The US$2.5-billion, two-year-old Mars Curiosity rover has come in last in a scientific review of NASA’s planetary missions, trumped by even the 10-year-old Opportunity rover.

The review evaluated seven working missions that are seeking funds for another two or more years of operations. The review panel, chaired by lunar scientist Clive Neal of the University of Notre Dame in Indiana, said that all seven proposals have high science value — ranked from ‘excellent’ to ‘very good’ — and that “all have important strengths.”

All were approved to continue after 1 October, although actual funding levels will depend on appropriations from Congress. But Curiosity came in for some of the report’s most scathing criticism. Among other things, the proposal for Curiosity’s next two years of operations “lacked specific scientific questions and testable hypotheses,” according to a summary of panel findings presented at a planetary science meeting today by NASA official William Knopf.

The panel also noted that project scientist John Grotzinger was present only by phone for the first round of discussions and not available for a follow-up round. “This left the panel with the impression that they were too big to fail,” the reviewers wrote. (Grotzinger says he had a pre-existing outreach commitment involving students and shares all mission responsibility with his deputy project scientists, one of whom attended in his stead.)

Curiosity has already rolled more than 9 kilometres across the surface of Mars, exploring an ancient lakebed within Gale Crater. In its next two years, mission planners had proposed sending it another 8 kilometres to visit four areas representing different stages of Mars’s climate history. According to Knopf’s overview, the instrument-laden Curiosity had planned to drill just eight samples during those two years, “which the panel considered a poor science return for such a large investment.” Instead, panel members recommended cutting back on the distance traveled and focusing on just two or three geologic areas.

NASA has asked the Curiosity team to revise its two-year plan, focusing on characterizing a particular geologic unit before going on to new ones or deciding whether to drill a sample. The agency has also asked for a stronger justification for how Curiosity supports NASA’s broader exploration goals, including its connections with orbiting spacecraft.

“The important thing to us as a mission is that they recommended the guideline budget we were asking for, so that we can continue to do operations,” says Grotzinger. He says the team constantly assesses the value of doing science in-place as opposed to driving to a new location, and that Curiosity’s sampling instruments are sophisticated enough that they often need relatively few drillholes to achieve science goals such as chemically analyzing rocks and soil.

Curiosity received a ‘very good/good’ rating from the panel. But the much older Opportunity got a higher rating of ‘excellent/very good’ for its extended mission plans. They include exploring ancient clay deposits near Endeavour crater, which may or may not be similar to other environments Opportunity has already encountered.

Of the other Mars missions reviewed, the Mars Reconnaissance Orbiter was extended with a particular nod to the number of scientific publications coming from researchers who are not part of the science team. The Mars Odyssey orbiter, soon to enter its sixth extended mission, was tapped for its instruments that probe the radiation environment and atmosphere of Mars, and their relevance to future human exploration. (Odyssey may, however, “be coming to the end of its productive science life as highlighted by declining rate of publications,” the panel reported.) And NASA contributions to the European Space Agency’s Mars Express mission will drop funding for its high-resolution camera but continue atmospheric measurements to support the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, which will arrive at the red planet later this month.

At the Moon, NASA will continue the Lunar Reconnaissance Orbiter but, on the panel’s recommendation, will terminate a radar instrument. Two other instruments suggested for cutting will be retained given that they measure lunar water and radiation, both of interest to NASA’s exploration goals.

And at Saturn, the highest panel ranking of all — ‘excellent’ — went to the Cassini orbiter. NASA has extended Cassini until 2017, when the spacecraft will plunge into the planet in a mission-ending finale.