _10.000 _100.000 _250.000

N

100

Dynamical prop. + composition + rotation

Rotation + composition + mass + albedo

All the above + non-grav. forces

Possible improvement: Orbit improvement Shape/pole Taxonomy Binary (Æmasses) Size Mass

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Problems to be solved

Object motion

Loss of observations during the transit on the focal plane Smearing of the signal

Finite size

Smearing

CTI – radiation damage

Alteration of the instrument response – memory effect

Identification (threading, parasites…)

Sparse observations to be linked toghether

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