Deep freeze: Technology to explore Jupiter's icy moons

For space scientists and other aficianados, there can be little doubt that 2014 was the year of the comet; the European Space Agency’s (ESA’s) achievement in landing the Philae probe on comet 67/P Churyumov-Gerasimenko dominated headlines, even though the team still don’t know exactly where the lander is.

By the same token, 2015 is set to be the year of the dwarf planet. Two NASA probes are to greatly expand our knowledge of these enigmatic miniature worlds: the probe Dawn is preparing for its encounter with the largest of the asteroids, Ceres; while in the far distance, New Horizons has emerged from a long hibernation before a close encounter with Pluto. Neither body has been known as anything more than a smudgy image up until now, and solar system specialists are eagerly looking forward to being able to study more detail on these bodies and learn what they can tell us about the formation of our cosmic neighbourhood.

But the nature of Solar System exploration means that, for engineers, the hard work of designing and building these spacecraft was completed many years ago; it takes them so long to complete their journeys that inevitably attention shifts to upcoming missions — at least until the planned encounters actually happen and instruments need to be operated. For NASA and ESA, the next big targets are in the Jovian system of our local gas giant, Jupiter, and its many moons, and the
design targets centred around their exploration.

Jupiter has a pivotal history in our understanding of space and our place in it. In the 17th century, Galileo Galilei’s discovery and subsequent observation of the four largest moons of Jupiter — now known as the Galilean moons — laid the foundations for his theories of planetary orbits: if these bodies circled their planet, he reasoned; it could not be assumed that Earth was the fixed centre of the universe and could conceivably itself be in orbit around the Sun. His writings on the subject earned him house arrest from the Inquisition, but form the basis for cosmology to this day (and his optical telescope remains an inspiration to engineers).

’Galileo Galilei’s discovery and subsequent observation of the four largest moons of Jupiter — now known as the Galilean moons — laid the foundations for his theories of planetary orbits

The moons of Jupiter are still of interest, because they are worlds to themselves with geological activity, atmospheres, weather and, most enticingly of all, water. Although distant from the Sun and so not receiving its heat, they are subject to immense tidal pulls from Jupiter itself, which pummels their cores and heats them from the inside: the combination of heat, water and organic molecules detected in their atmospheres by previous man-made visitors has raised the possibility that they might even harbour life, or at least its vital ingredients; they could therefore give us insight into the development of early life in Earth’s oceans.

The three largest Galilean moons — Ganymede, Callisto and Europa — are the targets for ESA’s next major scientific mission. Called JUICE (JUpiter ICy moon Explorer), this probe is set to leave Earth in 2022 ahead of its arrival in the Jovian system in 2030, eventually going into orbit around Ganymede. Although this might seem to be well into the future, such is the nature of deep-space missions that JUICE is very much active now. Its instrument payload was agreed in 2013 and the multilateral agreement between the 16 countries taking part in the mission — 14 in Europe, plus the US and Japan — was finalised last year.

’JUICE is set to leave Earth in 2022 ahead of its arrival in the Jovian system in 2030, eventually going into orbit around Ganymede

In simple terms, JUICE’s goals are to use the Jovian system as a model for gas giants and for the solar system itself, investigating the conditions for the formation of large multi-body orbiting systems, what conditions might influence the development of life and whether gas giants might be suitable locations for habitable worlds. As with most Solar System missions, this will expand our knowledge of the history of our own world and inform exploration of deep space, with the study of exoplanets and their neighbourhoods expected to accelerate and deepen in the next decades with the launch of more powerful telescopes both in space (such as the James Webb Telescope) and on Earth (such as the European Extremely Large Telescope in Chile and the planned 30m Telescope in Hawaii).

The JUICE spacecraft will use chemical rockets (carrying about three tonnes of fuel for the 25 planetary flybys, orbital insertions and gravitational assistance manoeuvres it will make during its three-and-a-half years in the Jovian system); a high gain antenna more than 3m across to handle transmission of up to 1.4GB of data back to Earth; and solar panels with an area of around 60–75m2 to maximise the amount of power from the dim sunlight; there is also a possibility of capturing electrons from surrounding space to generate current.

JUICE’s payload includes imaging and spectroscopy equipment working from the ultraviolet to sub-millimetre wavelengths, which will study clouds and characterise ice and minerals on the moons’ surfaces; a laser altimeter and a radar sounder to explore the surface of Ganymede, and how the tidal forces affect it; magnetometers to look at how Jupiter’s immense magnetic field affects the moons, and how it interacts with Ganymede’s own field (this will also study the subsurface oceans believed to exist on the moons); plasma instruments; and gravity field sensors.

