While NASA is counting on an act of Congress or a renegotiated deal with Russia to acquire plutonium for its next robotic deep space missions, the European Space Agency is considering alternative nuclear fuels to power its own probes traveling into the sun-starved outer solar system. NASA’s dwindling supply of plutonium-238 nuclear fuel will not be sufficient to power an orbiter to visit Jupiter’s moon Europa, NASA’s contribution to a planned $4.5 billion joint flagship mission between the U.S. space agency and Europe. That’s unless the U.S. Department of Energy, which supplies nuclear fuel for NASA missions, receives funding to restart domestic production of plutonium or successfully resolves a contract dispute with the Russian government, said Jim Adams, the deputy director of NASA’s planetary science division.