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Interstellar travel is the idea of travel from one star system or planetary system to another through means of either crewed or unscrewed spacecrafts. Such a feat would be exceptionally difficult. For example, interplanetary travel within our own solar system is generally under 30 AU or distances between the Earth and Sun. The distances for interstellar travel would be at a minimum hundreds of thousands of times further than any attempted missions of today’s age. Thus, this type of travel constitutes its own unit of measurement known as light years. The unit is expressed as a fraction of the speed of light as it is believed such travel would demand this speed or be subject to excruciatingly long travel times of decades or even millennia.
The excessive speeds required in order to complete interstellar travel within a human lifetime are beyond the present capabilities of spacecraft propulsion. Even in theoretical settings, the amount of energy needed to achieve near light speeds is lavish in comparison to even the most generous modern energy production techniques. To put this into perspective, the Voyager 1 space probe has only traveled 1/600 of a light year in 30 years at a speed of 1/18,000 the speed of light. The closest star with exoplanets is known as Proxima Centauri. At Voyager’s rate of speed, it would take about 80,000 years total for the probe to reach the system from Earth (Dunbar).
The required kinetic energy needed to reduce these travel times down to even a few decades for possible human travel is astronomical. Millions of times of energy would be required than presently possible to complete a trip within a lifetime. This is due to the velocity needed being many thousands of times more than the capabilities of any modern spacecraft. To further visualize this, the formula for kinetic energy is K=½mv^2 where K is kinetic energy, m is the final mass, and v is the velocity. To achieve even 1/10th of the speed of light in velocity, 125 terawatt-hours would be needed. This is relatively equivalent to the world-wide energy usage per year. This is an immense obstacle to overcome if interstellar travel is ever to occur. The onboard energy system would have to be unimaginably efficient and the fuel itself light enough to not hinder the kinetic energy formula’s viability.
Another issue regarding interstellar travel is the affects of interstellar dust and gas on a spacecraft. Traveling at near light speeds would increase the knitting energy of such particles and heighten any damage dealt and thus, must be taken into account when designing an interstellar vehicle. Micrometeoroids and other small space debris could be especially life threatening as they directly impact the crew. Furthermore, the unknown of larger space objects posses a more dangerous threat to any proposed mission. Fortunately, there have been some proposals as to how to mitigate these risks.
Another issue at hand is the psychological effects of isolation on any crew involved in an interstellar mission. Couple this with ionizing radiation exposure and the degradation of the body in weightless environments and there are many medical issues that need to be addressed.
The last problem to face when trying to validate interstellar travel is choosing the right time to launch a mission. Technology improves at such an impressively fast rate that many argue we should not pursue such travel at all if it can’t be accomplished within 50 years. This logic is based on the idea that if we did somehow figure out a method of interstellar travel, the vessel will likely be traveling at a relatively slow speed in comparison to something developed years afterword. Thus, a mission launched later on might pass the previously launched mission in transit if technology continues to improve at an exponential rate. One scientist to give this concept a great deal of thought is Andrew Kennedy. He states that if a journey has been calculated with the expectation of growth, a minimum travel speed can be derived in which any spacecrafts to leave after it will not supersede any that left prior.
Despite the aforementioned challenges, there are 59 known stellar systems within 40 light years of our Sun. Within these 59 systems, the ten closest have been identified as possible targets for interstellar travel. The first is the closest system, Alpha Centauri. It is roughly 4.3 light years away and contains three stars. One of these stars is very similar to the Sun and in August of 2016, an Earth-like exoplanet was discovered to be orbiting in the habitable zone. The second closest system is the Barnard’s Star which is located 6 light years from the Sun. It is a small red dwarf, but the second closest solar system to our own. Sirius is located 8.7 light years from the Sun and is comprised of two stars, one of which being a white dwarf. Next is the Esilion Eridani at 10.8 light years away. The system contains a single star that is smaller and less hot than the Sun, but also includes two asteroid belts. It is likely that this system has a solar-system type planetary system. The Tau Ceti system has a single star similar to the Sun and is also likely to contain a solar system type planetary system. At 11.8 light years away, it shows promise of five planets within two possibly habitable zones. The next system out is Wolf 1061 and is roughly 14 light years away. There is a planet there over 4 times the size of Earth believed to have a rocky terrain. Also, it is within a zone in which the possibility of water existing is likely. At 20.3 light years, Gliese 581 planetary system is a multi-planet system with one confirmed potentially habitable exoplanet. Gliese 667C is roughly 22 light years away and is believed to have a six-planet system with at least three of them being in a zone in which water is likely. At 25 light years, Vega is a relatively new system believed to be in the process of planetary formation. Lastly, TRAPPIST-1 is a 39 light year away system in which seven Earth-like planets exists.
