In the outer reaches of our Solar System, far beyond the orbit of Pluto, lies one of the most mysterious objects ever discovered, Sedna. This reddish dwarf planet follows such an extreme orbit that it takes over 11,000 years to complete a single journey around the Sun. Now, scientists are proposing a new mission to reach this distant world using a revolutionary propulsion technology.

Sedna represents far more than just another distant rock, though. It’s a new orbital class of object, the sednoids, and its extreme orbit suggests it may be the first known member of the inner Oort Cloud. Understanding Sedna could unlock secrets about the early Solar System’s formation and the gravitational influences that shaped it.

Sedna imaged from the Hubble Space Telescope (Credit: NASA)

Its surface is one of the reddest among Solar System objects, suggesting a complex chemistry that could reveal clues about organic compounds in the outer regions. At its current distance, temperatures never exceed −240°C, making it one of the coldest places in our Solar System.

This distant world is expected to pass through the perihelion of its orbit, its closest point to the Sun, in 2075–2076 and then move slowly away from the Sun. When Sedna reaches its closest approach at 76.19 AU (about 76 times Earth’s distance from the Sun), it will still be incredibly distant, nearly three times farther than Neptune. After this brief moment, Sedna will begin its long journey back toward the darkness again, not returning to this proximity for hundreds of years.

A new feasibility study has examined two cutting-edge approaches to technology that would reach Sedna within this narrow window of opportunity. The first involves the Direct Fusion Drive (DFD), a conceptual nuclear fusion engine, designed to produce both thrust and electric power. For the DFD, researchers assume a 1.6 MW system with constant thrust and specific impulse, representing a massive leap beyond current propulsion technology.

Schematic of a Direct Fusion Drive (Credit: WikiHelper2135)

The second approach involves an ingenious variation on solar sailing technology. Rather than relying entirely on solar radiation pressure, this concept uses thermal desorption instead. This is a process where molecules or atoms that are stuck to a surface are released when that surface is heated up, and it’s this process that produces the propulsion. It would be assisted by a gravity assist manoeuvre around Jupiter, using the planet’s immense gravitational field as a gravitational slingshot.

Artist impression of IKAROS, the first space-probe with a solar sail in flight (Credit: Andrzej Mirecki)

The analysis reveals surprising results about these two vastly different technologies. Results from the paper that was authored by a team led by Elena Ancona from Politecnico di Bari in Italy indicate that the DFD could reach Sedna in approximately 10 years, with 1.5 years of thrusting. Solar sails, on the other hand, assisted by Jupiter’s gravity, could complete the journey in 7 years. The solar sail’s superior travel time stems from its ability to continuously accelerate without carrying heavy fuel, while the fusion drive offers the advantage of enabling orbital insertion rather than just a flyby.

This speed difference highlights the fundamental trade-offs in deep space exploration. Due to the different performances, the DFD would enable orbit insertion whereas for the solar sail, only a flyby is envisioned. An orbital mission would allow for extended study of Sedna, mapping its surface, analysing its composition, and potentially discovering moons or other features. A flyby, while faster, would provide only a brief snapshot.

Both proposed technologies face significant development hurdles. The DFD remains largely conceptual for now, requiring breakthroughs in fusion containment and control that have eluded us for decades. The company’s modelling shows that this technology could propel a spacecraft with a mass of about 1,000 kg to Pluto in four years, but achieving such performance in reality remains, for now, uncertain.

Pluto’s image taken by New Horizons on July 14, 2015 (Credit: NASA/Johns Hopkins University Applied Physics Laboratory)

Advanced solar sailing with thermal desorption represents a more evolutionary approach, building on proven solar sailing principles while adding new capabilities. The technique’s reliance on precisely timed gravitational assists and innovative materials science presents its own challenges, but may be more achievable in the near term.

The window for reaching Sedna during its current approach is rapidly closing, though. Whether humanity will rise to meet this challenge depends on our willingness to invest in revolutionary propulsion technologies and embrace the risks inherent in pushing the boundaries of space travel.

Source: New Propulsion Systems Could Enable a Mission to Sedna