NASA and ESA have jointly announced their plan to send the next big joint planetary exploration mission to Europa and Ganymede, two of the four planet-sized moons of Jupiter. The decision follows years of anticipation in the planetary science community, where the last such big decision was made back in 1988 when NASA and ESA agreed to work together on the Cassini-Huygens mission to Saturn and Titan. That completed its primary mission phase in 2008 and is now in the extended mission, still in orbit around Saturn.

The decision this time came down to a choice between two mission concepts. The plan that was not picked was another mission to the Saturn-Titan system, which capitalized on the momentum gained through the huge and continuing success achieved by Cassini-Huygens. The anticipation among planetary scientists was evident at formal meetings and in coffee rooms, where it has been a major topic of conversation for months. Even Nature weighed in last month and ran a two-page special report and an editorial on the subject, giving their push to the Saturn-Titan mission, arguing that the technological breakthroughs planned for the mission, including a hot-air balloon to float in the sky of Titan, will further open up frontiers of space exploration.

The final pick by NASA and ESA, called the Europa Jupiter System Mission (EJSM), involves two spacecrafts launched separately by NASA and ESA. The plan calls for sending NASA’s orbiter to Europa, and ESA’s to Ganymede. The probes are, for now, called Jupiter Europa Orbiter (JEO) and Jupiter Ganymede Orbiter (JGO). The decision was based on, among many factors, the maturity of the mission idea. NASA has been studying mission concepts for Europa since the late 1990s, the first of which was the Europa Orbiter, developed under the Faster, Better, Cheaper strategy but subsequently canceled after a series of FBC-mission failures.

Visiting the moons

Next came the Jupiter Icy Moons Orbiter. JIMO was part of Project Prometheus, which aimed to launch a series of probes powered by nuclear reactors; however, in 2005, the plan was deemed too ambitious and the entire project was frozen indefinitely. Up to now, all missions to the outer planets have employed thermocouples driven by the heat generated by blocks of radioactive isotopes, as the dim sun makes solar panels an ineffective source of power in the outer solar system, but we've never sent a full-blown reactor.

The mission plan started converging toward the current EJSM design through the Europa Geophysical Explorer (EGE) plan in 2005 and the Europa Explorer (EE) in 2006. Both EGE and EE design concepts included an optional plan to land on the surface of Europa. Although the current design for NASA’s JEO seems to be directly based on the EE concept, JEO does not include any lander.

Europa makes an exciting scientific target because it is believed to harbor a deep ocean underneath a relatively thin (i.e., tens of kilometers thick) shell of ice at the surface. Its craterless surface suggests that it is refreshed by geological processes, most likely through convection in the ice shell, and scientists are seriously studying the potential for life in the vast salty ocean under the shell. The potential for life was recognized soon after the Voyager flybys of the Jovian system in 1979 returned close-up pictures of Europa, which generated a huge amount of excitement in the field and inspired science fiction works such as 2010: Odyssey Two, which featured alien organisms crawling out of icy crevasses on the moon’s surface.

Ganymede, the target of ESA’s JGO, is the largest satellite in our solar system, with a radius that is bigger than that of the planet Mercury. Like Europa, it is also believed to harbor a deep ocean underneath its icy crust, except that its ice shell is estimated to be much thicker than that of Europa. The surface exhibits a mixture of old, highly cratered terrain and relatively younger surfaces, suggesting that geological activities at least partially renewed its surface in its past.

The moon also has characteristic grooved terrains, which is believed to be “wrinkles” formed when the size of the moon changed as the inner ocean partially froze and thawed in the past—when the liquid water ocean freezes, its volume expands, and the radius of the moon increases; the radius shrinks when the ice thaws. Thus, the surface geological features carry important hints to the thermal history of the moon.

What to watch for

Both JEO and JGO will carry ice-penetrating radars to peer through the surface. The radar on JEO is designed to be sensitive to ice layers at 3-30km depth, thus enabling it to detect the boundary between the ice shell and the underlying ocean, at least where the ice shell is at its thinnest. As Ganymede’s ice shell is much thicker than that of Europa, the radar onboard JGO is not designed to sense the solid-liquid boundary but instead it will search for warm chunks of ice underneath the surface, which would provide hints of geologically actively spots.

Both spacecraft will also carry laser altimeters to observe how the shape of the satellites deform as they orbit in the strong gravitational field of Jupiter. And of course, they will carry a suite of imaging instruments that will take photographs at many different wavelengths of light.� These cameras will survey not only the moons the probes orbit around but the entire Jovian system.� In addition to JEO and JGO, the Japanese space agency JAXA is considering pitching in by launching its own orbiter, to be called Jupiter Magnetospheric Orbiter, which will take three-dimensional measurements of Jupiter’s extensive magnetosphere.

The new mission to the Jupiter system will be the latest addition to the short list of "flagship" missions NASA has launched to the outer planets. The flagship-class missions are quite ambitious, typically costing several billion dollars (JEO is currently projected to cost NASA about $3.8 billion, while JGO will cost ESA �650 million). Since the 1970s, there has been about one flagship mission to the outer planets every decade: Pioneer 10 and 11 visited Jupiter and Saturn in mid-1970s; Voyager 1 and 2 toured the outer solar system in the 1980s and continue to explore the outer fringe of the solar system today; Galileo was in orbit around Jupiter from 1994-2003; and Cassini has been in Saturnian orbit since 2004. The launches of the EJSM spacecrafts are currently planned for 2020 and should arrive at the Jupiter system in 2026.

As for the Saturn-Titan mission concept that was not picked this time, none of the efforts that went into forming the mission will be wasted. NASA is forwarding the results of the mission-design studies to the National Research Council, which is conducting the Planetary Science Decadal Survey that will recommend a broad plan of solar system exploration for the next decade, 2013-2022—the report is due in March 2011. The exploration of Europa was the single top flagship priority listed by the previous decadal survey released in 2002. The well-defined Saturn-Titan mission concept is sure to get the attention of the new survey, and hopefully the mission will get a green light in the next round, probably in 10 years or so.

These large flagship missions have the potential to define the careers of many planetary scientists. For example, I am a recent PhD specializing in the atmospheres of Jupiter and Saturn, and I will be in my late 40s if JEO and JGO arrive at their destinations as planned in 2026. If I am privileged to stay in the field until then, the data they return will surely keep me busy for the rest of my scientific career, and I am hopeful that the Saturn-Titan mission will also materialize before i retire.

In the meantime, the development and construction of the spacecraft hardware will soon begin, and scientists will continue to study the Jupiter systems using existing data to predict what the mission will discover, and to better refine the requirements for the instruments that will be onboard JEO and JGO in order to ensure that their capabilities are optimally tuned for what they will observe. There’s tons of work to be done ahead of us, and many exciting discoveries to look forward to!

Further reading:

The original announcement by NASA and ESA



NASA's Outer Planet Flagship Mission website



The reports on the two finalists