Frequently Asked Questions about Gaia

. What is the goal of Gaia?

The Gaia mission will survey about one billion stars in the Milky Way Galaxy to create the largest and most accurate three-dimensional map of the Galaxy ever obtained. In so doing, it will also detect new asteroids and extragalactic sources such as quasars, find new exoplanets and even provide some tests of Einstein’s theory of general relativity.

. Why is the mission important?

From the information obtained, astronomers will be able to understand much more about the structure, contents and evolution of our Galaxy, how it came into being and why it is the way it is.

. What are the primary mission objectives?

Gaia measures the position and velocity of more than one billion stars in the Milky Way – about 1% of the stars in the Galaxy – charts the three-dimensional distribution of these stars and determines their brightness, temperature, composition and motion through space.

. Are additional discoveries expected?

Scientists also expect to discover thousands of exoplanets beyond the Solar System, tens of thousands of failed stars (brown dwarfs) and more than 20,000 exploding stars (supernovae). And by watching the large-scale motion of stars within the Galaxy, they probe the distribution of dark matter. Gaia also measures some 500,000 distant quasars, providing a connection to the reference frame currently defined in radio wavelengths. In future, Gaia’s quasar measurements will redefine the fundamental reference frame used for all astronomical coordinate systems.

. When was Gaia started?

Gaia was approved in 2000 as a European Space Agency Cornerstone Mission within ESA’s Horizon 2000 Plus science programme. It is a purely European mission.

. Which ESA Member States are participating in this mission and how?

As an ESA Science Programme, contributions are mandatory, so all member states are taking part. Companies from 15 member nations were awarded contracts to build the spacecraft. Most member states also have a role in the science portion of the mission as part of the Data Processing and Analysis Consortium (DPAC) created to process and analyse scientific mission data.

. Initially Gaia was supposed to be an interferometry mission. Why was this changed?

The interferometer concept was considered at the very inception of the mission but was quickly abandoned in favour of the current optical telescope design, which makes it possible to collect more signals and therefore to measure fainter stars.

. How does Gaia’s performance compare with previous astrometry missions like Hipparcos?

Hipparcos (1989-1993) catalogued more than 100,000 stars to a high precision and more than a million to lesser precision. Gaia charts 10,000 times as many stars as Hipparcos, measuring their position and motion with 100 times greater precision.

. How long will the mission last, and could it be extended?

After 6 months’ commissioning, the nominal mission will last 5 years, ending in 2019. Consumables (mainly fuel) have been sized to extend the mission by at least 1 year.

. How long beyond the nominal mission could Gaia expect to function, assuming all key systems remain in working order?

Assuming nominal fuel consumption, the mission could be extended at least a full year and possibly up to late 2023.

. Why was it necessary to create the Data Processing and Analysis Consortium? Who is taking part, who financed it, and how long is it expected to function?

A primary motivation behind the Data Processing and Analysis Consortium (DPAC) is the unprecedented amount of data Gaia generates: surveying 1 billion stars, 70 times each over five years amounts to an average of 70 million objects observed each day! This translates into 40 Gigabytes of information per day, or 73 Terabytes over the full, nominal life of the mission. Taking into account the additional data products that are created from the basic observations leads to a total volume of about 1 Petabyte (1 million Gigabytes) for the complete dataset. Such a huge amount of data requires a vast range of scientific expertise that only international networking can provide. DPAC brings together more than 450 specialists from throughout the scientific community in Europe. It will remain in place until around 3 years after the end of the mission, up to the release of the final product: the Gaia catalogue.

. Is access to data limited to DPAC members? If not, how is access regulated?

Access to raw data is limited to DPAC members, who are entrusted with the task of converting the telemetry data into scientifically meaningful information. DPAC members do not have any proprietary access to the data for scientific exploitation. Instead, once the data have been processed and validated they are released to the worldwide astronomical community in the form of intermediate data releases and a final catalogue.

. When can we expect initial mission results to be made public?

The first intermediate Gaia Data Release is planned for 14 September 2016. Even before this, Gaia Science Alerts – announcements to the science community of the detection of transient events such as supernovae and outbursting stars – have been active since September 2014.

