NASA and the European Space Agency (ESA) today unveiled their joint plans for exploring the outer planets, choosing to head to Jupiter and four of its moons before further exploring the moons of Saturn. Each space agency will launch a spacecraft in 2020 that will reach the Jovian system in 2026 and then spend three years exploring Jupiter and its largest natural satellites: Europa, Ganymede, Io and Callisto. ESA's craft will spend one of those years examining Ganymede, and NASA's probe will do the same for Europa.

Europa is considered one of the more habitable bodies in the solar system and so merits investigation as a possible abode for life. The Galileo spacecraft, which explored the Jovian system from 1995 to 2003, discovered what might be a subsurface ocean of liquid water under Europa's crust of ice. The moon was considered sufficiently compelling that when NASA concluded the Galileo program, the spacecraft was sent plunging to its doom in the atmosphere of Jupiter to avoid possible disruption or contamination of Europa and its ocean.

Even though Europa is better known, Ganymede is hardly a dullard: it's the largest moon in the solar system and the only one known to generate its own magnetic field. The presence of such a field, or magnetosphere, likely indicates a convecting molten core within Ganymede, which may in turn indicate a rich geologic history for the moon.

The other mission that had been competing for the space agencies' attention, an exploration of Saturn and its moons Titan and Enceladus, will be tabled for later.

The timing of the announcement dovetails nicely with the International Year of Astronomy (IYA2009), honoring the 400th anniversary of the year Italian astronomer Galileo Galilei turned his telescope to the skies—he discovered the four targeted moons, now known as the Galilean satellites, in 1610.

Photo of Europa from Galileo mission courtesy of NASA/JPL/DLR