Mars and Jupiter's moon Europa should be NASA's main areas of focus over the coming decade, according to a report from the National Research Council.
But, it warns, budgetary constraints may mean that these major missions should be delayed, while smaller ones such as those in its New Frontiers and Discovery programs should go ahead first.
"Our recommendations are science-driven, and they offer a balanced mix of missions - large, medium, and small - that have the potential to greatly expand our knowledge of the solar system," says Steven W Squyres, a Cornell professor and chair of the committee that wrote the report.
"However, in these tough economic times, some difficult choices may have to be made. With that in mind, our priority missions were carefully selected based on their potential to yield the most scientific benefit per dollar spent."
Top of the list should be the Mars Astrobiology Explorer Cacher (MAX-C), a Mars mission to examine whether the planet ever supported life and answer questions about its geologic and climatic history. But, says the report, the mission should go ahead only if NASA can keep its costs under $2.5 billion - $1 billion less than the independent estimates provided to the committee.
A mission to Jupiter's icy moon Europa and its subsurface ocean - one of the most promising environments in the solar system for supporting life - should be NASA's second priority, says the report. However, NASA will again need to get the cost down from initial estimates.
The third priority for large missions would be the Uranus Orbiter and Probe mission, which aims to investigate the planet's interior structure, atmosphere, and composition.
The report says that work on this mission could start between 2013 and 2022, but that careful tabs should be kept on the cost, and the mission should be reduced or cancelled if it goes much over its estimated $2.7 billion price tag.