Saturn’s largest moon, Titan, has been a source of mystery ever since scientists got their first up-close look at it in 1979 with the Pioneer 11 probe. The mystery has only deepened as additional spacecraft, like the Voyager probes and the Cassini-Huygens mission, acquired better images and more data on this moon. In addition to being the only body other than Earth to have a dense, nitrogen-rich atmosphere, Titan also has a methanogenic cycle similar to Earth’s water cycle. On top of that, it has a rich prebiotic environment and organic chemistry on its surface.
These properties have fueled speculation that life may exist on Titan, possibly in the form of simple life forms that dwell within its vast methane lakes. This is also why NASA is developing the Dragonfly mission, a nuclear-powered quadcopter that will begin exploring Titan’s atmosphere and surface in 2034. But what about crewed missions to Titan? According to a recent study by Explore Titan, a nuclear-fission propulsion spacecraft could enable the first crewed mission to the outer Solar System.
The study was conducted by William O’Hara and Marcos Fernandez-Tous. O’Hara is a researcher from the University of North Dakota, the Lunar Habitat Formulation Lead & Technical Program Director at Blue Origin, and an engineering consultant at Explore Titan, Inc. Fernandez-Tous is an Assistant Professor of Space Studies at the University of North Dakota. The paper describing their concept was submitted to the 56th Lunar Planetary Science Conference (LPSC 2025) that took place in The Woodlands, Texas, back in March.
Since NASA launched the Constellation Program in 2005, its long-term aim has been to resume sending humans beyond Low Earth Orbit (LEO). This would consist of a stepping-stone strategy, starting with the first crewed missions to the Moon during the Apollo Era. This was to be followed by the creation of infrastructure in deep space that would enable missions to Mars and beyond. Since transit times to Mars could take between 6 and 9 months, multiple space agencies are investigating propulsion methods that could make the trip faster.
Explore Titan is a non-profit organization dedicated to fostering a dialogue about where this momentum should lead humankind after Mars. Essentially, they hope to extend the Moon to Mars approach to include Titan in the near future. As they state in their proposal paper:
“Of the possible candidates, Titan, the largest moon of Saturn, is the obvious candidate. Explore Titan proposes the next mantra beyond Moon-to-Mars to be Mars-to-Titan, where technology developed for human missions to Mars could be extended or evolved for use in missions to Titan.”
However, the distance between Earth and Titan is far greater than between Earth and Mars—approximately 8.5 astronomical units (AU) vs. 0.5 AU. As a result, transit times must be significantly reduced to ensure the crew spends a minimum amount of time in microgravity and is exposed to less cosmic radiation. Research into nuclear propulsion can be divided into two broad categories: nuclear-thermal propulsion (NTP) and nuclear-electric propulsion (NEP).
For their study, the team assessed the feasibility of NTP and NEP options for a hypothetical human-class mission to Titan with transit times of one to two years (one-way). They then evaluated the total mass of these systems (engines and propellant) and their resulting impact on spacecraft design. This began with the NTP concept outlined in NASA’s Design Reference Architecture 5.0 (DRA 5.0), which describes a 56.25 metric ton (62 U.S. ton) crew-capable spacecraft that utilizes a uranium-235 reactor and hydrogen propellant to achieve a two-way mission to Mars in about 375 days.
Combined with surface operations of 540 days and an optimal launch window, this mission would last two and a half years. A mission of this duration would pose significant risks for crew health, especially when extended for missions to Titan. They also considered “Copernicus,” the larger NTP concept proposed in a 2013 study led by NASA Glenn scientist Stanley K. Borowski. This vehicle would have a 172 metric ton (190 U.S. ton) propellant capacity and could reduce one-way transit times to 150 to 220 days.
However, the authors note that “predictions have shown that even this deep space mission duration may exceed allowable limits for crew exposure to cosmic radiation.” While the Copernicus could conceivably reduce one-way transits to 90 days by adding more propellant tanks, this would considerably increase the spacecraft’s mass and cost. They also considered the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) concept proposed by the Ad Astra rocket company.
This concept relies on a nuclear magneto-hydrodynamic (MHD) reactor to power an electric thruster. According to a study by Franklin Diaz et al., incorporating this engine could reduce transit times to 149 days. Last, they considered the 2020 study by Marco Gajeri et al. that proposes how a Direct Fusion Drive (DFD) could enable a robotic mission to Titan that would take 2.6 to 2 years to achieve a round-trip. Properly adapted and optimized for a human-rated mission, these concepts could enable the next great leap in human space exploration. As the team summarized:
“From this review, we conclude that nuclear space propulsion systems may be the key enabler for future missions to Titan. For human missions, given the health risks of deep space human spaceflight, it could be the most critical element to their success.”
Further Reading: USRA