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SPACE SCIENCE sion included a lander that would puncture the surface ice, that feature has been dropped after engineers deemed the task too difficult NASA, ESA Choose King of Planets technically. But there will be challenges enough. One major obstacle is the intense For Flagship Missions in 2020 radiation surrounding . That hazard helped shoot down a Europa proposal a In classical mythology, Jupiter abducted the joint missions “a wonderful new explo- decade ago, but NASA’s Green says there both Europa and Ganymede. But last week ration challenge and a landmark of 21st cen- have been important advances since then in those jovian satellites turned the tables and tury .” hardening sensitive instruments and space- seduced NASA and Both Europa and Ganymede tantalize craft components against such radiation. (ESA) officials into picking them, instead of outer planetary scientists. The former has an Even that technical hurdle, however, , an equally intriguing of pales in comparison with what the backers of Saturn, as the next major missions to a Titan mission had proposed: a sophisticated explore the solar system beyond . lander and a French-built balloon to sail over If all goes as planned, two separate the surface of the mysterious moon. Jonathan spacecraft will head to the Jupiter sys- Lunine, a planetary researcher at the Univer- tem in 2020, with the NASA probe sity of Arizona, Tucson, who worked on the targeting Europa and the ESA probe Titan proposal, acknowledged its technical heading for Ganymede. complexity but said “people were impressed The decision marks the end of a by how far we took the study.” One consola- heated 2-year competition between the tion is that Cassini, launched in 1997, could two systems (Science, 19 December continue beaming back data from the Saturn 2008, p. 1780). The scientific payoff system until 2016 or 2017. from the Galileo mission to Jupiter and As scientists move forward, the burning the ongoing Cassini mission to Saturn question is whether NASA and the Jet made equally compelling cases for a Propulsion Laboratory in Pasadena, Califor- follow-up visit, says Ronald Greeley, a nia, which will lead the NASA portion of the planetary scientist at Arizona State effort, can keep costs under control. “We’ve University, Tempe, who co-chaired the all got [Mars Science Laboratory (MSL)] in team that defined the science for the the back of our minds,” says Fran Bagenal, a Europa-Jupiter mission. But there’s planetary scientist at the University of Col- money for only one flagship planetary orado, Boulder, who also chairs the board mission, say NASA officials, and the that advises NASA on outer planet science. technical maturity of the Europa pro- MSL is well over budget and behind sched- posal clinched the deal. “The science Home away? NASA’s Jupiter probe will investigate the habit- ule, forcing NASA to make cuts in other was absolutely outstanding on both,” ability of Europa. planetary efforts. “We must not go down the says James Green, NASA’s planetary same path,” she warns. sciences chief. “But a Europa mission has icy crust concealing an ocean of water that Ed Weiler, NASA’s science chief, empha- been studied for a long time.” could harbor the building blocks of life, or sized that a mission to Titan is not off the Both the Europa and Ganymede missions life itself. And Ganymede, the largest moon table and that the agency has simply still face hurdles. NASA must win approval in the solar system, is the only satellite with assigned Europa a higher priority. “The deci- from the Obama Administration and Con- an internally generated magnetic field. Its sion means a win-win situation for all parties gress for the estimated $3 billion cost of the surface may also conceal an ocean far below. involved,” he added. Says Lunine: “Clearly, Europa mission, while ESA will weigh its Neither probe would depend on the other NASA wants to fly both.” Ganymede mission—and its $1 billion price for data, but officials are expecting synergis- Even if that’s true, however, few of tag—against two astronomy projects in tic results, notes Greeley. “Both probes could today’s scientists are likely to be around to 2011 before it can start design work in look at the magnetosphere of Jupiter at the analyze data coming back from a Titan flag- earnest. Neither agency has room for addi- same time from different positions,” or image ship. The sibling spacecraft won’t reach tional scientific missions in its current the volcanic activity on Io from different Jupiter until 2026, and any mission to Sat- budget, and the decision to launch separate angles. The exact trajectory of the probes has urn would presumably arrive even further probes allows either side to back out without yet to be determined, but each spacecraft into the future. “You have to think in terms torpedoing the entire mission. Not that they would likely tour the Jupiter system before of your students and your grad students,” have any plans to do so, of course. ESA settling into orbit around its respective target. says Greeley. “This kind of work takes a lot

Director of Science David Southwood called Although initial plans for the Europa mis- of lead time.” –ANDREW LAWLER JPL/NASA CREDIT:

1154 27 FEBRUARY 2009 VOL 323 SCIENCE www.sciencemag.org Published by AAAS