Between 1967 and 1977 medical researchers and engineers in two separate federally-funded U.S. programs tackled the technological complexity of designing a radioisotope-powered mechanical heart, one in which the primary power source was heat generated by radioactive decay, rather than fission. Its development highlights the technological optimism of scientists and engineers, the intersection of science and government, and the broader context of public debates about risk and uncertainty in this period. Medical researchers and engineers claimed that atomic hearts were feasible and practical and the technological complexities surmountable. However, political and social concerns arising in the context of a heightened sense of risk awareness in the 1970s ultimately played the biggest role in shutting down the atomic heart programs, as strong public support for increased government control of both atomic energy and medical devices overrode scientific assertions that further development could produce a safe and practical atomic heart.