The journey – from Earth to Jupiter in a roundabout way

In all three mission scenarios, JUICE will be taken into a heliocentric orbit around the Sun directly after launch. This orbit is initially almost circular and is then increasingly stretched into an ellipse by close fly-bys of Earth and Venus. Its furthest point from the Sun will lead to the orbit of Jupiter at the end of the approach phase.

In the Jupiter system, the orbit will be changed in the years 2031 to 2034, to some extent in the opposite direction – from an elongated ellipse around Jupiter to an orbit that increasingly approaches a circular path. Finally, at the end of 2034, the spacecraft will move into an initially elliptical and then circular orbit around Jupiter’s moon Ganymede.

A launch directly into a solar orbit

The JUICE mission is scheduled to begin on 14 April 2023 at 14:14 (CEST) with the launch of an Ariane 5 ECA (flight number VA 260) on the ELA-3 launch pad at the European spaceport Kourou, French Guiana.

At a flight altitude of 70 kilometres, two minutes and 14 seconds after the launch, the solid-fuel boosters attached to the sides of the rocket are separated. They descend on parachutes into the Atlantic Ocean, are recovered and can be reused. After three minutes and nine seconds – at which point the Ariane has reached an altitude of 100 kilometres – the protective fairing around JUICE is opened and jettisoned at an altitude of 116 kilometres.

At an altitude of 219 kilometres, the main stage of the Ariane rocket has used all its propellant. It is separated after eight minutes and 44 seconds and falls into the Atlantic Ocean like the two boosters before it.

Five seconds after the main engine has shut down, the ESC upper stage ignites. Twenty-five minutes and 25 seconds after the launch at an altitude of 1011 kilometres, the ESC upper stage is injected into a heliocentric orbit. Twenty seconds later, the cryogenic upper stage also shuts down and is separated. It burns up in Earth’s atmosphere.

Six minutes later, ESA's Space Operations Centre in Darmstadt takes over the mission and the deployment of the solar panels to generate power is initiated. After various tests of all systems, all antennas and the booms for geophysical experiments are deployed 16 hours after launch.

After launch, JUICE is not placed in an Earth orbit, as is often the case with planetary missions, in order to check all the spacecraft systems and fire the engines again to reach the mission target. This is why the launch has to be precise to the second, because at the equator the Earth’s surface moves 450 metres every second due to the planet’s rotation and the planet travels 30 kilometres along its orbit around the Sun. After leaving the Earth, JUICE is in a solar orbit. There, the spacecraft does not yet have the necessary energy or speed to fly directly to the Jupiter system. For this reason, several close fly-bys of Earth – including both Earth and the Moon on the first occasion – and once of Venus are planned. During these manoeuvres, JUICE will be accelerated without expending propellant so that by January 2029 the spacecraft will then have enough energy to reach Jupiter.

Flyby or gravity assist manoeuvres are spectacular close passes of planets or moons from a physical point of view, but they are routine in spaceflight today. They are used to change the flight direction and speed of spacecraft solely by exploiting the gravitational pull of celestial bodies.

If spacecraft are to leave Earth’s gravitational field and reach a distant target in the Solar System, energy is always required for acceleration and changes in direction on the way to the target – but also for decelerating once there. This can be carried as fuel for rocket engines, but often not in the necessary quantity. This is because the launch vehicle has an upper limit to the mass it can carry into space through Earth’s gravitational field. How much propellant can be transported into space with it for the necessary manoeuvres is also an issue of economics.

An elegant orbital mechanical-technical solution results from close flybys of planets – in a sense, from the natural play of gravitational forces between bodies of different masses. If a spacecraft in the Solar System is moving towards a massive body, from a certain distance onwards the gravitational field of the body prevails over that of the Sun, which otherwise influences all movements.

In a sense, a flyby is the 'juggling' of two forms of energy: the kinetic energy of the spacecraft and the gravitational energy of the planet. With its mass, which is many times greater than the spacecraft's, the planet attracts the small spacecraft when it comes close to it. During this process, depending on how fast the spacecraft is moving and how close it gets to the planet, energy can be transferred from the planet to the spacecraft. The spacecraft then speeds up, and the planet slows down almost imperceptibly. Conversely, kinetic energy can be transferred from the spacecraft to the planet, which slows down the spacecraft and imperceptibly speeds up the planet.

