Spacefaring Requires Advancing Space Propulsion

The roar and rumble of fast chemical rockets excite us. However, chemical propulsion systems are insufficient for a long-term presence in space. We will not become truly spacefaring with chemical propulsion alone. Sustainable space transportation requires alternative and forward-thinking paradigms to space propulsion. 

Propulsion is one of my favorite nouns in both English and Spanish. It originates in medieval Latin. Space propulsion gets even more exotic. In addition to the conventional chemical varieties, space propulsion includes a wide range of non-chemical systems, including but not limited to electric, nuclear, and photonic propulsion. 

I have been fortunate enough to look in-depth at alternative paradigms for space propulsion in my professional trajectory. I have also learned about the myriad nuances, advantages, and disadvantages of different space propulsion technologies for robotic and crewed missions in cislunar and deep space.  

For this article, I examined two different approaches in Seattle, United States, and Helsinki, Finland. Both of them seek to advance the state and limitations of the space industry to empower us with options. 

Agile thinking inspires nuclear design: A startup that brings the next evolution of NTP engines 

Established in 2019 as an independent subsidiary of its parent company, the R&D mission at the Seattle-based USNC-Tech is clear. It focuses on advancing nuclear technologies and in-space nuclear propulsion systems, specifically nuclear thermal propulsion - NTP and nuclear electric propulsion - NEP. NTP uses the thermal energy from a nuclear reactor to heat a rocket propellant to extremely high temperatures for high thrust (mechanical force). NEP uses a nuclear reactor to generate electricity to power an electric thruster with comparably less thrust than NTP. For a more in-depth, side-by-side comparison, consider reading these documents: A Comparison and Space Nuclear.

The foundation of USNC-Tech comes at a pivotal time in U.S. nuclear propulsion history. A decades-long bottleneck for nuclear space systems has been managing non-weapons grade uranium fuel capable of operating at extremely concentrated high temperatures. NTP is quickly becoming a favorite among U.S. government agencies for rapid innovation.

With sustained and continued support from the Executive and Legislative Branch across multiple administrations, two major nuclear thermal propulsion programs are already underway. The Defense Advanced Research Projects Agency - DARPA leads one under the Demonstration Rocket for Agile Cislunar Operations - DRACO program. Major companies such as Blue Origin, General Atomics, and Lockheed Martin received contracts to lead the demonstration missions of NTP systems. USNC-Tech supports some of these companies in the first phase of the DRACO program. It provides nuclear R&D expertise to Blue Origin. It also collaborates with General Atomics for key analysis capabilities and expertise.

The National Aeronautics and Space Administration - Space Technology Mission Directorate - (NASA STMD) leads the second major nuclear thermal propulsion program. USNC-Tech received a Phase I NASA Small Business Innovation Research program - SBIR grant in 2020 for a generator insulator for NTP systems. Most recently, NASA and the U.S. Department of Energy released a bid for proposals for a demonstration mission to Mars. USNC-Tech promptly embraced the opportunity and responded to the request for proposals.

NASA’s STMD also sponsored the 2021 Consensus Study Report, Space Nuclear Propulsion for Human Mars Exploration. Prepared independently by the National Academies of Sciences, Engineering, and Medicine, some key recommendations are summarized as follows:

[B]oth NEP and NTP systems show great potential to facilitate the human exploration of Mars. Using either system to execute the baseline mission, however, will require an aggressive research and development program… Because of the low and intermittent investment over the past several decades, it is unclear if even an aggressive program would be able to develop an NEP system capable of executing the baseline mission in 2039... For NTP systems, the fundamental challenge is to develop a system that can heat its propellant to approximately 2700 K, which is necessary to meet system performance requirements. Other key challenges are the long-term storage of liquid hydrogen in space with minimal loss, the lack of adequate ground-based test facilities, and the need to rapidly bring an NTP system to full operating temperature. An aggressive program [for NTP ] could overcome these challenges to achieve the baseline mission in 2039.  

