The Alternate Propellant Thermal Rocket (APTR) is a novel concept for propulsion of space exploration or orbit transfer vehicles. APTR propulsion is provided by utilizing a nuclear thermal reactor or solar thermal engine to heat a space storable propellant, preferably consisting of a volatile indigenous to the destination world, to form a high thrust rocket exhaust. Candidate propellants whose performance, materials compatibility, and ease of acquisition make them worthy of examination for APTR propulsion of exploration vehicles include carbon dioxide, water, methane, and methanol. An APTR utilizing indigenous CO2 propellant potentially offers high payoff to a robotic or manned Mars mission, both by sharply reducing the initial mission mass required in low Earth orbit, and by providing Mars exploration with unlimited mobility and global access. Additionally, an APTR could give nearly unlimited mobility to asteroid or outer solar system probes, while one using methane or nitrogen propellant could enable a Titan sample return mission. The APTR can also be used as the propulsion system for a high performance space storable orbit transfer system moving payloads from LEO to GEO or other orbits of commercial interest. In this case, leading candidate propellants include methane, ammonia, and methanol.