||This article may be expanded with text translated from the corresponding article in the French Wikipedia. (December 2013)|
Artist's impression of the Gaia spacecraft
|Mission type||Space observatory|
|Mission duration||5 years (planned)|
|Launch mass||2,029 kg (4,473 lb)|
|Dry mass||1,392 kg (3,069 lb)|
|Dimensions||4.6 m × 2.3 m (15.1 ft × 7.5 ft)|
|Start of mission|
|Launch date||19 December 2013, 09:12:14UTC|
|Launch site||Kourou ELS|
|Reference system||Sun–Earth L2|
|Apoapsis||up to 707,000 km (439,000 mi)|
|Band||S Band (TT&C support)
X Band (data acquisition)
|Bandwidth||few kbit/s down & up (S Band)
3-8Mbit/s download (X Band)
|ASTRO: Astrometric instrument
BP/RP: Photometric instrument
RVS: Radial Velocity Spectrometer
Gaia is an unmanned space observatory of the European Space Agency (ESA) designed for astrometry. The mission aims to compile a 3D space catalogue of approximately 1 billion astronomical objects (approximately 1% of the Milky Way population) brighter than 20 G magnitudes, where G is the Gaia magnitude passband between about 400 and 1000 nanometres light wavelengths. Successor to the Hipparcos mission, it is part of ESA's Horizon 2000 Plus long-term scientific program. Gaia will monitor each of its target stars about 70 times over a period of five years.
Gaia will create a precise three-dimensional map of stars throughout the Milky Way and map their motions, which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements will provide the detailed physical properties of each star observed, characterising their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to tackle a wide range of important questions related to the origin, structure and evolutionary history of our galaxy. Large numbers of quasars, galaxies, extrasolar planets and Solar System bodies will be measured at the same time.
Gaia was launched on 19 December 2013 by Arianespace using a Soyuz ST-B/Fregat-MT rocket flying from Kourou in French Guiana. The spacecraft will be operated in a Lissajous orbit around the Sun–Earth L2 Lagrangian point.
The Gaia space telescope has its roots in ESA's Hipparcos mission (1989–1993). Its mission was proposed in October 1993 by Lennart Lindegren (Lund University, Sweden) and Michael Perryman (ESA) in response to a call for proposals for ESA's Horizon Plus long-term scientific programme. It was adopted by ESA's Science Programme Committee as cornerstone mission number 6 on 13 October 2000, and the B2 phase of the project was authorised on 9 February 2006, with EADS Astrium taking responsibility for the hardware. The name ‘GAIA’ was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics. This reflected the optical technique of interferometry that was originally planned for use on the spacecraft. However, the working method has now changed, and although the acronym is no longer applicable, the name Gaia remains to provide continuity with the Hipparcos project.
The total cost of the mission is around €740 million (~ $1 billion), including the manufacture, launch and ground operations.Gaia was completed two years behind schedule and 16% above its initial budget, mostly due to the difficulties encountered in polishing Gaia's ten mirrors and assembling and testing the focal plane camera system, as well as other issues.
The Gaia space mission has the following objectives:
- To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the few ways to achieve this without physical assumptions is through the star's parallax. Ground-based observations would not measure such parallaxes with sufficient precision due to the effects of the atmosphere and instrumental biases.
- Observations of the faintest objects will provide a more complete view of the stellar luminosity function. Gaia will observe 1 billion stars and other bodies, representing 1% of such bodies in the Milky Way galaxy.All objects up to a certain magnitude must be measured in order to have unbiased samples.
- A large number of objects are needed to examine the more rapid stages of stellar evolution. Observing a large number of objects in the galaxy is also important in order to understand the dynamics of our galaxy.
- Measuring the astrometric and kinematic properties of a star is necessary in order to understand the various stellar populations, especially the most distant.
In order to achieve these objectives, Gaia has these goals:
- Determine the position, distance, and annual proper motion of 1 billion stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag.
- Determine the positions of stars at a magnitude of V = 10 down to a precision of 7 millionths of an arcsecond (μas) (this is equivalent to measuring the diameter of a hair from 1000 km away); between 12 and 25 μas down to V = 15, and between 100 and 300 μas to V = 20, depending on the colour of the star.
- About 20 million stars will be measured with a distance precision of 1% and about 200 million will be measured to better than 10%. Distances accurate to 10% will be achieved as far away as the Galactic Centre, 30,000 light-years away.
- Measure the tangential speed of 40 million stars to a precision of better than 0.5 km/s.
- Derive the atmospheric parameters (effective temperature, line-of-sight interstellar extinction, surface gravity, metallicity) for all stars observed, plus some more detailed chemical abundances for targets brighter than V = 15.
- Measure the orbits and inclinations of a thousand extrasolar planets accurately, determining their true mass using astrometric planet detection methods.
- Detect 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.
- Potential to discover Apohele asteroids with orbits that lie between Earth and the Sun, a region that is difficult for Earth-based telescopes to monitor since this region is only visible in the sky during or near the daytime.
- Detect up to 500,000 quasars.
