Lund Observatory: David Hobbs

Gaia is a European Space Agency Astrometry Mission due for launch in late 2012. Gaia is an ambitious mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

Combined with astrophysical information for each star, provided by on-board multi-colour photometry, these data will have the precision necessary to quantify the early formation, and subsequent dynamical, chemical and star formation evolution of the Milky Way Galaxy.

Additional scientific products include detection and orbital classification of tens of thousands of extra-solar planetary systems, a comprehensive survey of objects ranging from huge numbers of minor bodies in our Solar System, through galaxies in the nearby Universe, to some 500 000 distant quasars. It will also provide a number of stringent new tests of general relativity and cosmology.

Gaia will rely on the proven principles of ESA’s Hipparcos mission to create an extraordinarily precise three-dimensional map of more than a thousand million stars throughout our Galaxy and beyond. Gaia will also map the motions of stars, which encode their origins and evolution. Gaia will provide the detailed physical properties of each star observed, revealing luminosity, temperature, gravity and composition. This huge stellar census will provide the basic observational data to tackle an enormous range of important problems related to the origin, structure and evolutionary history of our Galaxy.

Gaia will achieve its goals by repeatedly measuring the positions of all objects down to magnitude 20 (about 400 000 times fainter than can be seen with the naked eye). Onboard object detection will ensure that variable stars, supernovae, other transient celestial events and minor planets will all be detected and catalogued to this faint limit. For all objects brighter than magnitude 15 (4000 times fainter than the naked eye limit), Gaia will measure their positions to an accuracy of 24 microarcseconds. This is comparable to measuring the diameter of a human hair at a distance of 1000 km. It will allow the nearest stars to have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic centre, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%.

Gaia's expected scientific harvest is of almost inconceivable extent and implication. Its main goal is to clarify the origin and evolution of our Galaxy. In addition, it will test theories of star formation and evolution. This is possible because low-mass stars are extremely long-lived and retain a fossil record of their origin in the composition of their atmospheres.

Gaia will identify which stars are relics from smaller galaxies long ago ‘swallowed’ by the Milky Way. By watching for the large-scale motion of stars in our Galaxy, it will probe the distribution of dark matter, the hypothetical substance thought to hold our Galaxy together. In addition, Gaia will establish the range of brightnesses that forming stars can possess; detect and categorise rapid evolutionary phases in stars; place unprecedented constraints on the age, internal structure and evolution of all stars, and classify star formation and kinematical and dynamical behaviour within the Local Group of galaxies.

Gaia will target exotic objects in colossal numbers: many thousands of planets around other stars will be discovered and their detailed orbits and masses determined; stellar oddballs such as brown dwarfs and white dwarfs will be identified in their tens of thousands; some 20 000 exploding stars will be detected and their details passed to ground-based observers for follow-up observations. Solar System studies will receive a massive impetus through the observation of hundreds of thousands of minor bodies. Amongst other results relevant to fundamental physics, Gaia will follow the bending of starlight by the Sun’s gravitational field, as predicted by Albert Einstein’s General Theory of Relativity, and therefore directly observe the structure of space-time.

The astrometric data:

Positions based on the angular separation of stars and on the observed parallax. Proper motions based on the observed displacements during the mission lifetime and on radial velocity measurements from the onboard spectrometer.

The photometric data:

Effective temperatures, abundances and reddening of stars across the HR diagram. Luminosities, spatial distribution, chemical abundance and age information. A map of the interstellar extinction of the Galaxy.

The radial velocity data:

Radial-velocity measurements will be made for stars brighter than ∼16.5 mag using a slit less radial-velocity spectrometer(RVS). Its resolution (R ≃11,500) and wavelength range(847–874nm) have been optimised to allow radial velocities to be measured, at the 1–15kms−1 accuracy level, for a wide range of spectral classes.

Gaia, The Space Telescope

The Payload Module contains the optical instrument and all electronics required to manage its operation and process its raw data. A Video Processing Unit, a Clock Distribution Unit (housing the atomic clock references) and the Payload Data Handling Unit in charge of data storage between two downlink sessions.

The Astrometric instrument (ASTRO) is devoted to star angular position measurements, providing the five astrometric parameters:

The Photometric instrument provides continuous star spectra for astrophysics in the band 320-1000 nm and the ASTRO chromaticity calibration.

The Radial Velocity Spectrometer (RVS) provides radial velocity and high resolution spectral data in the narrow band 847-874 nm. Each function is achieved within a dedicated area on the single focal plane.

The Mechanical Service Module contains all mechanical, structural and thermal elements supporting the instrument and the spacecraft electronics. In addition, the mechanical SVM comprises: 1) Micro-Propulsion system, 2) Deployable sunshield, 3) Payload thermal tent, 4) Solar arrays and 5) Harness

The Electrical Service Module offers support functions to the Gaia payload and spacecraft for pointing, electrical power control and distribution, central data management and Radio communications with the Earth.

