Direct Fusion Drive Could Take Us To Sedna During Its Closest Approach In 11,000 Years

In 2003, NASA-funded researchers spotted what was then the most distant object discovered in our Solar System. The dwarf planet, named Sedna after the Inuit goddess of the sea, was found over 12.9 billion kilometers (8 billion miles) from Earth, potentially making it an inner object of the hypothetical “Oort cloud” surrounding our Solar System. 

“Sedna is distinguished among other objects mentioned above by its highly elongated orbit and a much greater aphelion radius,” a 2021 paper on the topic explains, adding that the surface composition also shows similarities with Kuiper Belt objects.

“The discoverers have supposed that Sedna was created in the Solar System at the early stage of its evolution, and its orbit was changed because of dynamic effects that followed the Sun’s formation within a dense stellar cluster. According to other versions, Sedna’s orbit was changed by a stellar encounter (e.g., the passing Scholz’s star about 70 thousand years ago at a distance of 52 thousand au from the Sun), or Sedna was captured from a low-mass star or a brown dwarf in interstellar space.”

The inner edge of the Oort cloud is thought to begin around 2,000-5,000 astronomical units (AU) from the Sun, with one AU being the distance between the Earth and the Sun, and ending somewhere between 10,000 and 100,000 AU from the Sun.

When it was discovered, Sedna was around 13 billion kilometers (8 billion miles) from the Sun.

“The Sun appears so small from that distance that you could completely block it out with the head of a pin,” Dr Mike Brown, associate professor of planetary astronomy at the California Institute of Technology, said in a NASA statement when the discovery was reported.

But its unusually wide orbit can take it much further away. In fact, getting multiple observations of the dwarf planet and calculating its trajectory, astronomers found that it is on an 11,000-year journey to around 936 AU, or 150 million kilometers (93 million miles). Astronomers had spotted Sedna near its perihelion, its closest approach to the Sun.

“The last time Sedna was this close to the Sun, Earth was just coming out of the last Ice Age,” Brown added. “The next time it comes back, the world might again be a completely different place.”

That gives humanity a small window of opportunity to study a potential Oort cloud object. But it is a small window, given that the object is still really, really far away. In July 2076, it will make its closest approach to the Sun, but will still be around 76.37 AU from it. 

We have sent spacecraft beyond this distance (hello, Voyager 1 and 2), but over the course of decades. In a new paper, which has not yet been peer reviewed, a team looks at new concepts for getting a mission to the object before it slowly moves away from us for thousands of years.

“Given its orbital period of 11 thousand years, scientists have been proposing missions for launch in the next few years (around 2030), including a gravity assist that would allow reaching Sedna in time for its closest approach in 2075,” the team explains in their paper. 

“Considered methods include chemical propulsion with gravity assist maneuvers near Venus, Earth, Jupiter, and Neptune, as well as an Oberth maneuver near the Sun. In this work, we present possibilities to explore Sedna considering missions with alternative propulsion: using a thermonuclear Direct Fusion Drive (DFD) and solar sail with thermal desorption of the coating.”

Both of these systems, though still experimental, are thought to have their advantages over traditional propulsion systems. 

“Due to the limitations of traditional methods, innovative propulsion systems are crucial to reach distant targets like Sedna,” the team writes. “Chemical propulsion, while providing high thrust for launches, suffers from low efficiency and high fuel mass requirements for long-duration missions: the Voyager 1 spacecraft, launched in 1977, traveled with a cruise speed of 17 km/s (3.57 AU per year).”

The obvious advantage of solar sails is that the mission could have a long duration, given that it provides thrust through the “ever-present gentle push of sunlight” rather than propellant. This concept is being developed by NASA for potential use in space weather satellites, near-Earth asteroid reconnaissance missions, and potentially crewed exploration missions. Direct fusion drives, still in early development but potentially to be tested as early as 2027, have their advantages too. 

“Compact and clean-burning, each 1-10 MW Direct Fusion Drive (DFD) engine would produce both power and thrust with high specific power (low mass),” Princeton Satellite Systems explains. “Producing propulsion directly in the fusion engine is highly efficient, shortening trip times and increasing capability for a wide variety of space missions: robotic missions to the outer planets, human missions to the moon or Mars, missions to near interstellar space.”

Looking at both types of propulsion, the team found that they could deliver a spacecraft to the dwarf planet easily within the window. They found that a DFD could cut down the travel time by 50 percent compared to traditional propulsion systems.

“Results indicate that the DFD could reach Sedna in approximately 10 years, with 1.5 years of thrusting, while the solar sail, assisted by Jupiter’s gravity, could complete the journey in 7 years,” the team concluded, adding that for DFD, “a valid solution to reduce the time even more would be to evaluate a constant thrust (CT) profile; thus the disadvantage is that this would require additional propellant, and result in a 1/3 reduction of payload mass.”

While both could deliver payloads to Sedna, there are still trade-offs to be considered. DFDs could get there faster, but by using a constant thrust profile, it would increase the need for propellant, decreasing the amount of scientific equipment it can carry to the object. Nevertheless, the team believes it could still be used to deliver a payload of 1,500 kilograms (3,300 pounds) in under 10 years. 

Meanwhile, the solar sail mission could deliver smaller payloads of around 1.5 kilograms (3.3 pounds), which could include instruments like spectrometers, magnetometers, and highly sensitive cameras. While this would make the mission cheaper, and would be less complex than a DFD-driven spacecraft, the payload delivered by DFD would be much more comprehensive.

Mission proposals for reaching Sedna are underway. Fingers crossed one gets going soon, before we miss this once-in-11,000-years opportunity to visit – potentially – an object from the distant Oort cloud.

The study is posted to the pre-print server arXiv.