The Interrelation of Frequency, Laser Interferometry, and Gravitational Waves Towards Thrust Engine Development

1. Introduction: Setting the Stage for Gravitational Wave Propulsion

The universe, as described by Albert Einstein's revolutionary theory of General Relativity, is not a static stage but a dynamic fabric known as spacetime. One of the most profound predictions of this theory is the existence of gravitational waves, which are propagating disturbances in the curvature of spacetime, akin to ripples on the surface of a pond. These waves travel at the speed of light and are generated by the acceleration of massive objects. For decades, gravitational waves remained elusive, their existence only supported by indirect evidence. However, the groundbreaking development and operation of laser interferometers have allowed scientists to directly detect these faint signals emanating from cataclysmic cosmic events such as the mergers of black holes and neutron stars. This monumental achievement has not only validated a key aspect of Einstein's theory but has also opened an entirely new window through which to observe the cosmos. Given this newfound ability to observe and characterize gravitational waves, a compelling question arises: can the fundamental relationship between the frequency of these waves and the sophisticated technology of laser interferometry, which enables their detection, be harnessed to create a novel form of propulsion, specifically a gravitational wave thrust engine? This report aims to explore this ambitious concept, delving into the fundamental properties of gravitational waves, the intricacies of their detection using laser interferometry, the theoretical proposals that explore the possibility of gravitational propulsion, and the significant challenges that must be overcome to realize such a technology. By examining these aspects in detail, we can gain a comprehensive understanding of the prospects and realities of utilizing gravitational waves for propulsion.

2. Understanding Gravitational Waves: Frequency and Generation

Gravitational waves are a manifestation of the dynamic nature of spacetime, representing propagating oscillations in the gravitational field.¹ These waves are generated by the acceleration of any object with mass; however, the strength of the waves produced is typically minuscule unless the masses involved are enormous and the accelerations are extreme, such as those found in astrophysical phenomena.² The frequency of a gravitational wave is directly linked to the motion of its source.⁴ For instance, in a binary system where two massive objects orbit each other, the frequency of the emitted gravitational waves is closely related to the orbital frequency of the binary. Specifically, for a binary system in a circular orbit, the dominant frequency of the emitted gravitational wave is twice the orbital frequency.⁴ As the binary system loses energy through the emission of gravitational radiation, the two objects spiral closer together, causing their orbital frequency to increase. This increase in orbital frequency results in a corresponding increase in the frequency and amplitude of the emitted gravitational waves, producing the characteristic "chirp" signal observed by detectors.⁴ Similarly, rapid changes in the mass distribution of an object, such as during the collapse of a star in a supernova explosion, can also generate gravitational waves with frequencies characteristic of the timescale of the event.

Astronomers have categorized gravitational waves into four main types based on their sources and frequency characteristics.⁵ Compact Binary Inspiral Gravitational Waves are produced by the inspiral and merger of dense, massive objects like black holes and neutron stars. These waves exhibit an increasing frequency and amplitude as the objects approach their final collision.⁵ Continuous Gravitational Waves are expected from rapidly rotating, non-spherically symmetric neutron stars. These waves would have a relatively stable frequency determined by the star's rotation rate.⁵ Burst Gravitational Waves are short-duration signals originating from sudden, energetic events like supernovae, and they can have a broad range of frequencies.⁵ Finally, the Stochastic Gravitational Wave Background is a faint, persistent background of gravitational waves from numerous unresolved sources, including potentially the very early universe, and is expected to have very low frequencies.⁵ The spectrum of gravitational wave frequencies is vast, ranging from nanohertz, observed by monitoring the timing of pulsars ⁸, to kilohertz, detected by ground-based laser interferometers.⁸ This wide range of frequencies carries a wealth of information about the diverse astrophysical phenomena that generate them.

