Cracking The Ice Europa Clipper’s Quest for Extraterrestrial Life

By Paul Fisher

On the 7th of January 1610, an Italian mathematician pointed his home-made telescope at Jupiter and noticed three bright points of light near the planet. He thought they were background stars, but over a period of days a fourth point appeared and all four kept changing position relative to the planet. Galileo Galilei had discovered the four large moons of Jupiter and correctly deduced that they were in fact orbiting the planet.

Fast forward 413 years and humankind is on its way to the most indepth exploration of those moon ever attempted. From its spaceport on the coast of French Guiana, the European Space Agency (ESA) launched an Ariane 5 rocket carrying JUICE – the Jupiter Icy Moons Explorer. Over eight years, JUICE will traverse the Solar System, arriving in orbit around Jupiter in 2031.

The Moons

Jupiter has many moons – 95 at last count – but four of them stand out in terms of size and mass. These are the Galilean moons –Ganymede, Callisto, Europa, and Io, which are visible from Earth through a pair of binoculars.

The orbits of Io, Europa, and Ganymede form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes… The eccentricity of their orbits causes regular flexing of the three moons’ shapes, with Jupiter’s gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. The friction created by this tidal flexing generates heat in the interior of the moon.

Io is the innermost of the large moons and at around 3,600 km diameter is about the same size as Earth’s Moon. The gravitational flexing of its shape generates extreme heat in the interior of Io, and consequent volcanic activity has left the surface of the moon peppered with craters and covered with sulphurous deposits. With its volcanic nature and extreme radiation, Io is a most unlikely home for life as we know it.

On the other hand, Europa has long been considered a potential haven for life. Slightly smaller than Io, Europa has an icy surface covered in fissures and ridges. The surface is largely devoid of craters, indicating that the ice layer is constantly being resurfaced. While the inner parts of Europa are thought to be rocky, there is evidence of a sub-surface water ocean up to 100 km deep. Images taken by the Hubble Space Telescope in 2014 and 2016 indicate plumes of water (geysers) up to 100 km high. This combination of liquid water and thermal heating makes Europa an exciting candidate for some form of life.

Ganymede is the largest moon in the Solar System – with a diameter of 5,200km it is larger than the planet Mercury. Ganymede has a magnetosphere (the only moon to have one) which indicates the presence of a liquid iron core. The bulk of the planet is silicate rock and water ice, with the possibility of a substantial sub-surface ocean.

Callisto is the outermost of the Galilean moons, about the same size as Mercury. Callisto is the least dense of the Galilean moons and is composed of equal parts rock and ice. Like Ganymede and Europa, Callisto is thought to have a subsurface ocean. However, Callisto does not share orbital resonance with the other moons and therefore does not have the same amount of internal heating.

The Science

Ever since Galileo’s discovery 413 years ago, astronomers have pondered the nature of Jupiter’s moons. With improving telescopes, we have begun to understand the size of the moons and a little bit about their makeup, but it’s only by sending space probes that we have gained a more in depth understanding. Starting with Pioneer 10 (launched in 1972), a series of increasingly complex missions have flown past Jupiter, with the Galileo and Juno probes orbiting the planet.

Juice carries many instruments, allowing it to study Jupiter, its atmosphere and magnetosphere. It will study the four major moons and many smaller moons, gaining an insight into the composition of the moons, their surface and internal features, and the presence of subsurface oceans. Detailed mapping of each moon will use visible light, radar, ultraviolet sensors, and other techniques.

Ultimately, Juice will provide data that will allow scientists to determine if any of the moons is in fact a potential haven for life.

The Spacecraft Size and weight

The JUICE spacecraft is a large device with a central cuboid “main bus” to which are attached solar panels and various instrument booms and antennae. Folded up inside the Ariane 5 fairing, JUICE had dimensions of 4.09 x 2.86 x 4.35 m. But when fully deployed, it expanded to 16.8 x 27.1 x 13.7 m.

The dry mass (without fuel) is 2,420kg. The craft carries some 3,650kg of propellant – an unusually large amount for a probe of this type but required for the complex series of orbital manoeuvres planned once it reaches Jupiter.

Main bus

The main body of the spacecraft is roughly cuboidal, and supports the engines, reaction wheels, power supply, computers and science instruments. It is also the point of attachment for the solar panels and instrument booms.

Instruments

JUICE carries many instruments to support its science mission. These include magnetometers, radio and plasma wave investigation instruments, and Langmuir probes to analyse the plasma surrounding Jupiter. In addition, there will be cameras, spectroscopes, and a laser altimeter capable of measuring the surface elevation of Ganymede to an accuracy of 10cm.

Perhaps the most exciting instrument is the RIME (Radar for Ice Moons Exploration) ground-penetrating radar, which will search for liquid water under the surface of the moons. RIME uses a 16-metre-long antenna, which was folded down for launch. Unfortunately, unfolding this antenna initially proved problematic but after nearly a month of trying, the antenna has been successfully deployed. The scientific instruments are mounted on the main bus and on four x three metre booms and a 10.6 metre boom.

Power Supply

It takes a lot of energy to keep such a large and complex spacecraft operating in the cold depths of space. Sunlight at Jupiter has only 4% of the intensity of that at Earth. Consequently, JUICE has been equipped with the largest solar panels ever fitted to a planetary probe: ten panels with a total area of 85m2. The panels will generate a total of 850 Watts of electricity – just enough to run a small microwave oven. There is also a large backup battery to keep all systems going when JUICE is behind Jupiter with no access to sunlight.

Communications

To send scientific data and to receive instructions and operating procedures, JUICE has two large communications antennae – a 2.5m high gain dish and a smaller steerable dish. The high gain dish will also act as a heat shield during JUICE’s flypast of Venus. Juice will download more than 2Gb/day, with the deep space tracking station at New Norcia, Western Australia being a key link in the communications chain.

Propulsion and Navigation

Juice carries over 3.6 tonnes of propellant, feeding a main engine developing 425 newtons of thrust. This will provide adequate delta-V for the orbital insertion at Jupiter and manoeuvres around the Jovian system. The main engine uses hypergolic fuel, which ignites spontaneously when exposed to the oxidiser.

Pointing and attitude control will be achieved by a set of reaction wheels – heavy flywheels set on three axes. By spinning up the wheels in turn or causing them to brake, the whole spacecraft can be made to rotate about its centre of mass and point in any desired direction.

The Trajectory

The road to Jupiter is a long one, and spacecraft travelling to that distant planet must fight their way of the Sun and Earth’s gravity wells. A direct trajectory is extremely inefficient, so Juice will follow a complex path around the inner Solar System, swinging past Earth and the Moon, in towards Venus, then back to Earth, and finally onto its final long leg to Jupiter. At each of these flybys Juice will steal momentum from Earth, Moon, and Venus, giving it sufficient kinetic energy to fly out to Jupiter.

The trip to Jupiter, with its many planetary gravity assists, will take eight years, with Juice finally reaching Jupiter in 2031. On the way it will traverse the asteroid belt twice and is scheduled to pass close to asteroid 223 Rosa in 2029.

On arrival at Jupiter, Juice must shed momentum, so that it can enter orbit around the planet and its moons. Initially it will have a highly elliptical orbit around Jupiter, repeatedly firing its engine to slow down and circularise the orbit. This phase will include multiple gravity assists from Ganymede. It will then circle Jupiter, flying by the large inner moons and many smaller ones as well. Eventually, Juice will transfer its orbit from Jupiter to Ganymede, where it will remain forever.

Image Credit: M. Kornmesser/ESO.