2. Jupiter Europa Orbiter
Europa, with an ocean thought to lie below its icy crust, is a prime candidate for hunting for life beyond Earth. The Jupiter Europa Orbiter would spend 2-1/2 years orbiting Jupiter, with a special emphasis on answering questions about Europa's make-up raised by NASA's Galileo mission, which ended in 2003. The Jupiter Europa Orbiter is currently estimated to cost $4.7 billion in FY2015 dollars. This comes in at No. 2 on the NRC's big-mission list. But the panel recommends that it, too, move forward only if the mission's ambitions are cut back and NASA's space-science budgets increase to a point where MAX-C and JEO can proceed without affecting other priorities the panel has set.