European scientists are in the final stages of preparation for one of the most ambitious space projects ever undertaken, launching a 12-year mission to the outer solar system to investigate whether three of Jupiter’s moons might support life.
The European Space Agency’s €1.6bn Juice spacecraft will launch in April carrying 10 scientific instruments that will use different techniques to observe the Ganymede, Callisto and Europa moons and the space around Jupiter, the largest planet in the solar system.
The mission will examine Jupiter’s complex physical and chemical environment. The planet is a “gas giant” mostly composed of hydrogen and helium that will offer scientists clues about the billions of similar planets and moons believed to exist across the Milky Way galaxy. Juice, short for Jupiter Icy Moons Explorer, will also help scientists assess whether living creatures might thrive in the deep oceans expected to lie beneath the moons’ ice crusts.
“We will be testing for habitability,” said Günther Hasinger, ESA science director, “but Juice does not include an instrument capable of directly detecting biological molecules.” Nor does the complementary Clipper mission that Nasa is due to launch to Europa next year, he added. Including such an instrument — a mass spectrometer — would not have been practical or affordable.
A key issue for habitability is the structure of the moons’ oceans. “If the water is squeezed in between ice on top and ice below, then the conditions for life would be difficult because the nutrients are missing,” Hasinger said. “You need an ocean floor with silicate minerals that can provide nutrients.”
Mission controllers hope to launch the 6-tonne Juice craft on an Ariane-5 rocket from the European spaceport in French Guiana on April 14, when the alignment of planets will minimise the energy needed to reach its destination. The launch window extends to the end of April.
“Ariane-5 is a very powerful rocket but it can only give us about half the energy we need to get to Jupiter,” said Justin Byrne, head of science for Airbus, the mission’s lead contractor. “We get the rest by doing planetary fly-bys, each one giving us a gravitational assist through a slingshot manoeuvre.”
The “fly-bys” entail Juice flying past Earth three times and Venus once, on a 6.6bn-km journey lasting more than eight years, before going into orbit around Jupiter in July 2031. It will then spend three years exploring Ganymede, Callisto and Europa in a series of fly-bys before the final phase of the mission — orbiting Ganymede for a few more months at altitudes between 200km and 500km from the moon’s surface, depending on how much fuel is left, then crashing on to Ganymede in late 2035.
En route, Juice and its instruments will have to survive more extreme swings in temperature than any previous spacecraft — from 250C during the Venus fly-by down to -230C on the side of Jupiter furthest from the sun
Another hazard is the intense radiation in Jupiter’s magnetic field, which is 20,000 stronger than Earth’s, said Byrne. “We’ve had to put all the sensitive electronics inside a lead-lined vault in the spacecraft. Otherwise the radiation would kill the electronics.”
When Juice reaches Jupiter, a carefully choreographed series of observations will begin. “Each fly-by requires extremely intricate planning of which instrument is switched on when,” said Hasinger. “They have to work together to share the limited data volume.”
“Some are very sensitive to internal electromagnetic disturbances, so while they are working you sometimes have to switch off the other instruments ,” he added. “They are performing like an orchestra, not always playing at the same time but making a symphony together.”
Michele Dougherty, head of physics at Imperial College London, is responsible for a particularly sensitive instrument, the magnetometer, which is placed on a boom 10.6 metres long to isolate it from electromagnetic interference.
“ESA and Airbus have built the cleanest spacecraft I have known,” she said, “so I am not concerned about any magnetic interference from the spacecraft while we take our measurements.”
As well as measuring the environment around Ganymede — the only moon known to generate its own magnetic field from an internal dynamo mechanism — Dougherty hopes the magnetometer will provide vital information about the way water circulates around Ganymede’s hidden ocean.
“It’s a really difficult task because we will have to detect very weak magnetic signals from currents flowing in the ocean,” she said. “They arise because the water contains salts which make it electrically conducting.”
Dougherty has been involved in Juice from the earliest stages when she was part of the team that defined the mission in 2008. Its first close-up observations of Jupiter’s moons are expected to be transmitted to scientists 24 years later, early in 2032.
“It has been a very long road,” she said. “You need a lot of patience to take part in missions to the outer planets.”