1. Gaia A Stereoscopic Census of our Galaxy http://www.rssd.esa.int/Gaia November 2003 http://www.rssd.esa.int/Gaia

2. 2 Gaia: Design Considerations Astrometry (V < 20): –completeness to 20 mag (on-board detection)  10 9 stars –accuracy: 10-20  arcsec at 15 mag (Hipparcos: 1 milliarcsec at 9 mag) –scanning satellite, two viewing directions  global accuracy, with optimal use of observing time –principles: global astrometric reduction (as for Hipparcos) Radial velocity (V < 16-17): –application: third component of space motion, perspective acceleration dynamics, population studies, binaries spectra: chemistry, rotation –principles: slitless spectroscopy using Ca triplet (848-874 nm) Photometry (V < 20): –astrophysical diagnostics (5 broad + 11 narrow-band) + chromaticity   T eff ~ 200 K, log g, [Fe/H] to 0.2 dex, extinction

3. 3 Gaia: Complete, Faint, Accurate

4. 4 Stellar Astrophysics Comprehensive luminosity calibration, for example: –distances to 1% for ~20 million stars to 2.5 kpc –distances to 10% for 150 million stars to 25 kpc –rare stellar types and rapid evolutionary phases in large numbers –parallax calibration of all distance indicators e.g. Cepheids and RR Lyrae to LMC/SMC Physical properties, for example: –clean Hertzsprung-Russell sequences throughout the Galaxy –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 –luminosity function for pre main-sequence stars –detection and dating of all spectral types and Galactic populations –detection and characterisation of variability for all spectral types

5. 5 One Billion Stars in 3-d will Provide… in our Galaxy… –the distance and velocity distributions of all stellar populations –the spatial and dynamic structure of the disk and halo –its formation history –a rigorous framework for stellar structure and evolution theories –a large-scale survey of extra-solar planets (~10–20,000) –a large-scale survey of Solar System bodies (~100,000) –support to developments such as VLT, JWST, etc …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 -3 present)

6. 6 Planets: Expected Discoveries Astrometric survey: –monitoring of hundreds of thousands of FGK stars to ~200 pc –detection limits: ~1M J and P < 10 years –complete census of all stellar types, P = 2–9 years –masses, rather than lower limits (m sin i) –multiple systems measurable, giving relative inclinations Results expected: –10–20,000 planets (~10 per day) –displacement for 47 UMa = 360  as –orbits for ~5000 systems –masses down to 10 M Earth to 10 pc Photometric transits: ~5000?

7. 7 Asteroids etc: –deep and uniform (20 mag) detection of all moving objects –10 5 –10 6 new objects expected (65,000 presently) –taxonomy/mineralogical composition versus heliocentric distance –diameters for ~1000, masses for ~100 –orbits: 30 times better than present, even after 100 years –Trojan companions of Mars, Earth and Venus –Kuiper Belt objects: ~300 to 20 mag (binarity, Plutinos) Near-Earth Objects : –Amors, Apollos and Atens (442, 455, 75 known today) –~1600 Earth-crossers >1 km predicted (100 currently known) –detection limit: 260–590 m at 1 AU, depending on albedo Gaia: Studies of the Solar System

8. 8 Light Bending in Solar System

9. 9 Satellite and System Mass: 1700 kg (payload 800 kg) Power: 2000 W (payload 1200 W) ESA only mission Launch date: 2010 targetted Lifetime: 5 years Launcher: Soyuz Orbit: L2 Ground station: Perth or Madrid Data rate: 1 Mbps

10. 10 Payload and Telescope SiC primary mirrors 1.4  0.5 m 2 at 106° Superposition of fields of view SiC toroidal structure Basic angle monitoring system Combined focal plane (CCDs) Rotation axis

11. 11 Astrometric Focal Plane Total field: - area: 0.6 deg 2 - size: 75  60 cm 2 - number of CCD chips: 110+70 - CCDs: 4500 x 1966 pixels Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events Astrometric field: - pixel size: 10  30  m 2 - window area: 6  12 pixels - flush frequency: 15 MHz - readout frequency: 30 kHz - total read noise: 6e - Broad-band photometry: - 5 colour Star motion

12. 12 On-Board Object Detection Requirements: –unbiased sky sampling (mag, colour, resolution) –no all-sky catalogue at Gaia resolution (0.1 arcsec) to V~20 Solution: on-board detection: –no input catalogue or observing programme –good detection efficiency to V~21 mag –low false detection rate, even at very high star densities Will therefore detect: –variable stars (eclipsing binaries, Cepheids, etc) –supernovae: 20,000 –microlensing events: ~1000 photometric; ~100 astrometric –Solar System objects, including near-Earth asteroids and KBOs

13. 13 Sky Scanning Principle Spin axis 50 o to Sun Scan rate: 60 arcsec/s Spin period: 6 hours

14. 14 Radial Velocity Measurement Concept F3 giant S/N = 7 (single measurement) S/N = 130 (summed over mission)

15. 15 Comments on Astrometric Accuracy massive leap from Hipparcos to Gaia: –accuracy: 2-3 orders of magnitude (1 milliarcsec to 4 microarcsec) –limiting sensitivity: 4 orders of magnitude ( ~10 mag to 20 mag) –number of stars: 4 orders of magnitude (10 5 to 10 9 ) measurement principles identical: –two viewing directions (absolute parallaxes) –sky scanning over 5 years  parallaxes and proper motions instrument improvement: –larger primary mirror: 0.3  0.3m 2  1.4  0.5m 2,   D -(3/2) –improved detector (IDT  CCD): QE, bandpass, multiplexing control of all associated error sources: –aberrations, chromaticity, solar system ephemerides, attitude control…

16. 16 Technical Studies (2002-04) and Schedule Main activities: –two parallel system studies: Astrium & Alenia/Alcatel –CCD/focal plane development: Astrium + e2v – first CCDs produced –SiC primary mirror: Boostec – mirror prototype under production –high-stability optical bench: Astrium + TPD Delft – testing underway –payload data handling electronics: Astrium-D – breadboard starting –radial velocity instrument optimisation: MSSL/Paris –mission analysis: ESOC –also studied: FEEPs; transmitter; solar array deployment; refocus mechanism; ground verification/calibration; active optics (backup) Schedule: –implementation phase start: May 2005; launch: mid-2010 –overall system design advanced and stable since 2000 –no major identified uncertainties to affect cost or launch schedule –technology/science ‘window’: 2010-12

17. 17 Scientific Organisation Gaia Science Team: –12 members Scientific working groups: –16 groups focused on payload, specific objects, and data analysis –220 scientists active in the working groups at some level Community is active and productive: –regular science team/working group meetings: ~20 in both 2002 & 2003 –active archive of scientific working group reports: ~150 since 1 Jan 2003 –advance of simulations, algorithms, accuracy models, etc Data distribution policy: –final catalogue ~2018 –intermediate catalogues as appropriate –science alerts data released immediately –no proprietary data rights

18. 18 Data Reduction Principles Sky scans (highest accuracy along scan) Scan width: 0.7° 1. Objects are matched 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

19. 19 Schedule 2000 20042008 2012 2016 2020 Acceptance Technology Development Design, Build, Test Launch Observations Analysis Catalogue Early Data Concept & Technology Study ESA SCI 2000(4) Re-Assessment: Ariane  Soyuz To L2 Assumed start of Phase B2

20. 20 Huge and timely scientific impact Well-defined payload and spacecraft Technology, cost and schedule maturity Substantial and active ESA-based community