Gaia/Hipparcos: project interviews

Claus Fabricius (Copenhagen, 1954) spent the first part of his career at the Copenhagen University Observatory in Denmark, where he worked on one of the main instruments contributing to ground-based astrometry in the years preceding Hipparcos. In the first of this two-part interview, Claus talks about his involvement in the development and operation of the Brorfelde transit circle, later as the Carlsberg Automatic Meridian Circle in La Palma, and in the creation of the Tycho Catalogue and its associated initiatives.

In the second of our two-part interview, David Milligan, Spacecraft Operations Manager for Gaia, describes the challenges involved in the spacecraft commissioning, including the problems of 'icing' and 'straylight'. And he talks about keeping the satellite operational and optimised over almost a decade, including optimisation to allow the downlink of significantly more data, and the various tiny effects that can be detected, including bubbles in the propulsion system, micro-meteorites, and the effects of sunspots.

David Milligan (Blackpool, UK, 1971) joined ESA, at its operations centre, ESOC in Germany, in 2000. There, from 2008 to the end of 2020, he was the Spacecraft Operations Manager for Gaia, the leader of the team responsible for its operation, health, and safety. In the first of this two-part interview, David talks about the preparations for the satellite operations in the years leading up to launch, and the major tasks involved in sending it on its way to its intended L2 orbit, 1.5 million km from Earth.

Carme Jordi (Barcelona, 1958) is a senior research scientist at the University of Barcelona. She played a key part in designing the photometric measurements that could - and should - be made with Gaia, and played a key role in the preparation for the photometric data analysis. We talk about the scientific rationale for making photometric observations from the satellite itself, the considerations underlying the design of the photometric system, and how the photometric measurements compare with the original plans.

Erik Høg (Lolland, Denmark, 1932) joins me for a third time. In our first conversation we talked about his early involvement in ground-based astrometry, and his part in getting Hipparcos optimised and adopted by ESA in the 1970s–1980s. In the second, we talked about his involvement in Hipparcos, from its acceptance in 1980 to completion in 1997, including his ideas for the Tycho experiment. Today, we talk about his contributions to Gaia [recorded 9 February]. Erik's transcripts are at https://arxiv.org/abs/2208.09332

Martin Altmann (Pretoria, S. Africa, 1970) is an astronomer at the Centre for Astrophysics, University of Heidelberg. He leads the Ground Based Optical Tracking campaign (GBOT). Nightly high-accuracy measurements of the Gaia satellite itself, made with various telescopes on Earth, track Gaia's position and velocity. These measurements allow the satellite's position and velocity to be determined, at all times, to around 150m and 2.5 mm/s respectively; data necessary to reconstruct Gaia's star distances and asteroid motions.

Paolo Tanga (Saluzzo, Italy, 1966) is a senior researcher in planetary science at the Nice Observatory, France. His work on asteroids in our solar system focuses on their formation, their collisional evolution, and properties such as their sizes, shapes, and chemical composition. As the leader of the data processing task on solar system observations, we talk about the large numbers of them being observed by Gaia, and why these results will be so important for our understanding of the formation and evolution of our solar system.

Vincent Poinsignon (Macon, France, 1958) was, between 2006 and its launch in 2013, the overall industrial Project Manager for the Gaia satellite. His company, Airbus Defence & Space (a division of Airbus), located in Toulouse, southern France, played the central role in the detailed design, manufacture, assembly, testing, and eventually the launch of the Gaia satellite. Vincent joins me to reflect on the engineering and management challenges faced by this extremely demanding space science mission.

Rudolf Le Poole (Delft, The Netherlands, 1942) is an astronomer at the Sterrewacht, the Astronomy Department of Leiden University, in The Netherlands. Rudolf was active in all aspects of Hipparcos: as a member of my Hipparcos Science Team (1981–1997), of the Input Catalogue Consortium, of the FAST Data Analysis Consortium, and someone interested in, and always questioning, many aspects of the instrument design. We talk about these various activities, and his perspectives looking back on the `Hipparcos years'.

Leslie Budd (Hayes, Middlesex, 1949) is professor of Regional Economy in the Faculty of Business and Law, at the Open University, Milton Keynes, UK, where he is known for his work on regional and urban economics. He has been involved in studies of the socioeconomic impacts of Brexit, of Spaceport Scotland, and of space exploration, including the benefits of the ESA Exploration Roadmap. He joins me today to talk about how economists can go about quantifying the wider benefits of space programmes like Gaia.