The European Space Agency's Juice probe is to thread the needle this week with a first for a space mission - swings around the Moon and Earth that will result in the spacecraft coming approximately 700 km from the lunar surface.

Juice, ESA's mission to study Jupiter's icy moons, launched in April 2023. The probe is due to arrive in the Jupiter system in 2031 and will spend the following years touring the moons before settling into an orbit around Ganymede at the end of 2034.

To save propellant, the spacecraft will use four gravity assists, the first due tonight as the probe barrels toward the Earth-Moon system.

ESA has described it as "a world first."

While using gravity assists to speed probes on to their destinations is hardly new, Juice will be attempting a double gravity assist - using the Earth and Moon to alter the spacecraft's velocity and trajectory to send it on to its next flyby, which will be around Venus in August 2025.

The maneuver is inherently risky. As Juice's Spacecraft Operations Manager, Ignacio Tanco, put it: "It's like passing through a very narrow corridor, very, very quickly: pushing the accelerator to the maximum when the margin at the side of the road is just millimetres."

ESA's constant ground coverage of the mission began over the weekend. Tonight, the spacecraft's science instruments and monitoring cameras will be switched on to observe the Moon as Juice swings by. The two cameras were used to monitor the deployment of the spacecraft's booms, antennas, and solar panels after launch. Using the same cameras to take pictures of the Moon is therefore an experiment. ESA admitted: "We're not 100 percent sure where exactly the cameras are pointing."

According to ESA, at 2116 UTC tonight, Juice will make its closest approach to the Moon. To make matters more interesting, the spacecraft will be eclipsed by the Moon and out of contact with Earth during this time. At 2125 UTC, controllers expect to start receiving monitoring camera images on Earth, and shortly after, the science instruments will stop collecting data.

On August 20, the spacecraft will do it all again, this time around Earth. The spacecraft will pass Earth at approximately 6,800 km at 2157 UTC. This will be during a ground station visibility gap expected to last from 2138 to 2259 UTC, but, like the lunar flyby, if an error has been made in the calculations, there is little that can be done to rectify the situation at that point.

Eventually, at 0610 UTC on August 21, the spacecraft's monitoring cameras will be switched off, and controllers can take a well-earned breather before thinking about the Venus flyby next year.

In a post on X, the ESA Operations account noted that of the four opportunities to fine-tune the trajectory, only the first was required: "Flight Dynamics hit a bullseye on the first try!"

A bullseye might have been hit, but we imagine plenty of folk at mission control in Darmstadt being jittery as Juice races moonward. ®

https://www.theregister.com//2024/08/19/esa_juice_gravity_assist/