In classical mythology, Jupiter abducted both Europa and Ganymede. But last week those jovian satellites turned the tables and seduced NASA and European Space Agency (ESA) officials into picking them, instead of Titan, an equally intriguing moon of Saturn, as the next major missions to explore the solar system beyond Mars. If all goes as planned, two separate spacecraft will head to the Jupiter system in 2020, with the NASA probe targeting Europa and the ESA probe heading for Ganymede.<\/p>\n
The decision marks the end of a heated 2-year competition between the two systems (Science, 19 December 2008, p. 1780). The scientific payoff from the Galileo mission to Jupiter and the ongoing Cassini mission to Saturn made equally compelling cases for a follow-up visit, says Ronald Greeley, a planetary scientist at Arizona State University, Tempe, who co-chaired the team that defined the science for the Europa-Jupiter mission. But there’s money for only one flagship planetary mission, say NASA officials, and the technical maturity of the Europa proposal clinched the deal. “The science was absolutely outstanding on both,” says James Green, NASA’s planetary sciences chief. “But a Europa mission has been studied for a long time.”<\/p>\n
Both the Europa and Ganymede missions still face hurdles. NASA must win approval from the Obama Administration and Congress for the estimated $3 billion cost of the Europa mission, while ESA will weigh its Ganymede mission–and its $1 billion price tag–against two astronomy projects in 2011 before it can start design work in earnest. Neither agency has room for additional scientific missions in its current budget, and the decision to launch separate probes allows either side to back out without torpedoing the entire mission. Not that they have any plans to do so, of course. ESA Director of Science David Southwood called the joint missions “a wonderful new exploration challenge and a landmark of 21st century planetary science.”<\/p>\n
Both Europa and Ganymede tantalize outer planetary scientists. The former has an icy crust concealing an ocean of water that could harbor the building blocks of life, or life itself. And Ganymede, the largest moon in the solar system, is the only satellite with an internally generated magnetic field. Its surface may also conceal an ocean far below.<\/p>\n
Neither probe would depend on the other for data, but officials are expecting synergistic results, notes Greeley. “Both probes could look at the magnetosphere of Jupiter at the same time from different positions,” or image the volcanic activity on Io from different angles. The exact trajectory of the probes has yet to be determined, but each spacecraft would likely tour the Jupiter system before settling into orbit around its respective target.<\/p>\n
Although initial plans for the Europa mission included a lander that would puncture the surface ice, that feature has been dropped after engineers deemed the task too difficult technically. But there will be challenges enough. One major obstacle is the intense radiation surrounding Europa. That hazard helped shoot down a Europa proposal a decade ago, but NASA’s Green says there have been important advances since then in hardening sensitive instruments and spacecraft components against such radiation.<\/p>\n
Even that technical hurdle, however, pales in comparison with what the backers of a Titan mission had proposed: a sophisticated lander and a French-built balloon to sail over the surface of the mysterious moon. Jonathan Lunine, a planetary researcher at the University of Arizona, Tucson, who worked on the Titan proposal, acknowledged its technical complexity but said “people were impressed by how far we took the study.” One consolation is that Cassini, launched in 1997, could continue beaming back data from the Saturn system until 2016 or 2017.<\/p>\n
As scientists move forward, the burning question is whether NASA and the Jet Propulsion Laboratory in Pasadena, California, which will lead the NASA portion of the effort, can keep costs under control. “We’ve all got [Mars Science Laboratory (MSL)] in the back of our minds,” says Fran Bagenal, a planetary scientist at the University of Colorado, Boulder, who also chairs the board that advises NASA on outer planet science. MSL is well over budget and behind schedule, forcing NASA to make cuts in other planetary efforts. “We must not go down the same path,” she warns.<\/p>\n
Ed Weiler, NASA’s science chief, emphasized that a mission to Titan is not off the table and that the agency has simply assigned Europa a higher priority. “The decision means a win-win situation for all parties involved,” he added. Says Lunine: “Clearly, NASA wants to fly both.”<\/p>\n
Even if that’s true, however, few of today’s scientists are likely to be around to analyze data coming back from a Titan flagship. The sibling spacecraft won’t reach Jupiter until 2026, and any mission to Saturn would presumably arrive even further into the future. “You have to think in terms of your students and your grad students,” says Greeley. “This kind of work takes a lot of lead time.”<\/p>\n
Science 27 February 2009:
\nVol. 323. no. 5918, p. 1154
\nDOI: 10.1126\/science.323.5918.1154<\/p>\n