"I was very happy to see that people from many different countries were that interested in Gaia"

By Stefan Jordan, Astronomisches Rechen-Institut, Zentrum für Astronomie, Universität Heidelberg

Everyone involved in the Gaia project remembers last year's excitement: On 19 December 2013 at 10:12 Central European Time a Russian-built Soyuz-Fregat rocket lifted off from Sinnamary, close to Kourou in French Guiana, carrying the ESA satellite Gaia into space. Gaia was no longer a project consisting of engineering drawings or hardware standing in factory buildings or test facilities. If everything went well Gaia would be in space within the following few minutes.

I was not alone in my excitement. Together with about a hundred colleagues from the astronomy departments, people from the media, and some special guests we followed the event, knowing that many things could go wrong during a rocket launch. Many of us who spent years preparing the Gaia mission were very aware of this possibility.

We were extremely lucky that the sky in French Guiana was clear and that the cameras on ground could follow the launch until Gaia was in space. About two minutes after lift-off we could see the four boosters separating from the rocket. Shortly afterwards even the drop of the fairing which had protected Gaia during launch could be seen by the cameras. A few minutes later the second stage of the Soyuz rocket separated as well and after two subsequent burns of the Fregat upper stage, Gaia was on target for its destination, the Lagrange point L2 at a distance of 1.5 million kilometres from Earth.

Everything looked fine but it was not yet the time to celebrate since there was still one risky manoeuvre ahead: the unfolding of the large solar shield with a diameter of almost eleven meters. We knew that this was planned to be finished 88 minutes after lift-off. Minutes of uncertainty increased our excitement again because at that time ESA TV had stopped providing updates on the satellite status. Ten minutes later we found the awaited information on Twitter and a call to a colleague at ESOC in Darmstadt confirmed: the sunshield had unfolded. Relieved we clinked our glasses with sparkling wine. It was a very good start for the Gaia mission.

Gaia was finally in the sky, still 26 days away from its injection into an orbit around L2, becoming fainter and fainter for observers on Earth. While it was planned to precisely determine Gaia's orbit at L2 using observations with meter-class telescopes, we knew that Gaia should be visible to much smaller telescopes when it was still relatively close to Earth. My idea was to ask amateur astronomers around the world to take pictures of Gaia on its way to its final destination. Thanks to ESA, several colleagues and a number of amateur astronomers, the call for such observations was widely distributed a few days before the Gaia launch.

Six hours after lift-off an Australian amateur astronomer took the first image of Gaia with a 40-cm telescope. More images and videos followed from the Netherlands, the UK, Brazil, Spain, Chile, France, the USA and Germany. Several images can be found on Cosmos.

Timelapse of the Gaia spacecraft by Nick James from Chelmsford, UK. The images were taken about 16 hours after launch.

Unfortunately bad weather prevented me from performing such observations from the state observatory on the Königstuhl mountain in Heidelberg myself. Nevertheless I was very happy to see that people from many different countries were so interested in Gaia. This first outreach campaign after the Gaia launch was a big success.

It was not only an outreach campaign though. It was scientifically important as well. The idea was to use the observations to estimate Gaia's brightness at L2 where it would be professionally observed for orbit determinations. No reliable data for the reflectivity of the solar shield were available.

At the time of the first ground-based observations Gaia's solar shield was directly pointing towards the Sun. In the afternoon of 20 December, Gaia's sunshield was oriented towards its nominal value of 45 degrees with respect to the Sun. This was known to have an effect on the visibility of Gaia from the Earth, but the change was very dramatic: several small telescopes failed to find Gaia and many observers were surprised by Gaia's faintness. Even the experts for the planned ground-based observations were astonished that the drop of the brightness after the change of the solar aspect angle was of the order of a factor of 15. The reason must be the extreme smoothness of the surface of the solar shield. Extrapolating this value to the distance of L2, it became clear that larger telescopes would be needed than originally planned. It was extremely valuable that this was known so early on in the mission, thanks to the many amateur and professional astronomers who participated in our "Gaia in the Sky" project! Now Gaia is regularly observed by two-meter class telescopes and the team for the ground-based observations is precisely determining Gaia's orbit from these data.