Gaia (spacecraft)

Did You Know?

• The name 'Gaia' originally stood for Global Astrometric Interferometer for Astrophysics, reflecting the planned use of interferometry, though the acronym is no longer applicable.
• The mission was proposed in October 1993 by Lennart Lindegren and Michael Perryman in response to a call for proposals for ESA's Horizon Plus long-term scientific programme.

Spacecraft

Gaia lifted off on 19 December 2013 at 09:12 UTC from the Soyuz launch site in Kourou, French Guiana, aboard a Soyuz ST-B rocket with a Fregat-MT upper stage, operated by Arianespace. The spacecraft separated from the upper stage 43 minutes later, at 09:54 UTC. It then traveled to the Sun–Earth Lagrange point L2, roughly 1.5 million kilometers from Earth, arriving on 8 January 2014. This location offered a steady gravitational and thermal setting, and Gaia entered a Lissajous orbit that kept the Sun from being blocked by Earth, which could have reduced solar power and upset temperature balance. After launch, a 10-meter-wide sunshade unfurled. It always stayed at a 45-degree angle to the Sun while precessing to scan the sky, keeping the telescope parts cool and supplying power through solar panels on its surface. These design choices let Gaia operate in temperatures ranging from -170°C to 70°C. The spacecraft carries three main instruments. The astrometry instrument (Astro) measures angular positions of stars brighter than magnitude 20, enabling calculations of parallax, distance, and proper motion. The photometric instrument (BP/RP) collects luminosity data across 320–1000 nm for stars brighter than magnitude 20. The Blue Photometer (BP) covers 330–680 nm, and the Red Photometer (RP) covers 640–1050 nm, together revealing stellar temperature, mass, age, and elemental composition.

Mission progress

Gaia’s testing and calibration began on its way to the SEL2 point and ran until the end of July 2014, delayed by three months because stray light got into the detector. After six months of commissioning, the satellite started its planned five-year science mission on 25 July 2014 with a special scan pattern focused on the ecliptic poles. It switched to its normal, more uniform scanning mode on 21 August 2014. Originally meant to only observe stars dimmer than magnitude 5.7, commissioning tests showed Gaia could spot stars as bright as magnitude 3 on its own. When regular science began in July 2014, it was set to routinely handle stars from magnitude 3 to 20. For the 230 stars brighter than magnitude 3, special procedures were used to download raw data, and methods were developed to analyze it, with the goal of complete bright-end coverage and errors of a few dozen microarcseconds. On 30 August 2014, Gaia spotted its first supernova in another galaxy. On 3 July 2015, a star-density map of the Milky Way was released. By August 2016, the spacecraft had processed over 50 billion focal plane transits, 110 billion photometric observations, and 9.4 billion spectroscopic observations. The mission was extended in 2018 to 2020, then to 2022, with an indicative extension to 2025. The main limit was the cold gas nitrogen supply for the micro-propulsion thrusters; the chemical propellant could have lasted decades. In March 2023, the mission was extended through the second quarter of 2025, when the cold gas was expected to run out.

Data releases

Several Gaia catalogues have been released over the years, each with increasing information and better astrometry; early releases miss some stars, especially fainter ones in dense star fields and close binary pairs. The first data release, Gaia DR1, based on 14 months of observation, occurred on 14 September 2016. It 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; light curves and characteristics for about 3,000 variable stars; and positions and magnitudes for more than 2,000 extragalactic sources used to define the celestial reference frame. The second data release (DR2) on 25 April 2018, based on 22 months of observations, includes positions, parallaxes and proper motions for about 1.3 billion stars, positions of an additional 300 million stars, red and blue photometric data for about 1.1 billion stars, single colour photometry for an additional 400 million stars, median radial velocities for about 7 million stars, and data for over 14,000 selected Solar System objects. The third data release was split into two parts: EDR3 on 3 December 2020, and full DR3 on 13 June 2022, which includes Solar System data, variability information, results for non-single stars, quasars, extended objects, astrophysical parameters, and the Gaia Andromeda Photometric Survey.

Significant results

In July 2017, the Gaia–ESO Survey reported using Gaia data to find double-, triple-, and quadruple-star systems, identifying 342 binary candidates, 11 triple candidates, and 1 quadruple candidate. Nine of these had been identified by other means, confirming the technique. The possible quadruple star system HD 74438 was identified in a 2022 paper as a possible progenitor of a sub-Chandrasekhar Type Ia supernovae. In November 2017, scientists led by Davide Massari of the Kapteyn Astronomical Institute released a paper characterizing proper motion within the Sculptor Dwarf Galaxy and its trajectory through space using Gaia and Hubble Space Telescope data. The data showed Sculptor orbits the Milky Way in a highly elliptical orbit, currently near its closest approach at about 83.4 kpc, but can reach around 222 kpc. In October 2018, Leiden University astronomers determined orbits of 20 hyperrunaway star candidates from DR2, finding seven exiting the Milky Way and 13 approaching it, possibly from extragalactic sources or as halo stars. Independent measurements cast doubt on the highest Gaia radial velocity among these stars. Also in late October 2018, the galactic population Gaia-Enceladus, remains of a major merger with the defunct Enceladus dwarf, was discovered, associated with at least 13 globular clusters and the creation of the Milky Way's Thick Disk.