"Gaia... something out of a science fiction movie"

By Ronald Drimmel, Astrophysical Observatory of Torino

Launch, in a way, is like a birth. After all those years of preparation and construction, all the pieces are finally together, turned on, and then... violently thrown into the sky. Would Gaia survive? And would it "open its wings" and fly?

Not unlike other major space missions, Gaia had a very long gestation period. Initial studies of the Gaia mission concept began soon after the end of the Hipparcos mission, resulting in the publication of the Gaia Concept and Technology Study Report (sometimes referred to as the "Gaia Red Book") in the summer of 2000, and the formal approval of the Gaia mission by ESA in October of the same year. Then followed years of preparation, design studies, construction, programming, testing..., all finally culminating in the long awaited launch of Gaia.

I was in Italy, where my home institute is, when Gaia was launched one year ago. ASI (Agenzia Spaziale d'Italia) had organised a launch event at ALTEC in Torino, the site of DPCT, one of the six Data Processing Centres that are part of the Gaia Data Processing and Analysis Consortium (DPAC), responsible for turning the Gaia telemetry into the largest, most accurate and precise stellar census of our Galaxy that has ever been produced. It was from here that I watched and listened to the live feed from ESA, together with many other colleagues across Europe who had been involved in preparing for the Gaia mission. Thanks to Hipparcos the Gaia community has never been small, and I knew that there were many others like me, others who began working on Gaia even before it was an official ESA mission, impatiently watching the launch event and hoping for a success.

Having devoted nearly 20 years of my career to Gaia, it was hard not to feel a knot of anxiety in the pit of my stomach as the countdown proceeded, hard not to hold my breath as I watched the Soyuz Fregat rocketing its precious cargo into space. When the rockets shut down and a successful orbit insertion was announced, I distinctly remember feeling relieved. But my eyes and ears peeled onto the screen with the video coming from Kourou as I knew that there was still one last critical manoeuvre: the deployment of the sunshield. It was only after the successful completion of this milestone had been announced that I broke out in an irrepressible grin. Gaia's "wings" were indeed open, and she was on her way!

Remembering the launch of Gaia is somewhat strange for me. It seems as if it was only yesterday, as this first year has really gone by fast!  But it also seems like centuries ago, as so much has transpired. It has not been all smooth sailing, but a bit of a bumpy ride, as unexpected issues have presented themselves. As astrophysicist-turned-instrument-scientist, I've had a chance to wrestle with some of those issues. Just 15 days after launch, the data from Gaia started to arrive at DPCT in Torino, responsible for the daily processing and analysis of data from the BAM device (an on-board laser interferometer that monitors changes in the orientation of the two primary mirrors) and for selected astrometric observations to monitor their quality and the health of the instrument. Together with other Gaia Payload Experts at Torino, I've been "getting to know" this creature that came to birth a year ago ever since the data stream started flowing - fortunately with the support of a team of scientists, engineers and software developers. Between investigating possible causes of Gaia's unexpected quirks, and adapting the data processing software accordingly, we've certainly had our hands full, so it has turned out to be a very busy year.

In hindsight, we should perhaps not have been surprised that there were bumps in the road. The Gaia satellite is a technological feat, and nothing like it has ever been constructed and flown in space. The Gaia instrument is perhaps the most precise one that has ever been built and flown in space, and is as a consequence amazingly sensitive. Not only is Gaia observing on average 5000 stars per second, but it also detects micrometeoroids and micro-Kelvin temperature changes! As with any "pathfinder" mission breaking into new territory, one cannot expect to find a smooth highway to discovery.

In other respects, considering the actual performance of the satellite and the instrument, Gaia indeed seems a miracle, or something out of a science fiction movie. For example, the Gaia satellite lies approximately one and a half million kilometers from Earth, yet its velocity is tracked by ESA to within a few millimeters per second. The satellite controls its spin rate to 15 parts in a million, and does so automatically every 4 seconds. The laser interferometer on Gaia (the BAM device) is able to perform picometer (10-12 metre) level metrology to monitor changes in the orientation of its two main mirrors, that is, at a dimension corresponding to the size of a hydrogen atom!  

Bumpy road or not, the Gaia mission will undoubtable break new ground in terms of astrometric measurement, with final precisions expected to be about 100 times better than those of its predecessor Hipparcos. The Gaia Archive will contain over a billion stars, 10000 times more than can be found in the Hipparcos Catalogue. For Galactic studies the volume of the Milky Way sampled by Gaia will be even more significant. Hipparcos was only able to provide relatively accurate parallaxes out to about a hundred parsecs from the Sun, while Gaia will do the same out to a distance of about 5 kiloparsecs. (See the figure below for an image of the total volume that Gaia will sample in the disk of the Milky Way, with stars down to 20th magnitude.)

Figure 1: Expected sampling of Gaia over the Milky Way. The Blue/green area around the Sun approximately covers the volume sampled by Hipparcos in the Galactic plane

But we are not there yet. The five-year nominal mission just began at the end of last July, so we still have almost 8 years - at least - before the final Gaia Archive will be completed. However, to satisfy our hunger for data, the first Gaia data release is already expected about a year and a half from now. Between now and then there is still a lot of hard work to be done to get there.

Image credits

Figure 1: X. Luri and CU2/DPAC; artist's impression of the Milky Way: NASA/JPL-Caltech/R. Hurt