Gaia: Solar System observations

Transcription

1 Gaia: Solar System observations Paolo Tanga Observatoire de la Côte d Azur (France)

2 Gaia will also observe Asteroids (~ most known) Comets Mainly Main Belt Asteroids (MBA) Several NEOs Other populations (trojans, Centaurs,..) 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

3 How many asteroids will be seen by Gaia? 2006 Cumulated population 2001 H The large majority of asteroids observed by Gaia (~ ) will be known (F. Mignard)

4 Present situation Photometry shapes, poles rotation periods Satellites Low-res spectroscopy: surface composition Astrometry, orbit determination masses, σ<60% Size / albedos N ~100 ~1000 ~20 (MBA) ~1500 ~40 ~2000 (indirect method)

5 The scanning law Rotation axis movement Scan path in 4 days Scan path 4 days Spin axis 4 rotations/day Spin axis trajectory 4 days 45 Sole Sun trajectory, 4 months Spin axis trajectory, 4 months

6 Discovery potential and follow-up Observable region on the ecliptic plane Sun unobservable Discovery space: Low elongations (~45-60 ) Inner Earth Objects (unknown population) Other NEOs Gaia unobservable Need of groundbased follow-up (resp. W. Thuillot)

7 Properties of Gaia Solar System data

8 Focal plane 1 pixel 60 x 180 mas 106 CCDs (4.5 x 2 kpix) = 1 Gpixel SM1-2 AF1-9 BP RP RVS 420 mm 0.69 WFS WFS BAM FOV1 FOV2 BAM 0s s sec sec

9 Along and across scan proper speeds MB NEOs MB NEOs

10 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 AF1 AF2 Signal recorded

11 Gaia data for asteroids Astrometric Field Astrometry Accuracy 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 nm

12 Asteroid dynamics and physics by Gaia

13 Gaia andasteroiddynamics Astrometry ground-based Gaia single measurement arcsec mas Larger sensitivity to «small» effects: Mutual perturbations (<100 mas) among several bodies! Masses of ~100 objects Uncertainty < d for d > 2 km Orbit improvement (> 100) Shape effects (<0.1 x diameter) Photocenter-barycenter difference Non-gravitational accelerations Thermal emission (Yarkovsky, ~0.1 mas) Comet jets Relativity effects

14 Asteroid masses: today limited astrometric precision, long periods of observation perturbations by other unknown masses uncertainty > M (10-30% Ceres, Pallas, Vesta) ~40 asteroids at better than 60% (Mouret et al. 2007) Asteroid Mass (M ) Reference 10 Hygiea (4.7 ± 2.3) Scholl et al (5.6 ± 0.7) Michalak Parthenope (2.6 ± 0.10) Viateau Rapaport Eunomia (4.2 ± 1.1) Hilton 1997 (1.2 ± 0.4) Michalak 2001

15 Final statistics for mass determination N-body system of «unknown» masses The global solution (orbits + masses) must take into account the complete system. ~100 Larger perturbers better than 15%!! General improvement of SS dyn. model But: Reality will be better : ~10 times more objects observed Mouret 2007 Problem (opportunity): several encounters occouring «before» and «after» Gaia

16 Simulated Gaia photometry Orbit of 39 Laetitia λ p = 30 Δ(mag) wrt first observation β p = 60 b/a = 0.7 c/a = 0.5 P = 7 h.527 φ 0 = 0.4 A. Cellino, P. Tanga, M. Delbo

17 Inversion limits Ellipsoidal model inversion: when the problem has a solution? A. Cellino, P. Tanga

18 Binary asteroids - today Centroiding acc. Gaia pixel - AL Gaia window - AL radar/ lightcurves Imaging (AO)

19 Gaia and the asteroids: a new global picture N Orbit improvement Shape/pole Taxonomy Binary (masses) Size Mass Dynamical prop. + composition + rotation Rotation + composition + mass + albedo All the above + non-grav. forces Possible improvement: Ground based support

20 CU4 SSO: data flow (daily processing)

21 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 accuracy of Gaia position Need to be of the same level as the expected astrometric accuracy (~10-15 km)

22 Improving the scientific return Masses astrometric measurements before and after Gaia, on specific objects about 25 added + interferometry / AO size bulk density Non-gravitational effects (Yarkovsky thermal acceleration) astrometric measurements before and after Gaia, on specific objects ~50 expected