USNC-Tech leverages the R&D from its parent company to build the next generation of the Fully Ceramic Microencapsulated (FCM™) fuel. FCM fuel contains the U-235 uranium kernel that uses less than 19.75% of high-assay, low-enriched uranium (HALEU). The fuel packaging is also layered with ceramic coatings and encased within a fully dense silicon carbide matrix. This unique packaging allows it to operate with temperatures of the order of 2,700 degrees Kelvin. To use this fuel in nuclear thermal propulsion systems and heat its propellant at even higher temperatures, USNC-Tech innovates with a couple of different materials. Instead of using a silicon carbide matrix, for example, the company uses zirconium carbide. USNC-Tech also names their reactor cores with popular science fiction names, including Scotty, Padmé, and Sulu - Super Use for Low-Enriched Uranium. Crucial components of their nuclear systems are at a technology readiness level - TRL above three.

NTP engine design. Courtesy of USNC-Tech.

Paolo Venneri is the Founder and CEO of USNC-Tech. He leads 35 full-time staff distributed across Seattle, Texas, California, and Utah. Paolo believed early on in the importance of nuclear space propulsion. 

Paolo Venneri. May 2021. 

In our conversation, he recalled being impressed by South Korea's prolific engineering capacity to build nuclear power plants and reactors on time and budget in early 2010. Paolo undertook a doctorate in nuclear engineering at the Korean Advanced Institute of Science and Technology in Daejeon, South Korea. With a patent under his name for the control of reactivity in an NTP system and increasing its overall performance, Paolo shared the following:  

"NTP has a combination of high thrust and high specific impulse - Isp, which means you can take large payloads and get to destinations in cislunar space and Mars pretty quickly. Technology development has already progressed a lot for NTP. And second, nuclear propulsion is, in a certain sense, propellant agnostic. You can use hydrogen to get the highest Isp, but you can also use other propellants such as ammonia. You could even design a nuclear propulsion system that uses water as a propellant. This versatility opens up the possibility of having real in situ resource utilization of propellants. At that point, you're essentially taking care of the rocket equation.

However, for nuclear thermal propulsion to enable us to be a spacefaring species, it has to have a commercial future. We have done a considerable amount of work at USNC-Tech showing that non weapons-grade material can be used as the fuel for these propulsion systems. The really unique thing about where we are now in the space industry, compared to where we were back in previous decades, especially during the Cold War era, is that the nuclear fuel for propulsion systems can be owned and operated by commercial entities. A system that is demonstrated for a governed application can then be used for commercial systems to have a wider impact.

In this sense, the entrepreneurial nature of the nuclear space sector offers particular advantages. We can approach old problems with a new perspective. When we first started working with NASA, for example, we were fresh out of grad school. The approach that we took was not to deliver them a single concept that was super optimized and took millions of labor hours to do. Instead, we automated the design process so that we would be able to deliver to NASA millions of reactor designs in one single presentation. We were able to answer questions on the spot relating to changing factors and performance parameters. 

Often, the new folks who approach new problems don't have the history and baggage of those trying to do nuclear propulsion for the past 30 or 40 years. We're not daunted by the difficulties that are involved with it. This allows us to actually approach the challenges with new solutions and then sort all possible options to find the optimal solution that works for a particular use case. At USNC-Tech, we try things. If they don't work, we can pivot and move quickly on to the next potential solution. And this new entrepreneurial perspective and approach for nuclear space brings a huge amount of value in it.

Defined by a preference for agile-thinking, I asked Paolo about his personal and company motivation. He explained:

I have always been confident that nuclear power is critical and necessary to have a sustained, permanent presence in space. Being a part of that movement to develop this technology beyond the defense and government applications has continuously kept me going with the idea that I am on the right path. I've come to realize that the really distinctive thing about us at USNC-Tech is that we have reactor designers, thermal analysts, material scientists, spacecraft system designers sitting in the same room and communicating daily. We are united by this dream of building and deploying commercial space nuclear systems. If you look at larger companies with a longer heritage of doing nuclear, they're essentially characterized by silos. Different areas often don't really talk to each other or are not continuously iterating solutions in the same room. The fact that we're all in the same place and across all these different fields, constantly iterating solutions allows us to have this agile-type approach to systems design.

