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Artist's impression of the Gaia spacecraft
|Mission type||Astrometric observatory|
planned: 5 years; possible extension by one to four years|
elapsed: 4 years, 9 months and 1 day
|Launch mass||2,029 kg (4,473 lb)|
|Dry mass||1,392 kg (3,069 lb)|
|Payload mass||710 kg (1,570 lb)|
|Dimensions||4.6 m × 2.3 m (15.1 ft × 7.5 ft)|
|Start of mission|
|Launch date||19 December 2013, 09:12:14 UTC|
|Launch site||Kourou ELS|
|Reference system||Sun–Earth L2|
|Periapsis||263,000 km (163,000 mi)|
|Apoapsis||707,000 km (439,000 mi)|
|Diameter||1.45 m × 0.5 m (4.8 ft × 1.6 ft)|
|Collecting area||0.7 m2|
S Band (TT&C support)|
X Band (data acquisition)
few kbit/s down & up (S Band)|
3-8 Mbit/s download (X Band)
ASTRO: Astrometric instrument|
BP/RP: Photometric instrument
RVS: Radial Velocity Spectrometer
Gaia is a space observatory of the European Space Agency (ESA) designed for astrometry: measuring the positions and distances of stars with unprecedented precision. The mission aims to construct the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars among others.
The spacecraft will monitor each of its target objects about 70 times over a period of five years to study the precise position and motion of each target. The spacecraft has enough consumables to operate for approximately nine years, if its detectors are not degrading as fast as initially expected. The mission could therefore be extended. The Gaia targets represent approximately 1% of the Milky Way population with all stars brighter than magnitude 20 in a broad photometric band that covers most of the visual range. Additionally, Gaia is expected to detect thousands to tens of thousands of Jupiter-sized exoplanets beyond the Solar System, 500,000 quasars and tens of thousands of new asteroids and comets within the Solar System.
Gaia will create a precise three-dimensional map of astronomical objects 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 all stars observed, characterizing their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to analyze a wide range of important questions related to the origin, structure, and evolutionary history of our galaxy.
Successor to the Hipparcos mission, the telescope is part of ESA's Horizon 2000+ long-term scientific program. 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 currently operates 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. While the working method evolved during studies and the acronym is no longer applicable, the name Gaia remained to provide continuity with the 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.
The Gaia space mission has the following objectives:
In order to achieve these objectives, Gaia has these goals:
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 headed towards the Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth, arriving there 8 January 2014. The L2 point provides the spacecraft with a very stable gravitational and thermal environment. There it uses a Lissajous orbit that avoids 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 keeping all telescope components cool and powering Gaia using solar panels on its surface.
The Gaia payload consists of three main instruments:
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, but with a precision one hundred times better, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle of 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:
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.
The expected accuracies of the final catalogue data have been calculated following in-orbit testing, taking into account the issues of stray light, degradation of the optics, and the basic angle instability. The best accuracies for parallax, position, and proper motion are obtained for the brighter observed stars, apparent magnitudes 3-12. The standard deviation for these stars is expected to be 6.7 micro-arc seconds or better. For fainter stars, error levels increase, reaching 26.6 micro-arc seconds error in the parallax for 15th magnitude stars, and several hundred micro-arc seconds for 20th magnitude stars. For comparison, the best parallax error levels from the new Hipparcos reduction are no better than 100 micro-arc seconds, with typical levels several times larger.
The overall data volume that will be retrieved from the spacecraft during the nominal five-year mission at a 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 an InterSystems Caché database. The responsibility of the data processing, partly funded by ESA, is entrusted to a European consortium, the Data Processing and Analysis Consortium (DPAC), which was 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 Gaia's final catalogue scheduled for 2020.
Gaia sends back data for about eight hours every day at about 5 Mbit/s. ESA's three 35-metre-diameter radio dishes of the ESTRACK network in Cebreros, Spain, Malargüe, Argentina and New Norcia, Australia, receive the data.
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 motivated their replacement and reverification once incorporated into Gaia. The rescheduled launch window was from 17 December 2013 to 5 January 2014, with 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 Sun-Earth L2 Lagrange point (SEL2), about 1.5 million kilometers from Earth.
In 2015 the Pan-STARRS observatory discovered an object orbiting the Earth, which the Minor Planet Center catalogued as object 2015 HP116. It was soon found to be an accidental rediscovery of the Gaia spacecraft and the designation was promptly retracted.
