ESA’s Juice swung past Earth and picked up 3.5 kilometres a second

Juice passed 8,640 kilometres above the Indian Ocean at 13:45 CEST on September 28, 2026, and left Earth moving 3.5 kilometres a second faster than it arrived. The European Space Agency’s Jupiter Icy Moons Explorer also came away on a path bent by 20 degrees from the one it followed before the encounter, and it did so while spending only a small part of the propellant set aside for the flyby.

The 3.5 km/s is ESA’s own figure for the speed increase, as is the 20 degree deflection. Both come from the agency’s report on the encounter, and both describe the effect of Earth’s gravity on the spacecraft rather than any burn from its engines, which is the entire appeal of the technique for a probe with eight years of travel ahead of it.

The gain from a gravity assist

A gravity assist borrows a sliver of a planet’s orbital energy. The spacecraft is pulled around the planet and leaves with a different speed and heading relative to the Sun, so the change costs no fuel in itself, but it has to be aimed with great care because a small error on approach grows into a large one on departure. In ESA’s report on the flyby, the agency states that the pass deflected the spacecraft by 20 degrees and increased its velocity by 3.5 km/s.

Closest approach came at 11:45 UTC, over the Indian Ocean. At 8,640 km the spacecraft stayed far above the atmosphere and the crowded low orbits, yet close enough for Earth’s pull to do the work.

Four weeks of tracking, one correction

The delicate part was the approach. Angela Dietz, Juice’s Spacecraft Operations Manager, described the flyby as requiring ultra-precise navigation in real time, and said that thanks to careful planning only a small amount of the propellant reserved for the encounter was used. ESA reports that in the four weeks before the pass, operators performed one of the six course-correction manoeuvres that had been set aside, and that careful tracking began on August 17 and runs through October 10.

The saving matters because the same tanks must later carry the spacecraft through its science mission at Jupiter. A preview published on Astrobiology.com before the flyby quoted Dietz as saying only one manoeuvre at minus four weeks was needed, and as calling it relatively small but extremely effective. The pass itself bore that out, since the one manoeuvre was the only one of the six needed in the final month.

Ten instruments and a camera test

Juice used the encounter as a rehearsal. ESA says the spacecraft ran all 10 of its science instruments during the pass, collecting imagery and data from Earth’s magnetotail, the stretched-out region of Earth’s magnetic field on the night side of the planet. Those observations complement measurements from the Smile mission, which studies Earth’s magnetic environment near the poles, so two European spacecraft looked at the same magnetic system from very different places, and the calibration data gathered between September 23 and October 3 per the preview gives Juice’s teams a known target against which to check their instruments.

The cameras also produced pictures. As Sky at Night Magazine reported, a monitoring camera known as JMC1 captured Earth as the spacecraft departed, with Africa and Madagascar in view and the Moon as a small grey dot beside the planet.

The ten instruments are grouped by ESA’s factsheet into remote-sensing, geophysical and in-situ packages, which means the Earth pass exercised every kind of measurement the spacecraft will need at Jupiter, from imaging to sampling the charged particles and fields around it. Scientists on the mission also used the encounter to test optical navigation with the onboard navigation camera, a technique that lets a spacecraft fix its position from pictures of nearby bodies rather than relying only on radio tracking from the ground.

The route still ahead

This was the third of four gravity assists on the route, according to ESA’s Juice mission page, which lists a Lunar-Earth assist in August 2024, a Venus assist in August 2025, this Earth pass in September 2026 and a second Earth pass in January 2029. Arrival at Jupiter follows in July 2031, after a launch on an Ariane 5 from Kourou on April 14, 2023, as the ESA factsheet records.

Once there, Juice is planned to make 35 flybys of Ganymede, Europa and Callisto, and then to become the first spacecraft to orbit a moon in the outer Solar System when it settles around Ganymede. The spacecraft carries solar wings covering 85 square metres to power that tour so far from the Sun. Every kilogram of propellant left in the tanks after Earth is a kilogram available for that work, and the January 2029 pass will have to hand the spacecraft one more push toward Jupiter before the long coast begins.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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