NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

NASA is on the verge of releasing its long-awaited prioritization of planetary missions, meant to guide the agency if tight budgets force it to switch off an operating spacecraft. But two missions that had been considered to be on the verge of closure — the Mars Opportunity rover and the Lunar Reconnaissance Orbiter (LRO) — have each received a reprieve of another two years of operations, scientists close to the projects have confirmed.

Although NASA officials had insisted otherwise, Opportunity and the LRO were considered particularly vulnerable because funding for them was included in a supplement to the White House’s annual budget request to Congress, rather than as part of the main planetary-sciences division budget.

In a decade of operation, Opportunity has rolled more than 40.6 kilometres across Mars, exploring areas including the most ancient habitable environment known on the planet. The rover has several mechanical issues as well as problems with its flash memory that have triggered computer resets in recent weeks. Opportunity, which costs on the order of US$13 million annually, is heading for a region called Marathon Valley, where scientists think clay minerals formed in a watery environment.

The LRO finished its main task in 2010: mapping possible locations for astronauts to return to the Moon. More recently it has focused on studying changes on the lunar surface, such as those from fresh meteorite impacts.

The complete ‘senior review’, encompassing five other planetary missions, will be released at a planetary sciences advisory group meeting in Washington DC on 3 September.

Of the five other missions, two are big-ticket items — on the order of $60 million annually — that are considered shoo-ins for approval. The Curiosity rover landed on Mars two years ago and is still heading for its ultimate goal, Mount Sharp. (The harsh rocks of Mars have taken a toll on Curiosity, however, and the rover recently had to backtrack out of a sandy valley so as not to get stuck, as well as give up on drilling what would have been its fourth hole on Mars.)

The Cassini mission has been orbiting Saturn since 2004, but as seasons change it has been observing new phenomena on the planet. “In many ways it’s a brand-new mission,” project scientist Linda Spilker, of NASA’s Jet Propulsion Laboratory in Pasadena, California, said earlier this month. Cassini engineers are planning for a ‘grand finale’ in 2017, when the probe will dive repeatedly between the gaseous planet and its ring system to make unprecedented close-up measurements. “It will be 7 seconds of terror every 22 days,” Spilker said.

The three remaining missions under scrutiny are the Mars Reconnaissance Orbiter, which costs around $30 million annually and has a crucial communications-relay role at Mars; the 13-year-old Mars Odyssey orbiter, at $12 million annually; and a $3-million contribution for an instrument aboard the European Space Agency’s Mars Express spacecraft, launched in 2003.

Jim Green, head of NASA’s planetary-sciences division, has said repeatedly that the agency will work within its budgetary constraints to try to fulfill the recommendations of the senior review panel. The big unknown is how much the agency will have to spend for each of the extended missions. NASA typically allocates around $1.3 billion annually to planetary sciences, but Congress has yet to decide the numbers for the 2015 fiscal year, which begins on 1 October.