Plutonium 238 has no central role in nuclear arms. Instead, it is valued for its steady heat, which can be turned into electricity. Nuclear batteries made of it are best known for powering spacecraft that go where sunlight is too dim to energize solar cells. For instance, they now power the Cassini probe exploring Saturn and its moons.

Federal and private experts unconnected to the project said the new plutonium would probably power devices for conducting espionage on land and under the sea. Even if no formal plans now exist to use the plutonium in space for military purposes, these experts said that the material could be used by the military to power compact spy satellites that would be hard for adversaries to track, evade or destroy.

"It's going to be a tough world in the next one or two decades, and this may be needed," said a senior federal scientist who helps the military plan space missions and spoke on the condition of anonymity because of the possibility that he would contradict federal policies. "Technologically, it makes sense."

Early in the nuclear era, the government became fascinated by plutonium 238 and used it regularly to make nuclear batteries that worked for years or decades. Scores of them powered satellites, planetary probes and spy devices, at times with disastrous results.

In 1964, a rocket failure led to the destruction of a navigation satellite powered by plutonium 238, spreading radioactivity around the globe and starting a debate over the event's health effects.

In 1965, high in the Himalayas, an intelligence team caught in a blizzard lost a plutonium-powered device meant to spy on China. And in 1968, an errant weather satellite crashed into the Pacific, but federal teams managed to recover its plutonium battery intact from the Santa Barbara Channel, off California.

Such accidents cooled enthusiasm for the batteries. But federal agencies continued to use them for a more limited range of missions, including those involving deep-space probes and top-secret devices for tapping undersea cables.