3. Laser Interferometry: The Precision Tool for Detecting Gravitational Waves

The direct detection of gravitational waves relies on the remarkable precision of laser interferometry.¹ At its core, a laser interferometer works by splitting a highly stable laser beam into two paths that travel along two very long, perpendicular arms.¹⁷ At the end of each arm, the laser beams are reflected by mirrors and travel back to the point where they were split, where they are then recombined.¹⁷ When the two beams recombine, they create an interference pattern that is extremely sensitive to any difference in the path lengths traveled by the two beams.¹⁷ This interference pattern arises from the superposition principle of waves: if the peaks of the two light waves arrive at the same time, they interfere constructively, resulting in a brighter light; if the peak of one wave arrives at the same time as the trough of the other, they interfere destructively, resulting in a dimmer light or even cancellation.¹⁷

The passage of a gravitational wave through a laser interferometer causes a minuscule change in the lengths of the two arms.¹ Specifically, a gravitational wave stretches space in one direction and compresses it in the perpendicular direction, with the effect oscillating at the frequency of the wave.⁴ This differential change in the arm lengths causes the laser beams to travel slightly different distances before they are recombined.¹⁷ Even though these changes in length are incredibly small – for example, LIGO's 4-kilometer arms are expected to change in length by as little as 10^-18 meters due to a passing gravitational wave ⁷ – they are sufficient to cause a measurable shift in the interference pattern.¹ This shift in the interference pattern, which manifests as a change in the intensity of the recombined light, is the fundamental signal that gravitational wave detectors are designed to measure. Facilities like LIGO, with its two observatories in the United States, and Virgo, located in Italy, are incredibly complex instruments featuring arms kilometers in length, operating in ultra-high vacuum to minimize laser scattering, and employing sophisticated vibration isolation systems to prevent terrestrial noise from masking the faint gravitational wave signals.⁷ Other interferometric detectors, such as GEO600 in Germany and TAMA in Japan, also contribute to the global network of gravitational wave observatories.⁷ Furthermore, the planned space-based Laser Interferometer Space Antenna (LISA) will utilize interferometry on a much larger scale, with arms millions of kilometers long, to detect lower frequency gravitational waves that are inaccessible to ground-based detectors due to seismic noise.⁷

4. The Interplay of Frequency and Laser Interferometry in Gravitational Wave Detection

The frequency of a gravitational wave is directly encoded in the time evolution of the interference pattern observed by a laser interferometer.¹ As a gravitational wave passes through the interferometer, the periodic stretching and compressing of spacetime causes the relative path length of the two laser beams to oscillate at the frequency of the wave.⁴ This oscillating difference in path length results in a corresponding oscillation in the phase difference between the two recombining beams, which in turn leads to a time-varying intensity of the interference pattern detected at the output of the interferometer.¹⁷ The rate at which this interference pattern changes – the frequency of the detected signal – is precisely the frequency of the incoming gravitational wave. A higher frequency gravitational wave will induce more rapid oscillations in the arm lengths and consequently a more rapidly varying interference signal.

Current gravitational wave detectors, such as LIGO and Virgo, have specific frequency sensitivity ranges that are optimized to detect gravitational waves from particular types of astrophysical events.¹⁵ For instance, the advanced LIGO detectors are designed to be most sensitive to gravitational waves in the frequency range from approximately 10 Hz to 10 kHz ¹⁶, which corresponds to the expected frequencies of gravitational waves produced during the final stages of inspiral and merger of stellar-mass black holes and neutron stars.⁷ Advanced Virgo has a similar sensitivity range, spanning from around 10 Hz to 10000 Hz.³¹ The analysis of the detected gravitational wave signal, particularly the way its frequency and amplitude change over time, allows scientists to infer crucial properties of the astrophysical source that produced the wave.¹ For example, the characteristic increase in frequency and amplitude of the "chirp" signal from merging black holes provides information that can be used to determine the masses and spins of the black holes and the distance to the merger event.¹ The frequencies of the quasi-normal modes that are excited in the single black hole formed after the merger are also directly related to its mass and spin.³² It is important to note that current detectors have limitations in their sensitivity across different frequency bands due to various noise sources.⁹ Ground-based interferometers are particularly susceptible to seismic noise at lower frequencies (below about 10 Hz) and to quantum noise at higher frequencies.⁷ This motivates the development of future gravitational wave observatories, such as the planned space-based LISA mission, which will operate in a lower frequency range (around 10^-4 Hz to 1 Hz ¹⁵) where it will be sensitive to gravitational waves from different types of sources, such as the mergers of supermassive black holes at the centers of galaxies.

