When you’re thinking deep space, it’s essential to start planning early, at least at our current state of technology. Sedna, for example, is getting attention as a mission target because while it’s on an 11,000 year orbit around the Sun, its perihelion at 76 AU is coming up in 2075. Given travel times in decades, we’d like to launch as soon as possible, which realistically probably means sometime in the 2040s. The small body of scientific literature building up around such a mission now includes a consideration of two alternative propulsion strategies.

Because we’ve recently discussed one of these – an inflatable sail taking advantage of desorption on an Oberth maneuver around the Sun – I’ll focus on the second, a Direct Fusion Drive (DFD) rocket engine now under study at Princeton University Plasma Physics Laboratory. Here the fusion fuel would be deuterium and helium-3, creating a thermonuclear propulsion thruster that produces power through a plasma heating system in the range of 1 to 10 MW.

DFD is a considerable challenge given the time needed to overcome issues like plasma stability, heat dissipation, and operational longevity, according to authors Elena Ancona and Savino Longo (Politecnico di Bari, Italy) and Roman Ya. Kezerashvili (CUNY), the latter having offered up the sail concept mentioned above. See Inflatable Technologies for Deep Space for more on this sail. A mission to so distant a target as Sedna demands evaluation of long-term operations and the production of reliable power for onboard instruments.

Nonetheless, the Princeton work has captured the attention of many in the space community as being one of the more promising studies of a propulsion method that could have profound consequences for operations far from the Sun. And it’s also true that getting off at a later date is not a showstopper. Sedna spends about 50 years within 85 AU of the Sun and almost two centuries within 100 AU, so there is an ample window for developing such a mission. Some mission profiles for closer targets, such as Titan and various Trans-Neptunian objects, and other Solar System destinations are already found in the literature.

Image: Schematic diagram of the Direct Fusion Drive engine subsystems with its simple linear configuration and directed exhaust stream. A propellant is added to the gas box. Fusion occurs in the closed-field-line region. Cool plasma flows around the fusion region, absorbs energy from the fusion products, and is then accelerated by a magnetic nozzle. Credits [5].

The Direct Fusion Drive produces electrical power as well as propulsion from its reactor, and shows potential for all these targets as well as, obviously, more immediate destinations like Mars. What is being called a ‘radio frequency plasma heating’ method uses a magnetic field that contains the hot plasma and ensures stability that has been hard to achieve in other fusion designs. Deuterium and tritium turn out to be the most effective fuels in terms of energy produced, but the deuterium/helium-3 reaction is aneutronic, and therefore does not require the degree of shielding that would otherwise be needed.

The disadvantages are also stark, and merit a look lest we get overly optimistic about the calendar. Helium-3 and deuterium require reactor temperatures as much as six times higher than demanded with the D-T reaction. Moreover, there is the supply problem, for the amount of helium-3 available is limited:

The D-3He reaction is appealing since it has a high energy release and produces only charged particles (making it aneutronic). This allows for easier energy containment within the reactor and avoids the neutron production associated with the D-T reaction. However, the D-3He reaction faces the challenge of a higher Coulomb barrier [electrostatic repulsion between positively charged nuclei], requiring a reactor temperature approximately six times greater than that of a D-T reactor to achieve a comparable reaction rate.

So what is the outline of a Direct Fusion Drive mission if we manage to overcome these issues? The authors posit a 1.6 MW DFD, working the numbers on the Earth escape trajectory, interplanetary cruise (a coasting phase) and final maneuvers at target. In an earlier paper, Kezerashvili has shown that the DFD option could reach the dwarf planet Eris in 10 years. The distance of 78 AU matches with Sedna’s perihelion, meaning Sedna itself could be visited in roughly half the time calculated for any other propulsion system considered.

Bear in mind that it has taken New Horizons 19 years to reach 61 AU. DFD is considerably faster, but notice that the mission outlined here assumes that the drive can be switched on and off for thrust generation, meaning a period of inactivity during the coasting phase. Will DFD have this capability? The authors also evaluate a constant thrust profile, with the disadvantage that it would require additional propellant, reducing payload mass.

In the thrust-coast-thrust profile, the authors’ goal is to deliver a 1500 kg payload to Sedna in less than 10 years. The authors calculate that approximately 4000 kg of propellant would be demanded for the mission. The DFD engine itself weighs 2000 kg. The launch mass bumps up to 7500 kg, all varying depending on instrumentation requirements. From the paper:

The total ∆V for the mission reaches 80 km/s, with half of that needed to slow down during the rendezvous phase, where the coasting velocity is 38 km/s. Each maneuver would take between 250 and 300 days, requiring about 1.5 years of thrust over the 10-year journey. However, the engine would remain active to supply power to the system. The amount of 3He required is estimated at 0.300 kg.

The launch opportunities considered here begin in 2047, offering time for DFD technology to mature. As I mentioned above, I have not developed the solar sail alternative with desorption here since we’ve examined that option in recent posts. But by way of comparison, the DFD option, if it becomes available, would enable much larger payloads and the rich scientific reward of a mission that could orbit its target rather than perform a flyby. Payloads of up to 1500 kg may become feasible, as opposed to the solar sail mission, whose payload is 1.5 kg.

The two missions offer stark alternatives, with the authors making the case that a fast sail flyby taking advantage of advances in miniaturization still makes a rich contribution – we can refer again to New Horizons for proof of that. The solar sail analysis with reliance on sail desorption and a Jupiter gravity assist makes it to Sedna in a surprising seven years, thus beating even DFD. The velocity is achieved by coating the sail with materials that are powerfully ‘desorbed’ or emitted from it once it reaches a particular heliocentric distance.

Thus my reference to an ‘Oberth maneuver,’ a propulsive kick delivered as the spacecraft reaches perihelion. Both concepts demand extensive development. Remember that this paper is intended as a preliminary feasibility assessment:

Rather than providing a fully optimized mission design, this work explores the trade-offs and constraints associated with each approach, identifying the critical challenges and feasibility boundaries. The analysis includes trajectory considerations, propulsion system constraints, and an initial assessment of science payload accommodation. By structuring the feasibility assessment across these categories, this study provides a foundation for future, more detailed mission designs.

Image: Diagram showing the orbits of the 4 known sednoids (pink) as of April 2025. Positions are shown as of 14 April 2025, and orbits are centered on the Solar System Barycenter. The red ring surrounding the Sun represents the Kuiper belt; the orbits of sednoids are so distant they never intersect the Kuiper belt at perihelion. Credit: Nrco0e, via Wikimedia Commons CC BY-SA 4.0.

The boomlet of interest in Sedna arises from several factors, including the fact that its eccentric orbit takes it well beyond the heliopause. In fact, Sedna spends only between 200 and 300 years within 100 AU, which is less than 3% of its orbital period. Thus its surface is protected from solar radiation affecting its composition. Any organic materials found there would help us piece together information about photochemical processes in their formation as opposed to other causes, a window into early chemical reactions in the origin of life. The hypothesis that Sedna is a captured object only adds spice to the quest.

The paper is Ancona et al., “Feasibility study of a mission to Sedna – Nuclear propulsion and advanced solar sailing concepts,” available as a preprint.