Gaia and the Astrometric Global Iterative Solution
Gaia is a European
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.
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.
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
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
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
lifetime and on radial velocity measurements from the onboard
The photometric data:
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:
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 Gaia spacecraft is composed of 3 major modules:
The Payload Module contains
The Astrometric instrument (ASTRO) is
devoted to star angular position measurements, providing the five
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.
This effect is usually neglected in
optical systems but is critical for Gaia.
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.
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
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
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.
Wavelength range 847 - 874 nm
Resolution ~ 11 500
Gaia and AGIS
Data reduction involves processing newly
arrived TM and auxiliary data by:
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
Intermediate objects are matched with
sources already in the main database.
New sources are created when necessary, can be linked to multiple
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
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
The following AGIS process are normally
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
Simulated Results from DPAC
Lund Observatory, Box 43, SE-221 00 Lund, Sweden
Visiting address: Sölvegatan 27
Phone: +46 46 22 27 300, Fax: +46 46 22 24614
Last updated: August 29, 2010