AS WITH ANY journey, before you can set off on your space odyssey you must first decide exactly where you’re going and how you’re going to get there. You need a flight plan. