Gaia (spacecraft)

Gaia

Artist's impression of the Gaia spacecraft
General information
Organization	European Space Agency
Major contractors	EADS Astrium, e2v Technologies
Launch date	October 2013.
Launched from	Kourou, French Guiana
Launch vehicle	Soyuz
Mission length	5+1 years
Mass	2030 kg
Type of orbit	Lissajous orbit around Sun–Earth L2
Wavelength	Optical
Diameter	~11m after deployment of a roughly circular sunshade disc
Website	Gaia pages
Statistics as of July 2011.

Gaia is a space observatory to be launched by the European Space Agency (ESA) in October 2013. The mission aims to compile a 3D space catalogue of approximately 1 billion stars, or roughly 0.2% of stars in the Milky Way. 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 to a magnitude 20 over a period of 5 years. Its objectives comprise:

• determining the positions, distances, and annual proper motions of 1 billion stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag
• detection of tens of thousands of extra-solar planetary systems
• capacity 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 in the sky during or near the daytime
• detection of up to 500,000 distant quasars
• more accurate tests of Albert Einstein’s general relativity theory

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.

Arianespace expects to launch Gaia for ESA in October 2013, using a Soyuz rocket from its Guiana Space Centre (GSC) in French Guiana. It will be operated in a Lissajous orbit around the Sun–Earth L₂ Lagrangian point.

Satellite

Gaia (originally an acronym for Global Astrometric Interferometer for Astrophysics) will be launched on a Soyuz-FG rocket and will fly to the Sun–Earth 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 and also disturb the thermal equilibrium.

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' 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.

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.

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 star's parallax (from which distance can be calculated). The radial velocity of the star is measured using the Doppler effect by a spectrometer, which is integrated into the Gaia telescope system.

"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 3300–6800Å ; the Red Photometer (RP) covers the wavelength range 6400–10500Å."

It is hoped to detect and measure the orbital inclinations of extrasolar planets using astrometry.

Features

The Gaia payload consists of

• 1.45 x 0.5 m primary mirror for each telescope
• 1.0 x 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 x 1966 pixels.

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.^{citation needed} Officially, 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 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 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 is mandatory. Such processing is particularly complex when scanning dense stellar fields.

Mission

The mission was proposed in October 1993 by Lennart Lindegren (Lund University, Sweden) and Michael Perryman (European Space Agency) 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 authorized on 9 February 2006, with EADS Astrium taking responsibility for the hardware. The launch is planned for October 2013. The total cost of the mission is around 650 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, 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.

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 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. 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. Note that a billion stars represents less than 1% of the content of our Milky Way galaxy.
• Measuring the astrometric and kinematic properties of a 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 an apparent magnitude (V) 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

• Determine the distances to the nearest stars within 0.001%, and to stars near the galactic center, 30,000 light years away, within 20%
• Measure the tangential speed of 40 million stars to a precision of better than 0.5 km/s
• Measure the orbits and inclinations of a thousand extrasolar planets accurately, determining their true masses using astrometric planet detection methods

Among other results relevant to fundamental physics, Gaia will 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.

See also

• Hipparcos, astrometry spacecraft which located over 118,000 pre-selected stars to high precision (0.6-1.0 milliarc-sec), and mapped another 2 million to 20–30 milliarc-sec precision.

Further reading

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

External links

• ESA Gaia mission
• Gaia page at ESA Spacecraft Operations
• Gaia In Depth
  • Gaia Mirrors ready to shine
• Gaia pages for the scientific community
• Gaia library
• Gaia Study Report Summary of the scientific goals of the mission, March 2000 (DEAD URL Oct 2012)
• Origins Billion Star Survey (OBSS) A similar project proposal as collaboration between NASA and U.S. Naval Observatory.