ESA’s Gaia mission is building the most precise three-dimensional map of our galaxy ever created by measuring the position, distance and velocity of 1 billion astronomical objects – around 1% of its population – with unprecedented accuracy. RHEA engineers are involved in data processing and management, testing and cataloguing.

Gaia mission timescale

Mission launch: 19 December 2013
Mission duration: 5 years
Nominal mission end: July 2019, extended to December 2022
Extension: Possibly until December 2025

About the Gaia mission

Gaia is an ‘astrometric’ mission, meaning that is studying the position, parallax and ‘proper motion’ of celestial bodies. As such, it follows on the footsteps of Hipparcos, a pioneering European mission that mapped over 100,000 stars to high precision and over 1 million at lesser accuracy. This time, however, ESA’s aim is far more ambitious, with a target of mapping 1 billion objects at high precision.

As Gaia builds its 3D map of our galaxy, most of the objects observed will be stars. However, it will also measure the position of planets, asteroids, comets and quasars. The positions and velocities of all these astronomical objects as measured by Gaia relates to their positions and velocities when the galaxy formed. Therefore, the map that is generated will allow astronomers to better understand the origins and evolution of the Milky Way and from there learn more about the general processes responsible for galaxy formation and evolution.

Data from the Gaia mission is made available in data releases, the first of which was in September 2016, 1,000 days after launch.

About Gaia’s orbit

Gaia was launched on 19 December 2013 on board a Soyuz-Fregat from the European spaceport in Kourou, French Guiana.

Gaia reached its final orbit on 8 January 2014. This is a ‘Lissajous type’ orbit around the L2 Lagrange point of the Sun–Earth system, which is 1.5 million kilometres from Earth.

At the Lagrange points, the gravitational effects of two large bodies cancel each other out, enabling a third, small body to stay in equilibrium. In this case, the large bodies are the Sun and the Earth and the small body is Gaia.

Gaia’s main technical challenges

Gaia measures the position and movement of astronomical objects in the sky as tiny angles. For the brightest stars (magnitude larger than 10), Gaia measures angular positions with uncertainties of the order of 7 microarcseconds.

The position errors in the Hipparcos catalogue were 200 times larger. If Hipparcos could have measured the height of an astronaut standing on the Moon, Gaia would be able to measure the size of that astronaut’s thumbnail. Given that Hipparcos was launched in 1993, the Gaia team achieved drastic improvements in positioning accuracy in the 20 years that separate the two missions.

Another challenging aspect of the Gaia mission is data processing. The amount of data that had to be processed for the last release of the Gaia catalogue was around 100TB. This required significant computing power and the development of special algorithms to process the enormous volume of data.

Gaia's stellar motion for the next 400,000 years
How 40,000 stars, all located within 100 parsecs (326 light years) of the Solar System, will move across the sky in the next 400,000 years. These ‘proper motions’ were released as part of the Gaia Early Data Release 3 (Gaia EDR3) and are twice as precise as those in the previous Gaia DR2 because Gaia has now measured the stars more times and over a longer interval of time. © ESA/Gaia/DPAC; CC BY-SA 3.0 IGO.

Gaia’s scientific payload

Gaia’s payload consists of three instruments:

  • Astrometric instrument – precisely measures the angular positions of stars brighter than magnitude 20
  • Photometric instrument – performs luminosity measurements of celestial bodies over the 320–1,000nm spectral band
  • Radial velocity spectrometer (RVS) – measures radial velocities of the target.

During the 5 years of the nominal mission, each of the target celestial bodies was measured around 70 times on average. By combining the 70 measurements, scientists were able to calculate both the position of each celestial body and its parallax, and therefore its distance and proper motion (not to be confused with radial velocity).

From the photometric instrument it is possible to infer the temperature, mass and composition of the target objects.

Putting all this information together, Gaia scientists can generate the most detailed map yet of our own galaxy, the Milky Way.

Gaia data releases

Data from the Gaia mission is released in phases:

  • 14 September 2016 – Gaia Data Release 1
  • 25 April 2018 – Gaia Data Release 2
  • 3 December 2020 – Gaia Early Data Release 3
  • First half of 2022 – full Gaia Data Release 3

Due to the amount of data to be processed, Gaia Data Release 3 will be split in two to provide scientists earlier access to products that may not be complete, yet are usable and of high quality. The Early Release adds new data to Release 2: the number of sources has increased up to 2%, but the number of sources with known radial velocities or effective temperatures remains the same.

Initial scientific achievements

Gaia Enceladus stars across the sky crop. © ESA/Gaia/DPAC; A. Helmi et al 2018
Debris of another galaxy named Gaia-Enceladus that once collided with our own, indicated by direction of travel and chemical composition: purple indicates the nearest and yellow the most distant stars; white are globular clusters; cyan are variable stars. © ESA/Gaia/DPAC; A. Helmi et al 2018

Gaia is a treasure trove of information about our own galaxy and, by extension, on galaxies of the same type as our own Milky Way. These are just three of the important discoveries made so far based on Gaia data:

  • There is a very faint dwarf galaxy on the outskirts of the Milky Way named Antlia 2 that has the lowest surface brightness of any known galaxy.
  • Some stars in the Milky Way rotate in the opposite direction to most other stars and display a different chemical composition, telling us that another galaxy once merged with our own.
  • The disc of the Milky Way is not completely flat but slightly warped. However, we now know the warp is not static but changes over time. Astronomers are convinced this is also the result of a collision with another galaxy in the distant past.

How RHEA is contributing to the Gaia mission

RHEA has a significant presence at the Gaia Science Operations Centre (SOC) at ESAC, with four operations engineers and one instrument operations scientist. Two of the operations engineers also double as test engineers.

Operation engineers are responsible for reception and initial processing of the Gaia data and its daily distribution to Gaia data processing centres across Europe. They are also responsible for management of databases with extremely large volumes up to 100TB.

Test engineers are responsible for organizing all the testing infrastructure associated with the data processing pipelines. Because of their complexity, most of the testing is performed automatically. However, when something raises a red flag, it needs to be checked by a human.

Milky Way s precessing galactic disc
Data from Gaia shows the warped galactic disc of the Milky Way precesses, or wobbles, faster than previously expected, completing one rotation in 600 to 700 million years. The speed of the warp’s precession led astronomers to believe that it must be caused by something powerful, such an ongoing collision with a smaller galaxy. © ESA Credit: Stefan Payne-Wardenaar

Main image: Artist’s impression of Gaia mapping the stars of the Milky Way. © ESA/ATG medialab; background: ESO/S. Brunier