Juice Spacecraft Flies Past Earth at 8,640 Kilometers

The European Space Agency’s Juice spacecraft, built to study Jupiter’s icy moons, flew past Earth today, September 28, at 1:45 p.m. Central European Summer Time (11:45 GMT), the same time as in Serbia. The closest approach, at a distance of just 8,640 kilometers from the planet’s surface, happened above the Indian Ocean. It was the probe’s third gravity assist maneuver on its route to Jupiter.

The flyby bends the spacecraft’s trajectory by about 20 degrees and adds roughly 3.5 kilometers per second to its speed, all without burning a drop of fuel. Rather than flying straight toward Jupiter and braking with its engines on arrival, which would call for a massive rocket and enormous amounts of propellant, Juice is instead using the gravity of Earth, the Moon and Venus to gradually bend its path and pick up speed.

An Eight Year Journey

Juice launched from Kourou in French Guiana. According to ESA and EarthSky, the launch took place on April 14, 2023. Universe Today cites April 23 in its own coverage, without specifying a year, a date that does not match the other two sources.

The spacecraft’s first gravity assist was a double flyby of the Moon and Earth on August 19 and 20, 2024, which sent Juice on course toward Venus. The Venus flyby followed on August 31, 2025, giving the probe another boost of speed. Today’s Earth flyby puts it on track for one more pass by the planet in January 2029, its fourth, before Juice finally reaches Jupiter in July 2031. It is due to enter orbit around Ganymede, Jupiter’s largest moon, in 2034, and along the way the mission will also study the moons Europa and Callisto.

Preparations for today’s maneuver, known as the navigation campaign, began on August 17. ESA had scheduled four windows for small trajectory corrections, at four weeks, two weeks, one week and three days before the flyby, to make sure the spacecraft arrived at exactly the right angle and speed for an optimal gravity assist.

“So far only one maneuver was needed, minus four weeks before the encounter. It was relatively small, but extremely efficient,” said Angela Dietz, Juice spacecraft operations manager at ESA, in a statement from the agency.

A final backup slot for an emergency correction was held open for September 28 at 8 a.m., in case of a critical navigation error or a risk of collision with another satellite, but it was not needed.

The maneuver came with an added complication. From 9:24 p.m. on Sunday, September 27, until 6:03 a.m. on Monday, September 28, the spacecraft sat in Earth’s shadow and ran on battery power alone, so the team switched on only its most essential instruments to keep the batteries from draining completely.

Between 15 and 30 minutes before closest approach, Juice crossed over Australia, moving from the continent’s northeast toward its northwest. Amateur astronomers with a suitable telescope or binoculars were able to track it across the sky.

All ten of the spacecraft’s science instruments were switched on from September 23 to October 3 for imaging and calibration passes over Earth and the Moon, just as they were during the 2024 flyby. Juice will also spend several days inside Earth’s so called magnetotail, a structure shaped by charged particles streaming from the Sun, and scientists plan to compare the data it gathers there with readings from the joint European Chinese Smile mission. The probe’s navigation camera, which will later help guide it around Jupiter’s moons, was also tested against the Moon’s horizon to check a new technique for more precise navigation.

“Every flyby of a planet carries risk, requires very careful planning and constant monitoring. But the upcoming Earth flyby is operationally much simpler than the 2024 maneuver past the Moon and Earth,” said Claire Vallat, Juice project scientist at ESA, in a statement from the agency.

Two monitoring cameras on board filmed the entire flyby, and the last images from this encounter are expected on September 29. ESA is publishing them on its website and social media channels; no live broadcast was planned.

Even though the maneuver went off without needing an emergency correction, that does not mean the spacecraft has reached its destination. Juice still has almost five years of travel ahead before it gets to Jupiter, including one more Earth flyby in January 2029, before it enters orbit around the planet in 2031 and, only in 2034, around Ganymede. As Claire Vallat herself pointed out, each flyby still ahead carries its own risk and demands the same careful planning as today’s.