The hottest competition in the solar system is over. NASA and the European Space Agency (ESA) said today that each agency will launch a probe to Jupiter  in 2020 to explore the moons of the solar system's largest planet—edging out a proposed mission to the ringed giant. The multibillion-dollar missions—no estimate of the cost was offered—will examine Europa, which has an icy surface covering a liquid water ocean, and Ganymede, which is the only moon in the solar system with an internally generated magnetic field. Both potentially harbor life or its building blocks. NASA would build and launch the Europa probe, whereas ESA would be responsible for the Ganymede orbiter. The schedule calls for each spacecraft to orbit Jupiter (pictured, Credit: NASA) in 2026 before splitting off and operating for 3 years.
In 2007, NASA invited teams to prepare proposals for missions to one of four outer planet satellites before choosing two finalists. The runner-up in the competition is Saturn and its intriguing moon Titan. That mission would have involved a NASA orbiter and an ESA lander and balloon. But officials from the two agencies determined that the proposal was not as technically advanced as the Jupiter mission. NASA and ESA officials said they remain open to a Saturn mission, although constrained budgets make such a trip a long shot.
ESA science chief David Southwood issued a statement calling the decision "a landmark of 21st century planetary science. … The cooperation across the Atlantic that we have had so far and we see in the future, between America and Europe, NASA and ESA, and in our respective science communities is absolutely right. Let's get to work."