My first Magazine Sky & Telescope - 03.2019

Twirling Telescopes like WMAP and Planck. But its mission is to build a detailed 3D map of our galaxy, plotting the positions and distances of stars in the sky. Before the 1990s astronomers could only estimate star positions and distances with ground-based telescopes. Yet this meant they had to deal with the confounding effects of the atmosphere, placing a limit on how accurate they could be. Gaia’s predecessor Hipparcos, launched in 1989, solved this issue, managing to plot positions and distances to some 120,000 stars 100 times more accurately than ever before. Gaia’s start in 2014 marked another jump: The second data release from the ﬁ rst 22 months of Gaia operation now gives astronomers a distance, or more precisely absolute parallax in astronomers’ parlance, that is 100 times more accurate than Hipparcos to some 1.3 billion objects. To collect this huge pot of data, Gaia has been performing a WMAP-esque dance around the Sun but much more slowly. Positioned at the now familiar L 2 point, Gaia traces large rings on the sky L 4 every 6 hours, with the space- craft wobbling full circle at 45° from the direction away from the Sun every 63 days. These three 45° Satellite spin axis Precession of the spin axis in 63 days Sun Line of sight 1 Earth Gaia Consecutive great circles traced by lines of sight 106.5° Line of sight 2 types of rotation allow the spacecraft to scan every object it sees about 70 times over the course of ﬁ ve years. Like a toddler trying to make sense of the world, the tele- scope makes no assumptions about the space around it. It has no sky map to guide it. All Gaia records is where a star shines in its ﬁ eld of view at a given point in time. In essence, Gaia doesn’t measure where things are but just when they are seen during its careful dance. Gaia then ﬁ res these measurements down to Earth, where they are added to a vast calculation called the Astrometric Global Iterative Solution (AGIS), involving billions of param- eters. AGIS gradually ﬁ ts the data together like a jigsaw puzzle. “Gaia and Hipparcos are very, very elegant mathematical mis- sions,” explains Michael Perryman (University College Dublin, Ireland), a scientist who alongside Lennart Lindegren (Lund University, Sweden) proposed the Gaia mission. “They spin, but a lot of their beauty is in the mathematical methods that are actually used to reconstruct the data-analysis problem.” With repeated observations of the same objects at different times and from different perspectives, AGIS gradually makes deeper insights on the ground using what Gaia sees from space. Crucially, this includes the distance to every object — something astronomers using ground-based and non-spin- ning scopes could only ever dream of. 60° L 3 L 1 60° L 2 L 5 24 M A RCH 2 019 • SK Y & TELESCOPE t LAGRANGIAN POINTS In a system of two massive bodies orbiting each other (such as the Sun and Earth), fi ve gravitational “balance points” exist where a third, much smaller object can orbit in a constant pattern. L 4 and L 5 are stable if one of the two bodies is at least 24.96 times more massive than the other. On the other hand, L 1 , L 2 , and L 3 are unstable on a time scale of about 23 days, so spacecraft at these locations need regular course and attitude corrections (for this reason, they often circle the Lagrangian point). L 1 is ideal for solar observations, whereas a craft at L 2 can always keep Sun, Earth, and Moon behind itself and have a clear view of deep space. S&T ILLUSTR S&T u GAIA’S PATH The Gaia spacecraft whirls around on its axis 1° per minute, scanning the sky simultaneously along two lines of sight that trace great circles on the celestial sphere. The rotation axis also moves, maintain- ing a 45° angle from the Sun as it slowly precesses around the Sun-to-Earth direction. As the craft orbits the Sun, it observes long over- lapping strips of sky, building an all-sky map (far right).

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