Nasa is about to propose "irrational" major cuts to its exploration of other planets, especially Mars, scientists claim.
Budget constraints mean Nasa executives have had to decide whether to invest in space exploration in the neighbouring planet or the far-off cosmos.
Scientists who have been briefed on the 2013 budget, which will be published next week, fear the US space agency will cancel two planned joint missions with Europe to explore Mars in 2016 and 2018. Nasa had agreed to pay $1.4bn for the project. Some Mars missions will continue, but the fate of future flights is unclear.
Two scientists, who asked not to be named, said the cuts to the Mars missions were part of a proposed reduction of about $300m in Nasa's $1.5bn planetary science budget. More than $200m of those cuts are in the Mars programme, they said. The current Mars budget is $581.7m.
"It is totally irrational and unjustified," said Edward Weiler, who until September was Nasa's associate administrator for science. "[The US is] the only country on this planet that has the demonstrated ability to land on another planet, namely Mars. It is a national prestige issue."