Astrobiologists regard Europa, which is about 1,900 miles (3,100 kilometers) wide, as one of the best bets in our solar system to host life beyond Earth. The moon is believed to harbor a large ocean of liquid water beneath its icy shell. Further, this ocean is likely in direct contact with Europa’s rocky mantle, raising the possibility of all sorts of interesting chemical reactions, Senske said. The irradiation of Europa’s surface and tidal heating of its interior also mean the moon likely has ample energy sources — another key requirement for life as we know it.
NASA has long been interested in exploring the icy moon and its ocean. Several years back, the agency drew up an ambitious mission concept called the Jupiter Europa Orbiter (JEO), which would have made detailed studies of Europa and the incredibly volcanic Jupiter moon Io.
The science returns from such a mission would have been impressive, according to the 2011 Planetary Science Decadal Survey, which outlined the scientific community’s goals in the field over the coming decade. The decadal survey ranked JEO as the second-highest priority among large-scale missions, just behind Mars sample-return. But the report said its $4.7 billion price tag was just too high.
“The recommendation was, immediately go and do a de-scope,” said Senske, who was also involved in JEO. “They loved the science; the science was great. But focus it.” So researchers got to work developing a leaner, cheaper Europa mission that would fit under a firm $2 billion cost cap. They came up with two main options: the clipper and a Europa orbiter (a lander was ruled out as premature).
Because of the intense radiation environment around Europa, the orbiter would have to be heavily shielded, adding weight and cost. Even with this armor, the concept initially called for a nominal design life at Europa of just 30 days, with later versions boosting that up to 109 days, Senske said.
While the orbiter would gather a great deal of interesting and valuable information, it falls short of what the flyby concept could deliver on a dollar-to-dollar basis, Senske said. For example, a $2 billion orbiter would not be able to carry an instrument that could investigate the composition and chemistry of Europa’s surface and atmosphere (and, by extension, its ocean). “In terms of an apples-to-apples comparison, Clipper really does rise to the top,” Senske said.
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