The concept of space travel has fascinated us for generations, and is a well-explored theme in science fiction. But what about in the real world? Do we have cause to hope that one day we’ll be able to travel amongst the stars, getting up-close and personal with exotic phenomena such as supernovae, quasars, and black holes?
If we’re ever to make interstellar travel a reality, there are a number of associated challenges we need to overcome. First off, the scale of astronomical distances is, well, astronomical! The distance between our star system and its nearest neighbour, Alpha Centauri, is about 41.5 trillion kilometres. Even travelling at the speed of light (3 x 108 m/s) that means it would take about 4 and a half years to reach it!
There are two main approaches to overcoming this challenge; we can develop a method of travelling faster than light, or we can plan to spend a long time travelling. Unfortunately, the box below will explain to you how Einstein’s theory of special relativity appears to prevent us from travelling faster than the speed of light. One option remaining to us is generation ships, missions launched with the understanding that those aboard who reach the intended destination will be the descendants of the original crew. A similar, yet more technologically advanced option, is sleeper ships, where the crew remain in some form of suspended animation during the trip.
Special relativity and mass
The equation below describes how the relativistic mass (m) of an object changes with its velocity (v), as derived by Einstein’s theory of special relativity. As the speed of an object increases its mass increases compared with its rest mass (m0), the value of its mass when its velocity is zero. This increase in mass results in a greater energy requirement to accelerate the object. As the object reaches an appreciable fraction of the speed of light (c), the energy requirement becomes prohibitively enormous, and eventually, infinite. This explains why nothing can travel faster than the speed of light.
Either way, the resource and energy requirements for such a trip would be enormous. The average person uses approx. 45m3 of water a year, to say nothing for the amount needed to grow our food, much more than we could possibly carry into space on a long journey. We would therefore need an incredibly efficient recycling system, conserving limited supplies by converting urine, shower water, and breath vapour back into drinking water. Inevitably however, inefficiencies in the system will result in losses, necessitating significant initial stores be included in the ship’s manifest, or a course which takes us past suitable sources of water (such as perhaps the ice which forms on some comets). The International Space Station (ISS) already has some advanced water recovery systems, which you may like to read more about. They are currently able to recover 70% of their wastewater, though they still rely on regular shipments of water from Earth.
If a person uses 125m3 of water, and 85m3 of it can be recycled, what is the percentage efficiency of the system?Click to reveal answer
Answer: 85m3 / 125m3 x 100 = 68%
Historically, all the food that astronauts consume has been grown and processed on Earth, then sent up into space. On a long journey however, we would need to ensure that food could be grown on-board in order to meet the on-going demand. The ISS have been experimenting with space-grown vegetables, which you can read more about here. They’ve found that there are a number of difficulties unique to growing veggies in microgravity, such as ensuring that they don’t float away from their ‘soil’, as well as watering limitations, and the ability to wash their food! However, we’re currently making good progress in this area.
One appealing alternative would be to use matter-energy converters, which would convert energy directly into edible matter. If we were able to replicate the structures of simple sugars, carbohydrates, proteins and fats, it might be possible to design your favourite dessert, and have it immediately assembled inside a ‘replicator’, avoiding the need to grow its components at all! The ‘science’ of such a suggestion comes from something called mass-energy equivalence, which stems again from special relativity.
Remember to consider how you’d eat it without gravity!
The famous equation below describes the relationship between the energy (E) and mass (m) of an object at rest. From it, we can see that c2 is a proportionality factor between equivalent amounts of energy and mass.
E = mc2
therefore, E ∝ m
where ∝ means “is proportional to”
Mass and energy can be viewed as two forms of the same underlying, conserved quantity. Thus, conservation of mass and conservation of energy instead become the conservation of mass-energy. This view requires that if either mass or energy decreases in a closed system, corresponding energy or mass is gained in accordance with the equivalence relation above.
Aside from life support, there are energy considerations involved in whatever kind of propulsion the spacecraft employs; we must be able to carry or collect enough ‘fuel’ to continue running the spacecraft’s propulsion systems, or we’ll end up merely floating in space!
Propulsion methods can be divided into two main categories; reaction engines, and those without internal reaction mass. Reaction engines expel material, which via Newton’s third law of motion provides an “equal, but opposite, force” on the craft, propelling it forwards. These include conventional rocket engines, Hall effect thrusters and ion drives. On the other hand, some engines require little or no expelled material, such as those that take advantage of gravitational fields, magnetic fields, electromagnetic waves, and solar radiation.
On a long journey it is likely to be impossible to set out with enough reaction mass (material to expel) for the entire trip, so we must consider sources of reaction mass which we might encounter in deep space, or options which don’t require internal reaction mass. One obvious example is solar energy; wherever the light from stars reaches us we should be able to harness it to generate electricity. However, we’re not likely to be regularly close to a star (given the vast distances between them), and since the intensity of light falls off according to the inverse-square law with respect to our distance from a light source (see below), we would need a very large collection surface. Solar sails use large ultra-thin mirrors, which harness the radiation pressure from incident starlight. Japan launched a solar powered spacecraft called IKAROS in 2010, successfully demonstrating the concept. Other options include magnetic sails, which deflect charged particles from solar wind using magnetic fields, thereby imparting momentum to the spacecraft.
The inverse-square law
The intensity of a point source of light is inversely proportional to the square of the distance between the source and the observer, as shown below.
Intensity ∝ 1distance2
Light from a point source is radiated evenly outward from a point in three-dimensional space. The surface area of a sphere is given by the equation below, which is proportional to the square of its radius. As the emitted radiation spreads further from the source, the area is it distributed over therefore increases with the square of the distance. Therefore, the intensity varies inversely with the square of the distance, giving us our inverse-square law.
Surface Area = 4πr2 Intensity = EnergyArea ∝ 1r2
Any attempt at manned interstellar travel will also have to contend with the hostile environment that deep space presents. Collisions with interstellar dust and gas can have particularly dangerous effects on the spacecraft and its crew, as well as exposure to high-energy cosmic radiation. The large relative speeds of the spacecraft with respect to the interstellar media (material between the stars) means that significant damage can be caused by collisions with even the smallest of grains of dust. Significant shielding is necessary to protect vital systems, and to prevent a hull breach.
One proposed method of protecting astronauts from cosmic radiation utilises magnetic fields to deflect incoming energetic particles away from the spacecraft, mimicking the Earth’s protective effects on humanity. It’s possible to protect against impact events by increasing the thickness of the hull, though this has obvious drawbacks in terms of significantly increasing the craft’s mass. A much smarter method is called Whipple shielding, where thin sacrificial walls are mounted on top of the external walls of the craft. Acting in much the same way as a car bumper, the Whipple shield absorbs the collision of the incoming material, resulting in the impact being spread across a wider section of the hull.
Hopefully this article has given you a little insight into the challenges involved in travelling between the stars. If you didn’t get chance on your first read through, don’t forget to have a go at the challenges and let us know how you got on in the comments!
Over to you...
What would your spaceship look like? Write a paragraph in the comments below, including the main features of your spaceship. Remember to consider how it will be powered, and how it will help to keep its crew alive!