Sky & Telescope - March 2019 - 24

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 ﬁrst 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 L2 point,
Gaia traces large rings on the sky
L4
every 6 hours, with the spacecraft wobbling full circle at
45° from the direction
away from the Sun every
63 days. These three
60°

60°

M A RCH 2 019 * SK Y & TELESCOPE

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 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 ﬁres 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.

t LAGRANGIAN POINTS In a system of two massive bodies orbiting each other (such as the Sun
and Earth), five gravitational "balance points" exist where a third, much smaller object can orbit in a
constant pattern. L4 and L5 are stable if one of the two bodies is at least 24.96 times more massive
than the other. On the other hand, L1, L2, and L3 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). L1 is ideal for solar observations, whereas a craft at L2 can always keep
Sun, Earth, and Moon behind itself and have a clear view of deep space.

G AIA IM AG ES: C. CA RRE AU / ESA (2); G AIA PATH: S&T ILLUSTR ATION,
SOURCE: ESA; L AG R A NGIA N: LE A H TISCIONE / 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, 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).

Sky & Telescope - March 2019

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