Europe’s Juice Mission Launches to Jupiter and Its Moons

Jupiter, king of the solar system, will be getting a new robotic visitor.

The Jupiter Icy Moons Explorer, or Juice, launched on Friday morning from the Guiana Space Center in Kourou, French Guiana, on the northeastern coast of South America. The original launch, scheduled for Thursday, was delayed after lightning was detected in the vicinity of the launch site.

On Friday the weather improved and the spacecraft, aboard an Ariane 5 rocket, lifted off flawlessly. A half-hour later, Juice separated from the rocket’s second stage and embarked on its long journey.

Jupiter, the largest planet orbiting the sun, is fascinating unto itself, but its massive moons are the ultimate prize. Some of them are hunks of icy rock that may hide life-harboring oceans beneath their surfaces. Juice, from the European Space Agency or ESA, aims to closely study three of Jupiter’s satellites: Callisto, Europa and Ganymede.

“This is one of the most exciting missions we have ever flown in the solar system, by far the most complex” said Josef Aschbacher, the head of ESA.

Weighing in at six tons, the European spacecraft carries 10 advanced scientific instruments to study the moons and capture images. Jupiter is not the mission’s primary target. Instead, it aims to probe Ganymede, the largest moon in the solar system, and two other moons, Europa and Callisto.

But reaching Jupiter will take Juice more than eight years, with a series of swings or gravitational assists past Venus, Mars and Earth to give the spacecraft the push it will need to enter Jupiter’s orbit in July 2031.

When Juice at last reaches Jupiter, it will repeatedly fly past the three moons on a looping orbit, staying outside the giant planet’s dangerous radiation belts as it gathers data. In total, 35 flybys are planned as the spacecraft searches for magnetic signals and other evidence to confirm the presence and size of oceans sloshing under the moons’ surfaces. It will also track how the exteriors of the moons move in response to Jupiter’s gravitational pull, possibly influenced by the subsurface oceans.

The moon that may be most promising in the search for life is Europa. Astronomers think its ocean is directly in contact with a rocky floor, which could provide food and energy for life as hydrothermal vents burst upward. Juice will perform two flybys of Europa.

The spacecraft will also perform 21 flybys of Callisto, which may also possess a salty ocean but is thought to be less capable of supporting life.

But the Juice mission’s primary objective is the study of Ganymede, a moon so large it is bigger than the planet Mercury. The spacecraft’s path around the Jovian system should allow the spacecraft to be captured into orbit around Ganymede in December 2034 — the first spacecraft to orbit a moon in the outer solar system. Beginning at about 3,100 miles above the surface, the spacecraft’s altitude will gradually be lowered to just over 300 miles in 2035 — and perhaps lower, fuel permitting.

“If we have enough propellant, which means we had a good trip to Jupiter without too many problems, we will reduce the orbit to” an altitude of about 150 miles, said Giuseppe Sarri, the project manager for Juice at ESA.

Orbiting Ganymede will allow scientists to intricately understand the moon’s characteristics. It is the only moon in the solar system known to have its own magnetic field, possibly from a liquid iron core like our own planet’s. “If you’re standing on the surface of Ganymede and you had a compass needle, it will point to the north pole like on Earth,” said Michele Dougherty from Imperial College London, who leads the magnetometer instrument on Juice. “We want to understand why.”

Juice should be able to discern the interior structure of Ganymede, including the size and extent of its ocean. It should even be able to measure the salt content of the ocean resulting from minerals that circulate within, which could provide life with sustenance. “We’re trying to understand where the salts came from,” Dr. Dougherty noted.

Ganymede’s ocean differs significantly from Europa’s, but it may still be habitable.

“For habitability you need liquid water, a heat source and organic materials,” Dr. Dougherty said. “If we confirm or deny those three things, we’ve done what we said we were going to do.”

The mission will end in late 2035 with a crash landing onto Ganymede’s surface, unless a discovery is made during the mission that suggests this might contaminate the moon’s ocean.

What other missions will study Jupiter?

Juice is not the only mission investigating Jupiter and its moons.

Juno, a NASA mission, has orbited Jupiter since 2016. Its focus has been the planet itself rather than its moons, although it has recently completed some close flybys of Europa and Ganymede, and soon will swoop past volcanic Io.

But Juice is also expected to be beaten to Jupiter by another new NASA mission, Europa Clipper, which is launching in October 2024. It is scheduled to arrive at the Jovian system in April 2030, owing to its more powerful launch vehicle, a SpaceX Falcon Heavy rocket. But there is no competition; the two missions are intended to work together.

“There will be two spacecraft at the same time looking at Jupiter and its moons,” Dr. Aschbacher said. “There’s a lot of science to be gained from that.”

The two missions were born in 2008 in response to exciting results from NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003.

“Galileo found this very intriguing magnetic signal that suggested there was a conductive ice layer beneath the shell of Europa,” said Louise Prockter of the Johns Hopkins Applied Physics Laboratory, who is part of the Europa Clipper team.

Scientists now think that was a sign of a global ocean encompassing Europa’s interior.

Observations by the Hubble Space Telescope in 2018 suggest Europa may occasionally spurt its ocean up in plumes through cracks in its icy shell, at least 10 miles thick. This could provide a novel way to directly study the ocean and look for signs of life as Clipper darts over the moon’s surface, sometimes at an altitude as low as about 15 miles.

“We could potentially fly through a plume,” Dr. Prockter said.

The results of both Juice and Clipper will reveal whether to attempt a landing on a moon of Jupiter on a future mission, likely at Europa, to directly look for life in the ocean, something NASA has proposed. Such a mission could be two decades away, but its scientific value is immense. Dr. Aschbacher said Europe was interested in something similar.

“We have discussed a sample return mission from one of the icy moons,” he said, which would bring materials back to Earth for closer study. “What we learn from Juice will be an extremely important input to that.”

For now, the spotlight is Juice’s, the first of a new era of spacecraft specifically designed to hunt oceans on alien worlds. “I can’t wait,” Dr. Dougherty said. “This is the next step.”