Gaia: science essays
In these short (mostly 2-page) weekly 'essays', I have picked out some of the scientific highlights of the Gaia mission as they are emerging, or as they caught my attention. They offer a snapshot of some of the discoveries that Gaia is making across all of astronomy. I've also included some essays on related topics, including the history of astrometry, and some more technical, managerial, or developmental aspects of both the Hipparcos and Gaia missions. In each, I have included a footnote DR1, DR2, EDR3, DR3, etc to indicate which of the (latest) data releases the essay refers to (described in essays #10 and #76), with DR0 signifying technical or historical material not connected with any specific data release. Who are they written for? Anyone who might have a general interest in science and astronomy, including amateur astronomers, young scientists starting out on their careers, mid-career scientists looking in on Gaia for the first time to get a feeling of what is possible, and specialists looking in from different areas of astronomy, or physics more generally. My thanks go to many people: to all those I worked with on the Hipparcos and Gaia projects over almost 30 years, to those now dedicating huge reserves of their time, energy, and skill to the ongoing data processing, and to those who have entered into the Gaia catalogue and published the results described here. Click on the access PDF icon to access the file. Only a few references are included, and these are 'discreetly' hyperlinked for those who want to read more... where references appear in the form (Einstein 1908) or www.gaia.com, clicking on the text (even though generally not highlighted!) should lead to the relevant online article. In a few cases, I've recorded an interview on the subject (see science interview page).
New: As of early July 2025, have converted Essays 1–130 into audio "discussion-type" podcasts, entirely using generative AI. They are available at my Gaia Essay YouTube channel, and I describe their construction in Essay 227.
As of July 2025, my essays will be monthly (on the first Monday of the month) until further notice.
This table page lists all essays, updated to the end of June 2025 (1–235 inclusive), in tabular form. It includes a simple search on the title field.
New: This Gaia Science Tree (v3.0, July 2025) presents essays 1–235 (Jan 2021–Jun 2025) as a hyperlinked "mind map"
* all end-nodes are hyperlinked to the given essay number (links are to "legacy" copies at the CERN-Zenodo site)
* catalogue content topics are at top right, background material at bottom left, otherwise moving "outwards" clockwise in the diagram
* I have prepared this as a didactic tool. Please feel free to make use of it as you wish
Please make use of this subscribe page to receive an email (usually Monday) when each new essay is published
Essays through to the end of 2023 (1–156 inclusive) also appear in a hyperlinked indexed form in the Bulletin of the American Astronomical Society (BAAS Vol. 56, Issue 1, 15 March 2024): ADS 2024BAAS...56a.008P
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10. Catalogue data releases
The data releases so far (to EDR3)
Gaia is more than six years into a possible 10-year data collection phase. Three data releases have been issued to date. Each successive catalogue supersedes the previous. Each comprises more observations, and an increased temporal baseline, and an improving accuracy of the astrometric parameters as a result.
8 March 2021
9. Gaia and GDP
Does it have any impact on economic growth?
Scientists are keen to emphasise that their contributions are essential for the advancement of human knowledge, and that they often have unexpected benefits much further down the line. Some economists are actually trying to quantify the returns on a country's investment made in the areas of fundamental or basic science.
1 March 2021
8. Why radial velocities?
And why they aren't easy to acquire
Classical astrometry measures star positions in two angular coordinates. But star motions along the line-of-sight can't be determined in this way. Great efforts were made to measure radial velocities from the ground for Hipparcos. Gaia is in the process of tracking more than a million of these radial motions directly from space.
22 February 2021
7. On-board detection for Gaia
How stars are detected on-board
Gaia uses a combination of advanced optical imaging and powerful onboard processing to detect every star that enters its field of view, down to a million times fainter than observable with the human eye. Each of these more than two billion stars are then tracked with its high-precision measurement system as the satellite scans the sky.
15 February 2021
6. Galactic tracers, by design
Establishing the mission goals
The design of the Gaia mission targeted very specific measurement goals, which were formulated in terms of limiting magnitude and astrometric accuracy. Gaia's limiting magnitude of around 20–21 mag, for example, was a requirement resulting from an examination of what types of stars, in different regions of our Galaxy, should be measured.
8 February 2021
5. An Input Catalogue for Hipparcos
Why it was needed for Gaia's predecessor
ESA's precursor to Gaia, Hipparcos, observed more than 100,000 stars in the early 1990s. The observing programme had to be pre-planned, in an effort that occupied many scientists for several years. That experience assisted the development of the techniques that Gaia uses today to detect everything that it observes directly on-board.
1 February 2021
4. Hipparcos: the push to space
The background to selecting Hipparcos
By the second half of the twentieth century the steady advance in the accuracy of stellar positions from the ground was running into a number of insurmountable barriers. The biggest problem was the bending and twinkling effects of the atmosphere. The answer was to carry out these measurements from space.
25 January 2021
3. A history of astrometry
Gaia in the big historical picture
The motions of the stars and planets perplexed the curious minds of the ancient world. Great advances in understanding our place in the Universe came in the Middle Ages, driven in part by the practical demands of navigation. Instrumental advances over the past few centuries have continued to drive scientific enquiry ever since.
18 January 2021
2. Why measure star positions?
For those with little knowledge of astrometry
The positions of stars in the sky vary minutely with time for a number of reasons. Accurately and repeatedly measuring them over months and years can discern tiny motions which prove central to understanding their nature, as well as providing a wealth of information on the structure and origin of our Galaxy and beyond.
11 January 2021
1. The measurement of angles
For those with little knowledge of astrometry
The basis of astrometric measurements is the accurate measurement of tiny angles that divide up the sky. Dividing a circle, whether on paper or on an imaginary sweep of the celestial sky, is a task well-posed in principle. Practical techniques for doing so aside, it is only necessary to agree on the unit of subdivision.
4 January 2021
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