Propellantless Solar Electric Propulsion for Smaller Payloads: The Promise for Deep Space Missions  

Pekka Janhunen has been innovating space propulsion concepts for more than two decades. A theoretical physicist by training, Pekka has always been thinking about the future. Originally from Finland, he has published 100+ scientific papers that detail his inventions. Pekka is widely recognized for two main innovations: electric solar wind sail (E-sail), a patent he owns, and plasma brake propulsion technology.  

An electric solar wind sail (E-sail) is meant to be lightweight to travel beyond a planet's magnetic field (i.e., magnetosphere) into the Solar System and deep space. In lieu of propellant, the E-sail uses several long and thin charged tethers (wires), centrifugally stabilized to extract energy from the solar wind's charged particles for thrust. Solar winds are surges of plasma and ionized particles that vary in intensity but are always present. In fact, despite significant variations in solar wind density, the propulsive thrust generated by these surges varies much less than the solar wind itself. 

E-sail model. Photo courtesy: Sini Merikallio. Finnish Meteorological Institute.

Propellantless solar sail propulsion systems have also been referred to as light sails or photon sails. Light sails use the sun's radiation pressure to propel it forward instead of charged tethers. In contrast, the vision for the E-sail integrates a solar-powered electron gun to keep the spacecraft and the tethers with a high positive charge (approximately 20 kV). E-sails are strong candidates for supporting cargo and refueling needs for crewed Mars missions. Pekka co-wrote an entire paper on this anticipating an e-sail's payload capacity between four to ten tonnes. E-sails require several days for maneuverability and adjustment. They cannot land on celestial bodies with tethers open.

Plasma brake propulsion technology uses a single stabilized charged tether device to deorbit a small satellite after completing its mission. This concept aims to offer a solution to the growing debris problem in low Earth orbit - LEO. Plasma break technology could be conceptualized with centrifugal-spinning tethers for larger satellites as well. However, according to Pekka's research, the proposed baseline idea is to stabilize the single tether by using Earth's gravity gradient. There is no need for an electron gun in plasma brake propulsion. It's enough for this technology to have an electrically conducting surface area that absorbs the natural thermal electron current from the surrounding plasma. A negative tether polarity works in Earth's ionosphere. A positive tether polarity could also work with solar winds.

I met Pekka recently. In addition to serving as Research Manager at the Finnish Meteorological Institute, Pekka is also the Senior Technical Advisor to the Finnish startup Aurora Propulsion Technologies. Aurora joined the European Space Agency Business Incubation Centre - ESABIC Finland program in 2019. It focuses on propulsion needs for small spacecraft. Under a non-exclusive licensing agreement with Pekka, Aurora also seeks to commercialize the E-sail and the closely related plasma brake technology. 

Pekka Janhunen. May 2021

As part of its ambitious plans, Aurora proposes the North Star mission. North Star would represent the first international in-space demonstration of an E-sail. The small spacecraft weighing roughly under 25kg would exit the Solar System with a small scientific payload by way of the North Celestial pole (North Star). 


During our virtual meet, Pekka voiced the following:

E-sails could contribute to crewed missions so that we could move the cargo or propellants for augmentation. The North Star mission would show a potentially cheap way to launch small payloads at high speed out of the solar system. From my perspective, it's under-appreciated the fact that this type of mission could potentially be relatively cost-efficient. Instead of 1 billion euros, it might be 10 million only or even less for the first mission. If it becomes routine, it would be much more affordable than that because we would only need to launch 25 kilograms to any kind of escape orbit to exit the Solar System. 

What's also not widely known is that E-sails can piggyback with any launch vehicle directed at any orbit, which goes either to escape orbit or very high elliptical Earth orbit. It's very different from the chemical routes where launch vehicles target the desired destination. This is economically rather important because there will be increasingly more opportunities to launch E-sails heading towards Jupiter or anywhere in the solar systems regardless of the spacecraft mission: Mars, lunar, etc. 

Given the state of materials science, the maximum tensile strength of materials and conductivity limits the quantity and length of the tethers that we can use. Let's say 1000 km at maximum capacity, and that limits total thrust to about 1 newton (N) or less. Of course, we can try to push these boundaries by improving materials science. 

