— NASA and the European Space Agency (ESA) have elected to continue studying a pair of flagship-class missions to the Jupiter and Saturn systems, placing the jovian explorer first in the tentative queue because of its higher level of technical readiness, the agencies announced Feb. 18.

Each mission calls for sending two probes – one supplied by NASA, the other by ESA – to explore the gas giants and their unique moons, such as Jupiter’s ice-covered Europa and Saturn’s shrouded moon Titan.

ESA has not yet agreed to undertake the Jupiter mission, tentatively slated for launch in 2020. But with the NASA-ESA agreement, the Jupiter proposal will now be entered into ESA’s ongoing mission competition process as the choice for outer planets exploration.

The Jupiter mission will compete with two other proposals. One is the International X-ray Observatory, and the other is the Laser Interferometer Space Antenna. ESA has set aside 650 million euros ($836 million) for one of the three missions, with a decision expected in 2011, said Fabio Favata, head of ESA’s science planning and community coordination office.

“It’s just a remarkable effort that I see,” Jim Green, director of NASA’s planetary science division, said in a teleconference with reporters. “The communities have really come together on both sides of the pond.”

NASA and ESA made the announcement following meetings between representatives of the agencies in
the week of Feb. 9.

“Although the Jupiter system mission has been chosen to proceed to an earlier flight opportunity, a Saturn system mission clearly remains a high priority for the science community,” Ed Weiler, NASA associate administrator for science, said in a prepared statement.

Dubbed the Europa Jupiter System Mission, the Jupiter-bound expedition would send two spacecraft to study the planet and its large moons Io, Europa, Ganymede and Callisto in unprecedented detail, NASA officials said.

NASA would build one orbiter, the Jupiter Europa, while ESA would provide the other, the Jupiter Ganymede, or Laplace. The spacecraft would launch from different spaceports with the goal of reaching Jupiter by 2026 and spending three years studying the planet and its moons, NASA officials said.

NASA would contribute up to $3 billion to the mission. While neither mission is currently fully funded, NASA is setting aside about $10 million to continue studying design challenges for its Jupiter Europa orbiter, Green said.

NASA’s last dedicated mission to Jupiter was Galileo, which spent eight years studying the planet and its moons before intentionally plunging into the gas giant in 2003. The next probe slated to fly to the planet is NASA’s Juno, which is scheduled for an August 2011 launch.

“For the Jupiter Europa mission, the overarching goal is to investigate the emergence of potentially habitable worlds around gas giants,” said Curt Niebur, NASA’s program scientist for the outer planets. “We’ll take a close look at Europa, to better define its habitability.”

Jupiter’s moon Europa is covered with a thick crust of ice that hides what astronomers believe to be a vast liquid ocean beneath its frozen exterior. In addition to studying Jupiter itself and flying by its other large satellites, NASA’s Jupiter Europa spacecraft would be able to orbit Europa and build global maps of the moon’s surface, topography and composition. A ground-penetrating radar and gravity-measuring sensors would also probe Europa’s interior to obtain definitive proof whether the underground ocean exists.

“We all firmly believe that there’s an ocean under the ice of Europa,” Niebur said. “This mission is going to verify that using three different lines of inquiry.”

‘s jovian probe would mirror NASA’s in-depth scrutiny of Europa at Ganymede, which is the largest of Jupiter’s moons, as well as the largest natural satellite in the solar system. The moon is larger than the planet Mercury. While NASA’s spacecraft will end its mission in orbit around Europa,
‘s would do so circling Ganymede, NASA officials said.

Like the proposed Jupiter mission, the Saturn expedition would consist of both NASA and European spacecraft.

Dubbed the Titan Saturn System mission, the flagship flight would include a NASA-built orbiter to study Saturn and its moons, as well as a European lander and research balloon to continue the exploration of the planet’s cloud-covered moon Titan. Saturn’s moon Enceladus, which harbors ice-spewing geysers, is also a major target for that mission. ESA officials have referred to their mission to Saturn and Titan as Tandem.

The daunting technical hurdles involved in assembling the mission will require more study and technology development before the flight can go forward, NASA officials said. Those hurdles, which include trying to keep spacecraft trim enough to fit on their rockets, led NASA and ESA officials to propose flying the Jupiter mission first, they added.

“Titan will not be forgotten,” Green said.

Under NASA’s current plan, the Titan Saturn System mission would take about 10 and a half years to reach Saturn, depending on when it is launched. NASA’s orbiter would spend about two years circling Saturn to study the planet, Enceladus and other moons, and then spend about one and a half years in orbit around Titan, Niebur said.

said Titan’s major draw is its chemistry, which appears to have many parallels to Earth’s ancient past. Images and data from the NASA-ESA Cassini orbiter have found evidence of liquid methane lakes and rain on the cloudy Saturnian moon.

“Titan is felt to be a very Earth-like world in terms of the processes going on,” Niebur said.

“This joint endeavor is a wonderful new exploration challenge and will be a landmark of 21st century planetary science,” David Southwood, ESA’s director of science and robotic exploration, said in a prepared statement. “What I am especially sure of is that the cooperation across the
that we have had so far and we see in the future, between
, NASA and ESA, and in our respective science communities is absolutely right. Let’s get to work.”

Peter B. de Selding contributed to this article from Strasbourg, France.