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Scientists have suggested a novel propulsion technique that may enable traversing the enormous distances needed for interstellar exploration within a human lifespan.
The primary obstacle in reaching another star system is determining how to produce and transfer sufficient energy to a spacecraft both effectively and affordably. The inherent restrictions of contemporary spacecraft present considerable difficulties for reaching interstellar space within a human lifetime, particularly as there is limited space onboard for fuel or batteries. If we aspire to attain the incredible velocities essential to bridge interstellar distances within a human lifetime, we must explore unconventional solutions.
Introducing relativistic electron beams consisting of electrons traveling close to the speed of light. “Transmitting energy to the vessel has long been acknowledged as one method to acquire more energy […] than we can bring with us,” Jeff Greason, Chief Technologist of Electric Sky, Inc., and chairman of the Tau Zero Foundation, conveyed to Space.com. “Energy is power [multiplied by] time — thus, to receive a specific amount of energy from a beam, you either require extremely high power or must remain in the beam for an extended period.”
One proposed solution involves utilizing electron beams accelerated to almost light speed to propel spacecraft, a strategy that could surmount the vast distances separating Earth from the nearest star. “For interstellar travel, the key challenge is that the distances are so vast,” Greason elaborated. “Alpha Centauri is 4.3 light-years away; about 2,000 times farther from the sun than the Voyager 1 spacecraft has reached — the furthest spacecraft we’ve ever dispatched into deep space thus far. No one is likely to finance a scientific expedition that requires much more than 30 years to yield the data — meaning we need to move quickly.”
A study authored by Greason and Gerrit Bruhaug, a physicist at Los Alamos National Laboratory, published in the journal Acta Astronautica, demonstrates that achieving practical interstellar velocities depends on the capability to deliver adequate kinetic energy to the spacecraft in a cost-effective manner.
“Interstellar travel necessitates gathering and managing immense quantities of energy to reach speeds sufficiently fast to be beneficial,” stated Greason. “Chemical rockets currently in use, even with the additional speed boost from planetary flybys, or from […] slingshotting around the sun for an increase, simply do not possess the capacity to scale to viable interstellar velocities.”
Most theoretical investigations concerning “beam riders” for interstellar navigation have concentrated on laser beams, which consist of particles of light known as photons. Prominent examples include laser-powered interstellar ramjets and laser sails. Ramjets propel spacecraft by compressing hydrogen gas sourced from the interstellar medium, aided by energy from a laser beam directed from a distant origin. Conversely, laser sails utilize the momentum of photons from the laser beam to thrust the spacecraft forward.
While both ideas appear to be optimal solutions, numerous constraints limit their effectiveness. Interstellar ramjets encounter issues such as the sparse density of the interstellar medium and immense energy and fusion necessities. Laser sails, although simpler in design, face difficulties in maintaining beam alignment and intensity over extensive distances to guarantee sufficient power delivery.
In contrast, electrons are significantly more straightforward to accelerate to near-light speeds and provide distinct advantages, although they remain less investigated due to their own limitations. “As all electrons carry a negative charge, they repel one another, causing the beam to disperse,” explained Greason.
However, Greason and Bruhaug contend that there are strategies to mitigate this issue.
At relativistic speeds — namely, near the speed of light — time progresses more slowly, meaning the electron beam would not disperse significantly, thereby keeping the beam concentrated.
Another benefit arises from the fact that space is not devoid of matter. “There exists a very thin distribution of ionized gases referred to as plasma that occupies space, which contains its own electrons and ions drifting around,” clarified Greason. “When the electron beam traverses [this plasma], it repels the lighter electrons from this background gas, yet the heavier ions move more slowly and remain behind.”
As the electron beam travels through the plasma, it perceives a magnetic field due to the ions left behind from the space plasma; this magnetic field generates a force that constrains the electron beam, effectively compressing it and preventing it from dispersing. “This phenomenon is referred to as a ‘relativistic pinch,'” stated Greason. “If everything functions correctly, we can maintain the beam’s coherence in space over an extended distance — thousands of times the range between Earth and the sun — and that would supply the energy necessary to accelerate a spacecraft.”
In their research paper, the pair calculated that an electron beam moving at these velocities could produce sufficient power to propel a 2,200 lb(1,000 kg) probe — roughly the same dimensions as Voyager 1 — at up to 10% of the speed of light. This would facilitate reaching Alpha Centauri in merely 40 years, a notable enhancement over the existing 70,000 years it would necessitate.
Greason contends that instances of these compressed relativistic beams already exist in the cosmos, such as jets of charged particles emitted by black holes, suggesting it is theoretically feasible. “But can we artificially create those types of conditions?” he inquired. “Will the sun’s own magnetic field disrupt the beam? How would we initiate the electron beam? These are all inquiries that remain unanswered.”
In the document, the team proposes positioning a “beam-generating spacecraft” near the sun, where the powerful sunlight could deliver the energy required for the beam. “Although there is engineering involved in creating such a high-power beam, it’s not particularly challenging compared to other obstacles,” remarked Greason.
Extending an electron beam toward a spacecraft is also merely half the struggle — the energy generated must be capable of propelling a spacecraft. “That implies converting the energy of the beam into ejecting some form of propellant or ‘reaction mass,'” explained Greason. “This beam would convey a significant amount of power, and that conversion would need to produce minimal waste heat in the spacecraft to avoid melting!”
He mentions they have several concepts for how this might be achieved, but they are all presently hypothetical and need further exploration to clarify. They also require additional computer modeling studies to enhance their understanding of the beam’s behavior and how it might be initiated, followed by space-based experiments to yield concrete data for their endeavors. “For instance, a satellite far from Earth could beam power to the Moon to empirically verify that the outcomes align with those anticipated by the modeling,” stated Greason.
While securing funding may present difficulties, the researchers argue that, in comparison to alternatives like laser-driven sails, electron beams could accomplish 10,000 times the range, thereby necessitating less energy and being capable of propelling heavier spacecraft. “The expense associated with constructing a large beam scales with the power, so the relativistic electron beam strategy may be considerably more cost-effective,” noted Greason. “The efforts being undertaken on laser-driven spacecraft for interstellar travel are focusing on vessels weighing merely a few grams, which makes it extremely challenging to obtain scientific data in return. If we can propel larger spacecraft weighing tens of kilograms, we can incorporate additional power sources, instruments, and communication systems to relay the data back to Earth.”
The capacity to transmit energy over extended distances has extensive ramifications, ranging from enabling expedited travel within the solar system to delivering power from the sun to other sites like the Moon.
Although it remains a far-off aspiration, reducing the costs of interstellar travel could eventually allow humanity to embark on journeys to other stars, extending the frontiers of what was previously considered possible in space exploration.
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