Mar 19th 2025 by Anthony Nielsen
AI created, human edited.
When it comes to reaching Mars and beyond, conventional chemical rockets might not be up to the task. The latest episode of This Week in Space podcast features a fascinating deep dive into nuclear propulsion with Dr. Robert O'Brien, Director of the Universities Space Research Association’s Center for Space Nuclear Research. Hosts Rod Pyle and Tariq Malik explore how nuclear propulsion could revolutionize space travel, potentially reducing travel times and enabling missions that would otherwise be impossible with current technology.
Regaining Lost Capabilities
One of the most surprising revelations during the conversation is that the United States was actively testing nuclear rocket engines in the 1960s under the Rover-NERVA program. These engines weren't just theoretical—they actually worked.
"In the Apollo era, the nation invested incredibly in several programs," Dr. O'Brien explains. "The Rover-NERVA program saw real nuclear thermal rockets tested to a significant technology readiness level."
These tests weren't small endeavors. Engineers ran these nuclear engines for hours, compared to chemical rockets that typically fire for just minutes. This endurance demonstrated the potential for sustained propulsion that could dramatically change the equation for missions to Mars and beyond.
So what happened? According to Dr. O'Brien, the program wasn't canceled due to technical failures or safety concerns. Rather, it was abandoned when plans for crewed Mars missions were shelved after the Apollo program.
"The real driver for stopping the Rover-NERVA program is because we did not continue on to Mars," he says.
Nuclear Propulsion Options
Dr. O'Brien outlines two main approaches to nuclear propulsion currently being investigated:
Nuclear Electric Propulsion
This system uses a nuclear reactor or radioisotope system to generate electricity, which then powers electric propulsion systems. While providing less thrust than chemical rockets, nuclear electric propulsion can operate continuously for long periods, gradually building up substantial velocity.
"With nuclear electric propulsion, you can maintain steady propulsive operation and, over a period of time, build up a total change in velocity or delta V that could indeed leave the Earth's gravity well and go to the outer planets," Dr. O'Brien explains.
Nuclear Thermal Propulsion
Like a conventional rocket, nuclear thermal propulsion expels propellant through a nozzle to create thrust. The difference is in how the propellant is heated—using a nuclear reactor rather than chemical combustion.
"Just like we use water on Earth to cool a reactor, we use hydrogen or another propellant like ammonia or helium to cool that reactor," Dr. O'Brien says. "But in doing that, it too becomes a propellant, and we expel it through a nozzle."
The result? "The efficiency is at least several factors greater than conventional chemical rocket systems," he notes.
"Maneuver Without Regret"
One compelling phrase Dr. O'Brien uses to describe the advantages of nuclear propulsion is "maneuver without regret." With chemical rockets, fuel is precious; every unplanned maneuver or course correction could potentially leave a spacecraft without enough propellant to complete its mission.
"When we look at space operations, the regret comes when you make an unplanned maneuver or you have to burn too much propellant. You leave nothing in reserve, and that could lead to regret," he explains.
Nuclear propulsion systems could fundamentally change this equation, providing enough propulsive capability to handle contingencies without endangering the mission.
"This gives us the best chance to get people to Mars and bring them home quickly to cut down on dose and radiation dose that people might pick up with a long duration, transport and transit to the destination," Dr. O'Brien says.
Safety Considerations
Addressing the elephant in the room, the conversation turns to safety concerns regarding nuclear rocket technology. Dr. O'Brien notes that today's approach is drastically different from the open-air testing conducted in Nevada during the 1960s.
"The regulatory space was actually being defined as the technology was being developed back under the Rover-NERVA program," he says. "We would not do open air testing [today]."
Modern development includes comprehensive safety protocols, including full containment of exhaust during testing and safeguards to ensure reactor systems remain non-critical even in worst-case scenarios.
"We're actively working on key technologies to capture the exhaust, provide enclosures and full containment" for ground testing, Dr. O'Brien explains. "Even if the system catastrophically failed, there would not be a release to the public."
Project Orion: The Ultimate Nuclear Propulsion Concept
The conversation takes a fascinating detour into Project Orion, perhaps the most audacious nuclear propulsion concept ever proposed. Developed in the 1950s and 60s, Orion envisioned spacecraft propelled by a series of small nuclear detonations.
Rod Pyle, clearly excited about the topic, describes how these designs ranged from small vehicles to massive 8-million-ton spacecraft "the size of Glendale, California" he jokingly adds, capable of carrying 150 people at potential interstellar speeds.
Dr. O'Brien offers a more measured take: "I think that's safe to say that there's very little to no interest today in launching a system from the Earth's surface propelled by nuclear devices. But for interstellar space, for example, there could be some rationale to look at pulsed nuclear detonation propulsion."
He points out a fundamental limitation of the concept—the inefficiency of the energy transfer. "With spherical pulsed events, there's isotropy in where that energy goes," he explains. "Only a fraction of the energy that is generated by each device is actually captured on the system... at least 60% waste energy goes out to the other part of space."
The Future: AI and Fusion
Looking to future developments, Dr. O'Brien discusses how artificial intelligence could enhance nuclear propulsion systems through materials optimization, autonomous operation, and safety protocols.
"If you think about controls, if you are looking at robotic missions or deep space missions that have a ground control element, the time to send a command, receive that command, perform an operation can increase, of course, the further away from that control center," he explains. AI could help close this control loop without requiring human intervention.
Even more exciting is the potential of fusion propulsion. While nuclear fission is the immediate focus, Dr. O'Brien sees fusion as a promising future technology.
"Sustained ignition, sustained fusion, how you fuel a fusion system of tomorrow, I think is an interesting topic," he says, adding that fusion could be useful not just for propulsion but also for science applications like analyzing geological samples on the Moon or Mars.
Public Engagement and Education
Throughout the conversation, Dr. O'Brien emphasizes the importance of public education and transparent communication about nuclear technologies in space.
"In the vacuum of information, people create their own narrative," he observes. "We communicate with the public and do not treat everybody as though they have no clue what a safe system looks like or what their concerns are about safety." Clear communication is key.
This approach marks a departure from the secretive development of nuclear technologies in the past, recognizing that in today's connected world, "no government, organization or industry can hide in silence."
The Path Forward
The Universities Space Research Association’s Center for Space Nuclear Research is working to develop the expertise needed to advance these technologies, which Dr. O'Brien describes as "unicorn capabilities." Their approach combines practical program needs with teaching and learning experiences for students and early-career professionals.
As the conversation wraps up, there's a sense of optimism about nuclear propulsion's potential to transform space exploration—enabling faster travel times, greater operational flexibility, and ultimately, human expansion throughout the solar system.
Want to learn more about nuclear propulsion and other cutting-edge space technologies? Listen to the full episode of This Week in Space featuring Dr. Robert O'Brien, Director of the Universities Space Research Association’s Center for Space Nuclear Research, for discussions on NASA's latest developments, Saturn's newly discovered moons, and more insights into the future of space exploration. Available now on your favorite podcast platforms.
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