1. Carlos Allende Prieto Mullard Space Science Laboratory University College London The ESA mission Gaia

2. Gaia ’ s Three Elements Astrometry (V < 20) completeness to 20 mag  10 9 stars accuracy: 10–25  arcsec at 15 mag scanning satellite, two viewing directions principles: global astrometric reduction (as for Hipparcos) Photometry (V < 20) Low-dispersion spectrophotometry 0.3 - 1  m Radial velocity (V < 16–17) slitless spectroscopy near Ca II triplet (847–874 nm ) third component of space motion, perspective acceleration dynamics, population studies, binaries spectra: chemistry, rotation

3. Gaia: Complete, Faint, Accurate Quasars Radial vel. Target selection

4. Stellar Astrophysics Parallaxes and photometry imply a comprehensive luminosity calibration distances to 1% for ~10 million stars to 2.5 kpc distances to 10% for ~100 million stars to 25 kpc parallax calibration of all distance indicators e.g. Cepheids and RR Lyrae to LMC/SMC accurate parallaxes  accurate surface gravities and age

5. Stellar Astrophysics An unbiased survey implies a detailed Galactic census solar neighbourhood mass function and luminosity function e.g. white dwarfs (~200,000) and brown dwarfs (~50,000) initial mass and luminosity functions in star forming regions rare stellar types and rapid evolutionary phases in large numbers Statistics on variability across the board (~40 (RVS) - 100 (AS,XP) visits per object)

6. One Billion Stars in 6-d will Provide … in our Galaxy … the distance and velocity distributions of all stellar populations a rigorous framework for stellar structure and evolution theories a large-scale survey of extra-solar planets (~20,000) a large-scale survey of Solar System bodies (~ few 100,000) … and beyond definitive distance standards out to the LMC/SMC rapid reaction alerts for supernovae and burst sources (~20,000) QSO detection, redshifts, microlensing structure (~500,000) fundamental quantities to unprecedented accuracy:  to 10 -7 (10 -5 present)

7. Exo-Planets: Expected Discoveries Astrometric survey: monitoring of hundreds of thousands of FGK stars to ~200 pc detection limits: ~1M J and P < 10 years masses, rather than lower limits (m sin i) multiple systems measurable, giving relative inclinations Results expected: ~20,000 exo-planets (~10 per day) orbits for ~5000 systems masses down to 10 M Earth to 10 pc >1000 photometric transits Figure courtesy François Mignard

8. Asteroids etc.: deep and uniform (20 mag) detection of all moving objects ~ few 100,000 new objects expected (357,614 with orbits presently) taxonomy/mineralogical composition versus heliocentric distance diameters for ~1000, masses for ~100 orbits: 30 times better than present Trojan companions of Mars, Earth and Venus Kuiper Belt objects: ~300 to 20 mag (binarity, Plutinos) Near-Earth Objects : Amors, Apollos and Atens (2249, 2643, 406 known today) ~1600 Earth-crossers >1 km predicted (937 currently known) detection limit: 260–590 m at 1 AU, depending on albedo Movie courtesy of Jos de Brujne Studies of the Solar System

10. Satellite and System ESA-only mission Launch date: 2011 Launcher: Soyuz–Fregat Orbit: L2 Lifetime: 5 years Ground station: New Norcia and Cebreros Downlink rate: 4–8 Mbps Mass: 2120 kg (payload 700 kg) Power: 1720 W (payload 735 W) Figures courtesy EADS-Astrium

11. Payload

12. Payload and Telescope Two SiC primary mirrors 1.45  0.50 m 2 at 106.5° SiC toroidal structure (optical bench) Basic angle monitoring system Combined focal plane (CCDs) Rotation axis (6 h) Figure courtesy EADS-Astrium Superposition of two Fields of View (FoV)

13. Focal Plane Star motion in 10 s Total field: - active area: 0.75 deg 2 - CCDs: 14 + 62 + 14 + 12 - 4500 x 1966 pixels (TDI) - pixel size = 10 µm x 30 µm = 59 mas x 177 mas Astrometric Field CCDs Blue Photometer CCDs Sky Mapper CCDs 104.26cm Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Wave Front Sensor Basic Angle Monitor Wave Front Sensor Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discrimination Astrometry: - total detection noise: ~6 e - Photometry: - spectro-photometer - blue and red CCDs Spectroscopy: - high-resolution spectra - red CCDs 42.35cm Figure courtesy Alex Short

14. On-Board Object Detection Requirements: unbiased sky sampling (mag, colour, resolution) all-sky catalogue at Gaia resolution (0.1 arcsec) to V~20 Solution: on-board detection: good detection efficiency to V~21 mag FPA CCDs generate Gbps  windows

15. Sky Scanning Principle Spin axis 45 o to Sun Scan rate: 60 arcsec/s Spin period: 6 hours 45 o Figure courtesy Karen O’Flaherty

