Jupiter, king of the solar system, will receive new visitors. The largest planet orbiting the sun is interesting in itself, but its massive moons are the ultimate prize – some of them are chunks of icy rock that may hide life-supporting oceans beneath their surfaces.

The robotic mission that will depart for Jupiter on Thursday is Juice, or Jupiter Icy Moons Explorer, from the European Space Agency, or ESA, aimed at closely studying three of Jupiter’s satellites: Callisto, Europa and Ganymede.

“This is one of the most exciting missions we’ve ever done in the solar system,” said ESA chief Josef Aschbacher, and “by far the most complex.”

Here’s what you need to know about the Juice mission.

When will the launch take place and how can I watch it?

Juice is set to launch on April 13 at 8:15 a.m. EST. ESA will broadcast the launch live on its website and on its YouTube channel.

The spacecraft will head into space on an Ariane 5 rocket from the Guiana Space Center in Kourou, French Guiana, on the northeast coast of South America. The same type of rocket launched the James Webb Space Telescope from the European-run launch site in December 2021.

What is the Juice mission and what will it investigate?

Weighing six tons, the European spacecraft carries 10 advanced scientific instruments and takes images. Jupiter is not the main target of the mission. Instead, it aims to probe Ganymede, the solar system’s largest moon, 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 boost it needs to enter Jupiter’s orbit in July. 2031.

When Juice finally reaches Jupiter, he will repeatedly fly over all three moons in a looping orbit, staying out of the giant planet’s dangerous radiation belts while he collects data. A total of 35 flybys are planned as the spacecraft searches for magnetic signals and other evidence to confirm the presence and size of oceans lapping beneath the moons’ surfaces. It will also track how the exteriors of the moons move in response to Jupiter’s gravitational pull, possibly influenced by subterranean oceans.

The most promising moon in the search for life is Europa. Astronomers believe its ocean is in direct contact with rocky ground, which could provide food and energy for life when hydrothermal vents erupt upwards. Juice will make two overflights of Europe.

The spacecraft will also perform 21 flybys of Callisto. This moon is believed to be unable to support life in its ocean. Its surface is extremely old and covered in craters, and it appears to lack a solid core that could supply an ocean with the nutrients necessary for life.

“We don’t know why this is the case,” said Michele Dougherty of Imperial College London, who runs the magnetometer instrument on Juice.

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

“If we have enough propellant, which means we have 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 understand the characteristics of the moon in a complex way. It is the only moon in the solar system known to have its own magnetic field, possibly from a core of liquid iron like that of our own planet. “If you were standing on the surface of Ganymede and had a compass needle, it would point to the north pole like on Earth,” Dr Dougherty said. “We want to understand why.”

Juice should be able to discern Ganymede’s interior structure, including the size and extent of its ocean. It should even be able to measure the salt content of the ocean resulting from minerals circulating there, which could provide food for life. “We’re trying to figure out where the salts come from,” Dr. Dougherty noted.

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

“For habitability, you need liquid water, a heat source, and organic materials,” Dr. Dougherty said. “Whether we confirm or deny these three things, we did what we said we were going to do.”

The mission will end at the end of 2035 with a crash landing on the surface of Ganymede, unless a discovery is made during the mission suggesting it could contaminate the moon’s ocean.

What other missions will Jupiter study?

Juice isn’t the only mission investigating Jupiter and its moons.

Juno, a NASA mission, has been orbiting Jupiter since 2016. It has focused on the planet itself rather than its moons, although it has recently made a few close flybys of Europa and Ganymede , and will soon pass in front of the volcanic Io.

But Juice is also expected to be beaten to Jupiter by another new NASA mission, Europa Clipper, which will launch in October 2024. It is expected to arrive at the Jovian system in April 2030, thanks to its more powerful launch vehicle, a SpaceX Falcon. Heavy rocket. But there is no competition; the two missions are meant 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 drawn from this.”

Both missions originated 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 layer of conductive ice beneath Europa’s shell,” said Louise Prockter of the Johns Hopkins Applied Physics Laboratory, part of the Europa Clipper team.

Scientists now believe this was a sign of a global ocean encompassing the interior of Europe.

Observations from the Hubble Space Telescope in 2018 suggest that Europa can occasionally shoot its ocean in plumes through cracks in its icy shell, at least 10 miles thick. It could provide a new way to directly study the ocean and search for signs of life as Clipper flexes on the moon’s surface, sometimes at altitudes as low as around 15 miles.

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

The Juice and Clipper results will reveal whether a Jupiter moon landing should be attempted on a future mission, likely to Europa, to directly search for life in the ocean, which 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’ve been discussing a sample return mission from one of the icy moons,” he said, which would bring material back to Earth for further study. “What we learn from Juice will be a hugely important contribution to that.”

For now, the spotlight is on Juice’s, the first in a new era of spacecraft purpose-built to chase oceans on alien worlds. “I can’t wait,” Dr. Dougherty said. “That’s the next step.”