Stand on a coastal shore and watch how eagles, ravens, seagulls, and crows take flight in high winds. It’s an inspiring sight, to be sure. Additionally, it illustrates an important concept in aerial mechanics, like how the proper angling of wings can allow birds to exploit differences in wind speed to hover in mid-air. Similarly, birds can use these same differences in wind speed to gain bursts of velocity to soar and dive. These same lessons can be applied to space, where spacecraft could perform special maneuvers to pick up bursts of speed from “space weather” (solar wind).
This was the subject of a recent study led by researchers from McGill University in Montreal, Quebec. By circling between regions of the heliosphere with different wind speeds, they state, a spacecraft would be capable of “dynamic soaring” the same way avian species are. Such a spacecraft would not require propellant (which makes up the biggest mass fraction of conventional missions) and would need only a minimal power supply. Their proposal is one of many concepts for low-mass, low-cost missions that could become interplanetary (or interstellar) explorers.
The team included spaceflight researcher Mathias Larrouturou and mechanical engineering Professor Andrew J. Higgins from McGill University in Montreal, Quebec. They were joined by Jeffrey K. Greason, an electrical engineer, board chairman of the Tau Zero Foundation (an international non-profit dedicated to interstellar travel), and the chief technologist and co-founder of Texas-based Electric Sky Inc. Their proposal was detailed in a paper titled “Dynamic soaring as a means to exceed the solar wind speed” that recently appeared in the journal Frontiers in Space Technologies.
In addition to being a professor of mechanical engineering, Higgins is also the principal investigator of McGill’s Interstellar Flight Experimental Research Group (IFERG). This group is dedicated to advancing propulsion systems that would enable missions that could explore extrasolar planets in our lifetime. Conventional propellants (chemical rockets and electronic propulsion) are impractical and would take millennia to reach even the nearest stars. Moreover, telescopes are not nearly large enough to directly image exoplanets to the point that we could study their surfaces in detail.
Taken together, these two challenges are what motivate groups like IFERG to develop new propulsion systems that can allow for rapid transit and more efficient methods. As Larrouturou told Universe Today via email:
“Interstellar travel is primarily a problem of propulsion and – even more acute – power. The power required to send a Voyager-class probe to several percent of the speed of light would consume a significant fraction of our civilization’s capacity for power generation. Thus, the guiding motivation for our work is attempting to exploit sources of energy freely available in space for propulsion.”
To date, humanity’s only missions to interstellar space include the Voyager space probes, which were launched in 1977 and entered the interstellar medium (ISM) in 2012 and 2018, respectively. They will be joined in the coming years by the Pioneer 10 and 11 missions, which launched in 1972 and 1973 (respectively). In a study released in 2019, Dr. Coryn Bailer-Jones of the Max Planck Institute for Astronomy and Davide Farnocchia of NASA JPL calculated that the Voyager 1 and Pioneer 10 missions are expected to reach Proxima Centauri in about 16,700 and 18,300 years (respectively).
While this is certainly a long time from now, these estimated timelines are actually ahead of the curve. As we explored in a previous article, it would take between 19,000 and 81,000 years to reach Alpha Centauri using conventional methods. Since all methods that could achieve interstellar flight in just a few decades are still entirely theoretical (and/or require the existence of additional physics to work), researchers have been investigating solar sails and related concepts as an alternative. One of the greatest advantages of sails is that they do not require propellant, which makes up the most significant mass fraction of conventional rockets.
For example, consider the Space Launch System (SLS) that just sent NASA’s Artemis I mission on a circumlunar flight and is currently the most powerful launch vehicle in the world. The core stage of the SLS has a “dry weight” of over 85 metric tons (94 U.S. tons) but weighs almost 1,073 metric tons (1,182.5 U.S. tons) when fully fueled. When SpaceX’s Starship is ready to make orbital flights, it will weigh 4536 t (5,000 U.S. tons), of which only 77 t (85 U.S. tons) will not be its liquid methane and liquid oxygen propellant. That being said, solar sails also have their share of drawbacks. Said Larrouturou:
“A sail must be nearly solid to intercept the incoming photons, and this limits the accelerations that can be achieved. Even the thinnest solar sails are only capable of about 1 mm/s2, which is one ten-thousandth of a ‘gee’ of acceleration. A solar sail released very near the sun might—if it can withstand the temperatures—reach 1 percent of the speed of light as it leaves the solar system. Of course, sunlight falls off with the inverse square of the distance, so there is a fundamental limit to how fast solar sails can go.”
