Why Is Juice Taking Sooo Long? Straight Lines in Space Are a Massive Waste of Energy

Illustration of JUICE Spacecraft at Jupiter

Illustration of the JUICE spacecraft at Jupiter. Juice’s mission to Jupiter is a testament to intricate space navigation, leveraging gravity assists for efficient travel. Aiming for a 2031 arrival, it seeks to explore Jupiter’s moons and unveil cosmic secrets. Credit: ESA

Juice: Why’s It Taking Sooo Long

At their closest point in orbit, Earth and Jupiter are separated by almost 600 million kilometers (375 million miles). At the time of writing, five months after launch, Juice has already traveled 370 million kilometers (230 million miles), yet in time it’s only 5% of the way there. Why is it taking sooo long?

The answer depends on a variety of factors that flight dynamics experts at ESA’s Mission Control know well, from the amount of fuel used to the power of the rocket, the mass of a spacecraft, and the geometry of the planets.

Based on this, ESA’s flight dynamics experts design a route. The world of orbital mechanics is a counterintuitive place, but with a bit of patience and a lot of planning, it allows us to do a great deal of science with just a little fuel, as we’ll explain.

Jupiter and Europa Hubble 2020

This image of Jupiter was taken by NASA’s Hubble Space Telescope on August 25, 2020. It was captured when the planet was 406 million miles from Earth. Credit: NASA, ESA, STScI, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL team

Straight Lines in Space? Massive Waste of Energy

As such, a spacecraft launched from Earth already has a great deal of ‘orbital energy’ – the only unit that matters when determining the size of an orbit around a central body. Just after launch, a spacecraft is in more or less the same orbit as our planet is around the Sun.

To break free from this orbit and fly in the shortest possible straight line from Earth to Jupiter, would need a big rocket and a lot of fuel. But it can be done. The next problem is, that you’d then need even more fuel to brake and go into orbit around Jupiter and not zip right past it.

Propulsion Orbits Traveling Through Space

The launch of a spacecraft is the start of a new mission and the only means to reach the depths of the Solar System. With reusable rockets becoming a reality, what is ESA doing to advance propulsion technology and make it greener? Credit: ESA/S. Berna

Targeting Empty Space

Jupiter and Earth are always moving with respect to each other. At their farthest apart, on opposite sides of the Sun, they are separated by 968 million kilometers (601 million miles). The shortest distance between the two planets is when Earth and Jupiter are on the same side of the Sun with just under 600 million kilometers (375 million miles) between them. But they’re in this position just for a moment before the distance grows again, never remaining at a constant distance.

The planets are all moving at different rates in their orbits around the Sun. Imagine throwing a ball at a moving target from a moving vehicle. Engineers must calculate the ideal time to make the jump on a circular path from Earth’s orbit to where Jupiter will be when the spacecraft arrives, not where it is when the spacecraft leaves Earth.

So, assuming we have the most powerful launcher available, and we launch on the shortest trajectory at the right time when the planets are aligned correctly, how long would it take?

Early space missions, such as the Voyager and Pioneer probes, made the journey in less than two years, and the fastest any object has traveled to Jupiter was the New Horizons mission. Launched on January 19, 2006, New Horizons made its closest approach to Jupiter on February 28, 2007, taking a little over a year to reach the planet. All these missions continued onwards, excellent examples of determining how long it takes for a Jupiter flyby on the way to somewhere else.


This animation depicts Juice’s journey to Jupiter and highlights from its foreseen tour of the giant planet and its large ocean-bearing moons. It depicts Juice’s journey from leaving Earth’s surface in a launch window of April 5–25, 2023, and performing multiple gravity assist flybys in the inner Solar System, to arrival at Jupiter (July 2031), flybys of the Jovian moons Europa, Callisto, and Ganymede, orbital insertion at Ganymede (December 2034), and eventual impact on this moon’s surface (late 2035). Credit: ESA/Lightcurve Films/R. Andres

The Longer the Stay, the Slower the Approach

To get into orbit around the huge planet to study it from all sides and over time, perhaps even get into orbit around one of its moons – a Juice ‘first’ – you’ll need to lose some energy. This ‘deceleration’ will require a lot of fuel for a large orbit insertion maneuver. If you don’t want to launch with vast amounts of fuel, you take the scenic route, with a transfer duration of 2.5 years.

This is where we see the mass of the spacecraft as a crucial factor in determining the time it takes to get anywhere. Engineers need to control the spacecraft’s mass, balancing the amount of fuel with the instruments it needs to carry to complete its mission. The more mass the spacecraft has, the more fuel it needs to carry, which increases its weight and makes it more difficult to launch.

