Ground-based lasers have the potential to propel spacecraft towards distant stars at an accelerated pace.

The future of space exploration entails ambitious plans to venture farther from Earth than ever before. In addition to the current proposals for establishing infrastructure in cis-lunar space and conducting crewed missions to the Moon and Mars, there are also plans to dispatch robotic missions to the outer reaches of the Solar System, to the gravitational lens of our Sun, and even to the nearest stars in order to explore exoplanets. Achieving these objectives necessitates the development of next-generation propulsion systems capable of providing high thrust and consistent acceleration.

One avenue of investigation involves focused arrays of lasers, also known as directed energy (DE), and lightsails. Initiatives such as Breakthrough Starshot and Swarming Proxima Centauri are extensively exploring these concepts. Furthermore, a team from McGill University in Montreal has proposed a novel type of directed energy propulsion system for Solar System exploration. In a recent publication, the team shared the preliminary findings of their Laser-Thermal Propulsion (LTP) thruster facility, which indicate that this technology holds promise in delivering both high thrust and specific impulse for interstellar missions.

The research team, led by Gabriel R. Dube, an Undergraduate Research Trainee with the McGill Interstellar Flight Experimental Research Group (IFERG), and Associate Professor Andrew Higgins, the Principal Investigator of the IFERG, collaborated with Emmanuel Duplay, a graduate researcher from the Technische Universiteit Delft (TU Delft); Siera Riel, a Summer Research Assistant with the IFERG; and Jason Loiseau, an Associate Professor with the Royal Military College Of Canada. The team presented their findings at the 2024 AIAA Science and Technology Forum and Exposition, as well as in a paper published in the AIAA journal Aerospace Research Central (ARC).

In a 2022 paper published in Acta Astronautica, Higgins and his team initially introduced this idea. The paper, titled “Design of a rapid transit to Mars mission using laser-thermal propulsion,” drew inspiration from interstellar concepts such as Starshot and Project Dragonfly, as reported by Universe Today. However, the focus of Higgins and his colleagues from McGill University was on utilizing the same technology to facilitate expedited journeys to Mars within a mere 45 days, as well as throughout the entire Solar System. They contended that this approach could not only validate the involved technologies but also serve as a crucial stepping stone towards interstellar missions.

Higgins informed Universe Today via email that the idea for their concept arose during the pandemic when they were unable to access their laboratory. During this time, their students conducted a comprehensive conceptual study on utilizing large laser arrays, similar to those proposed for the Breakthrough Starshot project, for a more immediate mission within the Solar System. Instead of a 10-km-diameter, 100-GW laser as envisioned for Breakthrough Starshot, they opted for a 10-m-diameter, 100-MW laser. Their study demonstrated that this laser configuration could effectively provide power to a spacecraft up to the distance of the Moon. By heating hydrogen propellant to extremely high temperatures, reaching tens of thousands of Kelvin, the laser enables the achievement of both high thrust and high specific impulse, which is considered the ultimate goal.

This concept bears resemblance to nuclear-thermal propulsion (NTP), a technology currently being developed by NASA and DARPA for expedited transit missions to Mars. In an NTP system, a nuclear reactor generates heat, causing the expansion of hydrogen or deuterium propellant. This expanded propellant is then directed through nozzles to generate thrust. In the case of Higgins’ concept, phased-array lasers are focused into a hydrogen heating chamber, and the resulting heated hydrogen is expelled through a nozzle, achieving specific impulses of 3000 seconds. Higgins mentioned that since he and his students have regained access to the lab, they have been working towards experimentally validating their idea.

Although they do not possess a 100 MW laser at McGill, Higgins stated that they now have a 3-kilowatt laser set-up in their laboratory, which is already quite formidable. They are currently investigating how the laser can effectively transfer its energy to a propellant, initially using argon due to its easier ionization compared to hydrogen. The AIAA paper provides a comprehensive account of the design, construction, and initial testing of their 3-kW laser facility.

Higgins and his team constructed a device that consisted of 5 to 20 bars of static argon gas for their experiments. Although the final design will use hydrogen gas as a propellant, they opted for argon gas during the tests due to its ease of ionization. They then emitted pulses of a 3-kW laser at a frequency of 1070 nanometers, which corresponds to the near-infrared wavelength, in order to determine the minimum power required for Laser-Sustained Plasma (LSP). Their findings revealed that approximately 80% of the laser’s energy was transferred to the plasma, aligning with previous studies.

Additionally, the pressure and spectral data they collected provided insights into the peak temperature of LSP when using the working gas. However, they emphasize the need for further research to draw conclusive results. They also highlight the necessity of a dedicated apparatus to carry out forced flow and other LSP experiments. Lastly, the team intends to measure thrust later this year to assess the acceleration (delta-v) and specific impulse (Isp) that a laser-thermal propulsion system can offer for future missions to Mars and other celestial bodies within the Solar System.

If this technology proves capable, we could witness a system that can transport astronauts to Mars in a matter of weeks instead of months! Other concepts chosen for the NIAC this year involve evaluating hibernation systems for extended missions in microgravity. Whether used individually or in combination, these technologies have the potential to enable rapid-transit missions that require less cargo and supplies, while minimizing the exposure of astronauts to microgravity and radiation.

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