16 AUSTRALIAN SKY & TELESCOPE April 2019
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 first 22 months of Gaia operation now gives
astronomers a distance, or more precisely absolute parallaxin
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 every 6 hours, with
the spacecraft wobbling full circle
at45°from thedirection
away from the Sun every 63
days. These three types
of rotation allow the
spacecraft to scan every60°L 5L 3L 4L 1 L 260°WLAGRANGIAN POINTSInasystemoftwomassivebodiesorbitingeachother(suchastheSun
and Earth),fivegravitational‘balancepoints’existwhereathird,muchsmallerobjectcanorbitina
constant pattern. L 4 and L 5 arestableifoneofthetwobodiesisatleast24.96timesmoremassive
than the other. On the other hand, L 1 ,L 2 and L 3 areunstableonatimescaleofabout23days,so
spacecraftattheselocationsneedregularcourseandattitudecorrections(forthisreason,they
often circle the Lagrangian point). L 1 isidealforsolarobservations,whereasacraftatL 2 can always
keep Sun,EarthandMoonbehinditselfandhaveaclearviewofdeepspace.GaiaEarthSunConsecutive
great circles
traced by lines
of sight106.5°45°Line of
sight 2Line of
sight 1Satellite
spin axisPrecession of
the spin axis
in 63 daysobject it sees about 70 times over the course of five years.
Like a toddler trying to make sense of the world, the
telescope 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 field 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 sends these measurements down to Earth,
where they are added to a vast calculation called the
Astrometric Global Iterative Solution (AGIS), involving
billions of parameters. AGIS gradually fits the data together
like a jigsaw puzzle. “Gaia and Hipparcos are very, very
elegant mathematical missions,” 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-
spinning scopes could only ever dream of.XGAIA’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, maintaining
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
overlapping strips of
sky, building an all-
sky map (far right).GAIA IMAGES: C. CARREAU / ESA (2); GAIA PATH:S&TILLUSTRATION,SOURCE: ESA; LAGRANGIAN: LEAH TISCIONE /S&TTWIRLING TELESCOPES
Gaia has no sky map
to guide it ...