Heading back to Jupiter’s moons

When NASA crashed the Galileo spacecraft into Jupiter in September 2003 (see “The long goodbye,” Astronomy, October 2003), it marked the end of a spectacular eight-year mission that provided strong evidence that oceans exist under the icy crust of three of the Galilean moons. Galileo already had far exceeded its design lifetime, but that doesn’t mean NASA has nothing left to explore.

To continue the exploration of Jupiter’s water worlds, NASA is thinking about the Jupiter Icy Moons Orbiter (JIMO). This sophisticated spacecraft would orbit Europa, Ganymede, and Callisto to investigate their makeup, their history, and their potential for sustaining life.

Galileo discovered that all of Jupiter’s large icy moons appear to have the three ingredients considered essential for life: water, energy, and the necessary chemical contents. Galileo’s evidence suggests that liquid water has reached Europa’s surface in geologically recent times, and it still may lie relatively close to the surface. Closer observations of Callisto and Ganymede would provide key comparisons for understanding the evolution of all three moons.

Current plans call for JIMO to use electric propulsion powered by a nuclear fission reactor. This would raise the technology bar for NASA, making it possible to conduct a realistic mission to orbit all three moons one after the other. Just as important, it also would open the rest of the outer solar system to detailed exploration in later missions.

The spacecraft would spend many months spiraling toward each moon, at least one full month in orbit, and another several months spiraling away before heading to its next target. Detailed observations would be made throughout this extended period, but the highest quality observations would come during the low-altitude, high-inclination orbit of each moon.

JIMO scientists have set three major goals for the mission. First, they want to study the oceans: verifying their existence, finding their locations within the moons, studying the structure of their icy crusts, and assessing the internal processes that keep the oceans liquid. Second, they want to study the possible role of the oceans in astrobiology: determining the types of volatile substances and organic compounds on and near the surfaces, and understanding the processes involved in their formation and modification. And third, they want to investigate interactions within the jovian system: studying the atmospheres of the satellites as well as the interactions among Jupiter and the surfaces and interiors of its satellites.