The Optical Assembly

Gaia contains two identical telescopes, pointing in two directions separated by a 106.5 degree basic angle and merged into a common path at the exit pupil. The optical path of both telescopes is composed of six reflectors (M1-M6), the last two of which are common (M5-M6). Both telescopes have an aperture of 1.45m ? 0.5m and a focal length of 35m.

The telescope elements are built around a hexagonal optical bench with a ~3m diameter, and provides structural support. Diffraction effects with residual aberrations induce systematic chromatic shifts of the diffraction images and thus of the measured star positions.

The systematic chromatic displacements will be calibrated as part of the ground data analysis using the color information provided by the photometry of each observed object.

The Orbit

Gaia will be inserted into an orbit at the second Legrange (L2) point, which lies 1.5 M-km from the Earth on the Sun-Earth line in the direction opposite to the Sun.

L2 is a gravitational saddle point of the Earth Sun system where the gravitational pull of the two large masses provides precisely the centripetal force required to rotate with them.

At L2 there is a radius (<13000 km) where the Sun is eclipsed by the Earth. To avoid reduced power to the solar panels Gaia will be placed in large Lissajous orbit ( >300000 km) to ensure that it stays out of the eclipse zone during the mission. The orbit also adds stability against external perturbations.

The Nominal Scanning Law

Gaia's measurement principle relies on the repeated observation of star positions in its two fields of view. The spacecraft rotates in 6 hours or at an angular rate of 1 degree per minute perpendicular to those two fields of view. With a basic angle of 106.5 degree separating the astrometric fields of view, objects transit the second field 106.5 minutes after the first one. The spacecraft rotation axis makes an angle of 45 degrees with the Sun direction. This is a compromise between astrometry requirements (large angle) and implementation constraints (payload shading and solar array efficiency).

The scan axis also describes a slow precession motion around the Sun-to-Earth direction, with an average period of 63.12 days. This allows the scanning law definition to be independent from the orbital position around L2.

Some regions of the sky, like the Baade's window, Omega Centauri or other globular clusters, the star density is excessive. In these cases, a modified scanning law may be temporarily followed in order to increase the number of successive transits in these regions.

The Focal Plain

ASTRO includes 62 CCDs, where the 2 FOV are combined onto the AF. The CCDs are used in TDI mode, synchronized to the scanning motion of the SC. Stars entering the FOV pass across the SM CCDs, where each object is detected. The object's position and brightness is processed on board in real-time to define a windowed region around it to be read by subsequent CCDs.

The photometer merges with the AF and uses low dispersion optics in the common path of the 2 telescope apertures: one for short wavelengths (BP: 320-660nm) and one the long wavelengths (RP: 650-1000nm).

The RVS instrument is a near-infrared, medium-resolution, integral-field spectrograph dispersing all the light entering the FOV. It is integrated with the AF and PF and also uses the common two telescopes.

Gaia and AGIS

Data Reduction

Reformatting data to create raw objects for permanent storage in the raw data base. Raw objects represent original measurements and do not change.

Derive initial values from observables, e.g. transit times, fluxes, producing intermediate objects. Intermediate data are updated as better calibrations and source data become available.

Intermediate objects are matched with sources already in the main database. New sources are created when necessary, can be linked to multiple sources.

First Look monitors the quality of science data after IDT so corrective measures may be taken without delay. First Look carries out a great circle solution referred to as a One Day Astrometric Solution (ODAS) followed by an evaluation of the results in the Detailed First Look (DFL). IDT runs daily, on 10s of GB of TM, to keep up with the data volume and to allow health checks via First Look. IDU runs at a much later time than IDT, at intervals of six months. In IDU the processing of the observations is repeated as better source and calibration parameters are available. The details of the processing is different for the astrometric, the photometric, and the spectroscopic observations.

The Astrometric Global Iterative Solution

The Astrometric Global Iterative Solution (AGIS) is the core astrometric processing of raw Gaia data, in which several processes are executed cyclically until convergence is achieved.

Source Update: determination of the astrometric parameters of a astrometric source (each source)
Attitude Update: determination of the celestial orientation of the instrument axis (each 12h interval)
Calibration Update: determination of the instrument calibrations (each CCD)
Global Update: determination of any model parameters that are constant throughout the mission (entire mission)

These processes are iterated as each one needs data from the others. Similar iterative schemes can be applied to core photometric and spectroscopic processing, although there is no equivalent to the attitude update in those cases. A direct solution taking the dependencies into account seems to be impractical by many orders of magnitude

Gaia and the Astrometric Global Iterative Solution