Gaia was launched by Arianespace, using a Soyuz ST-B rocket with a Fregat-MT upper stage, from the Ensemble de Lancement Soyouz at Kourou in French Guiana on 19 December 2013 at 09:12 UTC (06:12 local time). The satellite separated from the rocket's upper stage 43 minutes after launch at 09:54 UTC. The craft is heading toward the Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth. The L2 point will provide the spacecraft with a very stable thermal environment. There it will describe a Lissajous orbit that will avoid blockage of the Sun by the Earth, which would limit the amount of solar energy the satellite could produce through its solar panels, as well as disturb the spacecraft's thermal equilibrium. After launch, a 10-metre diameter sunshade was deployed. The sunshade always faces the Sun, thus cooling all telescope components and powering Gaia using solar panels on its surface.
The Gaia payload consists of three main instruments:
- The astrometry instrument (Astro) precisely determines the positions of stars of magnitude 5.7 to 20 by measuring their angular position. By combining the measurements of any given star over the five-year mission, it will be possible to determine its parallax, and therefore its distance, and its proper motion —the velocity of the star as it moves on the plane of the sky.
- The photometric instrument (BP/RP) allows the acquisition of luminosity measurements of stars over the 320–1000 nm spectral band, over the same magnitude 5.7–20. The blue and red photometers (BP/RP) are used to determine stellar properties such as temperature, mass, age and elemental composition. Multi-colour photometry is provided by two low-resolution fused-silica prisms dispersing all the light entering the field of view in the along-scan direction prior to detection. The Blue Photometer (BP) operates in the wavelength range 330–680 nm; the Red Photometer (RP) covers the wavelength range 640–1050 nm.
- The Radial-Velocity Spectrometer (RVS) is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). The measurements of radial velocities are important to correct for perspective acceleration which is induced by the motion along the line of sight." The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum.
In order to maintain the fine pointing to focus on stars many light years away, there are almost no moving parts. The spacecraft subsystems are mounted on a rigid silicon carbide frame, which provides a stable structure that will not expand or contract due to heat. Attitude control is provided by small cold gas thrusters that can output 1.5 micrograms of nitrogen per second.
The telemetric link with the satellite is about 3 Mbit/s on average, while the total content of the focal plane represents several Gbit/s. Therefore only a few dozen pixels around each object can be downlinked.
Despite its name, Gaia does not actually use interferometry to determine the positions of stars. At the time of the original design, interferometry seemed the best way to achieve the target resolution, but the design later evolved into an imaging telescope. Similar to its predecessor Hipparcos, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle (106.5°) between them. The spacecraft rotates continuously around an axis perpendicular to the two telescopes' lines of sight. The spin axis in turn has a slight precession across the sky, while maintaining the same angle to the Sun. By precisely measuring the relative positions of objects from both observing directions, a rigid system of reference is obtained.
The two key telescope properties are:
- 1.45 × 0.5 m primary mirror for each telescope
- 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each.
Each celestial object will be observed on average about 70 times during the mission, which is expected to last five years. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time (motion) and lastly, the star's parallax from which distance can be calculated. The radial velocity of the brighter stars is measured by an integrated spectrometer observing the Doppler effect. Because of the physical constraints imposed by the Soyuz spacecraft, Gaia's focal arrays could not be equipped with optimal radiation shielding, and ESA expects their performance to suffer somewhat toward the end of the five year mission. Ground tests of the CCDs while they were subjected to radiation provided reassurance that the primary mission's objectives can be met.
Accuracy will range from 20% for stars near the centre of the Galaxy, some 30,000 light-years away, to 0.001% for the stars nearest to our Solar System. 
The overall data volume that will be retrieved from the spacecraft during the five-year mission assuming a nominal compressed data rate of 1 Mbit/s is approximately 60 TB, amounting to about 200 TB of usable uncompressed data on the ground, stored in the InterSystems Caché database. The responsibility of the data processing, partly funded by ESA, has been entrusted to a European consortium (the Data Processing and Analysis Consortium, or DPAC) which has been selected after its proposal to the ESA Announcement of Opportunity released in November 2006. DPAC's funding is provided by the participating countries and has been secured until the production of the Gaia final Catalogue scheduled for 2020.
Gaia will send back data for about eight hours every day at about 5 Mbit/s. ESA's two most sensitive ground stations, the 35 m diameter radio dishes in Cebreros, Spain, and New Norcia, Australia, will receive the data.
Launch and orbit
In October 2013 ESA had to postpone Gaia's original launch date, due to a precautionary replacement of two of Gaia's transponders. These are used to generate timing signals for the downlink of science data. A problem with an identical transponder, on a satellite already in orbit, necessitated their replacement and reverification once incorporated into Gaia. The rescheduled launch window was from 17 December 2013 to 5 January 2014, and Gaia slated for launch on 19 December.
Gaia was successfully launched on 19 December 2013 at 09:12 UTC. About three weeks after launch, on 8 January 2014, it reached its designated orbit around the SEL2 point. Testing and calibration phase, started while Gaia was en route to SEL2 point, will continue until May 2014.
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