As described at the annual meeting of the American Geophysical Union in San Francisco, NASA plans to launch by 2012 at the earliest a hulking nuclear-powered probe to orbit each of Jupiter’s three icy, planet-sized moons in turn for months at a time and determine whether or not they have the potential to harbor life. The Jupiter Icy Moons Orbiter, or Jimo, would circle the Jovian moons Callisto, Europa and Ganymede, all of which are believed to have vast oceans beneath thick covers of ice. The unmanned JIMO probe, far larger and more powerful than any other sent to explore the outer solar system, would spend years studying the moons’ makeup, geologic history and potential for sustaining life, as well as Jupiter itself.

“Our primary theme is to see if there is liquid water beneath the icy crust,” notes Ronald Greeley of Arizona State University. “The second consideration is, is the right chemistry there? We’ll be able to determine what the composition of the ice is. And then the third consideration is the presence of an energy source; all life requires energy.”