At a quarter to one in the afternoon, British time, on Monday 28 September, a spacecraft bound for Jupiter slipped past our planet just 8,640 km above the Indian Ocean. It took some photographs, and left travelling faster and in a new direction.
It burned hardly any fuel to do so. Earth did the heavy lifting.
The spacecraft is the European Space Agency's Jupiter Icy Moons Explorer, known as Juice. According to ESA, the flyby deflected Juice by 20 degrees compared with its previous path and increased its speed by 3.5 km/s. That is roughly 12,600 km/h of extra speed, gained with very little propellant.
How a slingshot works
A gravity assist is one of the most elegant tricks in spaceflight. Earth is itself racing around the Sun, and a spacecraft that swings close by is tugged along by our planet's gravity. Its path bends, and if it leaves in the right direction it carries away a tiny share of Earth's own motion.
Think of a ball bouncing off the front of a moving train: it comes away faster than it arrived. Earth slows by an amount far too small ever to notice.
The alternative is not appealing. ESA says that flying straight to Jupiter would have needed an enormous rocket and an unthinkable amount of fuel, with a great deal more to stop Juice sailing straight past on arrival. Instead, the agency is using Earth, the Moon and Venus to adjust the spacecraft's speed and course along the way.
A very small nudge
None of this works unless the spacecraft arrives in exactly the right place. ESA set aside six opportunities in the four weeks before the flyby to correct Juice's course. Only one tiny nudge was needed.
"The flyby required ultra-precise navigation in real time. Thanks to our very careful planning, we used only a small amount of the propellant reserved for this flyby," says Angela Dietz, Juice's Spacecraft Operations Manager. "This gives us more to use at Jupiter to carry out observations of the planet's icy moons."
Earth as a test site
Changing course was the main purpose, but Earth also served as a practice target. ESA says this was the third chance to try out Juice's 10 science instruments on a real surface in space, after the 2024 lunar-Earth flyby and observations of Comet 3I/ATLAS in 2025.
"Juice's journey to Jupiter provides only a few opportunities to calibrate and validate the instruments under well-understood environmental conditions," explains Juice project scientist Claire Vallat, who led the effort.
Not everything could run at once. Juice spent part of the flyby in Earth's shadow, relying on battery power alone, so the team had to choose. "During this flyby, calibration activities were prioritised based on their relevance to preparing the instruments for their work at Jupiter, while also considering whether an opportunity is unique or can be scheduled later in the journey," Vallat says.
Juice also spent several days flying through Earth's magnetotail, the part of our planet's magnetic field that stretches away from the Sun, measuring magnetic fields and charged particles far from home.
Pictures to come
ESA expects to publish images and spectra from some of Juice's instruments in the coming weeks, once the data have arrived and been evaluated. These include high-resolution images of the Moon and Earth from JANUS, the spacecraft's scientific camera.
Juice launched in April 2023 and is not finished with us yet. It returns in January 2029 for one last Earth flyby, which will put it on the final track to reach Jupiter in 2031.
Once there, it is due to make 35 flybys of the giant moons Ganymede, Callisto and Europa before settling into orbit around Ganymede. ESA describes the three ocean-bearing moons as potential habitats for past or present life.
It is a long road, and Earth has just given Juice a helpful push along it.