Several concepts call for the use of high-powered direct energy arrays (lasers) to accelerate a sail and gram-scale “nanocraft” to a fraction of the speed of light (relativistic speed). Examples include Breakthrough Starshot, a concept being researched by Breakthrough Initiatives, and Project Dragonfly — a design study and mission concept hosted by the Institute for Interstellar Studies (i4is). While promising, the power requirements and engineering challenges are considerable, not to mention the associated costs.
To achieve 20 percent of the speed of light (0.2 c), the Starshot nanocraft (Starchip) and sail would require a 100 Gigawatt (100 billion watts) laser array to push the sail for a sustained period of 10 minutes. One of the most powerful lasers in the world today, the Zetawatt-Equivalent Ultrashort pulse laser System (ZEUS), is capable of generating 30 terawatts (30 trillion watts). While exponentially more powerful, ZEUS can only fire for 25 femtoseconds (25 quadrillionths of a second). In the meantime, said Larrouturou, scientists have largely neglected solar wind as an alternative since the power is much less than that of the photons that make up sunlight.
However, the charged nature of solar wind, when paired with magnetic fields, is considered a plausible means of acceleration. Recent research into electric and magnetic sails, said Larrouturou, has opened up new possibilities since they demonstrate how a small structure can interact with an enormous volume of wind:
“A spacecraft using a parachute-like drag device called the plasma magnet in the solar wind might be able to accelerate at 1 m/s2, or about one-tenth of a gee, much greater than a solar sail. Also, as the solar wind becomes increasingly tenuous as you move away from the Sun, the region that the plasma magnet interacts with naturally expands to intercept more of the flow of particles, so the drop-off in thrust is not as severe as with solar sails. Alas, such a drag device can only be dragged up to the maximum speed of the solar wind, which is about 700 km/s, not fast enough for interstellar travel.”
In their paper, the team argues that a solar sail spacecraft could also take advantage of lifting trajectories to achieve “dynamic soaring.” This technique, used by seabirds and glider pilots, consists of extracting energy from wind shear by passing between different regions with different air speeds. For the sake of their paper, Larrouturou and his colleagues explored the different regions of wind shear in the Solar System. This included the fast and slow solar winds that emerge from the Sun’s polar and equatorial regions (respectively).
“The termination shock and heliopause that surrounds the outer solar system are also promising,” he said. “Here, the solar wind comes to an abrupt stop, permitting us to perform dynamic soaring on these different regions of wind. Using these naturally occurring structures in our solar system, we show it is possible to reach speeds of 6000 km/s or about 2 percent of the speed of light.”
Compared to light sail concepts that require powerful lasers, a dynamic soaring solar sail has minimal power requirements. And, like its electric and magnetic counterparts, a solar sail that can generate lift from solar wind does not require propellant, making it a much lighter and more cost-effective method than other interstellar concepts. Lastly, it also has certain advantages over electric, magnetic, and other concepts that try to harness the solar wind as a means of propulsion.
“The ability to generate lift is an improvement over other solar wind sail concepts such as the magnetic sail and electric sail, as those are predominately drag-only devices, which can only accelerate outward, away from the Sun,” added Larrouturou. “In addition to dynamic soaring, the ability to generate lift opens new kinds of trajectories that enable rapid transit missions within the Solar System.”
The team also noted that their dynamic soaring sail concept is only the first stage of a multi-stage approach to interstellar travel that the team, IFERG, and Tau Zero are working on (independently and together). Other concepts include a “wind-pellet shear” spacecraft that interact with both a stream of high-velocity macroscopic pellets and the mass of the interplanetary or interstellar medium. There’s also the “q-drive,” which relies on power harvested from the surrounding medium to shoot reaction mass onboard backward to generate thrust.
In the meantime, Greason and an international team have proposed a technology demonstrator called the Jupiter Observing Velocity Experiment (JOVE) that would conduct a flyby of Jupiter 30 days after launch. This proposed CubeSat would rely on a combination of Solar Electric Propulsion (SEP) and a magnetic drag device (the “Wind Rider”) to accelerate using solar wind plasma. Greason and other colleagues are also developing a Wind Rider Pathfinder Mission that would travel to deep space, where it could use the Sun as a gravitational lens to study the Trappist-1 system.
This and other Solar Gravitational Lens (SGL) proposals are vital precursors to interstellar exploration missions, as they will help space agencies and scientists prioritize which systems to send spacecraft to. By leveraging solar sails, dynamic soaring, and advanced optics, low-cost spacecraft could obtain very high-resolution images of nearby exoplanets — like Proxima Centauri, Ross 128, TRAPPIST-1, and other stars that have confirmed “Earth-like” exoplanets. In the event that any show strong signs of biosignatures or indications of life itself, we will know exactly where to send our spacecraft.
Those missions may also be using Wind Rider or similar technologies to get there!