. What will be included in the first public data release?

Gaia Data Release 1 includes the positions and G magnitudes – a broad, visible light passband spanning 330 nm to 1050 nm – for more than one billion stars using observations taken between 25 July 2014 and 16 September 2015.

In addition, for a subset of data – about 2 million stars in common between the Tycho-2 Catalogue and Gaia – there will be a five-parameter astrometric solution, giving the positions, parallaxes, and proper motions for those objects. This is referred to as the Tycho-Gaia Astrometric Solution (TGAS).

Photometric data for a few thousand RR Lyrae and Cepheid variable stars that were observed frequently during a special scanning mode that repeatedly covered the ecliptic poles will also be made public.

The data release also includes the positions and G magnitudes for more than two thousand quasars – extragalactic sources – for which there are optical counterparts. These quasars are used to align the Gaia data with the current best celestial reference frame.

. When will the final product – the Gaia Catalogue – be available?

The Gaia Catalogue will be based on the complete mission dataset and for this reason will be available three years after the end of science operations. It is expected to be published in the early 2020s.

In the meantime, starting in September 2016, intermediate data releases will be published, based on increasingly longer stretches of observations and with additional subsets of data, leading up to the final catalogue.

This final catalogue will contain astrometric and photometric measurements for more than 1 billion objects; radial velocity measurements for about 150 million stars; light curves and characteristics for tens of millions of variable stars; orbital solutions for tens of millions of multiple star systems; classification and astrophysical parameters for stars, unresolved binary systems, galaxies and quasars; a list of stars hosting exoplanets; individual observations for all sources; and all ground-based observations that were made for data-processing purposes.

. Will Gaia work in tandem with other current astronomy missions like Kepler, JWST or Euclid?

We can expect that all Kepler stars will at some point in time obtain their distance data from Gaia. Similarly, the pointing for all visible JWST targets will most likely be based on coordinates provided by Gaia. It can also be assumed that some JWST proposals will be based on Gaia discoveries. Euclid will benefit from the precise positions of stars provided by the Gaia survey, and in turn will provide complementary information (infrared colours and spectra) for every Gaia star it observes. Gaia astrometry and photometry will also be an important input to future large-scale, ground-based surveys, such as those of the Large Synoptic Survey Telescope (LSST) and the European Extremely Large Telescope (ELT).

. How and when was Gaia launched?

The Gaia spacecraft was launched by an STB/Fregat MT Soyuz rocket – a Europeanized version of Russia’s Soyuz – from Europe’s spaceport in Kourou, French Guiana, on 19 December 2013 at 09:12 GMT (10:12 CET).

. What is the total cost of the mission? How does the final cost compare to the initial cost forecast?

The total cost of the mission, from the beginning of preliminary studies to the end of operations, is 740 M€. This does not include expenditures related to the DPAC consortium, which are covered by the member states participating in the consortium, not ESA. These costs are currently estimated at around 250 M€. The price tag for the spacecraft itself is 450 M€. Compared to estimates determined at the start of the industrial phase, the mission cost exceeds the original forecast by 16%.

The Spacecraft

. What kind of instrument package does Gaia have, and how does it compare with those used in previous astronomy missions?

The instrument package comprises two identical optical telescopes/imaging systems, a radial velocity spectrometer, and blue and red photometers. The payload features a 106-CCD focal plane array totalling nearly 1 billion pixels making it the largest digital camera ever used in space.

. What was the mission launch weight?

The Gaia spacecraft weighs 2,030 kg, including 710 kg of payload, a 920 kg service module (including the sunshield) and 400 kg of propellant.

. How big is the spacecraft?

The spacecraft body (payload and service module) is 4.3 metres in diameter and 2.3 metres in height. With the sunshield/solar array assembly fully deployed the spacecraft measures more than 10 metres across.

. Who were the mission contractors?

