An elaborate choreography of multiple spacecraft will play out among Jupiter and its Galilean moons in the decade of the 2020s, if plans now taking shape at NASA and other space agencies get the go-ahead in the next several years. The lion's share of these coordinated and collaborative ventures will focus on orbiting—and possibly landing on—Europa, a scientifically intriguing world where evidence of a watery ocean beneath the moon's icy crust points to a possible abode for extraterrestrial life.

In February, NASA selected a Europa voyage as a priority for the agency's "flagship" mission to the outer solar system, edging out a proposed visit to Saturn's moon Titan that was deemed to require further study and technology development.

The overall plan, currently known as the Europa Jupiter System Mission, or EJSM, includes a NASA spacecraft dubbed the Jupiter Europa Orbiter(JEO), which would settle into orbit around Europa after conducting a broader tour of the Jovian environs. Also, a European Space Agency (ESA) orbiter, launched separately, would focus on the planet's largest moon Ganymede, while a Japanese probe would monitor Jupiter's magnetosphere.

A further possibility—albeit one still in an early stage of discussion—is that a separately launched Russian spacecraft (pdf) would be sent to land on Europa. This probe might be timed to arrive soon after JEO has gathered data about possible landing sites, with the lander's precise target to be programmed en route.

A landing on Europa is "not an easy thing to achieve, primarily because we don't really know what the surface is like on the meter and submeter scale," says Ronald Greeley, an Arizona State University planetary geologist and co-chair of NASA's science definition team for the Europa mission. "We don't know how rough the ice is or how chopped up it might be or what the slopes are like." Collecting such information has been a central goal of the planned orbiter, whether for the possible Russian probe or subsequent landers.

"Europa is a fascinating object," Greeley says. "It's essentially a rocky world like our moon but then it has this shell of water around it that's some 100 miles [160 kilometers] thick. The surface is frozen. It could be frozen all the way down—we don't think so. We think that it's a relatively thin ice shell, but that's what we want to find out." He notes that Europa may have three times the amount of liquid water found on Earth.

Europa's surface is an extremely hostile environment, as a result of constant exposure to Jupiter's intense radiation belts. The topmost ice layer would have been "sterilized," observes Karla Clark, NASA study manager for the Europa mission. Thus, the search for possible life will require going deeper. "If there were a lander or series of landers," she says, "the ultimate goal would be to get underneath the ice." Jupiter's radiation also will shorten the active operation of any lander as well as that of JEO, which may function in orbit only for several months, despite heavy shielding of its electronics.

JEO, estimated to cost $2.7 billion, requires congressional approval, which NASA officials hope will be forthcoming in the next year or two. Similarly, other spacecraft proposed in the EJSM will require OKs from their respective governments.

Under a provisional NASA timetable, JEO would launch in 2020, reach the Jupiter system in 2025 and begin circling Europa the following year. The NASA probe would also make close flybys of another Jovian moon, Io, while the ESA orbiter, dubbed Laplace (named for the French mathematician and astronomer), would investigate Callisto as well as Ganymede. Having three orbiters (including the Japanese craft) would facilitate a thorough survey of Jupiter's magnetosphere.

Among the benefits of a Russian lander is that its surface data would provide what scientists call "ground truth" to calibrate the Europa orbiter's remote global measurements. In addition, a U.K. scientific team is proposing to put a penetrator (pdf), about half a meter long and carrying a small scientific payload, on board either the NASA or ESA orbiter, to be detached and sent crashing into the surface of Europa or Ganymede.

Some scientists see the EJSM's international collaboration as a model for future robotic space exploration. "Flagship missions are very expensive. And if we can start to collaborate at the international level, it's certainly going to enhance the science and the frequency with which we can do this sort of investigation," says Reta Beebe, a professor of astronomy at New Mexico State University.

Moreover, with separate launches and funding, parts of the overall plan could yield valuable data even if other parts fail to materialize or experience in-flight failures. "If the ESA orbiter does not get confirmed, the NASA orbiter can go alone and do fantastic science," NASA's Clark says. "And vice versa, if the NASA orbiter ends up not getting through the process, then the ESA orbiter can go alone and again do fantastic science on its own, as well."