Jovian moons get top priority for next flagship mission
BY STEPHEN CLARK
Posted: February 18, 2009
Europa and Ganymede, two moons of Jupiter believed to harbor underground oceans of liquid water, are the highest-priority targets for the next robotic mission to the solar system's outer planets, scientists announced Wednesday.
"Europa is just a tremendously exciting water world. It has an underground ocean with probably more water in it than the Earth does," said Jim Green, director of the planetary science division at NASA headquarters.
"Ganymede is indeed the largest moon in the solar system, larger than Titan and significantly larger than Mercury," Green said.
Leaders from both agencies met in Washington last week to review reports on both missions. Officials considered the scientific and technical merits in their decision.
The missions offered exciting scientific returns for the agencies' objectives to learn more about potentially habitable worlds in the outer solar system.
"We're investigating the questions of whether or not Jupiter harbors habitable worlds and what processes are occurring there that could affect those worlds," said Curt Niebur, NASA's program scientist for the outer planets.
Both missions scored high in the science arena.
"This was something of a dead heat on both sides of the Atlantic," said David Southwood, director of ESA's science programs.
"Speaking personally as a scientist from this area, I felt this was a just outcome," Southwood said. "However, both systems come up well in throwing light on issues of solar system evolution and the search for water."
Officials deemed the Jupiter mission to be the most ready for implementation based on technical assessments, which highlighted deficiencies in spacecraft design, the propulsion system, the trajectory to Titan, and low mass margins, officials said.
Europa has long been an attractive target in the outer solar system, and the icy moon has consistently been ranked as the highest priority for future science missions to the gas giants.
NASA has completed on-and-off studies of a robotic probe to Europa for nearly a decade. Lessons learned from those studies gave the Europa mission an advantage, Niebur said.
"We're simply at a point now where we can recoup the investment that we've made in the past. And that investment means that the Europa concept is very mature, and we have a high level of understanding about the mission implementation," Niebur said.
A Titan mission has been studied for a shorter period of time, and scientists' understanding of that project is "correspondingly less," Niebur said.
NASA would field a Europa orbiter, weighing up to 11,100 pounds at launch, which is tentatively scheduled to launch in February 2020. Engineers are baselining the $3.8 billion nuclear-powered mission to launch aboard the largest version of the expendable Atlas 5 rocket, with five solid rocket boosters and a five-meter payload fairing.
Europe's solar-powered 7,700-pound Ganymede orbiter, also called Laplace by ESA, would launch the following month aboard an Ariane 5 rocket.
Japan has expressed "major interest" in adding an additional orbiter to launch with Laplace for magnetic field studies at Jupiter, but there is no formal agreement yet, Green said.
The probes would arrive at Jupiter in late 2025 and early 2026 to begin multi-year tours of the gas giant's four largest moons.
Both spacecraft would enter orbit around Europa and Ganymede in 2028 for in-depth observations.
But ESA's contribution still must overcome several more hurdles before being approved.
The Laplace mission is now one of three "L-class" projects being considered under ESA's Cosmic Vision program. European officials expect to choose a mission for implementation in November 2011. L-class missions are designed to cost less than 650 million euros, or about $815 million.
Laplace is competing against a new X-ray space telescope and a mission to hunt for black holes.
NASA's Europa orbiter is designed to fly to Jupiter alone if ESA selects another mission under the Cosmic Vision program, according to Green.
"Our understanding of the icy moons, Ganymede, Callisto and Europa, will be significantly advanced from this mission alone," Green said.
"Europa is in a harsh radiation environment. There's been a lot of major technical progress in that area, but we have much more modeling and understanding to do in that," Green said.
The Titan team will present their findings during the next planetary science decadal survey, a National Academy of Sciences poll that ranks priorities for future NASA missions.
"Titan will not be forgotten," Green said. "We believe there's just nothing like Titan that could supersede it in that prioritization."
Work on the Europa orbiter is still in the "pre-Phase A" period, meaning the project is still undergoing conceptual analysis.
Officials plan to begin soliciting the science community for instrument proposals in late 2011.
"Our next step would be to look at instrumentation," Green said. "Much like Cassini, which is a joint mission, had both NASA and ESA instruments on it, we believe that both of these orbiters will have those kinds of opportunities for the community."
Scientists have already broadly defined the scientific goals of the Jupiter system mission.
After several fly-bys of Ganymede, Callisto and Io, the Europa orbiter would remotely probe the moon's liquid water ocean hidden beneath a thick shelf of fractured ice.
"We firmly believe that there is an ocean under the ice of Europa. This mission is going to verify that using three independent lines of inquiry, so that we can very well understand and characterize it," Niebur said.
The spacecraft would also create multi-spectral global maps of Europa, which would aid future missions tasked with landing on the moon's icy surface.
"Before you're able to land on a body, you need to first understand the surface. And we don't have sufficient data of what the surface of Europa is like, in terms of boulders, crevasses, and things like that," Niebur said.
The Ganymede orbiter would enter a resonant orbit at Jupiter, allowing it to focus its observations on Callisto and Ganymede.
After entering orbit around Ganymede, the probe would study moon's hypothesized subsurface ocean, map its icy surface and measure the interaction of Ganymede's magnetosphere with Jupiter.
The orbiters will collect much more science data than the Galileo probe, which circled Jupiter for nearly eight years until it was intentionally crashed into the planet in 2003.
Galileo's high-gain communications antenna failed to open after launch, narrowing the stream of data the craft could transmit back to Earth.
"The Europa orbiter will do a lot more than Galileo did for us. It will be a much more capable spacecraft with more advanced instruments and a larger instrument suite as well. But most importantly, it will be able to transmit a tremendous amount of data back to Earth, something Galileo could not do because of its antenna problem," Niebur said.
Galileo conducted 34 fly-bys of Jupiter's four largest moons, including 11 visits to Europa and eight trips past Ganymede, according to NASA.