Gaia probe
Encyclopedia
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 March 2013, using a Soyuz
rocket from CSG
. It will be operated in a Lissajous orbit
around the Sun
-Earth
L2 Lagrangian point for a planned 5 year mission.
Gaia will compile a catalogue of approximately one billion star
s to magnitude
20. Its objectives comprise:
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 quasar
s, galaxies
, extrasolar planet
s 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.
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 eclipse
s 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.
, 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 (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.
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
.
The radial velocity of the star is measured using the Doppler Effect
by a spectrometer, which is integrated into the Gaia telescope system.
Gaia contains 3 separate instruments:
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.
taking responsibility for the hardware. The launch is planned for March 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, not 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
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 European Space Astronomy Centre (ESAC), one of the ESA centres 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.
Gaia is expected to:
Among 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.
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
(ESA) astrometry
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
space mission, and a successor to the ESA Hipparcos
Hipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
mission. It was included within the context of the ESA Horizon 2000 Plus long-term scientific programme in 2000. Arianespace
Arianespace
Arianespace SA is a French company founded in 1980 as the world's first commercial space transportation company. It undertakes the production, operation, and marketing of the Ariane 5 rocket launcher as part of the Ariane programme....
expects to launch Gaia for the ESA in March 2013, using a Soyuz
Soyuz (rocket)
The Soyuz was a Soviet expendable carrier rocket designed by OKB-1 and manufactured by State Aviation Plant No. 1 in Samara, Russia. It was used to launch Soyuz spacecraft as part of the Soyuz programme, initially on unmanned test flights, followed by the first 19 manned launches of the...
rocket from CSG
Centre Spatial Guyanais
The Guiana Space Centre or, more commonly, Centre Spatial Guyanais is a French spaceport near Kourou in French Guiana. Operational since 1968, it is particularly suitable as a location for a spaceport due to its proximity to the equator, and that launches are in a favourable direction over water...
. It will be operated in a Lissajous orbit
Lissajous orbit
In orbital mechanics, a Lissajous orbit, , named after Jules Antoine Lissajous, is a quasi-periodic orbital trajectory that an object can follow around a Lagrangian point of a three-body system without requiring any propulsion. Lyapunov orbits around a libration point are curved paths that lie...
around the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
-Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
L2 Lagrangian point for a planned 5 year mission.
Gaia will compile a catalogue of approximately one billion star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s to magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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 (microarcsecondMinute of arcA minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....
) at 15 mag, and 200 µas at 20 mag - spectrophotometric measurements, providing multi-epoch observations of each detected object
- radial velocityRadial velocityRadial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
measurements.
Gaia will create an extremely precise three-dimensional map of stars throughout our Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
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
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
, effective temperature
Effective temperature
The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...
, gravity
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
and elemental
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
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 quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s, galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s and solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
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.
Satellite
Gaia will be launched on a Soyuz-FGSoyuz-FG
The Soyuz-FG launch vehicle is an improved version of the Soyuz-U, from the R-7 family of rockets, designed and constructed by TsSKB-Progress in Samara...
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
Lissajous orbit
In orbital mechanics, a Lissajous orbit, , named after Jules Antoine Lissajous, is a quasi-periodic orbital trajectory that an object can follow around a Lagrangian point of a three-body system without requiring any propulsion. Lyapunov orbits around a libration point are curved paths that lie...
which will avoid eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
s of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
by the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, 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 HipparcosHipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
, 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 (LOS). The spin axis in turn has a slight precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
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
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
.
The radial velocity of the star is measured using the Doppler Effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
by a spectrometer, which is integrated into the Gaia telescope system.
Features
The Gaia payload consists of- a 1.4 x 0.5 square metre primary mirror for each telescope
- A 1.0 x 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDsCharge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
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.
- 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 is mandatory. Such processing is particularly complex when scanning dense stellar fields.
Mission
The mission was adopted by ESA as cornerstone mission number 6 on 13 October 2000 and the B2 phase of the project was authorized on February 9, 2006, with EADS AstriumEADS Astrium
Astrium Satellites is one of the three business units of Astrium, a subsidiary of EADS. It is a European space manufacturer involved in the manufacture of spacecraft used for science, Earth observation and telecommunication, as well as the equipment and subsystems used therein and related ground...
taking responsibility for the hardware. The launch is planned for March 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, not funded by ESA, has been entrusted to a European consortium (the Data Processing and Analysis Consortium, or DPAC
DPAC
The Gaia Data Processing and Analysis Consortium is a group of over 400 European scientists and software engineers formed with the objective to design, develop and execute the data processing system for ESA's ambitious Gaia space astrometry mission...
) which has been selected after its proposal to the ESA Announcement of Opportunity released in November 2006.
DPAC
DPAC
The Gaia Data Processing and Analysis Consortium is a group of over 400 European scientists and software engineers formed with the objective to design, develop and execute the data processing system for ESA's ambitious Gaia space astrometry mission...
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 European Space Astronomy Centre (ESAC), one of the ESA centres 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 billion stars represents less than 1% of the content 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.
Gaia is expected to:
- Measure the astrometric properties of over a billion stars down to a magnitudeApparent magnitudeThe apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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
Among other results relevant to fundamental physics, Gaia will follow the bending of starlight by the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
’s gravitational field, as predicted by Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
’s General Theory of Relativity, and therefore directly observe the structure of space-time.
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 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.