Earth today, Jupiter tomorrow: An insight into Juices recent Earth flyby
On 28 September, ESA's Juice mission reached another important Jupiter-bound milestone with its penultimate gravity assist manoeuvre: a flyby of Earth. This encounter, however, was not only an essential step on the road to Jupiter, but an opportunity to rehearse the observations and operations that will define Juice's exploration of the Jovian system.
Since its launch on 14 April 2023, Juice has been making its way towards Jupiter and its icy moons, where it will arrive in 2031. But getting there is not straightforward.
"If you want to go directly to Jupiter, you need to reach a certain velocity," explains Arnaud Boutonnet, ESA Juice Mission Analyst. "This is why you need gravity assist manoeuvres."
These manoeuvres use a planet's gravity to alter a spacecraft's course and speed. With its recent Earth gravity assist, Juice has now completed three of four planned flybys.
Following its Venus flyby, Juice had enough speed relative to Earth, but was not yet heading in the right direction. This latest flyby effectively steered the spacecraft around 20 degrees towards Jupiter, ahead of a final adjustment during its last Earth flyby in 2029.
But the milestone was about more than navigation. It also gave mission teams a valuable opportunity to rehearse for the operational and science challenges awaiting Juice at Jupiter.
This past August, many witnessed the spectacular total solar eclipse that swept over Greenland, Iceland, Spain and parts of Portugal. For observers on Earth, totality lasted just over two minutes.
Spacecraft encounter eclipses too.
During its Lunar Earth gravity assist in 2024, Juice experienced a 32 minute long eclipse. Its recent Earth flyby, however, broke a new mission record.
Before closest approach, Juice slipped into Earth's shadow, spending three hours in full umbra and six in partial shadow. More importantly, the eclipse closely resembled conditions Juice will encounter at Jupiter, giving the Flight Control Team a rare opportunity to test future operational strategies years before arrival.
"Flybys have become fairly routine for the mission, but they still require a great deal of preparation. We've been preparing for this one for more than a year," says Juice Spacecraft Operations Manager Angela Dietz.
The eclipse turned that preparation into a rehearsal for Jupiter.
Juice is powered by two large solar arrays, which together make up 85 squared metres. This is roughly half the area of a volleyball court. Those arrays will be essential at Jupiter, where sunlight is just 3% as intense as it is at Earth, and eclipses can last up to five hours.
"This was our first opportunity to test those prolonged eclipse conditions in flight," says Angela.
Months in advance, the Flight Control Team at the European Space Operations Centre (ESOC) rehearsed eclipse and contingency scenarios while Science Operations Teams from the European Space and Astronomy Centre (ESAC) prepared instruments for observations and calibration activities.
On the evening of 27 September, as Juice entered the penumbra at 21:30 CEST, flight operators at ESOC monitored the drop in solar-array power and rise in battery use as the telemetry came in via ESA's Cebreros Estrack deep space antenna.
After the three-hour umbra phase, the battery power dropped to 54% before the solar arrays resumed power generation at 04:14 CEST. With the operational limit set at 35% and the spacecraft behaving nominally, the result exceeded expectations.
"The power data we got from this flyby is very important because it will be used for future Jupiter eclipse simulations," explains Paula Betriu, Juice Science Operations Engineer working at ESAC for Starion Spain.
But the eclipse was about more than simply keeping the spacecraft alive.
At Jupiter, Juice will need to continue carrying out science even during long periods of darkness.
"Initially we thought we would not have enough power for the payloads. But after we looked into it further, we realised we could keep several instruments active and make full use of the opportunity," recalls Angela.
All ten science instruments contributed to the flyby campaign, though not all operated simultaneously during the eclipse.
"It was a challenging campaign," says Pilar Esquej, Juice Science Operations Engineer working at ESAC for Starion Spain. "The eclipse imposed tight constraints, so we had to carefully prioritise observations and find compromises."
The eclipse also provided an opportunity to test another key system needed at Jupiter - Juice's enhanced autonomous guidance through limb extraction (EAGLE) software.
EAGLE uses the spacecraft's two navigation cameras to improve pointing accuracy during close moon flybys by tracking a moon's illuminated edge, helping Juice determine its position more precisely and keep observations on target.
For its first exercise in flight, the Earth-Moon geometry was used to replicate that of Jupiter and Io, preparing EAGLE for future Galilean moon flybys. The results will be analysed in the coming months.
Despite its challenges, the eclipse also proved advantageous. Juice is currently in a hot cruise environment, where temperatures restrict observations. With the Sun temporarily blocked by Earth, this allowed the spacecraft to point freely and perform observations that would otherwise have been impossible.
Together, these activities turned the flyby into far more than a trajectory adjustment. They gave Juice its first taste of the challenges of the Jovian system and a chance to rehearse for them years in advance.
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