Interstellar travel will be needed in order to verify the presence of life or the possibility for colonization on any of these planets within the above systems. Therefore, methods of accomplishing such have been proposed. Firstly, in accordance with modern propulsion technologies, the idea of relatively slow, unmanned missions have been developed. Such concepts include Breakthrough Starshot and Project Dragonfly, Longshot, Icarus, and Daedalus. These probes would be similar to the Voyager program and take an incredibly long time to reach their targets. Furthermore, the limitations of the longevity of onboard technology is a concern.
Another method tackles the problem of needed speed by greatly reducing the mass of the space vehicle. These, “nano probes” are being developed at the University of Michigan and need a nanoparticle-based propellant in order to work. The light weight means they would require significantly less energy to accelerate and plans for onboard solar cells would be used to persistently accelerate them. Although conceptually possible, there is still a lot of work to be done before this concept comes to fruition. Furthermore, the small size will pose other obstacles to overcome. The probes would be subject to magnetic fields and veer off course for example.
There are some proposals for manned missions under the assumption near light speed travel is not feasible. The first concept is known as generation ship or world ship. It is an interstellar ark in the sense that those arriving at the destination would be descendants of those whom began the mission. Although this would solve the time issue, it presents a whole lot of other challenges. Firstly, constructing a vessel that large is not currently possible. Furthermore, the amount of energy needed to launch it would be astronomically expensive. Also, even if the journey began smoothly, biological and sociological issues are bound to surface as the spacecraft ventures towards its destination.
Theoretical solutions to these aforementioned problems include the sci-fi ideas of suspended animation or cryonic preservation. Neither of which are currently possible, but by leaving passengers inert for the bulk of the journey, the psychological and sociological issues of being contained to a ship could be avoided. Furthermore, less resources would be needed on the journey assuming the state of the passengers is hibernation-like. Another idea is a combination of robotic travel and human cargo. Theoretically, space colonization could occur if embryos were frozen as cargo and brought to their destination by automated spaceflight. A multitude of other problems would surface with this method though such as there not being parents to raise the children and the development of artificial uteruses.
Assuming significant improvements on spacecraft acceleration in the future, a few benefits of near light speed travel will arise. One of which is time-dilation. This essentially reduces the time a traveler experiences as they increase their velocity. For example, a clock on an interstellar ship would run slower than an identical clock on Earth if the ship was accelerating at a constant rate. This phenomenon would allow for a round trip to nearly anywhere in the galaxy within 40 years ship-time at a constant acceleration of 1g. However, upon returning, the time experienced on Earth would have been significantly longer. For example, traveling to the Milky Way which is 30,000 light years from Earth and back in 40 years ship time would account to over 60,000 years on Earth. Despite these perceived negatives, the occurrence allows for travel beyond the initially thought 20 light year maximum and benefits interstellar travel possibilities.
Achieving time dilation requires constant acceleration, something that presents huge challenges but would account to the fastest travel times. Enough fuel would have to be kept on board to allow for constant acceleration for the first half of the journey and enough fuel for constant deceleration for the second half. This is due to Newton’s Laws of motion as the spacecraft would need to stop at its destination. Not only would this make the journey relatively fast, but it would provide a sense of gravity for those onboard. However, such a quantity of fuel would be exorbitantly expensive. Another problems with the aforementioned hyperbolic motion is that the crew’s gravitational field would gradually reverse midway through the journey. This certainly poses a logistical challenge. Also, any communications with Earth would be subject to time dilation. This means that the relatively short journey onboard would be perceived to take a very long time by those involved back on Earth.
The main issue with interstellar travel is providing enough kinetic energy to get a spacecraft near light speed and reduce travel time to within a lifetime. This requires immense improvements in modern day propulsion technology. We know that all spacecrafts are subject to the rocket equation. Essentially, the ratio of thrust to vehicle mass is the main challenge when setting trajectories light years away. However, even if this is solved, another issue is heat transfer. Especially with constant acceleration models, the issue of heat is imperative. Protecting the interior of the vehicle presents an immense engineering challenge.
A method of propulsion that might work is ion engines. In these engines, electric power charges particles and accelerates them at exceptionally fast velocities. Traditional fuel engines are limited by their storage capacity and thus, have top speeds. Ion engines on the other hand are only limited by the electrical power available which can be regenerated through means such as solar panels throughout the journey. Another prominent method of achieving interstellar travel is through the use of nuclear fission. The amount of energy capable by such methods greatly overshadow traditional fuels. However, not all methods of nuclear fission would bring a spacecraft close to the speed of light. For example, nuclear-electric or plasma engines, although much more powerful than traditional fuel, would only be capable of producing small accelerations with a predicted maximum velocity of 15% the speed of light. Furthermore, fission-fragment rockets would only be capable of reaching a mere 5% of light speed. Even nuclear pulse technology, such as the methods proposed for Project Orion would likely only reach 10% the speed of light. Thus, propulsion technologies beyond our present capabilities will be needed in order to travel further than the closest systems to our own Sun in a relatively short time frame.
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