In a report submitted to NASA's Director of Planetary Science, the panel charged with reviewing mission extension requests has recommended that all seven currently operating major missions continue to operate. That doesn't mean they all fared equally well, though. Let's a take a look at the results and what they mean for space exploration in the coming years.
First, what does the NASA Planetary Senior Review panel do? When a mission is originally proposed to NASA, the team sets a lifetime for the mission. You might wonder why all missions don't just set really long lifetimes, like ten years or something. There are two big reasons. For one, money to run every year of the mission must be included in the initial budget. It costs about $20 million a year to operate small missions and $60 million a year to operate large missions (currently Cassini and Curiosity). So a ten-year mission means that you'd need another $200-600 million in an already tight budget. The other reason comes back to money, too. The spacecraft must be engineered to have a high likelihood of surviving for your mission lifetime. A spacecraft designed to last for ten years (like the Voyagers), is much more expensive than one which only needs to last a few months (like Opportunity). So, in the end, missions tend to err on the side of being short. This ranges from the super-short (Opportunity was only expected to run for three months on Mars) to the medium (four years for Cassini).
However, if we're spending hundreds of millions or even billion on these spacecraft, we're not just going to junk them at the end of a semi-arbitrary period of time. Hence the process known as senior review. Once a project reaches the end of its primary (budgeted) mission, it can petition NASA for a new lease on life. In order to do this, the team must show that their spacecraft is in good health and that additional time would lead to substantial new science and exploration. The team basically re-proposes the mission with a new budget and list of objectives and the Senior Review panel decides whether to give them more money. This go around, all seven eligible missions were recommended for extension. But, the panel also has the power to make recommendations about changes to how the mission runs going forward and those aren't always so glowing.
The Cassini mission at Saturn was the only project to receive a unanimous rating of "Excellent" and the panel recommended funding it for a final three year extension. They praised Cassini's exceptional track record over the last ten years and an extensive list of new science objectives the team plans to study. This will be Cassini's last extension because, in three years, the spacecraft will have expended all of its remaining fuel during the dramatic Grand Finale close-in flybys of the planet and rings.
(Full disclosure: I work on the Cassini project - big smile on my face this morning!)
Perhaps the biggest surprise of the seven was the Opportunity rover on Mars. It's in its 11th year driving on the surface of the Red Planet, having lived more than thirty times longer than originally expected. With NASA's budget tight, this would have been an easy place to cut. The panel, however, had high praise for the project's ongoing science output.
These sorts of reports are very bureaucratic in nature, filled with lots of carefully-worded statements and dry language. That made it quite eye-opening to read some of the comments the panel had regarding the Curiosity rover, which has spent the last year driving towards nearby Mount Sharp. The panel felt that driving was being prioritized over making scientific observations and recommended that the team plan more stops for investigation. Most surprising of all, however, were the blunt comments they had regarding the attitude of the team. They described Curiosity's leaders as considering the mission "too big to fail" and criticized the project's top scientist for not even attending the review. They recommended that NASA work to refocus the team - words you rarely hear in official documents.
The other missions recommended for extension were Lunar Reconnaissance Orbiter, Mars Reconnaissance Orbiter, Mars Express, and Mars Odyssey. You can read the full report here.