Gaia mission

Gaia is a European Space Agency (ESA) astrometry space mission, and a successor to the ESA Hipparcos mission. It was included within the context of the ESA Horizon 2000 Plus long-term scientific programme in 2000. Arianespace expects to launch Gaia for the ESA in the Spring of 2012, using a Soyuz rocket.[1] It will be operated in a Lissajous orbit around the Sun-Earth L₂ Lagrangian point.

Gaia will compile a catalogue of approximately one billion stars to magnitude 20. Its objectives comprise:

• astrometric (or positional) measurements, determining the positions, distances, and annual proper motions of stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag
• spectrophotometric measurements, providing multi-epoch observations of each detected object
• radial velocity measurements.

Gaia will create an extremely precise three-dimensional map of stars throughout our Milky Way galaxy and beyond, 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 problems 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 will also be capable of discovering 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 in the sky during or near the daytime.^[1]

Satellite

Gaia will be launched on a Soyuz-FG rocket and will fly to the Lagrange point L2 located approximately 1.5 million kilometers from Earth. The L2 point will provide the spacecraft with a very stable thermal environment. There it will describe a Lissajous orbit which will avoid eclipses of the Sun by the Earth, which would otherwise limit the amount of solar energy the satellite can retrieve through its solar panels.

Measurement principles

Similarly to its predecessor Hipparcos, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle between them. The spacecraft rotates continuously around an axis perpendicular to the two telescopes line of sight (LOS). 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.

Each celestial object will be observed on average about 70 times during the mission, which is expected to last 5 years. These measurements will help determine the astrometric parameters of stars: 2 corresponding to the angular position of a given star on the sky, 2 for the derivatives of the star's position over time (motion) and lastly, the stars parallax.

One measurement that is however lacking by the above telescopes is the radial velocity of the star. This is obtained separately using the Doppler Effect by a spectrometer that is also onboard Gaia.

Features

The Gaia payload consists of

• a 1.4 x 0.5 square metre mirror for each telescope
• A 1.0 x 0.5 m focal plane array on which light from both telescopes are projected. This in turn consists of 106 CCDs of 4500 x 1966 pixels. Some of these CCDs are equipped with filters.

Gaia contains 3 separate instruments:

• The astrometry instrument (ASTRO), which is dedicated to measuring the angular position of the stars of magnitude 5.7 to 20.
• The photometric instrument, which allows the acquisition of spectra of stars over the 320-1000 nm spectral band, over the same magnitude 5.7-20.
• The high-resolution spectrometer to measure the radial velocity of the stars by acquiring high-resolution spectra in the spectral band 847-874 nm (field lines of calcium ion) for objects up to magnitude 17 ,

The telemetric link with the satellite is about 1 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. This means that detection and monitoring of objects on board mandatory, with such processing particularly complex when scanning dense stellar fields.

Mission

The mission was adopted by ESA as cornerstone mission number 6 on 13th October 2000 and the B2 phase of the project was authorized on February 9 2006, with EADS Astrium taking responsibility for the hardware. The launch is planned for Spring 2012. The total cost of the mission is around 450 million euros, including the manufacture, launch and ground operations.

The overall data volume that will be retrieved from the spacecraft during the 5-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. The responsibility of the data processing, not funded by ESA, has been entrusted to the a European consortium (the Data Processing and Analysis Consortium, abridged in DPAC) which has successfully been selected after from its proposal to the ESA Announcement of Opportunity released in November 2006.

The DPAC is a collaboration of about 400 astronomers and IT engineers from 20 European countries, including a significant participation of an ESA group based at the the European Space Astronomy Centre (ESAC), one of the ESA centre in Europe located near Madrid. The funding is provided by the participating countries and has been secured until the production of the Gaia final Catalogue scheduled for 2020.

Objectives

The Gaia space mission has the following objectives:

• To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the only ways to achieve 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. We must observe all the objects up to a certain magnitude in order to have unbiased samples.
• You need a large amount of objects 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. Note that a billon stars represents only 1% of the content of our galaxy.
• Measuring the astrometric and kinematic properties of star is necessary in order to understand the various stellar populations, especially the most distant.

Gaia is expected to:

• Measure the astrometric properties of over a billion stars down to a magnitude of V = 20
• 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 color of the star
• Measure the tangential speed of 40 million stars to a precision of better than 0.5 km / s

References

Further reading

• Thorsten Dambeck in Sky and Telescope, Gaia's Mission to the Milky Way, March 2008, p. 36 - 39

External links

Wikimedia Commons has media related to: Gaia (satellite)

• ESA Gaia mission
• Gaia in Depth
• Gaia page at ESA Spacecraft Operations
• Gaia pages for the scientific community
• Gaia Study Report Summary of the scientific goals of the mission, March 2000
• Origins Billion Star Survey (OBSS) A similar project proposal as collaboration between NASA and U.S. Naval Observatory.

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