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The Feasibility of Transport Outside of Our Solar System
On April 12, 1961 the first ever EVA was performed by Yuri Gagarin during a 108-minute flight orbiting the earth for just over one full orbit. Eight years later the first manned mission to the moon which was Apollo 11 took place. Neil Armstrong and Buzz Aldrin were the first men to have ever stepped foot onto to another planetary body, and it was the first all-night broadcast on British television. Thousands of people across the world watched the broadcast, a truly iconic moment in the world of space travel.
Countless more incredible feats have taken place since, with an unmanned mission to mars landing in the very hour I write this sentence. Not only this, but we are only 5 years away from when Space-X is planning to have started their first manned mission to mars. It is a wonderful time to live in for space exploration and I, myself, have been inspired by all these accomplishments.
This caused me to start thinking and I think many other people who are interested in this topic, what is the next step? There are still many planets in our solar system that we have not yet gathered data on and are still a long way off visiting with manned missions, but I am going to be taking this one step further. In this essay I am going to be writing about travel outside of our solar system. I am going to talk about some of the problems that arise from traveling such huge distances and whether we can overcome them using current and theoretical future technology. By the end of this essay I will have judged, if possible, how feasible transport outside of our solar system is.
Introduction of Destination
The earth is located in the Orion arm of the milky way which is around 28,000 light years away from the centre of the galaxy where the supermassive blackhole Sagittarius A lies. We orbit around this at around 828000 km/h with an orbital period estimated to be between 225 and 250 million years. The diameter of our galaxy is around 100,000 light years however the closest star to us is Proxima Centauri which is located in the alpha Centauri star system. The star system also consists of Alpha Centauri a and Alpha Centauri b. Proxima Centauri is around 4.2 light years away from our sun and about 0.2 light years away from the centre of the alpha Centauri start system. Not only is this the closest star system, but it is also believed that there is a good chance of there being habitable planets that are similar to earth with the slight possibility of there being life on these planets. A researcher named Javiera Guedes at the university of California ran multiple computer simulations of the system first 200 million years. In every case even when using different parameters a planet formed similar in size to that of earth. As well as this in a lot of these cases the planet fell within the inhabitable zone of the star. This gives plenty of incentive to visit this system but when are we going to be able to, if ever.
Orbital Mechanics
In order to send a spacecraft to another planetary body outside of our solar system the first step we will need to take is to get our spacecraft in orbit around earth. This involves launching the craft upwards and then beginning a gentle gravity turn just after launch. As the atmosphere becomes thinner the gravity turn becomes steeper until the craft burns horizontally at the height of its orbit. This burn will continue until the periapsis of the orbit becomes equal to the apoapsis giving a circular orbit around the orbit. Ignoring drag the delta-v required to reach a low earth orbit (2000km) is around 9.4 Km/s however atmospheric and gravity drag adds around 1.3-1.8 Km/s resulting a total delta v of 10.7-11.2 Km/s. In order to transger between to bodys usually you would use a method called a Hohmann transfer. This is the most efficient method of transfer. Firstly, the craft will burn prograde until its apoapsis is equal to the of the destination body. When the craft reaches apoapsis you would burn prograde until the craft periapsis is then equal to the destinations body. Both the ship and the destination will have the same orbit around the central body meaning that the craft will begin to orbit the destination body. In order for the craft and the destination body to be in the same place when the orbits intercept the craft must be launched at the correct time called a transfer window. However, this method starts to have a few flaws when used in a galaxy. Firstly, alpha centauri and the sun have incredibly similar orbits around the galaxy meaning that if a Hohmann transfer was attempted the prograde burn would be very small meaning the crafts speed would be very slow meaning that the time taken for the transfer would be very long. A Hohmann transfer to another star would have an orbital period of about half the orbit around the centre of the galaxy which is about 115 million years. As well as this, we would have to wait for the correct transfer window meaning probably around another 115 million years. Not only is this method unfeasible but very hard to do even with a craft that could withstand this period as the calculation become a lot more complicated for galaxys for Hohmann transfer. A big problem is that the mass is concentrated into the centre of the galaxy meaning that taking a mass of a central body and our orbital radius doesnt really work as there may be stars close by that have small masses but a large gravitational pull due to their distance. This means that calculation an orbital transfer requires having orbital data for all stars close by. As we dont even have an accurate measure of the mas of the whole galaxy calculating an orbital transfer like this is just too difficult with our current knowledge.
Although a Hohmann transfer may not work this doesnt mean there arent other methods. As our orbits are so similar the best method to transfer between the 2 systems is to burn directly towards the alpha centauri system with half the delta v and then burn retrograde for the next half accounting for the movement of the galaxy by aiming just ahead of its orbit. Although this method seems as if it would be incredibly inefficient it isnt actually that bad. As our orbits are so similar. This is the same method used for docking crafts in orbit around earth that have similar orbits giving us proof that it could possibly work. The distance between the sun and alpha centauri is roughly 4 x 10^16 metres meaning we can work out the time this would take with current engines with the Tsiolkovsky rocket equation.
