On 17 November 2023, ESA’s Juice probe carried out one of the largest and most important maneuvers on its eight-year journey to Jupiter.
Using its main engine, Juice changed its orbit around the Sun to put itself on the correct trajectory for next summer’s Earth-Moon dual gravity assist – the first of its kind.
The maneuver lasted 43 minutes and burned almost 10% of the spacecraft’s entire fuel reserve. It’s the first part of a two-part maneuver that could mark the last time Juice’s main engine will be used until its arrival in the Jupiter system in 2031.
Mission to Jupiter gains speed
ESA’s Jupiter Icy Moons Explorer (Juice) was launched from the European Spaceport in French Guiana on 14 April 2023.
The mission is to make detailed observations of the giant gas planet and its three large oceanic moons – Ganymede, Callisto and Europa.
But Juice will not begin his investigations into the nature and possible habitability of the Jupiter system until his arrival in 2031.
Why does it take so long to get to Jupiter? Well, the short answer is that it has less to do with the distance between Earth and Jupiter and more to do with fighting the Sun’s enormous gravitational pull as you venture through the Solar System.
Missions to giant gas planets, such as Juice, Europa Clipper, Galileo or Juno, would be little more than gigantic fuel tanks if they had to store all the energy needed to overcome the Sun’s gravity on their own.
Instead, they use “gravity assist” or “flyby” maneuvers to gain energy, swinging through the strong gravitational fields of several planets in their path.
Shooting for the Moon
Juice’s first boost will come from its home planet when it returns to Earth in August 2024, more than a year after launch.
In fact, in an unprecedented flyby, Juice will fly past the Moon first to give it an extra boost and make the Earth flyby that takes place 1.5 days later even more effective.
But even with this extra boost, to make the most of the gravitational assist, Juice has to arrive at the Earth-Moon system at precisely the right time, at the right speed, and traveling in the right direction.
That’s where today’s maneuver comes in.
On November 17, at 4:10 pm (CET), Juice’s main engine suffered a burnout that lasted around 43 minutes.
“This maneuver consumed around 363 kg of fuel – or almost exactly 10% of the 3,650 kg of fuel with which Juice was launched,” says Julia Schwartz, Flight Dynamics Engineer at ESA’s ESOC mission control center in Germany.
This is Juice’s biggest stunt yet. To date, Juice has only used about 10kg of fuel – mostly as part of a series of short burns used to help free his stuck RIME antenna.
“It was the first part of a two-part maneuver to put Juice on the correct trajectory for next summer’s encounter with the Earth and the Moon. This first burn did 95% of the work, changing Juice’s speed by almost 200 m/s ”, adds Julia.
“Juice is one of the heaviest interplanetary spacecraft ever launched, with a total mass of about 6,000 kg, so it took a lot of force and a lot of fuel to achieve this.”
“In a few weeks, after analyzing Juice’s new orbit, we will perform the second, much smaller part of the maneuver. Splitting the maneuver into two parts allows us to use the second engine run to correct any inaccuracies in the first.”
An additional, much smaller maneuver using Juice’s smaller thrusters could be performed in May 2024 for final adjustment during the approach to Earth.
Last use of main engine until 2031
For a mission on an eight-year journey, burning 10% of your fuel reserve in just 43 minutes may seem crazy. But investing all this fuel now will pay off in the coming years.
“If all goes well with both parts of this maneuver, we probably won’t need to use the main engine again until we enter orbit around Jupiter in 2031,” said Ignacio Tanco, Juice spacecraft operations manager. “For small trajectory corrections until then, we will use Juice’s smaller thrusters.”
But that doesn’t mean nothing interesting will happen until Juice arrives at Jupiter. Quite the contrary, the journey is interesting why allows Juice to reach Jupiter without firing its main engine again, reducing the amount of fuel the spacecraft needs and allowing it to be packed with scientific instruments.
After the double Earth-Moon flyby in 2024 (known as the Lunar-Earth Gravity Assist; LEGA), Juice will first do a flyby of Venus in 2025 and two more flybys of Earth in 2026 and 2029 (both without the additional boost from the Moon). .
“Today’s maneuver will ensure that Juice reaches the Earth-Moon system at the right time next year for the double flyby,” adds Ignacio.
“And thanks to the intelligent trajectory designed by our Mission Analysis team, this flyby will align almost perfectly with everyone else, without us having to fire up the main engine again.”
With each flyby, the spacecraft will gain more energy than could be obtained by burning a reasonable amount of fuel – energy that will help it climb toward Jupiter against the pull of the Sun’s gravity.
“It was very important that we carried out this maneuver today. Otherwise, the cost – how much fuel we would need to burn to reach the new orbit we need – would start to skyrocket dramatically,” says Ignacio.
Today’s burn also gave teams the opportunity to ensure Juice’s main engine was working properly. It was first tested shortly after launch, but until today it had not been used for such a large maneuver in deep space.
“There were some things we couldn’t test before. For example, we only had an estimate of how the liquid in the fuel tanks would move as the spacecraft accelerated. It’s very important to know this accurately, because if the fuel behaves differently than we expect, it could cause the spacecraft to veer off course during the burn. So we are monitoring it closely.”
Next stop: Jupiter!
The next time Juice will absolutely have to fire up its main engine will be during its ‘Jupiter Orbit Insertion’ in 2031. This is the most important maneuver that ESOC teams will oversee.
Just 13 hours after passing Ganymede and entering the Jupiter system, the spacecraft will have to slow down by about 1 km/s – five times the change in speed achieved today.
“This makes today’s maneuver also an important test for Jupiter insertion – the sooner we know if we have a problem with the main engine, the better,” says Ignacio.
Once in orbit around the gas giant, Juice can begin exploring the Jupiter system. ESOC teams will lead Juice through a series of 35 flybys of oceanic moons. Where flybys once occurred annually, on Jupiter they will be held once every two weeks.
These close-ups of the icy moons will allow spacecraft and scientists on Earth to gather the data needed to better understand these mysterious alien worlds.
Juice – updated and behind the scenes
Want to know more about how Juice was imagined, designed, built and launched? The feature film, ‘The Making of Juice’, premieres on the ESA YouTube channel at 18:00 CET on 23 November 2023.