Which Rocket to Space

Teams at Mission Control match the needs of ESA missions with the perfect rocket. The choice of rocket depends primarily on the mass of the payload and where it needs to go. The further from Earth a spacecraft needs to be lifted, and the more massive it is, the more fuel that is needed. Credit: ESA

And this is where the launching rocket’s performance comes in. The spacecraft needs to be launched with sufficient velocity to escape Earth’s gravity and be flung on its way to the outer Solar System. The better the shove, the easier the trip.

Juice is one of the heaviest interplanetary probes ever launched, at just over 6000 kg, with the largest suite of scientific instruments ever flown to Jupiter. Even the massive boost from the Ariane 5 heavy-lift rocket wasn’t enough to send Juice directly there in a couple of years.

Therefore, missions such as Juice and Europa Clipper, or like Galileo and Juno in the past, have to make use of ‘gravity-assist’ or ‘flyby’ maneuvers to pick up extra speed. The more powerful the rocket, the shorter the transfer.

Trading Energy With the Solar System

Pluto, at the edge of the Solar System, travels in a much larger orbit than Mercury, the innermost planet. Although Pluto moves more slowly with respect to the Sun, its orbital energy is far, far greater than Mercury’s.

To get a spacecraft into orbit around another planet, we must match its orbital energy. When BepiColombo was launched, its orbital energy was the same as Earth’s. It had to lose energy to fall closer to the center of the Solar System and did so by shedding excess orbital energy by flying close to neighboring planets.

Juice's Europa Flyby

During the tour of the Jovian system, Juice will make two flybys of Europa, which has strong evidence for an ocean of liquid water under its icy shell. Juice will look at the moon’s active zones, its surface composition, and geology, search for pockets of liquid water under the surface, and study the plasma environment around Europa. Credit: ESA

The same works in reverse to voyage to the outer Solar System. To get into a larger orbit, farther from the Sun, Juice is on a path that will let it steal orbital energy from Earth, Venus, and Mars.

Depending on the relative direction of motion of the planet and the spacecraft, a gravity assist can either speed up, slow down, or change the direction of the mission. (The spacecraft also deflects the planet, but by such a miniscule amount as to be insignificant. Nonetheless, Newton’s third law of motion has been preserved: ‘To every action, there is an equal and opposite reaction’.)

Juice will use a series of flybys of Earth, the Earth-Moon system, and Venus to set it on course for its July 2031 rendezvous in the Jovian system.

Juice ESA Mission Control

For months, engineers have been flying a fake Juice spacecraft that keeps going wrong. In just a couple of weeks, they fly the real thing. What they are doing now, helps ensure this bold mission’s success. Credit: ESA

Orbit on a Knife’s Edge

The most challenging part for the ESA’s flight control team comes when Juice finally arrives at Jupiter in 2031 and during its tour of Jupiter’s planetary system.

Juice’s challenging trajectory involves multiple gravity assists on the way to Jupiter – including the first ever Lunar-Earth flyby – and, once there, an impressive 35 flybys of its Galilean moons Europa, Ganymede, and Callisto. The final focus will be on Ganymede, making Juice the first spacecraft ever to orbit a moon other than our own.

The single most important maneuver that teams at ESA’s mission control in Germany will oversee, will be the slowing down of Juice by about 1 km/s only 13 hours after a Ganymede gravity assist, and ‘taking the exit’ to enter the Jupiter system, inserting the spacecraft into orbit around the gas giant.


Watch the full sequence of Juice’s journey to, and tour of, Jupiter and its icy moons. Juice will start its science mission about six months prior to entering orbit around Jupiter in 2031, making observations as it approaches its destination. It makes a first flyby of Ganymede a few hours before Jupiter orbit insertion. Once in the Jovian system and in orbit around Jupiter, a series of further gravity-assist flybys Ganymede will help Juice reduce its orbital energy as needed. Credit: ESA/Lightcurve Films/R. Andres

Getting into orbit around another celestial body is hard. A spacecraft must approach with the perfect speed, from a precise angle, then execute a vital, big maneuver at just the right moment, in a specific direction, and of the correct size.

Approaching too fast or slow, too shallow or steep, or maneuvering at the wrong time, with the wrong amount or direction, and you’re lost in space. Or you’re far enough off track that it will take a lot – perhaps too much – fuel to correct your path.

Juice will get close to Jupiter’s moons, trading energy with them that they’ve held onto for billions of years, to get a view of these environments like never before. Could there be life under the frozen oceans of Ganymede, Callisto, or Europa? What can we learn about the formation of planets and moons throughout the Universe? Through the wonder of flight dynamics, by trading energy with the Universe, we will soon(ish) find out.


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