5. Theoretical Frameworks for Gravitational Thrust Using Waves

The concept of utilizing gravitational waves for propulsion stems from the fundamental understanding that these waves carry both energy and momentum as they propagate through spacetime.³⁵ This is analogous to electromagnetic waves, which can exert a force through radiation pressure. The observation that binary star systems lose orbital energy at a rate precisely matching the energy radiated away as gravitational waves provides compelling evidence for the energy-carrying nature of these waves.³⁵ Furthermore, the emission of gravitational waves during highly energetic events like black hole mergers can also result in a net loss of linear momentum from the system, causing the merged black hole to recoil or "kick" with significant velocity.³⁵ This transfer of momentum in astrophysical settings naturally leads to the question of whether it might be possible to artificially generate and direct gravitational waves to exert a propulsive force on a spacecraft.

Several theoretical proposals have explored this possibility, focusing on different methods for generating gravitational waves and harnessing their momentum.³⁶ One approach involves the generation of high-frequency gravitational waves (HFGWs) through the rapid acceleration or vibration of massive objects, particularly if these objects possess significant electric charge or an asymmetric mass distribution.³⁸ The idea is that by inducing extremely rapid changes in the mass quadrupole moment of a system, it might be possible to generate detectable gravitational waves at higher frequencies. Another theoretical avenue explores the possibility of focusing gravitational waves, perhaps using gravitational lensing techniques or by creating specific spacetime geometries, to create a region of enhanced gravitational field that could then interact with a spacecraft to produce thrust.³⁷ Some researchers have also considered the potential of using quantum mechanical systems to generate gravitational waves with tailored properties that might be more amenable to propulsion applications.³⁹ A particular concept that has been explored in patents by Robert M. L. Baker Jr. involves the generation of gravitational waves through "jerk," which is the third derivative of motion with respect to time.³⁶ These patents propose using electromagnetic or nuclear forces to create rapid, jerk-like motions in masses, which are theorized to generate gravitational waves that could be directed and used for propulsion.³⁶

Despite these theoretical explorations, significant challenges exist in generating gravitational waves with sufficient intensity and directionality to achieve practical propulsion.³⁹ Gravity is the weakest of the four fundamental forces by an enormous margin, making the artificial generation of gravitational waves comparable in strength to those observed from astrophysical sources an incredibly demanding task. Calculations based on Einstein's field equations suggest that the energy requirements for producing even a modest amount of thrust using gravitational waves would likely be astronomical, far exceeding the total energy currently available to humanity.³⁹ Furthermore, effectively focusing and directing these waves to impart momentum to a spacecraft presents another major hurdle.

6. High-Frequency Gravitational Waves: Properties and Propulsion Potential

High-Frequency Gravitational Waves (HFGWs) are generally defined as gravitational waves with frequencies exceeding 100 kHz.⁴³ This frequency range is significantly above that of the gravitational waves typically detected from astrophysical sources by current ground-based observatories, which primarily operate in the 10 Hz to 10 kHz range. The study of HFGWs has garnered interest due to their potential applications beyond just astronomical observation, particularly in the fields of communication and propulsion.⁴²

One of the primary theoretical advantages of HFGWs for propulsion is their predicted weak interaction with ordinary matter.⁴³ Unlike electromagnetic waves, which can be absorbed, scattered, or reflected by matter, HFGWs are expected to pass through almost all materials with minimal attenuation. This property could potentially allow for the generation of a thrust force that is not easily shielded. Furthermore, the relatively short wavelengths associated with high-frequency gravitational waves, compared to the much longer wavelengths of low-frequency astrophysical waves, theoretically offer the possibility of generating more tightly focused and directed beams.³⁹ Such focused beams would likely be necessary to achieve efficient transfer of momentum to a spacecraft for propulsion.

Several theoretical mechanisms have been proposed for generating HFGWs in a laboratory setting.³⁸ One approach involves using piezoelectric materials or film bulk acoustic resonators (FBARs) that are driven by high-frequency electromagnetic sources like magnetrons.³⁶ The rapid oscillations of these materials are theorized to generate gravitational waves. Another concept involves inducing extremely rapid accelerations in charged particles or asymmetric mass distributions.³⁸ Some more speculative ideas explore quantum mechanical phenomena, such as the development of a "Gravitational-wave LASER" (GASER), which would aim to produce coherent beams of gravitons at high frequencies.⁴² Patents by Robert M. L. Baker Jr. also focus on using electromagnetic or nuclear forces to create rapid changes in motion ("jerk") to generate HFGWs for propulsion and other applications.³⁶

However, the field of HFGW generation and application is not without significant skepticism and challenges. A report by the JASON group, an independent advisory group that provides scientific and technical advice to the U.S. government, critically examined many of the proposed applications of HFGWs and concluded that they are fundamentally flawed.³⁹ The report highlighted the enormous energy requirements for generating HFGWs with any practical application, particularly for propulsion, where the power needed to achieve even a modest acceleration was found to be many orders of magnitude greater than the world's total power consumption.³⁹ The JASON report emphasized the need for rigorous scientific vetting of any claims related to HFGW technology.

