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NASA is feeling the pinch in its plutonium supplies.

Many spacecraft are powered by the radioactive decay of plutonium-238, but the US no longer produces the material. Instead, NASA relies on its shrinking stockpile, topped up with purchases from Russia.

Previous estimates suggested the decline would not affect solar-system exploration until after 2020, but NASA is already tightening its belt. Candidates for NASA鈥檚 next 鈥溾�� mission, which aims to launch an exploratory spacecraft by 2018, will not be allowed to rely on plutonium for power, effectively limiting the candidate probes to solar power only.

That puts a number of destinations off-limits, says Jim Green, head of . These include targets beyond Jupiter or even darkened regions closer to the sun, like the polar regions of Mars. 鈥淲ithout the plutonium, there鈥檚 just a huge dimension of science we鈥檙e going to be missing,鈥� Green told New 女生小视频.

NASA is also relying on Russia for some plutonium-238 that it needs for its next major mission to the outer solar system 鈥� to explore Jupiter and its moon Europa. The US Department of Energy is currently analysing what will be required to restart plutonium-238 production, but new fuel may not be ready in time for the mission to launch as planned in 2020.