By Prasad Ganti

Thanks to Surabhi for pointing me to this topic from the Astronomy magazine. I collected some more information and summarized the same here. Summarizing and packaging information helps me learn. The Gaia spacecraft was launched in 2013 by ESA (European Space Agency) to precisely measure the position, distance and motions of about a billion stars in our Milky Way galaxy and its neighbors. Gaia stands for “Global Astrometric Interferometer for Astrophysics”.

Gaia is the goddess of Earth in Greek mythology. Gaia also refers to a concept put forth by James Lovelock in the early 1970s which suggests that the Earth functions as a single breathing and living organism, due to the presence of life and the consequent dynamism. The first time I heard of Gaia was while reading Isaac Asimov’s “Foundation’s Edge”, a science fiction classic. Humans, having branched off to distant parts of our galaxy, are not able to find Earth in their ship’s navigation system, but can find “Gaia”, a world where the consciousnesses of all the living beings are interconnected!

The spacecraft of the same name was conceived and constructed by several European partners. It was launched in 2013 to be positioned at the L2 Lagrangian point on the Earth-Sun axis, about a million miles away from Earth on the opposite side from the Sun. The L2 point provides the spacecraft with a very stable gravitational and thermal environment. The gravitational pull of the Earth and Sun cancel each other out, so the spacecraft can maintain its position with minimal fuel. It was in an orbit around the L2 point called the “Lissajous orbit”. This orbit provides enough shade for astronomy while still getting enough sunlight to power the solar panels.

Gaia carried two twin telescopes, pointing in different directions, each with a primary mirror of about 7.5 square feet, which is much smaller than James Webb Telescope’s primary mirror. Gaia did a survey of a billion stars and produced a catalog. The catalog is still pending the last 2 datasets which will be released in the future.

Supplementing the primary mirror is an astrometric instrument which measures the position and movements of stars using the parallax method. The same star is observed from 2 positions on opposite sides of the Sun six months apart as the Earth revolves around the Sun. The distance is derived using the triangle formed by the 2 separate positions and the star. The accuracy of the distance depends on the accuracy of the atomic clocks onboard.

The second instrument is a photometric instrument that records the colors of the star. The colors tell us a star’s temperature, composition, and mass. The third instrument is a spectroscopic instrument which measures the doppler shift of stars, also known as redshift, which compares the known position of the colored bands with the observed ones. The greater the redshift, the faster the star is moving away from us.

Using these three instruments, Gaia has produced the most accurate maps of the Milky Way to date, and has also made some discoveries such as that of a 9,000 lightyear long ribbon of gas which oscillates like a wave in the plane of the galaxy. It has been named the Radcliffe Wave. The nearest point of this structure is only 500 light-years from Earth, in our local spiral arm. Gaia also discovered many stars that appear close together in the sky from the Earth but are actually separated by many light-years, lying at vastly different distances away from us. In addition, Gaia found some new exoplanets, with more exoplanets potentially lurking in the yet to be released data.

Another discovery in November 2018 was the neighboring galaxy Antlia 2. It is similar in size to another neighboring galaxy the Large Magellanic Cloud but is 10,000 times fainter. After all these magnificent discoveries, the mission came to an end in 2025. The limitation was the fuel, cold nitrogen, required to power the micro propulsion system which is needed to stabilize the spacecraft to a high degree of accuracy. If the spacecraft itself is wobbly, it cannot accurately determine the position and movement of the stars. The fuel ran out and the mission had to come to a great end.