Gaia was designed and built by Astrium (now Airbus Defence and Space), with a core team composed of Astrium France, Germany and UK. The industrial team included 50 companies from 15 European states, along with firms from the US. Some 80 contracts were placed with European companies and three with those in the US. The spacecraft was launched by Arianespace. Between 2,500 and 3,000 people in all are involved in the mission.

. What were the most challenging engineering issues, and did any of them affect mission cost? Did any other factors impact the cost of the mission?

Gaia uses a highly precise cold gas micropropulsion system to maintain attitude control and keep the telescopes spinning at a constant rate. Difficulties involved in designing this element as well as some key elements of the payload accounted for about two thirds of the cost overrun. The remainder was mainly due to an increase in the cost of the Europeanized Soyuz launch vehicle in the years since mission kickoff.

. In developing Gaia, has European industry gained technology expertise that can be reused in the commercial space arena? If so, in what areas?

Industry has benefited from the mission in many areas, including high accuracy detection systems (from Gaia’s CCDs and focal plane array), optical technologies (telescope mirrors), high stability materials (silicon carbide used in telescope construction), communications (phased array antenna), high precision actuation systems (micropropulsion system), and low-vibration gyroscopes (attitude and orbit control system).

. The telescope CCDs were delivered in 2008. Given the enormous technical progress in this area since then, are they still suitable for a mission in 2013?

Yes. The charge coupled device technology used for optical observation is still valid, and a Gaia designed now would also use CCDs. Moreover, thanks to Gaia, we now have a much better understanding of CCD behaviour in a radiation environment.

. What has been done to protect the CCDs from radiation damage?

From the very outset of the mission, the issue of CCD radiation sensitivity was known to be one of the key risk areas. ESA invested 3 M€ in development and testing to identify specific radiation risks and provide a way to obtain the performance required to mitigate against them. The solutions adopted include use of appropriate shielding, operation at very low temperatures, and the introduction of special CCD design features (supplementary buried channel) and operational modes (periodic charge injection). In addition, the effect of radiation on the CCDs was “calibrated” as a function of different operating parameters. These calibration figures are an important input to the data processing. So far, the observed damage remains significantly below the value predicted.

. How does the mission function?

Gaia’s two telescopes monitor each of its target stars about 70 times over a five-year period, spinning slowly to sweep the entire celestial sphere. As the telescopes repeatedly measure the position of each celestial object, they detect the combination of the apparent motion caused by the parallax effect and the true motion of the object. By combining the measurements for all objects viewed, it is possible to obtain the parallax and proper motion for each object targeted.

. Is the spacecraft operating as planned?

The spacecraft is working well and most of the spacecraft systems that are crucial for the success of the mission have behaved as well as, or even better than expected. These include the focal plane assembly, onboard data handling system, onboard detection of sources, the phased array antenna, and the attitude control and micro-propulsion subsystems.

However, during the commissioning phase a number of unforeseen problems arose. These include contamination of the telescope optics due to ice, stray light (due to fibres on the edge of the sunshield) infiltrating the focal plane, micro-clanks – these are small mechanical vibrations arising from thermal effects, and larger-than-expected variations in the basic angle between the two Gaia telescopes. Detailed investigations by ESA, DPAC, and prime contractor Airbus Defence and Space have identified means of mitigating these issues. They are considered to have only a limited impact on the mission’s ability to reach its science objectives.

. How precise are Gaia’s measurements?

Gaia detects and measures celestial objects (stars, galaxies, quasars and Solar-System bodies) down to magnitude 20.5, about 650,000 times fainter than an unaided eye can see. The precision of the measurements – astrometric, photometric, and radial velocity – is a function of the type of object and their magnitude, with brighter objects being measured more precisely than fainter objects.

The measurements provided in the first Gaia Data Release are substantially more precise than those in existing catalogues and these will improve dramatically as increasingly longer stretches of Gaia data are used.

In the final Gaia catalogue, expected in the early 2020s, brighter objects (3-13 magnitude) will have positions measured to a precision of 5 microarcseconds, parallaxes to 6.7 microarcseconds, and proper motions to 3.5 microarcseconds per year. For the faintest stars (magnitude 20.5), the equivalent numbers are several hundred microarcseconds. (To aid in visualising this consider that ten microarcseconds is the size of a euro coin on the Moon as viewed from Earth.)

