Gaia’s final chapter: 11 years of mapping the Milky Way

The European Space Agency’s Gaia space telescope will be turned off after eleven years of observing from deep space. A team at the Institute for Astronomy, who have worked with the spacecraft during the entire operational phase, reflect on this milestone, the successes of the mission so far and the work still to be done.

Eleven years in deep space

The European Space Agency’s Gaia spacecraft was launched on 19 December 2013 and has been operating almost continuously for the last eleven years, scanning the sky with its twin telescopes as part of its mission to build a detailed map of the Milky Way. It resides in deep space at the second Lagrange point in the Sun-Earth system (‘L2’), a gravitationally neutral point that is a favourable location for making observations, and which hosts numerous space telescopes. The space environment is harsh, and Gaia has been subject to continuous radiation exposure, solar flares, and micro-meteorite impacts during its lifetime. However, it was built to be resilient, and it has remained operating almost without fault for the entire eleven years – more than twice as long as originally planned. Continual near-realtime monitoring of the health of the onboard instruments has been carried out by a team at the Institute for Astronomy as part of their contributions to the mission data processing.

‘Passivation’

Gaia has now run out of the cold gas propellant that is used to maintain its orientation, and the final science observations were collected in mid January. Since then the spacecraft has been undergoing a series of end-of-life tests to better understand the behaviour of its instruments and the status of numerous components after more then a decade in space. In late March it will be finally turned off, a process referred to as ‘passivation’. This involves first moving it away from L2 and putting it into a heliocentric orbit that keeps it safely away from operational satellites. On the 27 of March the final passivation steps will be carried out – the remaining propellant will be dumped, all electronic components turned off and the onboard software that controls the spacecraft will be deleted.

Work still to do

Although the spacecraft itself will be retired, the on-ground work of processing the raw data, production of the star catalogues and preparation for their release to the scientific community will continue for several years to come. Gaia observations are used to measure the distances to stars by triangulation, and although interim star catalogues have been published over the last few years based on partial datasets, the nature of the calculation means that the final, best accuracy cannot be reached until all the data is available. At the Wide Field Astronomy Unit, part of the Institute for Astronomy, a team works on the detailed characterisation of the scientific instruments, development of the calibration models and the creation of the data processing software. These are crucial inputs for the conversion of the raw data to science-ready catalogues of stars. With each successive publication of data from the mission the entire Milky Way map is redrawn, benefiting from improvements to the instrument models, data processing systems and increased number of data points. Now that the raw science dataset is complete, the production of the final star catalogue can begin. This will be released by the end of 2030.

Dr Nick Rowell, Senior Sky Survey Researcher at the School's Institute for Astronomy said:

The Gaia mission continues to deliver unprecedented insights into the history of our galaxy the Milky Way, as well as many other branches of astronomy. The Gaia space telescope has survived more than eleven years in the harsh environment of space where it has been battered by solar flares, cosmic radiation and regular meteor strikes. It has now run out of propellant and reached the end of its lifetime, so must be put into a disposal orbit where it won't pose a risk to operational satellites. The telescope has delivered huge volumes of data that we will be working hard to prepare and release over the next few years. As yet, only around a quarter of the observations have been included in the data released to the scientific community, and there is a lot of work still to do.

Dr Nigel Hambly Senior Researcher at the School's Institute for Astronomy said:

Gaia has proved to be remarkably resilient - a testament to all the engineers and scientists involved. It is a privilege for our team at the Institute for Astronomy to continue to play a significant role in the mission as we move into the post-operational phase. Preparation of the final data release products for the full 10 year mission will keep us occupied for several years to come and we look forward to further exciting discoveries as the world astronomical community continues to exploit the Gaia data releases.