Juice to fly past Earth for third gravity assist

Getting a spacecraft to the outer Solar System is no easy feat. Juice’s journey began almost 3.5 years ago, with its April 2023 launch from Europe’s Spaceport in French Guiana. Since then, flight controllers on Earth have successfully guided it through the first ever lunar-Earth flyby (a ‘braking’ manoeuvre to take Juice on a shortcut to Jupiter via Venus) and then a Venus flyby (that bent its orbit and gave it a little extra speed).

But there’s still a long way to go. To avoid needing enormous amounts of fuel to get it onto the right final track, Juice will make two further flybys of Earth. On 28 September 2026, the spacecraft will come within 8640 km of our planet. This interaction will set it up for a final Earth flyby in January 2029, putting Juice onto the optimum path to intercept Jupiter in July 2031.

Next week’s flyby will deflect the trajectory of the spacecraft by about 20 degrees and increase its velocity by about 3.5 km/s. The closest approach will be over the Indian Ocean at 13:45 CEST (11:45 UTC).

Between 15 and 30 minutes before closest approach, Juice will pass over Australia, travelling from the northeast of the country to the northwest. Amateur astronomers using the right telescope or binocular equipment may be able to track it. Trajectory data can be found here.

Why does Juice need gravitational help from the planets?

Why not just fly straight to Jupiter and then brake hard when we get there? True, this would have been much quicker. But it would have also required an enormous rocket and an unthinkable amount of fuel to pick up enough speed for the journey. And then a huge amount more fuel to prevent Juice from flying straight past Jupiter when it gets there.

Instead, flight controllers devised a clever strategy to make the most of the planets and their gravity. Thanks to Earth, the Moon and Venus, Juice is picking up some speed and adjusting its path in just the right way – almost entirely for free.

All this means that Juice will have plenty of fuel left in its tanks to enter orbit around Jupiter in 2031, and then giant moon Ganymede in 2034.

Find out more about Juice’s complex journey to Jupiter.

How are we making sure the flyby is a success?

What our spacecraft operators call the ‘navigation campaign’ started on 17 August. Six timeslots were set aside – including at four weeks, two weeks, one week, and three days before the flyby – to make tiny corrections to Juice’s trajectory to ensure that it arrives with exactly the right angle and velocity for an optimum gravity assist.

“Until now, only one manoeuvre at minus four weeks was needed,” says Angela Dietz, Juice Spacecraft Operations Manager. “It was of relatively small size, but extremely effective. It put Juice in the right spot for the flyby. Our Flight Dynamics Team did an amazing job to prepare it!”

At 08:00 CEST on 28 September, flight controllers will be prepared to make a final emergency manoeuvre to fix critical navigation errors or avoid a collision with other satellites. All going well, this manoeuvre will not be needed.

This flyby comes with an additional challenge. Just before the closest approach, from 21:24 CEST on 27 September to 06:03 CEST on 28 September, Juice will be in Earth’s shadow. The lack of sunlight means that the spacecraft will be working on battery power alone. To avoid depleting its batteries completely, the team is prioritising the most important instrument observations with Juice’s science instruments during this time.

Bonus science

Just like during the 2024 lunar-Earth flyby, Juice’s 10 science instruments will be switched on to collect data on Earth and the Moon between 23 September and 3 October.

This provides a great opportunity for instrument teams to collect and analyse data from an actual surface in space. By calibrating their instruments and preparing data analysis tools now, teams will be ready to collect the best scientific data at Jupiter and its icy moons.

“All flybys are risky, requiring very careful planning and continuous monitoring. But the upcoming Earth flyby is much simpler from an operational perspective than the 2024 lunar-Earth flyby, where a flyby of the Moon set Juice up for a flyby of Earth the next day,” says Claire Vallat, Juice Project Scientist.

“This is great news for science, because it means we will be able to turn Juice in all different directions to point its science instruments at different parts of Earth and the Moon.”

One interesting scientific aspect of this flyby is that Juice will spend several days in the tail of Earth’s magnetic field. Stretched out away from the Sun like a windsock, this ‘magnetotail’ is a very complex structure created by charged particles flying towards us from the Sun. Juice is likely to get some very interesting magnetic field measurements that could later be combined with data from the European–Chinese Smile mission.

Meanwhile, Juice’s navigation camera – which will ultimately help the spacecraft steer around Jupiter’s moons – will take the opportunity to point towards the horizon of our own Moon. The images it takes will test out a new technique to improve the accuracy of its optical navigation.

Juice’s two onboard monitoring cameras will be snapping photos throughout the flyby, which we will be sharing via social media and the ESA website. Their last images of this event will be taken on 29 September.

Follow @science.esa.int on Bluesky for the latest updates.

About Juice

ESA’s Jupiter Icy Moons Explorer, ‘Juice’, is humankind’s next bold mission to the outer Solar System. It will make detailed observations of gas giant Jupiter and its three large ocean-bearing moons – Ganymede, Callisto and Europa. This ambitious mission will characterise these moons with a powerful suite of remote sensing, geophysical and in situ instruments to discover more about these compelling destinations as potential habitats for past or present life.

Juice will monitor Jupiter’s complex magnetic, radiation and plasma environment in depth and its interplay with the moons, studying the Jupiter system as an archetype for gas giant systems across the Universe.

Juice launched on an Ariane 5 from Europe’s Spaceport in Kourou in April 2023. It has an eight-year cruise with flybys of Earth and Venus to slingshot it to Jupiter. It will make 35 flybys of the three large moons while orbiting Jupiter, before changing orbits to Ganymede.

Juice is a mission under ESA leadership with contributions from NASA, JAXA and the Israel Space Agency. It is the first Large-class mission in ESA’s Cosmic Vision programme.

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