In the case of the specific mechanisms of solar winds, the solar wind varies but is always there. The propulsive thrust generated varies much less than the solar wind itself. One reason is that when the solar wind is more tenuous, the effective electric width of the charged tethers increases i.e., the virtual sail area increases, which compensates part of the decrease of the dynamic pressure. Another mechanism is that low solar wind density allows us to increase the tether voltage without increasing the power consumption, which again cancels part of the thrust decrease. A third mechanism is that low solar wind density usually comes with higher solar wind speed, increasing the dynamic pressure. Despite significant variations in solar wind density, all three mechanisms tend to equalize the thrust when the device's power consumption is held constant. 

Pekka’s steadfast commitment to advancing space propulsion and technologies certainly inspires. I asked him to discuss his personal journey in the space sector. 

I guess this forward-thinking is somehow related to my personality type. I have always been living in the future. History wasn't my favorite subject in school. I was very excited about space when I was about maybe six years old. But then I kind of forgot about it for many years. I accidentally drifted to working with space plasma physics because there was a job opportunity to apply my skills as a theoretical physicist. I can deal with formulas and computers to model things. I did this for purely scientific reasons for about ten years. It's been a good position to be in at the Finnish Meteorological Institute. I have been relatively free to do what I want compared to many other researchers and professors in the world. When I invented this E-sail concept as a spinoff of space plasma physics, I was already working on it. It turned out that it provided technological value. I gradually started to learn about space technologies, chemical and electric propulsion. I have slowly drifted towards the more practical and engineering aspects of space. For the last years, for example, I have started writing articles about orbital space settlements.


Finald is a rising player in the new space wave. It has earned notoriety for its consistency in building private-public alliances. The National Space Strategy, revised in 2018, established bold milestones for nurturing and attracting commercial businesses. Pekka offered his insights about this:  


The startup scene has been strong in Finland for about 15 years in gaming, biotechnology, medicine, and IT. But not in the domains of space. Investors were not interested in the Finnish space industry for a long time. But then ICEYE happened. ICEYE is a successful startup story in Finland. They are now becoming the world’s leading commercial provider in radar imaging satellites. The benefit of the radar is that it can monitor everything on a surface and even through clouds. This advantage is certainly valuable because no other method can see through clouds. The success of ICEYE opened the floodgates of funding to other space startup companies in Finland around 2018.

Architectural explorations - RED (2017). Tarmo Juhola. 

Sustained Space Travel Depends on Advanced Propulsion Systems

My fascination with the term space propulsion goes beyond semantics, of course. To be genuinely multiplanetary, we have to travel faster and further into space. This sense of urgency is not optional. It is crucial to give support and visibility to the space engineering efforts that equip us with more options for space transportation

Paolo and his nimble team at USNC-Tech in Seattle focus on nuclear space systems. They work in alliance with the U.S. government, NASA, and commercial partners. Nuclear thermal propulsion systems promise the quickest technology readiness level for commercial space travel. 

Pekka serves double duty as a full-time researcher in Helsinki and advisor to Aurora Propulsion Technologies in Espoo. Pekka hopes that deep space exploration becomes more accessible and affordable. A historic in-space demonstration of his invention, the E-sail, can potentially pave the way for more missions.

We live in a golden era for space exploration. Entrepreneurs and researchers far and wide work hard to develop propulsion systems and technologies that allow us to break the silos around Planet Earth, the Moon, and Mars. Startups harness unprecedented private-public alliances to go quickly beyond the proof of concepts. If we continue to welcome and embrace innovation across the board in space propulsion, I trust that we will surprise ourselves and future generations. 

Return to base (2020). Tarmo Juhola.

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The featured artist is Tarmo Juhola. A Finnish architect, illustrator, and concept artist, Tarmo is based in Helsinki. He has been awarded multiple times in architectural competitions. I selected Return to base (2020) and Architectural explorations - RED (2017) because of the exquisite detail in the artwork. Tarmo surprises us with unique ways to think about spacefaring civilizations and advanced space propulsion systems.