Mission

Gaia mapping the stars of the Milky Way

Gaia will conduct a census of a thousand million stars in our Galaxy, monitoring each of its target stars about 70 times over a five-year period. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and failed stars called brown dwarfs. Within our own Solar System, Gaia should also observe hundreds of thousands of asteroids.

Additional scientific benefits include the 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 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.

Spacecraft

At its heart, Gaia contains two optical telescopes that can precisely determine 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. The service module contains the propulsion system, the communications units and other 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 called the ‘thermal tent’. Inside are the two telescopes, each consisting of three curved, rectangular mirrors, a beam combiner and two flat rectangular mirrors. The largest mirror in each telescope is 1.45 m by 0.5 m. The two telescopes focus their light onto the focal plane, which features three different zones associated with the science instruments: Astro, the astrometric instrument for detecting and pinpointing celestial objects; the Blue and Red Photometers (BP/RP), used to determine stellar properties such as temperature, mass, age, elemental composition; the Radial-Velocity Spectrometer (RVS), used to determine the velocity of celestial objects along the line of sight. Each instrument uses a set of Charge Coupled Devices (CCDs) as detectors. This information will all be combined to give a three-dimensional picture of how each celestial object is moving through space.

During its five-year mission, the spacecraft spins slowly, sweeping the two telescopes across the entire celestial sphere. As the detectors repeatedly measure the position of each celestial object, they will detect any changes in the object’s motion through space.

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

Gaia is expected to communicate with Earth for an average of 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 at an extremely high data rate (~ 5 Mbit/s) from a distance of 1.5 million km. ESA’s most powerful ground stations, the 35 m-diameter radio dishes in Cebreros, Spain, and New Norcia, Australia, will intercept the faint signal.

Gaia will always point away from the Sun. After launch, it will unfold a ‘skirt’ that consists of a dozen separate panels. These will deploy into a roughly circular disc, just over 10 m in diameter that performs two functions. The first is as a sunshade, which will permanently shade the telescopes in the payload module and allow their temperatures to drop to –100°C. In this way, the stability of the telescopes will be maintained. The other function is to generate power for the spacecraft. The underside of the shield will always be facing the Sun, so its surface will be partially covered with solar panels to generate electricity.

Journey

Gaia will map the stars from an orbit around the Sun, at a distance of 1.5 million km beyond Earth’s orbit. This special location, known as the L2 Lagrangian point, keeps pace with Earth as we orbit the Sun.

L2 offers a stable thermal environment, a clear view of the Universe because the Sun, Earth and Moon are always outside the instruments’ fields of view, and a moderate radiation environment. An operational lifetime of five years is planned.

History

ESA’s Hipparcos mission exceeded all expectations, cataloguing more than 100 000 stars to high precision, and more than a million to lesser precision. Hipparcos was so sensitive that it could have measured the diameter of a human hair at a distance of 20 km.

Now that technology has improved, it is time for another mission with similar cataloguing aims as Hipparcos but with a much more ambitious payback.

Gaia will catalogue a thousand million stars, producing 10 000 times more data than Hipparcos. It will have the equivalent sensitivity of measuring the diameter of a human hair at 1000 km. If printed, the 160 000 volumes of Gaia data would stretch the equivalent distance of Paris to Amsterdam.

Gaia was approved in 2000 as an ESA Cornerstone mission, and will be launched in 2013.

Partnerships

The Gaia Data Processing and Analysis Consortium (DPAC) will process all the raw data from the satellite into calibrated astrophysical quantities to be published in various Gaia catalogues. The computations are shared between six different data processing centres, which in turn receive code and algorithms from nine topically organised scientific coordination units. In all, DPAC consists of more than 400 individuals who will contribute some 2000 person-years of effort to the Gaia data processing exercise. Although the final Gaia results will not be published until 2020, the astronomical community will be served by science alerts from the early routine phase onward and with intermediate releases from 2015.

The Gaia Science Team (GST) provides advice to ESA during all phases of the mission. This committee comprises seven people who 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 and ESA member state involvement in Gaia.