In January 1957, the United States Air Force and the Atomic Energy Commission launched Project Pluto at the Lawrence Radiation Laboratory in Livermore, California. The objective: build a nuclear-powered ramjet engine capable of propelling a cruise missile at Mach 3, at treetop altitude, for virtually unlimited range. The lead engineer was Ted Merkle, and his team spent seven years turning this nightmare into hardware that actually worked.

The concept was elegantly brutal. A conventional rocket booster would accelerate the missile to ramjet ignition speed. Then, incoming air would be heated directly by an unshielded nuclear reactor — no turbines, no combustion, no fuel tanks. The reactor was the engine. Air entered the front, passed through the reactor core, expanded violently from the heat, and exited the rear as thrust. The missile, designated SLAM (Supersonic Low-Altitude Missile), would carry up to 16 hydrogen bombs, dropping them on individual targets along a pre-programmed route that could span tens of thousands of miles.

Merkle's team built two test reactors. Tory II-A first ran on May 14, 1961, producing enough thrust to validate the concept. Its successor, Tory II-C, ran on May 20, 1964, at the Nevada Test Site. It operated at 513 megawatts and generated 35,000 pounds of thrust for five minutes — a fully functional nuclear ramjet. The physics worked. The engineering worked.

So why was it canceled on July 1, 1964, barely six weeks after its triumphant test? Several reasons converged:

• ICBMs made it redundant. Minuteman missiles could reach Soviet targets in 30 minutes. SLAM's low-altitude cruise would take hours. By 1964, the ICBM fleet was operational and reliable.
• It was an environmental catastrophe in flight. The unshielded reactor spewed radioactive exhaust across everything in its flight path. Even a test flight over friendly territory would irradiate the ground below.
• It was diplomatically destabilizing. An invulnerable, unlimited-range weapon that poisons the earth as it flies was, even by Cold War standards, difficult to justify.
• There was nowhere to test it. You cannot flight-test a radioactive cruise missile over Nevada and pretend nothing happened.

The total program cost was approximately $260 million (roughly $2.5 billion in 2026 dollars). The hardware ended up in warehouses. Merkle went on to other projects. The reactors were never flown.

Here is where the modern case gets interesting — not for weapons, but for space propulsion. The core achievement of Project Pluto was a compact, high-temperature reactor that could directly heat a propellant gas. This is precisely the principle behind Nuclear Thermal Propulsion (NTP), which NASA and DARPA are currently pursuing under the DRACO program (Demonstration Rocket for Agile Cislunar Operations), with a planned orbital test targeting the late 2020s.

Modern advances change the equation dramatically. High-assay low-enriched uranium (HALEU) fuels reduce proliferation concerns. Advanced ceramics and carbide fuel elements — descendants of work done at Los Alamos for Project Rover/NERVA in the 1960s — can withstand higher temperatures with better containment. Computational fluid dynamics allows reactor core optimization that Merkle's team could only approximate with physical mockups.

The specific impulse of a nuclear thermal rocket is roughly double that of the best chemical engines. For crewed Mars missions, this translates to shorter transit times and reduced radiation exposure for astronauts — an ironic redemption for a technology originally designed to irradiate everything beneath it.

Project Pluto proved in 1964 that nuclear ramjets work. The engineering was sound. It was the application — a doomsday weapon — that doomed it. Strip away the warhead, move it to vacuum, and you have the most promising propulsion technology for deep-space exploration that we've validated but never deployed.