7. Low-Frequency Gravitational Waves: Characteristics and Thrust Applications

Low-Frequency Gravitational Waves (LFGWs) typically refer to gravitational waves with frequencies ranging from nanohertz up to a few hertz.⁵ These waves are primarily produced by some of the most massive and energetic events in the universe, such as the inspiral and merger of supermassive black holes found at the centers of galaxies.⁵ There is also the possibility of detecting primordial gravitational waves from the very early universe in this frequency range.⁵

Generating low-frequency gravitational waves artificially for propulsion presents even greater challenges than generating high-frequency waves. The production of gravitational waves at these frequencies typically requires extremely large masses undergoing significant accelerations over vast distances or extended periods. For instance, the low-frequency gravitational waves detected by pulsar timing arrays are thought to originate from the slow orbital motion of binary supermassive black holes, a phenomenon occurring on scales of light-years.⁸ Engineering systems on Earth or even in space that could replicate such conditions to generate detectable LFGWs for propulsion seems practically impossible with our current technological capabilities and understanding of physics.

While it is conceivable to speculate about indirect ways in which LFGWs might be harnessed for propulsion, such as through resonant effects with extremely large structures or by exploiting potential interactions with other fundamental fields, there are currently no established theoretical frameworks or proposed mechanisms that describe how this could be achieved in a controlled and efficient manner. The primary focus of current scientific research on low-frequency gravitational waves is their detection and study as a tool for gaining fundamental insights into cosmology, galaxy evolution, and the behavior of supermassive black holes.⁵ The development of future space-based gravitational wave observatories like LISA is driven by the immense scientific potential of observing these low-frequency signals from the most energetic events in the universe.

8. The Role of Laser Interferometry in Enabling Gravitational Propulsion Concepts

Laser interferometry, while primarily developed as an incredibly sensitive tool for detecting the minute spacetime distortions caused by gravitational waves, could potentially play a role in the development of future gravitational propulsion systems, albeit likely in ways beyond directly generating thrust.

One crucial area where laser interferometry could contribute is in the precise measurement and characterization of any artificially generated gravitational waves. If, in the future, a device capable of producing directed gravitational waves for propulsion is developed, laser interferometers could be used to accurately measure the frequency, amplitude, polarization, and directionality of the emitted waves.¹ This detailed characterization would be essential for understanding the performance of the propulsion system and for optimizing its design and operation. For instance, multiple interferometers strategically placed around a gravitational wave generator could map the spatial distribution of the emitted gravitational field, providing valuable data for controlling and focusing the wave.

Furthermore, laser interferometry might be adapted to serve as a highly sensitive diagnostic tool for monitoring the effects of gravitational wave propulsion on a spacecraft or its surroundings. By using interferometers to measure minute changes in distance or relative motion, it might be possible to detect the subtle forces exerted by artificially generated gravitational waves. This could be particularly relevant in experiments aimed at demonstrating the principle of gravitational thrust. It is also conceivable that interferometric techniques could be incorporated into a feedback control system for a gravitational thrust engine. By continuously monitoring the generated gravitational waves and their interaction with a target object or the surrounding spacetime, the system could make real-time adjustments to the gravitational wave generator to maintain a stable and directed thrust.

However, it is important to acknowledge that current laser interferometer designs, such as those used in LIGO and Virgo, are specifically optimized for detecting extremely weak gravitational waves originating from vast cosmic distances.¹ They are not designed to interact with or generate strong, localized gravitational waves for propulsion. Achieving this would likely require fundamentally different approaches and potentially entirely new types of interferometric devices or techniques. While the exquisite sensitivity of interferometers to minute changes in spacetime is a key asset, how this sensitivity could be directly leveraged to create a propulsive force remains a highly speculative question with no clear answers based on current scientific understanding.