One thing JUICE won’t do is send anything down to the surface of the moons, but this is an aspiration for future missions. In fact, the teams that built instruments for Philae are waiting with barely restrained enthusiasm to come up with ideas for landers to explore these strange worlds. ‘It’s a fair assumption that these exploratory missions will pave the way for landers, and these moons are likely to be fantastic places to explore; like nothing we’ve ever seen before,’ said Dan Andrews, one of the team at the Open University responsible for Philae’s mass spectrometer, Ptolemy.

In fact, by the time exploratory missions come along, they may be like something we’ve seen before. The asteroid Ceres is an icy planetoid and may also have liquid water under its ice crust. Moreover, we do of course have experience of exploring water environments below ice, where stringent conditions of cleanliness have to be observed: in recent expeditions to drill into lakes in the Antarctic.

So enticing is the prospect of exploring the moons of our gas giant neighbours that several groups are working on concepts for missions to the Jovian satellites and even the more distant moons of Saturn.

Drill systems for icy worlds are among the best-developed ideas. These might be placed in the nose of a ‘penetrator’, such as those that were planned to crash into the Moon’s surface in the UK’s MoonLITE mission. Ice penetrators have advanced as far as trials, carried out last year by UCL’s Mullard Space Laboratory at Pendine Sands in Wales, where 20kg steel shells were fired at a 10-tonne cube of ice along a rocket track used by Qinetiq to test missiles. The penetrator and
its internal components survived the impact.

A penetrator has the advantage of simplicity over a ‘soft-lander’; it doesn’t have to be slowed down as it drops to the surface, so doesn’t need to carry ablative heatshields, parachutes or airbags. In the case of ice, this is a double advantage, as sinking into the surface helps to get drilling started. On Europa, the upper levels of ice are thought to be effectively sterilised by solar radiation and particle bombardment, so investigation has to take place at depth. Europa’s ice could be anything from a few to tens of kilometres deep; there could be a slushy layer before liquid begins, and nobody has any idea how deep the water might be.

’A penetrator has the advantage of simplicity over a ‘soft-lander’; it doesn’t have to be slowed down as it drops to the surface, so doesn’t need to carry ablative heatshields, parachutes or airbags

This effectively limits the ambition of a drilling project, because of the need for cabling to connect the drill and its associated instruments to the surface; it would quickly become too heavy to be practical for spaceflight. The same problem, unfortunately, would almost certainly put paid to any ideas of sending submersibles to explore extraterrestrial oceans.

Drills for use on icy moons would have to combine thermal and mechanical operation, to melt through the ice and clear away rocky material embedded in it. Antarctic drills generally work by melting ice to generate a ‘reservoir ‘ of working liquid, which is then heated and pumped to the tip of the drill; this ensures that no foreign substances are introduced to the ice, as it is essentially used to melt itself.

But other moons have no ice to drill through, instead having standing liquid on the surface. One idea to explore such a world is the Titan Explorer (TiME), a lander for Titan that is designed to ‘splash down’ in one of the methane lakes that have been imaged on the moon’s surface and to bob around in the liquid hydrocarbon, taking measurements, looking for evidence of microbial life and even exploring the shoreline.

TiME has already been rejected once as a proposed NASA mission in favour of the InSight Mars probe, which is to launch next year with a lander to study the Red Planet’s subsurface geology, possible plate tectonics and core; but its designers, including NASA chief scientist Ellen Stofan and principal investigator Ralph Lorenz of Johns Hopkins University in Maryland, are keeping the concept alive for future mission proposals. TiME would not have a propulsion system, but would need non-solar power, as Titan’s clouds are so thick that there’s no chance of the Sun penetrating them. The craft would have to be equipped with a nuclear battery, which generates electricity using the difference in temperature between the ambient conditions (very cold on Titan) and a lump of plutonium oxide.

With NASA’s budgetary constraints, TiME is unlikely to be practical before about 2040, as is Europe’s proposed equivalent, Talise, which would be equipped with rotating paddles to navigate Titan’s lakes. Spanish company Sener is investigating propulsion systems.

Another option is to fly; Titan’s dense atmosphere and low gravity mean flying would be easier and require less energy than on Earth. One concept from a Franco-US team proposes a prop-driven UAV, Aerial Vehicle for In-situ and Airborne Titan Reconnaisance (Aviatr), to investigate the landscapes on the moon. The team envisage AVIATR as part of a vessel including a space vehicle, entry vehicle, and an aircraft weighing 20–100kg and powered by a nuclear battery, which would cruise Titan’s skies surveying its surface and sampling its atmosphere. Such a craft would not require significant development beyond current autonomous UAV technology and would therefore be a relatively cheap way of surveying large parts of Titan’s surface. One goal could be to locate optimal sites for future landers such as the ones described above