The European space probe JUICE is actually on its way to the large icy moons of Jupiter. But before it reaches its destination, the probe must gain momentum several times by passing planets in the inner solar system. Today and tomorrow, JUICE will pass close to the moon and the earth. For the first time, it will perform a combined “Lunar-Earth Gravity Assist”: within 24 hours, the space probe will first fly close to the moon and then close to the earth – a double flyby of this kind has never been carried out in space travel before. This maneuver is made risky by the space probe’s high speed of around 15,000 kilometers per hour and the comparatively short distance during the overflights: at the moon it is 750 kilometers, at the earth around 6800 kilometers.
The JUICE (Jupiter Icy Moons Explorer) space probe was launched on April 14, 2023 on its 778 million kilometer journey to the Jupiter system. When the probe from the European Space Agency ESA arrives there in 2031, it will primarily investigate three objects in more detail: the three icy moons Ganymede, Europa and Callisto. All three of Jupiter’s moons probably have an ocean of liquid water beneath their icy crust and are therefore considered possible locations for extraterrestrial life in the solar system. But before that happens, JUICE must complete an eight-year journey of around 778 billion kilometers – almost ten times the average distance from Jupiter to Earth. This is necessary because a direct approach to the gas giant would require more fuel than the space probe can carry. That’s why JUICE uses several so-called gravity assist maneuvers to gain momentum and catapult itself onto the right course. The probe flies close to planets or moons so that it can be redirected and accelerated by their gravity like a slingshot.
Daring double maneuver
Over the next two nights, JUICE will perform one of these maneuvers, using the combined effect of the Moon and Earth for gravity assist for the first time. No other space agency has ever performed this double maneuver, ESA explains. The first part of this double flyby will take place from August 19 to 20, 2024. After the space probe first enters the radio shadow of the Moon at around 11:15 p.m. our time, it will fly over the lunar surface at an altitude of just 750 kilometers around half an hour later. The probe must fly at exactly the right speed at the right time in the precise direction for the maneuver to succeed. JUICE operations manager Ignacio Tanco from ESA compares this to driving a racing car through a very narrow alley: “We accelerate as much as possible, while there is only millimeters of space on either side of the road,” says Tanco. If this first maneuver is successful, it will accelerate the JUICE spacecraft to around 15,000 kilometers per hour while slightly changing its trajectory.
This diversion of the moon flyby will cause the space probe to next head towards Earth. On the night of August 20-21, it will fly over the Earth’s surface at a distance of around 6,800 kilometers. It will pass the western hemisphere at night and Southeast Asia and the Pacific Ocean during the day. According to ESA, from Europe, JUICE could even be seen as a point of light with good binoculars or a telescope. During this passage of Earth, the Jupiter probe’s trajectory will change a second time and be diverted towards the inner solar system. Given Jupiter’s position in the outer solar system, this initially seems counterproductive, but the maneuver will enable JUICE to head for its next diversion maneuvers – at Venus in 2025, and again at Earth in 2026 and 2029. Together, these loops will give the Jupiter probe the momentum and course that will take it to Jupiter without using much fuel.
Testing, calibration and troubleshooting
However, the two close flybys of the Moon and Earth are also important for the scientific objectives of the JUICE mission. During these maneuvers, the ten scientific instruments on board the spacecraft can be thoroughly tested and calibrated for the first time since its launch. “For some of the instruments, this is the only opportunity to carry out certain test measurements during the eight-year journey to Jupiter,” explains ESA. “This gives scientists and engineers the chance to calibrate their instruments, iron out a few final kinks and perhaps even gain new scientific insights in the process.” One of these instruments is the JANUS camera system, in which the German Aerospace Center (DLR) is also involved. During the flyby, the camera will record the lunar surface with a resolution of up to 13 meters per pixel. The photos covered a narrow strip along the moon’s equator, both on the far side of the moon and on its currently fully illuminated front side. During its flyover, the JNAUS camera will image parts of Madagascar, Thailand, Cambodia, the Philippines and Hawaii.
Also active is the laser altimeter GALA (Ganymede Laser Altimeter), which will initially record elevation profiles of the moons Callisto and Europa upon arrival in the Jupiter system. After entering orbit around Ganymede, the altimeter will create a global topographic map of the Jupiter moon and record the deformation of the moon by Jupiter’s tidal forces. This can help to verify the existence and nature of Ganymede’s subglacial ocean. To test the function of GALA, the laser altimeter will scan the surface of the moon with 30 laser pulses per second during the lunar flyby.
Source: European Space Agency (ESA), German Aerospace Center (DLR)