After 11 years of uninterrupted operation in space, the European Space Agency’s (ESA) Gaia spacecraft has completed its sky scanning. During this time, it collected over 3 billion observations of approximately 2 billion stars and other celestial objects. The data gathered by the Gaia mission has revolutionised our understanding of the Milky Way and the surrounding universe. Polish scientists from the Astronomical Observatory of the University of Warsaw actively participate in the Gaia mission.
The Gaia spacecraft is running out of fuel and can no longer actively survey the sky. In the coming weeks, technological tests will take place, after which Gaia will be moved to a “retirement orbit.”
“Although we are ending the scientific observation phase, we have much to celebrate. The mission’s success has exceeded our wildest expectations. The satellite operated for almost twice as long as originally planned and delivered more scientific discoveries than the Hubble and Webb Space Telescopes combined,” says Prof. Carole Mundell, ESA’s Director of Science.
A Map of the Milky Way
Gaia monitored the positions, distances, motions, brightness variations, chemical compositions, and many other characteristics of stars and other cosmic objects, observing them repeatedly with its instruments. This enabled the spacecraft to achieve its primary goal creating the largest and most accurate map of the Milky Way, showcasing our galaxy in a way no previous mission has done.
A Discovery Machine
Gaia’s repeated measurements of star distances, movements, and other properties are critical for conducting the so-called galactic archaeology of the Milky Way. These efforts uncover missing links in the complex history of our galaxy, helping us better understand humanity’s origins. Gaia has dramatically changed our perception of the Milky Way, revising fundamental concepts such as the rotation of the galactic central bar, the warp of the disk, the detailed structure of spiral arms, and the distribution of interstellar dust near the Sun.
While surveying the stars of the galaxy, Gaia also discovered other objects, from the smallest asteroids in the Solar System to massive galaxies and quasars bright, active galactic centres powered by supermassive black holesbeyond the Milky Way.
Gaia also revealed a new type of black hole, including one nearly 33 times the mass of the Sun, located in the constellation Aquila, less than 2,000 light-years from Earth. For the first time, such a massive stellar-origin black hole has been observed in the Milky Way. Polish scientists also contributed to this discovery.
Polish Contributions to the Gaia Mission
Astronomers from the Astronomical Observatory of the University of Warsaw have been involved in the Gaia mission for many years. Prof. Łukasz Wyrzykowski began his involvement in 2008 as a staff member at the Institute of Astronomy, University of Cambridge. There, he co-developed the Gaia Alerts system, which detects transient events such as supernovae, sudden stellar brightening, or disappearances.
The system has been issuing alerts since the mission’s start and continued doing so until today, the last day of data collection. Among the alerts were candidates for lensing black holes (e.g., Gaia18ajz) and other dark objects (e.g., Gaia16aye). A global network of small telescopes, BHTOM.space, was built to follow up on these alerts from Earth, providing thousands of observational measurements since the mission’s inception, involving both professional and amateur telescopes, says Prof. Wyrzykowski.
Dr. Milena Ratajczak joined the Gaia team in 2022. She specialises in analyzing photometric data, particularly binary systems where a star pairs with a black hole. The Gaia team has already published three such systems, including the closest black hole to Earth, Gaia BH1, located 1,500 light-years away.
Although Gaia is ending its sky scanning, the data collected will be analyzed for many years to come. We expect these observations to lead to further groundbreaking discoveries, including more surprising systems with black holes, adds Dr. Ratajczak.
Polish scientists also co-developed the Gaia Vari community science project, enabling astronomy enthusiasts to work with data previously accessible only to professional astronomers. Project participants supported scientists in classifying variable stars in the Milky Way and made numerous surprising discoveries.
More Discoveries Await
Even though Gaia has stopped collecting new data, scientific and engineering teams are working intensively on analyzing the data gathered so far and preparing Gaia’s fourth data release (DR4), scheduled for publication in 2026. The volume and quality of data have increased with each release, and Gaia DR4, containing approximately 500 TB of data, will be no exception.
Gaia DR4 will enable the discovery of many additional exoplanets and the expansion of its catalogue of binary stars, the largest of its kind ever created. Gaia’s unique ability to detect minute motions of closely orbiting celestial pairs has revealed many previously hidden companions around bright stars. New discoveries will also include additional black holes detected in binary systems with stars or causing gravitational lensing.
Interestingly, Gaia’s final observations on January 10 focused on the binary star system 61 Cygni. This iconic star attracted astronomers’ attention in the 19th century, providing the first measurements of proper motions and parallax, the techniques Gaia used to study billions of stars.
Gaia’s Retirement Plan
After several weeks of testing, Gaia will leave its current orbit around the Earth-Sun Lagrange Point 2 to be placed in a heliocentric orbit, far beyond Earth’s gravitational influence. The spacecraft will be permanently decommissioned on March 27, 2025, to avoid interference and potential hazards for other space observatories nearby.
During the technological tests, Gaia will be turned toward the Sun, temporarily becoming much brighter in the sky. This will allow the spacecraft to be directly observed from the ground one last time. Even small telescopes, including those in the BHTOM.space global observing network coordinated by Prof. Wyrzykowski under the ACME European Horizon Europe grant will be able to observe the satellite. Instructions for locating Gaia have been published here, and astronomy enthusiasts are invited to share their observations.
