Science1 publisher3 min readPublished
Earth's gravity gives Juice 3.5 km/s toward Jupiter on 28 September
ESA's Jupiter Icy Moons Explorer takes speed and a new heading from Earth's gravity on 28 September, the third such encounter since launch, and one more Earth pass is still needed before it reaches Jupiter.
The Scientist · Science desk
What happened
- Juice comes within 8640 km of Earth on 28 September 2026, the third planetary encounter of a route that already included a lunar-Earth flyby and a pass by Venus.
- ESA expects the pass to deflect the spacecraft's trajectory by about 20 degrees, with closest approach over the Indian Ocean at 13:45 CEST.
- A final Earth flyby in January 2029 is what puts Juice on the optimum path to intercept Jupiter in July 2031, following its April 2023 launch from French Guiana.
- All 10 of Juice's science instruments are switched on between 23 September and 3 October to collect data on Earth and the Moon.
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Why it matters
- constraint The overnight passage through Earth's shadow puts a ceiling on the calibration data: with the spacecraft on stored power, the team has to rank observations and drop the rest.
- decision Every correction window the flight dynamics team skips leaves propellant for Jupiter orbit insertion in 2031 and Ganymede capture in 2034.
- exposure Whatever targeting error survives 28 September is inherited by the January 2029 geometry, and that pass is the one that fixes the Jupiter intercept.
- capability Instrument teams get to exercise their analysis tools on a surface whose properties are already known, before the only chance they get at the icy moons.
ESA gives the reason for the detour in its own announcement. Flying straight to Jupiter would have been quicker. But it would have needed "an enormous rocket and an unthinkable amount of fuel" to build the speed, and a great deal more fuel to stop the spacecraft sailing past the planet on arrival [9]. Instead the pass adds roughly 12,600 km/h [1], and ESA says the speed and the course changes are coming "almost entirely for free" [10].
Launch to Jupiter orbit is about eight years and three months [2], and orbit around Ganymede follows about three years later, in 2034, with propellant still in the tanks for both insertions [11][7].
The navigation campaign opened on 17 August, 42 days before closest approach, with windows reserved at four weeks, two weeks, one week and three days out among others, each held for small corrections to the arrival angle and velocity [12][5]. "Until now, only one manoeuvre at minus four weeks was needed," said Angela Dietz, Juice Spacecraft Operations Manager [13]. "It was of relatively small size, but extremely effective. It put Juice in the right spot for the flyby" [14]. Five of the reserved windows stand unused [6].
Power is the tighter constraint on the day. Juice is in Earth's shadow from 21:24 CEST on 27 September to 06:03 CEST on 28 September, eight hours and 39 minutes on battery alone. The team is prioritising its most important observations so the batteries are not drained [16][3]. From 08:00 CEST, five hours and 45 minutes before closest approach, controllers will be ready with an emergency manoeuvre to fix critical navigation errors or avoid a collision with other satellites. ESA expects not to need it [15][4].
ESA's stated purpose for switching the payload on is calibration: a chance to collect and analyse data from an actual surface in space, and to have the analysis tools working before the icy moons [18]. "All flybys are risky, requiring very careful planning and continuous monitoring," said Claire Vallat, Juice Project Scientist. "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" [19]. Between 15 and 30 minutes before closest approach the spacecraft crosses Australia from the northeast to the northwest. ESA says amateur astronomers with the right telescope or binoculars may be able to track it [20].
How many assists this is depends on how you count 2024. ESA's sequence runs lunar-Earth, Venus, this Earth pass, and a last Earth pass in January 2029 [6][7]. Vallat describes the 2024 event as a Moon flyby that set up an Earth flyby the following day [19]. That gives four encounters across five bodies. The announcement does not include a propellant figure, so the claim that the velocity is nearly free cannot be checked here against a delta-v budget [21].
What to watch
- Whether controllers use the three-day correction slot, or the emergency manoeuvre held ready from 08:00 CEST on flyby day.
- Post-flyby tracking data showing the deflection and velocity gain actually achieved against ESA's 20 degree and 3.5 km/s figures.
- Whether instrument teams report gaps in the eclipse observations because stored power ran short.