NASA's future space missions may be delayed or cancelled to the scarcity of plutonium-238, which has been used by the space agency to fuel its manned spacecrafts for the past 50 years.

Scientists say that without additional stores of this fuel, the agency's ability to conduct future planetary science is in jeopardy, adding that it is something the United States simply cannot afford.

NASA's Mars rover Curiosity, scheduled to launch Saturday, is powered by this radioactive element. However, with the chemical getting scarce, Curiosity may be the last in a long line of spacecrafts to be powered by plutonium.

"It's like having a car and no gasoline in the car," said Ralph McNutt, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory and a project scientist for NASA's Messenger mission to Mercury. "The development of this power system has taken place in the U.S. over five decades, and we're on the verge of throwing it all away."

In 2009, the National Research Council reported that plutonium-238 has been and will continue to be "essential to the U.S. space science and exploration program." The council recommended that domestic production of the material be restarted in order to sustain NASA's planetary science program, and to avoid delays or even cancellations of future missions.

Plutonium-238 is a toxic substance that gives off heat that can be converted to electricity in the cold, dark depths of space. The United States produced this highly toxic chemical in facilities that supported the nuclear weapons program during the Cold War but they stopped making it in the late 1980s.

The NASA has used these plutonium-powered systems for famous missions like the Voyager probes. However, Jim Adams, deputy director of planetary science at NASA, told NPR that even with slow down in space exploration due to budget constraints, fuel for NASA missions is only up to around 2022.