Sky & Telescope - January 2025 - 58

ESA / CNES / ARIANESPACE
JANUARY 2025 OBSERVING
Pro-Am Conjunction by Diana Hannikainen
Goodbye
to Gaia
Catch a last glimpse of a
successful spacecraft.
A
ll good things must come to an end.
And so it is with the European Space
Agency's Gaia space observatory. Gaia
has carried the mantle of the ancient
and time-honored tradition of astrometry,
or measuring the positions and
motions of celestial objects. And it has
done so with unprecedented accuracy,
precisely recording the positions of more
than 1.8 billion targets since the start
of nominal operations shortly after its
launch in December 2013. The mission
has been extraordinarily successful -
you'd be hard-pressed to find an issue of
this magazine from the past decade that
doesn't mention Gaia somewhere in its
pages (see, e.g., S&T: Feb. 2023, p. 34 and
S&T: Apr. 2024, p. 12).
The spacecraft will have used up all
its fuel by mid-January 2025, signaling
the end of the space-based phase of the
mission. But before that, we'll have a
chance to say farewell.
Missing magnitudes. In order to
ensure accurate measurements of the
positions of celestial objects, Gaia scientists
have to calculate the spacecraft's
orbital parameters with high precision.
To do so, they established the Ground
Based Optical Tracking, or GBOT,
program to monitor Gaia's position on
the sky from Earth - a crucial parameter
for this task. Gaia doesn't glow per
se, but it does reflect sunlight back at
us from its position far beyond Earth's
nightside, the light bouncing off its
11-meter-wide (36-foot-wide) sunshield.
Initially, engineers estimated the spacecraft's
magnitude would be around 18.
58 JANUARY 2025 * SKY & TELESCOPE
As the mission got under way, a fleet of
1- to 2-meter-class telescopes across the
globe was poised to record the spacecraft's
celestial position daily.
As preliminary data rolled in, Gaia
kept clocking in at around magnitude
21 instead of the predicted 18. Bewildered,
the GBOT team had to rethink
their strategy - the diminished magnitude
forced them to limit their data
acquisition to telescopes 2 meters or
larger in aperture. But that compromised
coverage, as bigger instruments
are fewer and farther between and have
less flexibility in their scheduling.
After a decade of poring over the
data, engineers are still puzzled as to
why Gaia's apparent magnitude never
reached the pre-launch prediction. In
order to solve this mystery and also to
find out just how the deep-space environment
has affected the spacecraft, the
GBOT team plans to run a battery of
technical tests and experiments to help
them better understand this discrepancy.
Goodbye to Gaia. Space engineers
conduct procedures known as " end-oflife "
tests on spacecraft before decommissioning
so that they can anticipate
problems that might arise in future missions
and offer potential solutions. And
so, Gaia will undergo the same process,
which will, among other things, shed
light on the effects of the harsh atmosphere
of space on the craft's sunshield.
GBOT team leader Martin Altmann
(Heidelberg University, Germany)
outlines that one of these tests will be
to slew the satellite from its normal
pEngineers inspect Gaia's 11-meter sunshield
before launch from Kourou, French Guiana.
inclination to the Sun to angles more
favorable to making it light up for
ground-based observers. Starting in
mid-January, operators will guide the
spacecraft through a series of maneuvers
that will change the angle of the
sunshield from 45° to 90° and back
again over a period of several weeks.
During this time, Gaia's apparent
magnitude should leap to 14 or so, well
within the reach of amateur telescopes.
Altmann and his team are calling on
the amateur community to participate
in the end-of-life testing period, requesting
magnitude and color measurements.
For this, even an 8-inch scope should
suffice - and an astronomical camera
and filters are desirable for color
estimates. Head over to https://is.gd/
Gaia_EOL for information on how and
when to observe Gaia as it approaches
the close of its space operations. The
GBOT team is eager to hear from you
- but note that you'll have to reduce all
the data yourself before you upload your
measurements.
As you point your telescope at Gaia
and gather data that will help the GBOT
team unravel the mystery of the missing
magnitudes, see this as a fitting farewell
to an extraordinarily successful spacecraft.
Join Altmann and his colleagues in
what he calls " the time to say goodbye. "
¢ Observing Editor DIANA HANNIKAINEN
plans to wave farewell to Gaia in January.
https://www.is.gd/gala_eol https://www.is.gd/gala_eol
Sky & Telescope - January 2025

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