Gaia is on a mission to survey more than a billion stars, charting the largest three-dimensional map of our galaxy, the Milky Way. In so doing, the spacecraft is revealing the composition, formation and evolution of our galaxy, and a whole lot more.
For the last five and a half years, the spacecraft has travelled in an orbit designed to keep it out of Earth’s shadow, the second Lagrange point.
At 1.5 million kilometres from Earth – four times further than the moon – the ‘L2’ is a fabulous place from which to do science. As the sun, Earth and moon are all in one direction relative to the spacecraft, the rest of the sky is free to observe.
Placing Gaia in L2 has also ensured the star-catcher’s stability, because to this day it has never passed into Earth’s shadow. This has kept the spacecraft undisturbed by any change in temperature or varying infra-red radiation that would result from an Earth eclipse.
Although at the end of its planned lifetime, Gaia still has fuel in the tank and a lot more science to do, and so its mission continues. However, its eclipse-dodging path will not. In August and November of this year, without measures to change its orbit, the billion-star hunter will become partially shrouded by Earth’s shadow.
These two eclipses would prevent enough of the sun’s light reaching Gaia’s solar panels that the observatory would shut down. As well as affecting its stability and power, such shade would cause a thermal disturbance, impacting the spacecraft’s scientific data acquisition for weeks.
To keep Gaia safe from these shady possibilities, operators at ESA’s mission control are planning the ‘Whitehead eclipse avoidance manoeuvre’.
David Milligan, spacecraft operations manager for the mission, said, "We’ve named this operation after a great colleague of ours, Gary Whitehead, who sadly passed away last month after serving on the flight control team for more than 11 years."
On 16 July, Gaia will use a combination of its on-board thrusters to push it in a diagonal direction, away from the shadow, in a special technique known as 'thrust vectoring'.
"The manoeuvre will allow us to change Gaia’s orbit without having to turn the spacecraft body, keeping sunlight safely away from its extremely sensitive telescope," Milligan added.
Gaia is an incredibly stable spacecraft. In fact, it is many, many times more stable – and therefore precise – than any other spacecraft in operation today.
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