The dream of sending a spacecraft to another star system has captivated scientists and dreamers for generations. Despite the enormity of this challenge, organizations dedicated to developing technology that could make interstellar travel a reality are springing up. Two prominent players in this field are Breakthrough Starshot and the Tau Zero Foundation. Both organizations are innovating within the realm of beamed energy propulsion methods, aiming to pave the way for distant exploration.
A recent paper co-authored by Jeffrey Greason, chairman of the Tau Zero Foundation’s board, and physicist Gerrit Bruhaug from Los Alamos National Laboratory, delves into the intricacies of relativistic electron beams as a potential means of propelling a spacecraft. It draws attention to the myriad of factors that must be considered when attempting to design and execute an interstellar mission, signaling both challenges and opportunities.
One of the most significant limitations in designing a spacecraft for a journey to Alpha Centauri is the weight of the vehicle itself. Breakthrough Starshot envisions a lightweight spacecraft equipped with expansive solar sails to catch photons from a laser beam and accelerate to interstellar speeds. However, such a minimalistic design poses limitations; it lacks the sufficient instrumentation required to conduct meaningful scientific investigations upon arrival in the Alpha Centauri system.
In contrast, the paper emphasizes the concept of a more substantial probe, weighing in at around 1,000 kg — equivalent to that of the Voyager missions. Such a spacecraft could incorporate advanced technology and instrumentation, vastly improving the scientific return of the mission. However, the increase in weight introduces new challenges, particularly in defining the optimal energy source to propel the probe.
The Breakthrough Starshot initiative is primarily focused on utilizing laser beams operating in the visible spectrum to propel its lightweight spacecraft. However, the authors of the recent paper argue for an alternative approach that relies on a different form of energy, utilizing relativistic electron beams. Their hypothesis suggests that extending the duration of propulsion could enable a heavier probe to attain a significant fraction of light speed.
A primary concern arises when we consider the feasibility of maintaining beam coherence over long distances. Current laser technology limits effective propulsion capabilities to approximately 0.1 astronomical units (AU) from the laser source. Nonetheless, the paper proposes that electron beams could maintain their integrity over greater distances, potentially allowing propulsion efforts to extend to 100 or even 1,000 AU, far beyond the reach of conventional methods.
The envisioned mission, referred to as Sunbeam, hinges on harnessing the capabilities of relativistic electron beams to impart momentum to a spacecraft moving toward Alpha Centauri. The advantages of this approach stem from the ability to accelerate electrons to velocities near the speed of light easily. A remarkable characteristic of relativistic particles is the notion of “relativistic pinch” where the inherent repulsive forces among electrons are minimized due to relativistic effects, thus allowing for a coherent propulsion mechanism.
Calculations suggest that a 1,000 kg probe could achieve velocities of up to 10% of light speed, enabling it to reach Alpha Centauri in slightly over four decades—a promising timeframe within the realm of human possibility.
However, significant hurdles remain, one of the largest being the power requirements needed to produce effective beams at vast distances. Past studies indicate that a beam could require energy levels reaching up to 19 gigaelectron volts (GeV) to propel a spacecraft successfully at 100 AU. While this seems daunting, existing technologies such as those employed in the Large Hadron Collider are capable of generating much higher energies.
To capture this energy effectively, the authors propose a theoretical device known as a solar statite. Situated close to the Sun, this platform would utilize magnetic fields and solar radiation to maintain a stable position while avoiding the destructive pull of the Sun’s gravity. Cloaked behind a sun shield, the statite could serve as a stable energy source to push the spacecraft over extended periods.
Though much of what has been discussed remains speculative, it ignites hope and excitement for the field of space exploration. The discussion on the ToughSF Discord server showcases a common goal among enthusiasts: to make the impossible possible. The advancements proposed in this paper suggest that a scientifically valuable probe could be propelled to Alpha Centauri within a human lifespan, illustrating a paradigm shift in how we might think about interstellar travel. As the vision transitions from science fiction to a tangible possibility, we may be on the brink of unprecedented discoveries beyond our solar system.