Measurement Capabilities

• catalogue: of the order of 1 billion stars; 0.34×10⁶ to V=10 mag; 26×10⁶ to V=15 mag; 250×10⁶ to V=18 mag; 1000×10⁶ to V=20 mag, completeness to about 20 mag
• sky density: mean density approximately 25 000 stars deg^-2, maximum density: about 3 ×10⁶ stars deg^-2
• accuracies: median parallaxes of 4 µas at V=10 mag, 11 µas at V=15 mag, 160 µas at V=20 mag
• distance accuracies: from Galaxy models: 21 million better than 1 per cent, 46 million better than 2 per cent, 116 million better than 5 per cent, 220 million better than 10 per cent,
• radial velocities: 1 to 10 km s^-1 to V=16-17 mag
• photometry: to V = 20 mag in 4 broad and 11 medium bands

Summary of Scientific Goals

Galactic structure

• origin and history of our Galaxy
• tests of hierarchical structure formation theories
• inner bulge/bar dynamics
• disc/halo interactions
• dynamical evolution
• nature of the warp
• star cluster disruption
• dynamics of spiral structure
• star formation history
• chemical evolution
• distribution of dust
• distribution of invisible mass
• detection of tidally disrupted debris
• Galaxy rotation curve
• disc mass profile

Star formation and evolution

• dynamics of star forming regions
• luminosity function for pre-main sequence stars
• detection and categorization of rapid evolutionary phases
• complete and detailed local census down to single brown dwarfs
• identification/dating of oldest halo white dwarfs
• age census
• census of brown dwarfs in binaries

Distance scale and reference frame

• parallax calibration of all distance scale indicators
• absolute luminosities of Cepheids
• distance to the Magellanic Clouds
• definition of the local, kinematically non-rotating metric

Local group and beyond

• rotational parallaxes for Local Group galaxies
• kinematical separation of stellar populations
• galaxy orbits and cosmological history
• zero proper motion quasar survey
• cosmological acceleration of Solar System
• photometry of galaxies
• detection of supernovae

Solar system

• deep and uniform detection of minor planets
• taxonomy and evolution
• disruption of Oort Cloud

Extra-solar planetary systems

• complete census of large planets to 200-500 pc
• orbital characteristics of several thousand systems

Fundamental physics

• space curvature parameter, gamma, to 5×10^-7
• constraints on beta, on the rate of change of the gravitational constant, and gravitational wave energy

Specific objects

• 10⁶ galaxies
• 10⁵ supernovae
• 10⁵-10⁶ (new) solar system objects
• more than 50 000 brown dwarfs
• 30 000 extra-solar planets
• 200 000 disc white dwarfs
• approximately 1000 microlensed events
• TBC resolved binaries