Giuseppe 'Bepi' Colombo's ingenious trajectory calculation

Flyby manoeuvres were used for the first time on the Mariner 10 mission in 1974/75 – at that time along the orbit of Mercury to enable two more close flybys after the first flyby of this planet. The necessary calculations for Mariner 10's orbit were performed by the Italian engineer and mathematician Giuseppe 'Bepi' Colombo. The large European-Japanese Mercury mission was named in his honour – BepiColombo. Launched in 2018, it will make close flybys of Earth, twice of Venus and six times of Mercury to place it precisely in Mercury's orbit, where it will then be placed into orbit around the planet in 2025 with little propellant consumption. This elaborate approach is necessary because the Sun is only approximately 55 million kilometres away.

JUICE accelerates to Earth, Moon and Venus

In the first of JUICE's inner Solar System planetary flybys, the spacecraft will perform a unique flyby of the Earth-Moon system referred to as a Lunar-Earth Gravity Assist (LEGA). The LEGA manoeuvre will take place in August 2024 and will see JUICE fly past both the Moon and, 36 hours later, Earth, using the gravity of both celestial bodies in a single flyby manoeuvre. If the LEGA flyby is successful, JUICE will save a significant amount of fuel. This may give mission teams more options for flybys of Jupiter or a mission extension. The effect depends mainly on the launch date and would be maximum for the 13 April 2023 launch.

On the second flyby, scheduled for August 2025, JUICE will pass Venus and use the planet’s gravity to raise the aphelion, the furthest point from the Sun in its elliptical orbit. At that point, the planet will be only 96 million kilometres from the Sun. Special measures are needed to keep the temperature of the sensitive instruments and all the electronics under control. To do this, JUICE will be rotated so that the 2.5-metre antenna functions as a protective shield.After Venus, there will be two more flybys of Earth, planned for September 2026 and January 2029. The fourth flyby will then raise the altitude of the spacecraft’s aphelion to match that of Jupiter's orbit. This will give the spacecraft a trajectory that reaches all the way to the Jupiter system and, when it arrives there in 2031, will take it so precisely behind Jupiter that the planet’s immense gravitational pull will be intercepted. At the same time – using only a small amount of propellant – JUICE will enter an initially highly elliptical orbit around Jupiter.

If launched in mid-April 2023, JUICE will arrive in July 2031 after a journey of more than eight years. Six months earlier, the first scientific investigations of the Jupiter system will begin. However, the speed of the spacecraft will then still be far too high for it to be captured by Jupiter's gravity in just one targeted approach so that the mission can enter an orbit around the planet. First, it will fly past Ganymede very quickly for the first time at a distance of only 400 kilometres. This flyby will be used to collect scientific data and acquire images. It will also serve as a gravity assist for an initial deceleration manoeuvre.

This is followed by the most critical phase of the entire mission, as all manoeuvres in the inner Solar System will have been carried out without igniting the engine. JUICE will now get closer and closer to Jupiter and will require a two-hour deceleration burn by the engine. The spacecraft passes over Jupiter's cloud cover at an altitude of 808,000 kilometres and first enters a very elongated elliptical orbit with the furthest point of the orbit from Jupiter at 19 million kilometres away from the gas giant. This loop opens up the only opportunities during the mission to observe some of Jupiter's small moons, which orbit the planet at distances of up to 20 million kilometres.

After a second orbit, shortened by two-thirds, JUICE will turn its attention to the outermost Galilean moon, Callisto, after eleven months and four more Ganymede flybys between 400 and 5600 kilometres away in an equatorial phase of the mission. In July 2032, the only two close flybys of the moon Europa will take place, both 400 kilometres above the ice crust – two of the many major highlights of the mission. From the end of July 2032 to November 2033, a phase of intensive exploration of Callisto will follow, from different altitudes and also with variations in the inclination of the trajectory. This will prepare the final mission phase – the transfer from an orbit around Jupiter to an orbit around the moon Ganymede.

Initially, Ganymede will be observed from an elliptical orbit for one month at the end of 2034, then from a circular orbit with an altitude of 5000 kilometres above the moon at the beginning of 2035, which will be lowered to 500 kilometres after three months. If there is still fuel for another orbital manoeuvre, JUICE will descend to 200 kilometres. This orbit can be maintained for a month and would provide images with the highest resolution of the entire mission. Towards the end of 2035, JUICE will finally crash into Ganymede’s icy crust without any remaining propellant. This would mean that the Jupiter mission would have spent just under 1700 days or a good four and a half years in the Jupiter system performing scientific tasks, according to the planning status as of April 2023.