January 29, 2023

Exploring Jupiter’s Icy Moons


PEOPLE here on our humble planet, termed “the pale blue dot” due to the way the Earth looks when seen from space, usually get excited at human space flights time when they involve landings on the Moon or, potentially in preparation for a Mars landing sometime in the near future. Even moon landings, however,  soon ceased to electrify live audiences in the US and elsewhere when, just a couple of lunar landings after Neil Armstrong’s historic maiden landing on the Apollo 11 mission, they began to be viewed as rather routine, with even TV networks not relaying live telecasts! This rapid dwindling of public enthusiasm was a major reason for the cessation of manned lunar landings by NASA in the 1970s and the ensuing large budget cuts in NASA’s manned space flights, including to the International Space Station. 

At least partly due to this, the world’s biggest space agency NASA had been concentrating on robotic, that is, unmanned scientific exploration of outer space. The USA’s big space rival, the Soviet Union and its successor state, Russia, had anyway not pursued human landings on the Moon and were also, of course, compelled to cut back on space-related activities due to budget constraints. Other space-faring nations, notably China and India, both still on the upswing as regards robotic missions to the moon and Mars, manned space missions and potential lunar landings, are still driven by excitement and national pride , which is sure to last some while after the first few major human space missions happen.

Meanwhile, nations that have the technological capabilities and the financial resources are continuing with space exploration mostly within the solar system but also through missions which wander well beyond, sending back valuable information, photographs and other data which have contributed to evidence-based understanding of our own solar systems and its constituent planets and other bodies, of our own galaxy the milky way, of outer space, of other stars and galaxies, and of various phenomena and ideas about the origin of the universe.

The average person may not be able to fully appreciate the meaning or significance of all the information provided by these deep space missions. However, they continue to be mesmerised by the amazing photographs sent back by the space missions and space-based telescopes such as Hubble, James Webb and others. With effective science communication, more and more people will become more aware of the growing knowledge about our universe, and better informed about how science and technology have helped to generate this knowledge.

In that spirit, this article discusses the latest, and hitherto most ambitious European space mission, long over-shadowed by the exploits of their richer and technologically advanced US allies, to explore the icy moons of Jupiter, the largest planet in our solar system.


The European Space Agency’s (ESA) grand mission to explore Jupiter and especially its three main icy moons containing sub-surface oceans, namely Europa, Ganymede and Callisto, is now ready with the JUICE (Jupiter’s Icy moons Explorer) spacecraft fully tested, packed awaiting shipment any day from ESA’s main contractor, Airbus’s Toulouse base in France, to the launch pad in ESA’s spaceport in French Guiana in South America. The launch is expected someday in April this year, on board ESA’s workhorse Ariane-5 rocket, the last mission of this veteran rocket of numerous missions and satellite launches, after which it will be replaced by the Ariane-6.

But don’t hold your breath expecting to see photos of Jupiter soon after that. The journey to Jupiter will take eight long years, with the Juice spacecraft expected to reach Jupiter in 2031, conducting various flyby’s and other tasks around the giant planet and its three moons, and finally settling down to orbit around Ganymede in 2034.

The Juice spacecraft will reach the Jovian system by traveling through space not in a straight line as some may expect, but using a series of orbits and gravity assists from different planets and moons. Gravity assists are when the spacecraft comes close to a planet or moon and uses the latter’s gravitational forces to fling it back out into space at greater velocity. In the case of Juice, there would be a flyby of and gravity assist by the Earth-Moon system in August 2024, Venus in August 2025, Earth again in September 2026 and January 2029. When Juice arrives at Jupiter it will first adopt an elliptical orbit then gradually descend to a circular one, then conduct a flyby of Europa in 2032 in a highly elliptical orbit allowing study of Jupiter’s polar regions and its powerful magnetosphere.  In 2034 it will enter into elliptical orbit of 5000km around Ganymede, becoming the first spacecraft to orbit a moon other than our own, then descent to a circular orbit of 500km in 2035. When the spacecraft runs out of propellant in late 2035, it is proposed to allow it to crash into Ganymede.      


Jupiter is a gas giant, at the far cold outer reaches of our solar system, extremely far away from the Sun, at 770 million km compared to Earth’s 147 million km. Jupiter therefore receives only one-twenty-fifth of the sunlight reaching Earth. However, the gravitational forces between the giant planet and its large moons, and the resultant pushes and pulls, are believed to impart considerable energy such that its moons are warmed up sufficiently to allow these three moons to have oceans below their icy surfaces. NASA’s early Galileo probe (1989-2003), the first to orbit Jupiter, gave possible evidence of sub-surface salty oceans in Europa, apart from sending back maps of the four large moons, and information about Jupiter’s powerful magnetic fields and its intense radiation belts, which JUICE has been designed to withstand. Subsequent missions have confirmed these data. Ganymede, Europa and Callisto are all believed to have sub-surface oceans. Europa’s oceans are believed to hold half the water of all Earth’s oceans combined, but the intense radiation from Jupiter makes it a dangerous region for orbit, hence Ganymede was selected instead. The fourth of the set of “Galilean moons,” (more about this term later), Io is not believed to have oceans and instead has active volcanoes. Previous Missions have also raised the probability of these three moons of Jupiter having solid rock and iron cores beneath the oceans.

All these conditions lead to the belief that conditions exist for the possible existence of life on one of these Jovian moons. Water, the right degree of warmth, and organic matter, are known to be three conditions under which life may possibly be created and exist. And this is one of the main objectives of the JUICE mission.

JUICE will conduct detailed observations of Ganymede while in orbit, and supplement this data by data from Callisto and Europa gathered during the many flyby’s performed by JUICE.

For Ganymede, and to a lesser extent for Callisto, JUICE will study characteristics of the ocean layers and possible water reservoirs, mapping and composition of the surface,  physical properties of the icy crusts, internal mass distribution and dynamics of the interiors, Ganymede’s tenuous atmosphere, and study of Ganymede’s magnetic field and its interaction with Jupiter’s magnetosphere. Focus of study of Europa will be on its chemistry, especially of aspects relevant to life including organic molecules, the non water-ice materials in its crust and thickness of its crust.


It is no surprise that the JUICE spacecraft carries a plaque in honour of Galileo, which replicates several pages of his famous manuscript, Sidereus Nuncius in which he describes his pathbreaking observations of Jupiter’s moons. Galileo had fashioned his own version of a “spyglass” which made distant objects appear closer that was believed to have originated from the Arab lands, and used it to study Jupiter and nearby bodies. In early 1609, he discovered what he first thought were small stars but which further study showed to be four moons orbiting Jupiter, which were henceforth termed the Galilean moons. Galileo also studied Venus and came to the conclusion that Venus too, like earth, revolved around the Sun. These findings shattered many ancient myths and beliefs current for over a thousand years, namely that the Earth was the centre of the universe and therefore all celestial bodies moved around our planet.                  

The JUICE mission promises to unravel many more mysteries of Jupiter and its icy moons, which would have surprised and pleased Galileo himself. But we will have to wait for 2032 and a few years thereafter to find out!

(Courtesy: NASA/JPL)