Shortly after launch, ESA revealed that Gaia was suffering from a stray light problem. The problem was initially thought to be due to ice deposits causing some of the light diffracted around the edges of the sunshield and entering the telescope apertures to be reflected towards the focal plane. The actual source of the stray light was later identified as the fibers of the sunshield, protruding beyond the edges of the shield. This results in a "degradation in science performance [which] will be relatively modest and mostly restricted to the faintest of Gaia's one billion stars." Mitigation schemes are being implemented to optimise the mission performance. The degradation is more severe for the RVS spectrograph than for the astrometry measurements. It only affects the fainter stars.
The testing and calibration phase, which started while Gaia was en route to SEL2 point, continued until the end of July 2014, three months behind schedule due to unforeseen issues with stray light entering the detector. After the six-month commissioning period, the satellite started its nominal five-year period of scientific operations on 25 July 2014 using a special scanning mode that intensively scanned the region near the ecliptic poles; on 21 August 2014 Gaia began using its normal scanning mode which provides more uniform coverage.
Although it was originally planned to limit Gaia's observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen µas".
On 12 September 2014, Gaia discovered its first supernova in another galaxy. On 3 July 2015, a map of the Milky Way by star density was released, based on data from the spacecraft. As of August 2016, "more than 50 billion focal plane transits, 110 billion photometric observations and 9.4 billion spectroscopic observations have been successfully processed."
The Gaia catalogue is released in stages that will contain increasing amounts of information; the early releases will also miss some stars, especially fainter stars located in dense star fields. The first data release, Gaia DR1, based on 14 months of observations made through September 2015, took place on 14 September 2016 and is described in a series of articles published in Astronomy and Astrophysics. The data release includes "positions and … magnitudes for 1.1 billion stars using only Gaia data; positions, parallaxes and proper motions for more than 2 million stars" based on a combination of Gaia and Tycho-2 data for those objects in both catalogues; "light curves and characteristics for about 3000 variable stars; and positions and magnitudes for more than 2000 … extragalactic sources used to define the celestial reference frame". Data from this DR1 release can be accessed at the Gaia archive, as well as through astronomical data centers such as CDS.
The second data release (DR2), which occurred on 25 April 2018, is based on 22 months of observations made between 25 July 2014 and 23 May 2016. It includes positions, parallaxes and proper motions for about 1.3 billion stars and positions of an additional 300 million stars, red and blue photometric data for about 1.1 billion stars and single colour photometry for an additional 400 million stars, and median radial velocities for about 7 million stars between magnitude 4 and 13. It also contains data for over 14,000 selected Solar System objects. The coordinates in DR2 use the Gaia celestial reference frame (Gaia–CRF2), which is based on observations of 492,006 sources believed to be quasars, and which has been tied to the International Celestial Reference System. Comparison of Gaia–CRF2 with a preliminary version of the forthcoming ICRF3 shows a global agreement of 20 to 30 μas, although individual sources may differ by several mas. Since the data processing procedure links individual Gaia observations with particular sources on the sky, in some cases the association of observations with sources will be different in the second data release. Consequently, DR2 uses different source identification numbers than DR1.
The third data release potentially will include orbital solutions for many binary stars and classifications for spectroscopically "well behaved" objects, as well as improved positions, parallaxes and proper motions. The fourth data release potentially will include variable star classifications, complete Solar System results, and non single-star catalogues. The complete final Gaia catalogue is currently scheduled for 2022, three years after the end of the nominal five-year mission. It would be pushed back if the mission is extended to nine years. The number of releases between DR2 and the final release has not yet been decided.
An outreach application, Gaia Sky, has been developed to explore the galaxy in three dimensions using Gaia data.
In November 2017, scientists led by Davide Massari of the Kapteyn Astronomical Institute, University of Groningen, Netherlands released a paper describing the characterization of proper motion (3D) within the Sculptor dwarf galaxy, and of that galaxy’s trajectory through space and with respect to the Milky Way, using data from Gaia and the Hubble Space Telescope. Massari said, “With the precision achieved we can measure the yearly motion of a star on the sky which corresponds to less than the size of a pinhead on the Moon as seen from Earth.” The data showed that Sculptor orbits the Milky Way in a highly elliptical orbit; it is currently near its closest approach at a distance of about 83.4 kiloparsecs (272,000 ly), but the orbit can take it out to around 222 kiloparsecs (720,000 ly) distant.
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