Spacecraft that can't use solar power (e.g. when beyond Jupiter, landing on dusty surfaces like Mars, or bathed in long-duration shadows like lunar landers) depend on Plutonium-238 (Pu-238) to generate heat that is turned into electricity. NASA has used Radioisotope Thermoelectric Generators (RTGs) – basically large thermocouples – for decades to supply electricity, but those are inefficient. And since the 1990s NASA has been running low on plutonium-238.

The ASRG program was created to help extend the life of the remaining Pu-238 supply. It uses a stirling engine to generate electricity at four times the efficiency of a regular RTG. This means more missions to these harsh places using less Plutonium. While NASA has started to generate Pu-238 again, it won't be ready to use until 2019, and even then the Department of Energy will only produce about 1kg - 1.5kg per year. The New Horizons mission to Pluto used about 11kg, which would take anywhere from 7 - 11 years to generate under the current plan.

The cost of this program was contained within the Planetary Science Division's Technology budget, which has to pay for the entire cost of creating and maintaining the Department of Energy's Plutonium-238 infrastructure. Historically, this is unusual. But recent decisions by the White House and previous decisions by the Congressional energy committees have ensured that NASA is on the hook for the entire cost of creating and storing Plutonium-238. That new burden, combined with sequestration and the relentless desire to cut the planetary exploration by this White House, likely left NASA with little choice but to cancel this program. Creating Pu-238 is fundamentally more important than having an ASRG, even if that means fewer missions.