9. Existing Research and Patents on Gravitational Wave Propulsion

The concept of harnessing gravitational waves for propulsion, while still largely theoretical, has been the subject of research and has even led to the filing of patents.³⁶ Notably, Robert M. L. Baker Jr. has been a prominent figure in this area, with several patents describing gravitational wave generators designed for both communication and propulsion.³⁶ These inventions often propose using electromagnetic or nuclear forces to induce a rapid third-time-derivative motion, or "jerk," in a mass, which is theorized to generate high-frequency gravitational waves. The patents suggest that these directed beams of gravitational waves could then be used to propel a vehicle, either through the reaction force created by emitting the waves or by manipulating the local gravitational field around the vehicle.³⁶ Another patent in this domain describes a high-frequency gravitational wave generator that employs a gas-filled shell, microwave emitters, sound generators, and acoustic vibration resonant gas-filled cavities.⁵⁴

Beyond patents, theoretical research papers have explored various aspects of gravitational field propulsion and the manipulation of gravity for thrust generation.³⁷ These studies often delve into advanced concepts within General Relativity and sometimes explore more speculative ideas beyond the standard model of physics. For example, Robert Forward, a pioneer in advanced propulsion concepts, proposed the idea of a "Forward Coil" to generate gravitomagnetic acceleration fields.⁵⁸ Another concept involves using a Tokamak plasma quadrupole oscillator to generate gravitational waves that could potentially be rectified to produce thrust.⁵⁸ The possibility of using beamed gravitational waves, generated remotely, to propel a spacecraft has also been theoretically investigated.⁴⁰

It is important to note the historical context of "gravity control" propulsion research, which saw significant interest in the mid-20th century.⁶² While this early research did not yield any practically viable anti-gravity or gravity propulsion technologies, it reflects a long-standing human fascination with the idea of manipulating gravity for space travel. More recently, there have been claims and patents related to "propellantless" propulsion systems like the EmDrive and the "Exodus Effect".⁶³ However, these concepts have often faced significant skepticism from the mainstream scientific community due to a lack of clear theoretical grounding within established physics and, in some cases, failure to be independently replicated. Their connection to the controlled generation and harnessing of gravitational waves as described by General Relativity is generally considered to be weak or non-existent.

10. Challenges and Technological Hurdles in Realizing a Gravitational Wave Thrust Engine

The realization of a functional gravitational wave thrust engine faces a multitude of formidable challenges and technological hurdles, as evidenced by the current state of research and understanding:

• Extreme Weakness of Gravity: The fundamental weakness of the gravitational force compared to the other fundamental forces presents the most significant obstacle. Generating gravitational waves with sufficient intensity to produce a measurable thrust on a spacecraft requires incredibly massive objects undergoing extreme accelerations.³⁹ Achieving such conditions artificially on a scale suitable for propulsion is a monumental task.

• Enormous Energy Requirements: Theoretical calculations indicate that the energy required to generate even a small amount of propulsive force using gravitational waves would likely be astronomically high, potentially exceeding the total energy output of the world by many orders of magnitude.³⁹ This makes the practical realization of such a system with current or foreseeable energy sources seem highly improbable.

• Difficulty in Achieving Directionality and Focus: For propulsion to be effective, the generated gravitational waves would need to be focused into a directed beam that can impart momentum to the spacecraft in a specific direction.³⁷ Achieving this with gravitational waves, especially the low-frequency waves produced by natural sources, is a significant technological challenge. While high-frequency waves theoretically offer better focusing potential, their generation remains a major hurdle.

• Lack of Efficient Interaction Mechanisms: Even if strong, directed gravitational waves could be generated, scientists currently lack a fundamental understanding of how to efficiently interact with these waves to transfer momentum to a spacecraft in a controlled and sustained manner.⁴⁵ Any proposed propulsion system would need a mechanism to "catch" or react against the gravitational waves to produce thrust.

• Potential Violation of Conservation Laws: Some of the more speculative "propellantless" propulsion concepts that are sometimes associated with gravity manipulation appear to violate fundamental laws of physics, such as the conservation of momentum.⁶³ A viable gravitational wave thrust engine would need to operate within the established laws of physics.

