In all of human history, people have looked up to the stars and wondered what lurks just out of view; some have deified the stars, others have blamed them for all their misfortunes. As modern science continues its march through time, we are beginning to form an understanding of space and its intricacies. Yet that does not mean that we are any less enamoured with the stars as a species. During the Cold War, the exploration of space was a fundamental determiner of a country’s prowess. However, discussions at various dinner tables have transitioned from inquiry into discovery into an analysis of the colonisation of space. Scientists from many fields, as this is overwhelmingly a multi-disciplinary issue, have put huge amounts of research into the potential displacement of the human race. In terms of physics, the problems that I will focus on are the locating of habitable planets and the interstellar travel between Earth and the prospective planet. 

Planet B: 

Although we have been presented with a number of places within the solar system to which we can escape, none are truly viable. Sure, we could make a base on Mars, but such an experiment would require many supplies to be ferried from Earth, to a very hostile location, inevitably leading to a fragile colony with a very slow rate of growth. What scientists have also been considering is moving to an exoplanet with earth-like conditions. However, there are a number of conditions that a planet must fulfil to be considered a habitable, earth-like planet. 

The first condition is the location of the planet and its proximity to other celestial bodies. The type of star that an exoplanet orbits will have huge consequences for any attempts at sustaining life. Not only will this affect its habitability, but it will also make a difference to the viability of growing earth-type plants, which have adapted to the specific electro-magnetic frequency emitted by the sun. Moreover, if the exoplanet is orbited by a moon or rather multiple moons, this can create severe tidal phenomena that can be very destructive to any prospective colony. 

Habitable Zone of Main Sequence Stars 
Credits: PHL, University of Puerto Rico, Arecibo

The next consideration is the mass and as such the gravitational strength of the planet. This is crucial as an appropriate strength of gravity will facilitate and nurture growth, while a deviation from this optimum will cripple any human attempts at exporting civilization away from Earth. If there is too much gravity, we as humans will begin to be crushed and will develop various back and heart problems. All equipment destined for such an exoplanet will require special design to cope with the extra pressure, vastly increasing costs. On the other hand, if there is too little gravity, a dense enough atmosphere will not be able to be maintained, resulting in any colony being kept underground, bound to the archetypal science-fiction domes scattered across the surface of the planet. 

Finally, the habitability of an exoplanet is affected by its rotational speed and atmosphere. When a planet rotates slowly relative to Earth, it has two conflicting effects; it harms the viability of plants that find it difficult to last longer periods without photosynthesising, yet encourages the development of Hadley cells in the planet’s atmosphere. Due to the larger differential between temperatures on the dark and light sides of the planet, they develop currents of colder and warmer air that in turn create greater cloud cover, increasing the deflection of radiation from the sun. This vastly augments the size of the habitable zone, allowing much closer planets to potentially sustain a colony. Yet all of this is only relevant when the atmosphere is amenable to human and Earth-plant physiology, ideally with a substantial percentage of oxygen. Also, the majority should be an inert gas and at the right pressure to retain the fidelity of our body. 

Planetary atmosphere  
Credits: Thomson Higher Education  

Interstellar Travel: 

It is not good enough however, to just find a viable exoplanet that we can colonise; we must have a way to reach it and transport a huge amount of supplies and people at as fast a rate as possible. Professor Cameron Smith of the University of Oregon stated that 20,000-40,000 people would be required to provide the minimum genetic diversity to sustain the colony in the long term and even reach it in the first place due to potential catastrophes and the inability to increase population size during the journey. Moreover, multiple tons of supplies would be required per person per year until viable food, medicine, water and various other essential production systems were set up. As such, the two main problems that must be overcome are the time required to travel distances measured in light-years and the fuel requirements to travel that distance. 

As a species we are currently only touching the realm of possible methods of propulsion, so the methods that I will describe are mostly theoretical at this point. The first is an exception to this rule; electric propulsion and ion drives. The concept involves the use of electricity to create charged particles, often the gas xenon and eject them rapidly from the spacecraft. Although the acceleration is minimal, over long distances, the gradual increase in velocity can easily result in the spacecraft travelling at not insignificant fractions of the speed of light. The ability to use electricity is a much better alternative than having to collect fuel such as methane. 

The use of nuclear reactions to propel spacecraft is one that is debated widely in both scientific and political communities, seeing as it is banned under the 1963 Partial Test Ban Treaty, however it would be a viable method of propulsion. Both fission and fusion could power a rocket and achieve figures reaching 5% and 10% of the speed of light respectively, assuming fairly current rocketry technology for the rest of the rocket. Freeman Dyson, a member of Project Orion said that a 100,000 tonne spacecraft could reach Alpha Centauri in 130 years with comparable technology. 

A similar method, also using various minor explosions to accelerate, would be a matter/antimatter engine. This would entail the annihilation of matter and antimatter either to directly propel the rocket forward or to heat a working fluid that would then do the work. Yet there are clear issues in terms of the storage of the anti-matter, especially in turbulent conditions often present aboard spacecraft, and the gradual heating of the ship via the emission of gamma rays. However, much higher speeds would be attainable than with fusion or regular combustion. 

There are also several purely theoretical ideas that have been postulated by various scientists, including faster than light travel, the use of hawking radiation from artificial black holes, wormholes and Alcubierre drives, which contract space-time in front of the ship and expand it behind. 

However, to ground ourselves in the probable, we should look at the progress made by SpaceX, a pioneer in private space travel. They have created and are testing a spaceship that can carry 100,000kg of supplies to LEO (Low Earth Orbit) and is potentially reusable. And so, for the best self-sustaining colony, we should send people to a far away exoplanet, but with what we have access to, we can definitely content ourselves with a second home on Mars. Their spaceship uses regular combustion, a combination of cryogenic liquid methane and liquid oxygen, in order to propel itself through space, effectively a complex car engine. Moreover, SpaceX has been the first private company in history to create and use a shuttle to send people to outer space and specifically the ISS. 

SpaceX rocket launch 
Credits: Joe Raedle / Getty Images  

And so, whenever a little boy or girl sees a drawing of a little rocket ship and decides that they too want to conquer the stars, this is no longer a momentary whim of an excited child who has stayed up past their bedtime that will inevitably fail, it is a dream that very well might be fulfilled. Whether it is to expand our influence as a species or to escape some dreadful catastrophe, the exploration and colonisation of the stars is something that should be supported and discussed in great detail, especially where our new home would be.  

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