Engine ISP and Chemistry
The Tsiolkovsky rocket equation relates delta v, the dry mass of the rocket which is the mass of the rocket with no fuel, the wet mass of the rocket which is the mass of the rocket with fuel, the ispg0 of the engine or ve which is exhaust velocity which relates to hoe efficient the engine is. The highest specific impulse ever achieved was 542 seconds using tripropellant liquid engines. However these engines are very impractical and have not currently implemented into a rocket design as it requires liquid lithium, gaseous hydrogen, and liquid fluorine. This is difficult to achieve as lithium needs to be at a very high temperature to flow as a liquid, and liquid hydrogen is near absolute zero. As well as this, the exhaust fumes are incredibly toxic. Although, lets assume that a method of using this type of engine on a new craft is worked out we will be able to have an isp of 542 seconds. The mass of the payload we will ignore as it is negligible in comparison to the mass of the engines. We will assume that our payload would be around 30kg as the mass of the solar sail sunjammer is 32kg which is being designed to be sent to alpha centauri. The engine with the highest thrust to weight ratio is the Merlin 1D 158. Although no engine exists with an ISP and a thrust that is that high exists we will model the calculation with those specifications as it is much simple rather than choosing between engines and it will still give us a very good idea of the time that his mission would take. With all of this information now filled in we have only two variables remaining: the amount of engines which will be our dry mass, and the amount of fuel which will be part of our wet mass (we will also assume the mass of fuel tanks is negligible).
Cryostasis
Solar sails
Although these traditional methods of space travel do not seem to work on such vast scales it does not mean that there arent other solutions. A key problem that has had to be dealt with when launching spacecraft is a phenomenon called light pressure, and it results from the fact that light carries momentum. This means that when the light produced from other stars, mainly our sun when travelling within our solar system, it hits the spacecraft and some of the momentum is transferred over to the spacecraft. This effect influences the path of spacecraft quite dramatically ad it was realised that this problem that was previously having to be accounted for in the past could actually be used to fuel a spacecraft. The idea behind a solar sail is that it is an incredibly light spacecraft with a large surface area that is propelled by using light pressure. To make sure that this thrust is sufficient and in the correct direction the craft will make use of a laser array built on earth instead of the light from stars. This technology is revolutionary as it means that the energy required for the spacecraft does not have to be carried by the spacecraft. Due to F=MA this means that the acceleration of the craft will be much greater due to the decreased mass with identical thrust. Although currently there is no proof of concept for solar sails there is project named project Starshot that plans, to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri within the next generation. At this moment in time there are still issues with logistics that are unsolved but no problems have come up that seem unsolvable. A lot of engineering is still required but this may be the first method that actually seems feasible, and not only this but feasible within the near future. The infrastructure that is required to be built before these solar sails can be launched is quite large and will most likely take a decade to a few decades. A solar array that will need to be multiple of kilometres wide at a high altitude to avoid diffraction through the atmosphere. As well as this a mothership with multiple solar sails will need to be launched into low earth orbit. The time this takes will be dependent on how fast the engineering problems are solved.
The method of using light pressure to propel spaceships is revolutionary, however the journey will still take over 10 years to travel to the closest solar system in our galaxy. Although still impressive, the payload capacity is very small and there is no possibility of transporting humans using this method. Not only this but trying to travel to any other solar system within our galaxy, let alone outside of our galaxy is unfeasible with this method. With current technology and our current understanding of physics the speed these solar sails are travelling at is very close to the limit we are able to travel at. This is due to the fact that as an object approaches the speed of light exponentially more energy is required to accelerate it, with an object with mass requiring infinite energy to reach the speed of light. With our generic methods of travel travelling past our neighbouring solar systems is just beyond our reach. However, one method still exists that may be able to solver this seemingly unbsolvable problem: although we are unable to break past or even approach the speed of light we are able to move faster than the speed of light relative to other objects. This results from the fact that space is constantly expanding causing the gaps in-between objects to increase making it seem as if the object is moving away from us. Very little is known about why the universe is expanding but the current theory is that an energy called dark energy exists that causes it to expand. After this theory was proposed and generally accepted across the scientific community a device, the alcubierre drive, was thought of. This uses the idea of dark energy and negative energy, and shows that with our current model of physics if a negative energy density was able to be created then it would be able to shrink space in front of it and expand space behind it. The device is very theoretical and is believed by some to require exotic matter which is something predicted to exist however not yet discovered. However some believe that negative energy densitys found in other places such as the Casimir effect between to plates would suffice.
Limitations and Judgement
Current methods of transport around our solar system are unlikely to be able to be used for transport outside of our solar system. Using this method we are limited by the Tsiolkovsky rocket equation and when the distances are scaled up to the vast distances across our solar system the time is too long, and the cost is extortionate. It is important to consider that although slow, this method would complete the mission eventually, and if no other methods are feasible our current methods could be used if a colony was sent with the ability to create and use sustainable resources. As well as this, if cryostasis technology is created then this method is certainly viable. The concept of solar sails would definitely make transport outside our solar system feasible, however probably only to close by solar systems as even the closest to us will take around 10 years with the current design specification. As well as this there are still engineering problems that need solving, but the design seems possible and realistic with our technology. This method seems like the most likely method to work, and I think we will be seeing within the next thirty years. The alcubierre drive, although possible, I do not think will not be physically meaningful. The idea hinges on a lot of unknowns that we dont have the answers to and extrapolates our knowledge of physics very far. With our current model of physics there are already contradictions that need solving for example between general relativity and quantum mechanics; areas this device both heavily relies on. In conclusion, I definitely think transport outside of our solar system is feasible using solar sails, not only this but has a high chance of being achieved in the near future. As well as this even if there are complications in this design process that make solar sails unfeasible for this task then current methods of transport around our solar system. Although, the journey will be incredibly expensive, and the time taken will span across multiple generations the method may still happen quite far into the future. As far as travelling outside of our galaxy to our closest neighbouring galaxy
