Nasa's twin Voyager probes may have to close down in October to save money, the US space agency has said.
Nasa scientists are preparing themselves for the worst
Launched in 1977, Voyagers One and Two are now more than 14 billion and 11 billion km from Earth, respectively.
They are on their final mission to locate the boundary between the Sun's domain and interstellar space.
But the agency's Earth-Sun System division has had to cut its budget for next year from $74m to $53m, meaning that some projects will be abandoned.
Although the Voyager probes are thought to have another 15 years of life left in them, they are very expensive to run, costing Nasa about $4.2m a year for operations and data analysis.
Other missions like Ulysses, which was launched in 1990 to explore the Sun's polar regions, might also have to be abandoned after the end of the fiscal year in October.
Although the decision is not yet final, some Nasa scientists are preparing themselves for the worst. Voyager project scientist Edward Stone of the California Institute of Technology told Nature magazine: "We are currently developing a plan for shutdown."