The photometry measurements in the final catalogue will be precise at the level of milli-magnitudes. For the subset of objects for which radial velocity measurements are obtained these will be measured with a precision of 15 km per second for the fainter stars and as precise as 1 km per second for the brighter stars.

The accuracy of the distances obtained by Gaia at the end of the nominal mission will range from 20% for stars near the centre of the Galaxy, some 30,000 light-years away, to a remarkable 0.001% for the stars nearest to our Solar System.

. In what orbit is Gaia operating?

The spacecraft circles the Sun in a Lissajous-type orbit at the L2 Lagrangian point, 1.5 million km from Earth. This location has the advantage of a low radiation environment and high thermal stability.

. From where is the mission operated?

The mission is controlled from the European Space Operations Centre (ESOC) in Darmstadt, Germany using ground stations in Cebreros (Spain), New Norcia (Australia), and Malargüe (Argentina). Science operations are conducted from the European Space Astronomy Centre (ESAC) in Villafranca, Spain.

Launch-related

. Why did you choose the Russian Soyuz launcher and not the preferred European launch system, the Ariane 5?

The cost of the Ariane 5 is more than twice that of Soyuz, which makes the Russian launch vehicle significantly more attractive. Gaia was subsequently sized so it could be launched with Soyuz.

. Initially the mission was supposed to be launched in 2010 rather than 2013. Why the delay?

The reference launch date was 1 December 2011, the date formally established after selection of the prime contractor. (Previous dates, which did not take into account manufacturing details, were only indicative.) The actual launch date was 19 December 2013, two years behind the initial schedule. Two months of this delay are attributed to the need to fit the Gaia mission liftoff into the overall launch schedule at Kourou. A further one month delay was due to an unexpected problem during the launch campaign. The remaining 21 months were essentially due to payload production complications, notably with respect to the focal plane and the 10 telescope mirrors, and to the long process of aligning the telescopes.

. If the launch could not take place as planned, what other possibilities were there for launch?

The launch window opened on 17 December and closed on 5 January 2014, which meant there was a 16 day margin with respect to the planned 19 December launch date. The next available launch window opened on 16 January and closed on 3 February 2014. Gaia launched as planned on 19 December 2013.

. What were the chief launch risk elements, and what were the planned fallback scenarios in the event of a degraded mission?

One of the most critical elements in the mission is the sunshield, which permits the telescopes to operate at a suitably cold temperature (-110°C) and, thanks to solar panels mounted on its undershield, serves to power the spacecraft and its instruments. Due to its large size, the sunshield had to be designed in multiple panels that unfolded once the spacecraft was in orbit. The sunshield was successfully deployed on 19 December, about 1 hour after launch.

Another critical element is the payload bipods, which serve to take up payload stress during launch and had to be released once the spacecraft reached transfer orbit. If the bipods failed to release, the mission could continue but with some loss of capability. The bipods were successfully released as planned.

An additional launch-related risk was a manoeuvre two days after launch, when the spacecraft was to be placed on course to L2 and the subsequent insertion manoeuvre (19 days later) into L2 orbit. If these manoeuvres failed to take place as planned, there would also be a loss of mission capability, depending on how much fuel was expended in completing them correctly. Both manoeuvres proceeded as planned.

. Was the launch insured?

In line with general ESA policy with respect to science missions, the launch was not insured.

. In the event of a launch failure, would ESA be able to rebuild the spacecraft, and if so, how long would this have taken?

Since the launch was not insured, construction of a new spacecraft would depend on the willingness of ESA member states to fund it (as was done in the case of Cluster). This would probably take around 5 years, key schedule drivers being the time needed to produce the 106 CCD detectors and the 10 mirrors required for the mission.

. What would happen if one or more of the instruments on-board failed? What kind of failure would threaten the success of the mission?

All spacecraft systems are redundant, which means that the mission could only be lost after a double failure of the same unit. However, the critical focal plane is built in seven independent rows, meaning seven units would have to fail for the full plane to be lost.