16. Astrometric Data Reduction Principles Sky scans (highest accuracy along scan) Scan width: 0.7° 1. Object matching in successive scans 2. Attitude and calibrations are updated 3. Objects positions etc. are solved 4. Higher terms are solved 5. More scans are added 6. System is iterated Figure courtesy Michael Perryman

17. Light Bending in Solar System Movie courtesy Jos de Bruijne Light bending in microarcsec, after subtraction of the much larger effect by the Sun

19. Photometry Measurement Concept Figures courtesy EADS-Astrium Blue photometer: 330–680 nm Red photometer: 640–1000 nm

20. Photometry Measurement Concept Figures courtesy Anthony Brown RP spectrum of M dwarf (V=17.3) Red box: data sent to ground White contour: sky-background level Colour coding: signal intensity

21. Ideal tests Shot, electronics (readout) noise Synthetic spectra Logg fixed (parallaxes will constrain luminosity) G=18.5 G=20 S/N per pixel Bailer-Jones 2009 GAIA-C8-TN-MPIA-CBJ-043

22. (Spectro-)photometry ILLIUM algorithm (Bailer-Jones 2008). Dwarfs: G=15  ([Fe/H])=0.21  (Teff)/Teff=0.005 G=18.5  ([Fe/H])=0.42  (Teff)/Teff=0.008 G=20  ([Fe/H])=1.14  (Teff)/Teff=0.021 G=20

23. Figures courtesy EADS-Astrium Spectroscopy: 847–874 nm (resolution 11,500) Radial Velocity Measurement Concept

24. RVS grating 20.5x15.5 cm 302.11 grooves/mm Wavefront error < 8nm rms >80% efficiency 847-874 nm © Fraunhofer Institut fur Optik und Feinmechanik

25. Radial Velocity Measurement Concept RVS spectra of F3 giant (V=16) S/N = 7 (single measurement) S/N = 77 (40x3 transits) Field of view RVS spectrograph CCD detectors Figures courtesy David Katz

26. RVS S/N ( per transit and ccd) 3 window types: G 10 (R~4500)  √ (S + rdn 2 ) Most of the time RVS is working with S/N<1 End of mission spectra will have S/N > 10x higher G magnitude Allende Prieto 2009, GAIA-C6-SP-MSSL-CAP-003

27. Sample RVS spectra (mission end, black line) G=10.5G=12.3 G=15.8 B5V G2V Metal- poor K1III Allende Prieto 2009

28. RVS produce Radial velocities down to V~17 (10 8 stars) Atmospheric parameters (including overall metallicity) down to V~ 13-14 (several 10 6 stars) Chemical abundances for several elements down to V~12-13 (few 10 6 stars) Extinction (DIB at 862.0 nm) down to V~13 (e.g. Munari et al. 2008) ~ 40 transits will identify a large number of new spectroscopic binaries with periods < 15 yr (CU4, CU6, CU8)

29. RV performance Spec. for late-type stars 1 km/s at V<13 15 km/s down to V=17 Latest performance estimates V(mag) 11 13.5 17

30. Atmospheric parameters (Ideal tests) Solid: absolute flux Dashed: absolute flux, systematic errors (S/N=1/20) Dash-dotted: relative flux Allende Prieto (2008)

31. Scientific Organisation Gaia Science Team (GST): 7 members + ESA Project Scientist + DPAC Executive Chair Scientific community: organised in Data Processing and Analysis Consortium (DPAC) All software written in JAVA (two populations) ~375 scientists in 16 countries active at some level Community is active and productive: regular science team/DPAC meetings growing archive of scientific reports advance of simulations, algorithms, accuracy models, etc. Data distribution policy: final catalogue ~2019–20 intermediate catalogues as appropriate science alerts data released immediately no proprietary data rights

32. Ingestion, pre-processing, data base + versions, astrometric iterative solution ESAC + Barcelona + OATo Object processing + Classification CNES, Toulouse Photometry Cambridge (IoA) + Variability Geneva (ISDC) Spectroscopic processing CNES, Toulouse Overall system architecture ESAC Data simulations Barcelona From ground station Data Processing (simplified)

33. Status and Schedule Prime contractor: EADS-Astrium implementation phase started early 2006 preliminary design review completed June 2007 CDR in early 2010 (now delayed a few months to mid-2010) Main challenges: CCDs and FPA (including proximity electronics) SiC primary mirrors scientific calibration of CCD radiation-damage effects Schedule: no major identified uncertainties to affect cost or launch schedule launch in December 2011

34. Schedule Catalogue 2000 20042008 2012 2016 2020 ESA Acceptance Technology Development Design, Build, Test Launch Observations Data Analysis Early Data Concept & Technology Study (ESA) Re-assessment: Ariane-5  Soyuz Cruise to L2