Gaia mapping the stars of the Milky Way

Additional scientific benefits include detection and characterisation 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 about 500 000 distant quasars. It will also provide stringent new tests of Albert Einstein’s general relativity theory.
 
 
What's special?
 
Gaia will rely on the proven principles of ESA's Hipparcos mission to solve one of the most difficult yet deeply fundamental challenges in modern astronomy: to create an extraordinarily precise three-dimensional map of more than one thousand million stars throughout our Galaxy and beyond. In the process, Gaia will also map the motions of stars, which encode their origin and subsequent evolution. Through comprehensive photometric classification, Gaia will provide the detailed physical properties of each star observed: characterising their luminosity, temperature, gravity, and elemental composition. This massive 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 this by repeatedly measuring the positions of all objects down to V=20 magnitude. On-board object detection will ensure that variable stars, supernovae, burst sources, micro-lensed events, and minor planets will all be detected and catalogued to this faint limit. Final accuracies of 24 microarcsec at 15 mag, comparable to measuring the diameter of a human hair at a distance of 1000 kilometres, will provide distances accurate to 20% as far as the Galactic Centre, 30 000 light-years away.

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, by providing tests of the various formation theories, and of star formation and evolution. This is possible since low-mass stars live for much longer than the present age of the Universe, and therefore retain in their atmospheres a fossil record of their detailed origin. The Gaia results will precisely identify relics of tidally disrupted accretion debris, probe the distribution of dark matter, establish the luminosity function for pre-main sequence stars, detect and categorise rapid evolutionary stellar phases, place unprecedented constraints on the age, internal structure and evolution of all stellar types, and classify star formation and kinematical and dynamical behaviour within the Local Group of galaxies.

Gaia will pinpoint exotic objects in colossal and almost unimaginable numbers: many thousands of extra-solar planets will be discovered, and their detailed orbits and masses determined; brown dwarfs and white dwarfs will be identified in their tens of thousands; some 20 000 supernovae will be detected and details passed to ground-based observers for follow-up observations; Solar System studies will receive a massive impetus through the detection of many tens of thousands of new minor planets, and even new trans-Neptunian objects, including Plutinos, may be discovered. Amongst other results relevant to fundamental physics, Gaia will follow the bending of star light by the Sun, over the entire celestial sphere, and therefore directly observe the structure of space-time.
 
 
Spacecraft
 
At its heart, Gaia contains two optical telescopes that can precisely pinpoint the location of stars and split their light into a spectrum for analysis. The spacecraft itself can be divided into two sections: the payload module and the service module. The payload consists of the telescopes and three instruments which focus light on one common focal plane. The service module contains the propulsion system and the communications units, essential components that allow the spacecraft to function and return data to Earth. Beneath the service module and the payload module is the sunshield and solar array assembly.

The payload module is housed inside a geometrical, dome-like structure called the thermal cover. Inside are two telescopes, each telescope consisting of three curved, rectangular mirrors, followed by a beam combiner and two rectangular flat folding mirrors to focus the starlight onto the common focal plane. The largest mirror in each telescope system is 1.45 metres long. The focal plane features three different zones associated with the astrometric, photometric and spectroscopic instruments. Each instrument uses a set of electronic detectors (CCDs). The photometric instrument uses prisms to create a spectral distribution of the light received from the telescope. The spectroscopic instrument uses a grating with prisms to disperse star light.

The astrometric instrument is dedicated to the accurate measurement of stellar positions and brightnesses. The photometric instrument will measure the colours and brightness of all stars. This is valuable information which astronomers will use to determine the physical parameters of celestial objects. The spectroscopic instrument will detect whether celestial objects are moving towards or away from us. This information can then be combined with that from the astrometric instruments, to give a full picture of how the celestial object is moving through space.

During its life, the spacecraft spins slowly, sweeping the views of the two telescopes across the celestial sphere. As the detectors repeatedly measure the position of a celestial object over a number of years, they will detect, for example, that object's motion across the line of sight.

Situated between the sunshield and payload module is the service module. It will be constructed from aluminium, fashioned into a conical framework and clad in carbon-fibre-reinforced plastic panels. Inside will be housed the attitude and control, propulsion, communications, on-board data-handling and power systems, and some electronic units for the payload module.

Gaia is expected to communicate with Earth for, on average, eight-hours every day. During this time, it will transmit its science data and 'housekeeping' telemetry signal. Although Gaia's transmitter is weak, it will be able to maintain the transmission of an extremely high data rate (~ 5 Mbit per second) over a distance of 1.5 million kilometres. ESA’s most powerful ground stations, the 35-metre radio dishes in Cebreros, Spain, and New Norcia, Australia, will be used to intercept the faint signal transmitted by Gaia.

Gaia will always point away from the Sun. After launch, it will unfold a 'skirt' that performs two functions. The first is as a sunshade. This will permanently shade the telescopes in the payload module and allows their temperature to drop to -100°C. In this way, the stability of the telescope and its optical system will be maintained at precise levels.

The other function of the sunshield is to generate power for the spacecraft. As the underside of the shield will always be facing the Sun, its surface will be partially covered with solar panels that generate electricity from sunlight. The sunshield 'skirt' is the only deployable structure on Gaia. It consists of twelve separate panels that will be folded for launch. Once in space, the spacecraft will unfold these panels into a roughly circular disc, just over 10 metres in diameter.
 
 
Journey
 
Gaia will be placed in an orbit around the Sun, at a distance of 1.5 million kilometres further out than Earth. This special location, known as L2, will keep pace with the orbit of the Earth around the Sun - Gaia will map the stars from here.

This position in space offers a very stable thermal environment, very high observing efficiency (since the Sun, Earth and Moon are behind the instrument field of view) and a moderate radiation environment. An operational lifetime of five years is planned.
 
 
History
 
ESA's Hipparcos mission exceeded all expectations and catalogued more than 100 000 stars to very high precision, and more than 1 million to lesser precision. Hipparcos was so sensitive that it could have measured the diameter of a human hair at a distance of 20 kilometres. The mission produced 16 volumes of data.

In the meantime, the technology has improved. Detectors are better. On-board data handling, with the advent of more powerful computer processors, offers more possibilities. Optics have improved with the advent of silicon carbide technology.

Scientists realised that another mission could be sent into space with similar cataloguing aims as Hipparcos but with a much more ambitious payback.

A new mission could now catalogue one thousand million stars. It would have the equivalent sensitivity of measuring the diameter of a human hair at 1000 kilometres. The 16 volumes of Hipparcos would instead be 160 000 volumes and instead of filling one normal bookshelf, that bookshelf would have to stretch the equivalent distance of Paris to Amsterdam.

The successor mission to Hipparcos, Gaia, was approved in 2000 as an ESA Cornerstone mission to be launched around 2011.
 
 
Partnerships
 
The Gaia Science Team (GST) provides advice to ESA during the implementation phase of the mission. This committee comprises 12 people with a term of office next expiring mid-2007. The committee members take responsibility for specific tasks, for example, providing advice on the payload design, and coordinating contributions from others as required. The composition reflects scientific competence, but also reflects ESA member state involvement in Gaia.
 
 
Last update: 24 January 2008