The astrometry of solar system objects after Gaia. J.E. Arlot IMCCE/CNRS/observatoire de Paris Honolulu, IAU GA, August 2015

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1 The astrometry of solar system objects after Gaia J.E. Arlot IMCCE/CNRS/observatoire de Paris Honolulu, IAU GA, August 2015

2 The importance of the astrometry of the solar system Improving dynamics of the solar system helps to understand its formation and evolution. Astrometry of giant planets is possible only through their satellites (natural or artificial = space probes) The internal structure of large bodies (planets, major satellites) may be understood through the dynamics (tidal effects, ) Astrometry of solar system needs observations upon a long interval of time to quantify periodic effects

3 What will be the impact of Gaia on the astrometry of the solar system How Gaia will help? Gaia will provide about 50 observations of each body of the solar system (until mag 20) during a five-year interval of time with an accuracy better than one mas Is it sufficient? yes for a five-year ephemerides no for the extrapolation of the ephemerides no to understand complex dynamics with periodic perturbation terms

4 Ephemerides extrapolation: are very accurate observations necessary? The accuracy of the dynamical models and ephemerides depend more on the length of the period of observations than on the accuracy of the observations themselves This may be tested through the extrapolation of the ephemerides 4

5 The extrapolated accuracy of the ephemerides A long period of observations should be preferred to short period even with a better accuracy Case of Mimas: Left: 50 years 1547 obs. (accuracy: 0.30 ) preferable to Right: 30 years 2820 obs. (accuracy 0.15 ) due to short term libration (70 years) 5

6 The accuracy of ground based observations before Gaia Before Gaia: Ground based CCD observations are reduced with reference star catalogues The astrometric accuracy is limited by the accuracy of the reference star catalogue: for example about 60 mas using the UCAC4 catalogue

7 The accuracy of ground based observations after Gaia After Gaia: Ground based CCD observations will continue The GAIA catalogue of stars will provide very accurate positions: all stars until mag 20 to one mas accuracy New errors would be taken into account in order to reach one mas accuracy - measurement - center of mass - refraction anomaly - distorsion of the field

8 The accuracy of ground based observations made before Gaia, after Gaia After Gaia: Re-reduction of old observations An old observation is an observation made before the Gaia catalogue The accuracy of the proper motion of star in the Gaia catalogue will allow to reduce observations onecentury old.

9 9 The reduction of old observations

10 After Gaia: Making a new reduction of old photographic observations Uranians It will be observing in the past with today accuracy It will be essential for fast moving Objects such as natural satellites Saturnians Galileans

11 Cassini Camera ISS Field 0.35 Dione (1120 km) and Enceladus (512 km) Stars from UCAC2 (magnitude 9) 60 mas (UCAC2) = 3 km 1 mas (GAIA) = 50 m star star Space probe observations will benefit from the Gaia reference star catalogue star star star star

12 Pre-discoveries of small bodies (NEO, TNO, comets, ) on Schmidt plates

13 First tests made with UCAC2 catalogue The UCAC2 catalogue allows to reduce old plates until the 1960s First tests made with giant planet systems First results: - validation of ephemerides - need of the use of the same reference frame for all data: old reduced data introduce biases in the ephemerides - accuracy of 60 mas for observations made in the 1960s when the accuarcy provided by the old reduction made with old catalogues was 300 mas 14

14 Testing the quality of the planet Jupiter ephemerides using USNO plates Mean residual in a and in d and dispersion (in mas) ( ) 2200 observations from 1967 to 1998 Results using the UCAC2 catalogue

15 Conclusion: preparing the future: after Gaia era For natural fast moving satellites, GAIA will be useful through its astrometric catalogue Before and after Gaia, never stop observing satellites since they move fast: Preparing future observations Improving accuracy Search for old photographic plates of high quality: For selecting useful data For scanning and new reduction For pre-discoveries Observing in the past with today accuracy