The idea of space travel with parachutes and nuclear explosions will sound plenty familiar to
NASA alum. In 1946, only a year after the explosive and devastating debut of the nuclear age, Polish-American mathematician and physicist had an idea for a rocket that was so crazy it just might work. During the war, Ulam worked side-by-side with Robert Oppenheimer and Edward Teller on the creation of the atomic
bomb at Los Alamos, and now that peace was achieved, the future of the nuclear age was only just beginning. It was during this moment of possibility while still conducting research at the famous New
Mexico facility, that Ulam’s thoughts turned toward the stars: Could the explosive power of the atomic bomb be used for exploration instead of destruction? In other words, could nuclear bombs somehow become deep-space rocket engines? “It is a very ambitious but efficient way to undertake space explorations with a vehicle able to travel at high speeds with high payloads and an extremely good ratio of payload to total initial weight,” Ulam wrote in his 1976 memoir . “The spaceship could transport hundreds or thousands of people.” Earthlings want to send a probe light years away to spy on the San-Ti, but need an idea to get there, fast. Fast-forward to 2024 and Ulam’s nuclear daydreaming is enjoying a renaissance of sorts as the
Netflix series , based on a novel by Chinese sci-fi author Liu Cixin, cribs the idea as a fast-and-not-so-easy way to propel a payload to incredible speeds. To achieve this, the characters devise a “radiation sail” to be propelled by a thousand consecutive nuclear explosions in space until the
spacecraft reaches a velocity just a notch above 1 percent of lightspeed. But while the uses Ulam’s nuclear rocket for decidedly sci-fi ends, the 80-year-long story of this developing technology, known today as nuclear pulse propulsion (NPP), is very real — and it may be the future of space exploration. Every rocket humanity has ever sent skyward has been powered by chemical fuel, a combination of kerosene, oxygen, and hydrogen, or both, with enough “oomph” to escape Earth’s orbit and reach its otherworldly destination. That’s how the Apollo astronauts landed on the
moon, how the Space Shuttle crews built the
International Space Station, and how future space explorers will put bootprints on Mars. But similar to how fission (and especially fusion) represent efficient energy sources of the future, the same can also be said for rocketry. And while the idea of riding the concussive explosions of a nuclear bomb may seem strange, the idea isn’t quite as strange as you might think. “Your car is a pulsed system because the piston is compressing gas and air together, and then it explodes and pushes the piston apart,” Jason Cassibry, a professor at the Department of Mechanical and Aerospace Engineering at the University of
Alabama in Huntsville, tells . Cassibry is affiliated with the university’s where he recently worked similar to magneto-inertial fusion technology. “So every time you explode [a bomb] according to Newton's second law, it would give it an increase in momentum and accelerate it, just like you would driving your car and stepping on the gas.” NASA’s Project Orion involves a heavy-lift vehicle that used atomic bombs to detonate behind a pusher plate fitted with shock absorbers. Ulam recognized the beautiful simplicity of a pulsed system, and after mulling over the idea for years, he finally put his thoughts to paper stating that “the scheme proposed in the present report involves the use of a series of expendable reactors (fission bombs) ejected and detonated at a considerable distance from the vehicle.” Ulam eventually presented the idea to President Eisenhower’s scientific advisor George Kistiakowsky whose “reception of it was not enthusiastic,” Ulam wrote in his memoir. Despite initial skepticism, the idea eventually gathered steam , a heavy-lift vehicle concept that used atomic bombs, ranging from a few to several kilotons, to fitted with shock absorbers to limit the impact of that initial, explosive acceleration. While many of the early design challenges of the spacecraft were overcome, the project shut down in 1965 due to nuclear treaties that prohibited nuclear explosions in space. Chemical rockets had also become more powerful and were clearly NASA’s preferred chariot to the stars during the Space Race. But the idea didn't die and several projects carried the NPP flame with names like , , and . One of the most intriguing ideas was a spacecraft concept that altered the pulsed propulsion design using a lightweight sail (technically a spinnaker) to harness the pressure pulses of subsequent nuclear explosions, a concept that’s extremely similar to the one explored in the . Sadly, none of these concepts — including Cassibry’s own — ever made it to the launch pad, largely due to technological limitations and lingering concerns over detonating nuclear explosions in space. “There was a resurrection of Project Orion in the 90s when Clinton was in office, and when they got to the level to talk to some of his staffers…they said ‘no way are we putting
nuclear weapons in space,’” Cassibry says. “Now, they’re even more sensitive to it — even though the Cold War is over, there are still concerns.” However, not all nuclear propulsion systems are the same. While Cassibry worked on pulsed systems, ostensibly the great-grandchild of Ulam’s original vision, other systems include (NTP) and (NEP). While all three are based on nuclear technology, NTP and NEP use more traditional fission systems (i.e. heating up propellant, which turns to gas, and results in propulsion). So unlike NPP, which uses microexplosions for propulsion (much to the chagrin of international nuclear treaties), NTP and NEP systems can’t be weaponized. While all three technologies use different methods and have various use cases, nuclear propulsion — whether through fission heating or nuclear
explosion — has some pretty stark advantages over its chemical competitor. “There are certain missions to the ice giants — Uranus and Neptune — that could not be done, given all the constraints that you put on a mission that far out, with anything other than nuclear thermal,” Cassibry says. “[NTP] is more straightforward than some of the things we could do…it’s kind of low-hanging fruit in terms of advanced propulsion concepts.” NASA has taken notice and hopes to test its (DRACO) NTP rocket in 2026. Being three times more efficient than chemical rockets, that means Draco could travel from the Earth to
Mars in just 45 days or carry increased cargo loads in a more conventional timeframe. However, Cassibry still believes that nuclear pulse propulsion, using fission, fusion, or a combination of the two, will ultimately outperform even these nuclear-powered spacecraft decades down the road. “There’s a joke that crushes my soul every time I hear it that fusion is the technology of the future and it always will be,” Cassibry says. “But we started with really, really, terrible reactors and we’ve been making steady progress…we’ll see fusion propulsion being possible within about 20 to 30 years.” Whether on a popular Netflix show or in the minds of the leading minds on space travel, it would seem Ulam’s dream is alive and well.