• Extreme Technological Demands: The generation and manipulation of gravitational waves at the frequencies and intensities potentially required for propulsion would likely necessitate operating at the very limits of our current technological capabilities, possibly involving extreme energy densities, novel materials with exotic properties, or the harnessing of currently unknown quantum phenomena.³⁸

The current state of knowledge reveals a significant gap between the theoretical proposals for gravitational wave propulsion and the complete absence of any experimental demonstrations of a functional device. Overcoming these immense challenges would likely require fundamental breakthroughs in our understanding of gravity, spacetime, and our ability to manipulate them at a technological level.

11. Comparison with Other Advanced Propulsion Technologies

The concept of gravitational wave propulsion, while revolutionary in its potential, can be compared with other advanced space propulsion technologies currently under investigation:

• Ion Propulsion: This technology uses electric fields to accelerate ions, resulting in very high specific impulse but low thrust.⁶⁸ It is efficient for long-duration missions but unsuitable for rapid acceleration or escaping strong gravitational fields.

• Laser Propulsion: This method uses beamed laser energy from a remote source to heat a propellant or directly ablate a material, potentially offering higher thrust and specific impulse than chemical rockets.⁵⁶ It requires significant ground-based infrastructure.

• Photonic Propulsion (Solar/Laser Sails): These use the radiation pressure of photons from the Sun or lasers to generate thrust, offering propellantless propulsion but very low acceleration.⁷¹ They are suitable for long-duration, low-thrust maneuvers.

• Nuclear Propulsion: Utilizing nuclear reactions to heat a propellant, this could provide high thrust and high specific impulse, enabling faster interplanetary travel.⁸² It faces challenges related to safety and public perception.

• Directed Energy Propulsion: A broader category encompassing laser and microwave propulsion, aiming to beam energy to the spacecraft for various propulsion mechanisms, including thermal rockets and photonic thrusters.⁷¹

• Scramjets: Designed for hypersonic atmospheric flight, they are not applicable to space propulsion in vacuum.¹⁰⁶

Gravitational wave propulsion, if feasible, holds the unique potential for propellantless thrust by directly interacting with spacetime or through the momentum carried by the waves. High-frequency gravitational waves also offer the intriguing possibility of passing through matter unimpeded. However, compared to these other advanced methods, gravitational wave propulsion faces far greater fundamental challenges in terms of energy requirements, generation efficiency, and the lack of established interaction mechanisms. While other advanced propulsion technologies represent incremental improvements or address specific mission needs, gravitational wave propulsion remains a highly speculative concept requiring significant breakthroughs in fundamental physics and engineering.

12. Conclusion: Prospects and Realities of Gravitational Wave Propulsion

The relationship between the frequency of gravitational waves and laser interferometry is fundamental to our ability to detect and study these elusive phenomena. The frequency of a gravitational wave dictates the temporal signature of the spacetime strain it induces, which is precisely what laser interferometers are designed to measure with extraordinary precision. While this relationship has opened a new era in astronomy, the prospect of harnessing it to create a practical thrust engine for spacecraft propulsion remains a highly speculative endeavor.

The generation of gravitational waves strong enough to produce a measurable propulsive force is an incredibly challenging task due to the inherent weakness of gravity. The energy requirements for such an endeavor appear to be astronomically high, far beyond our current technological capabilities. Furthermore, efficiently focusing and directing these waves, and developing a mechanism to effectively transfer their momentum to a spacecraft, are major hurdles that require significant breakthroughs in our understanding of physics and engineering.

While theoretical proposals and even patents exist for gravitational wave propulsion, many of these concepts lack robust theoretical foundations or experimental validation. The scientific community remains largely skeptical, particularly regarding claims of easily achievable propellantless propulsion through gravitational wave manipulation. Compared to other advanced propulsion technologies that offer more incremental but tangible improvements, gravitational wave propulsion represents a much more radical and currently unattainable goal.

In conclusion, while the concept of a gravitational wave thrust engine is undeniably fascinating and continues to inspire theoretical exploration, its realization in the foreseeable future appears highly improbable based on our current scientific understanding and technological capabilities. Achieving gravitational propulsion would likely require revolutionary advancements in our comprehension of gravity and spacetime, potentially opening entirely new avenues of physics that are currently unknown. For now, the intricate relationship between gravitational wave frequency and laser interferometry remains primarily a tool for observing the universe rather than a pathway to a new era of space travel.

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