Farewell to Gaia

Well drat, I haven’t been this bummed out since Ingenuity was grounded for good. On January 15, 2025, the European Space Agency (ESA) announced the end of science operations for Gaia, one of the most precise astrometric (aka mapping) telescope ever launched, and it’s got me mourning.

I know on the surface a mapping telescope doesn’t sound nearly as exciting as, say, the sorts of observations carried out by Hubble or Webb, and Gaia’s findings haven’t been in the form of the show-stopping images that those two observatories produce. As a result, Gaia has never gotten as much attention as its flashier space telescope cousins.

But the list of things we know as a result of Gaia is huge, and its shutdown isn’t just a sad thing for me, it’s a sad thing for the entire astronomical community. So, in tribute to a fallen giant, let’s talk about Gaia.

GAIA to Gaia

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What eventually became the Gaia telescope was first proposed as a successor to the ESA’s highly successful star cataloging mission Hipparcos in 1993. It wasn’t until 2000 that the telescope was put into development. At this point it was GAIA, the Global Astrometric Interferometer for Astrophysics. 

“Interferometer”, when it comes to astronomy, refers to a specific way of observing, using multiple smaller telescopes as though they were a single, larger telescope. It tends to be more common in radio astronomy than optical. It is, for instance, how the Event Horizon Telescope which took the first images of the supermassive black holes at the centers of galaxies, works. It’s not actually one telescope at all but a series of radio observatories scattered across the Earth.

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When Gaia was GAIA it was going to be an interferometric telescope. Somewhere between that initial design and the actual building of the telescope (I tried to pin down when, but couldn’t find a date) the design changed, and it was felt that the acronym GAIA no longer properly applied. What to do then? The solution was delightfully straightforward. GAIA simply became Gaia, a telescope with a name that isn’t an acronym but is, instead, pure loveliness just because.

In December 2013 Gaia was loaded onto a rocket at the ESA’s launch site in French Guiana (which was also the departure point for the Webb Telescope in 2022) and lifted off into space. From Earth orbit it made its way to a point a million miles to the Sun-Earth Lagrange Point 2, or L2, an increasingly popular place to stick telescopes that can be affected by the radiation, space junk, and general noise of Low Earth Orbit (L2 is also Webb’s home).

(Ugh, I know, I know, I still haven’t posted a blog post about Lagrange Points, despite repeated promises. It’s half-written, I swear! I’ll finish it one of these days, other things just keep happening!) 

Mapping Mission

Gaia is not the kind of telescope Webb or Hubble is. It doesn’t look at one thing and figure out what makes that thing tick. Instead, Gaia takes wider views of the sky, checking on each target star in its catalog (of which there are nearly 2 billion) 14 times a year with its billion-pixel camera.

Each time it looks at a star Gaia maps its position and brightness with a precision that was unprecedented before its launch. Having done so for each star many, many times over its career, Gaia has an extremely detailed record of any tiny movement or changes in brightness for these objects over that time.

We don’t often think of it, but the stars of the Milky Way are all moving, the Sun included. They orbit the center of the galaxy at varying speeds. These motions aren’t so big that you’d notice them in, say, a human lifetime, or even several centuries. To our eyes, their movement is extremely slow and excessively small.

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But Gaia can see it. Which means it can tell which direction each of those nearly 2 billion stars are coming from, where they’re going, and how fast they’re moving. And that can tell us a whole heckuva lot about the history of the Milky Way. 

For instance, Gaia was able to figure out that our galaxy began pulling itself together about 13 billion years ago. It has actually been able to spot two streams of stars, nicknamed Shakti and Shiva, that may represent some of these early pieces that braided together to make the proto-Milky Way.

Gaia also discovered that 10 billion years ago the Milky Way underwent a huge merger with another galaxy (the last major merger it experienced—at least until the Andromeda Galaxy catches up to us in about 4.5 billion years) that added another 50 billion solar masses of stuff to the Milky Way. Gaia picked out the remnants of this colliding galaxy still moving around inside the Milky Way. This galaxy was given a name both tributary and amazingly ridiculous: Gaia-Sausage-Enceladus. Come on, that’s an amazing title!

The Data We Picked Up Along the Way

As Gaia was precisely mapping stars and teasing out our own galaxy’s history, it made several other discoveries along the way. Astronomers used its data to track asteroids in our own solar system, to watch the evolution of white dwarfs, to detect starquakes, which are exactly what you think they are, and to discover a number of new multi-star systems—hundreds of binaries, nearly a dozen trinaries and even a quaternary system. 

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Mind you, Gaia was not designed to do any of that. But that never stopped this telescope.

Gaia has found hypervelocity stars being fired out of and, unexpectedly, into the Milky Way. It created a more precise model of the movements of the Andromeda Galaxy, which is in the background of a number of its images. It has accidentally discovered exoplanets, star clusters, and the closest black hole to Earth.

I could keep going but as much as I’d like to create an exhaustive list of all of Gaia’s accomplishments (it is my techno-crush after all), you probably don’t want to read it. Let me sum it up this way: I have mentioned Gaia discoveries in no less than thirteen issues of Spacing Out and at least two blog posts, not counting this one. This telescope is a discovery machine.

The Trouble with Gaia

Being a space telescope always comes with a finite lifetime, alas. That’s especially true for telescopes out at L2, waaaaay too far away to even contemplate an upgrade mission ala Hubble, even if the world’s space agencies were still in the space-telescope-upgrading game. And, lest we forget, space is a rough environment, for all it looks so empty and peaceful. Even so, Gaia was chugging along pretty smoothly until this past spring when space decided to remind everyone that it doesn’t always play nice.

In April Gaia got pasted by a micrometeorite that punched into the protective cover over the telescope, preventing stray light from leaking in. It wasn’t much, but when you’re as sensitive to light as Gaia it doesn’t take much to throw off your readings. 

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Then in May one of Gaia’s CCDs failed. The reason for the failure probably has to do with the Sun. Remember that massive solar storm last May that created auroral displays that went all the way down to Puerto Rico? Well that’s about when the CCD failed, and it’s likely that the radiation from the storm simply overwhelmed the electronics. This all required a major refocus of the camera since the failed CCD helped root out false positives—suddenly Gaia’s detectors were screaming about stars that didn’t actually exist.

But none of that is what finally did Gaia in. Unlike Hubble, which uses spinning reaction wheels to point itself, Gaia uses thrusters. Thrusters need fuel, and a spacecraft can only carry so much. After 11 years in space, Gaia’s tank is running empty. It’s time to close the shop.

Not Done Yet

Although Gaia has ceased its science operations, it’s not quite done yet. ESA engineers are taking the opportunity to perform a few technical tests on the telescope that they didn’t want to do while it was in operation because of the effects on science observations. Now there’s nothing to lose and the results could help inform the building of future telescopes.

Once this is done Gaia will use its remaining propellent to move itself into a new orbit, away from the popular L2 point so that its drifting hulk will not pose a risk to any current or future missions stationed there. Then it will be shut down for the last time. 

Gaia’s data, of course, will live on. It produced so much that it’s expected that the last of it won’t be fully processed and ready for release before 2030. We’ll be learning new things from Gaia for many, many years to come.

Ad astra, Gaia. I’m gonna miss you.

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