OUR SPACE: Gaia heads into retirement

Published 2:04 pm Wednesday, January 29, 2025

On a clear dark night, when there is no moon in the sky, a good distance away from the city, you can see the Milky Way – our very own home galaxy. The faint band of light stretching across the heavens is truly a sight to behold, and while we have a much better understanding of its shape and properties now it wasn’t that long ago when people had no idea what that mysterious phenomenon was.

It took many individual discoveries building upon one another, and the ever-increasing quality of telescopes to tease out the different  puzzle pieces that shape our current knowledge of the Milky Way’s shape and size.

Since our solar system resides within the Milky Way it is impossible for us to observe its full shape – it’s a little bit like sitting inside a house and trying to discern its exact outline, or the type of roof it might have. However, there are clues within the architecture that lets you draw conclusions, and if you look out the window and see other houses you might get a better idea of your own house.

Most galaxies are too small and faint to see with the unaided eye, and early telescope observations merely showed them as faint smudges of light, like patches of fog, just like the Milky Way. It took quite large and powerful telescopes to first resolve individual stars in the Milky Way, and later on in other galaxies. We now know them to be a seemingly infinite group of incredibly diverse shapes and sizes of star accumulations, so naturally people wanted to know what our own galaxy looked like.

Math to the rescue!

Due to the Earth’s orbit around the sun it is possible to observe individual stars from two vantage points that are fairly far apart, and by measuring the tiny differences in locations of the target stars we can infer not only their positions but also their movements, a scientific principle called astrometry. Many decades of tireless calculations eventually led to a raw image of the Milky Way – it is in fact a barred spiral galaxy, and our solar system sits in one of its far-flung arms. The center of the galaxy lies in the direction of the constellation Sagittarius.

But Earth observations can only be precise to a certain degree, as atmospheric movement causes the stars’ light to flicker and move, which makes exact positioning impossible. An early space telescope named Hipparcos that operated from 1989 through 1993 made giant strides when it came to astrometry and hinted at the need for a longer duration mission with a far more powerful observatory.

Enter Gaia, a night sky observing telescope with a one billion pixel camera. Launched in 2013 and built by the European Space Agency (ESA), Gaia looks like a giant spinning top hat, with the bottom of the rim covered in solar panels. This is actually a sun shield, to keep the rest of the spacecraft cool and dark for better observations. Gaia is in orbit around the L2 Lagrangian Point where the gravitational pull of Earth and the Sun are in perfect balance and very little fuel is needed to keep the spacecraft there.

Gaia gathers enormous amounts of data and downloads them to receivers on Earth at specific times. These data dumps are so massive that it takes years to process them before they can be made available for study. Because Gaia looks at about a billion stars over and over you can imagine how much information it can gather.

But every good thing comes to an end, and on January 15 Gaia officially ceased observations. However, it’s not at the end of its mission — not by a long shot. There is enough data in Gaia’s onboard computers that it will take several more data dumps to download all the observations, and the final catalogue of measurements is not expected to be ready before 2030. Each pass Gaia made of the night sky was more precise that the previous, resulting in an astoundingly clear atlas of our galaxy, complete with movements of stars over time.

Gaia is having a modern day ideal retirement — the hard work is over, but now comes the fun part, and for a good long time to boot. The spacecraft will eventually move into a safe, stable orbit where there is no danger of collision with Earth or other missions when it runs out of fuel. And while we might bid Gaia a fond farewell now we will reap its benefits for many more years to come. We won’t get the calendar-worthy beauty shots of telescopes like Hubble, Webb or Chandra, but without Gaia’s precise map-making such missions would not be possible.

Learn more about Gaia and the enormous accomplishments of its mission at https://www.esa.int/Science_Exploration/Space_Science/Gaia.

Beate Czogalla is the Professor of Theater Design in the Department of Theatre and Dance at Georgia College & State University. She has had a lifelong interest in space exploration and has been a Solar System Ambassador for the Jet Propulsion Laboratory/ NASA for many years. She can be reached at  our_space2@yahoo.com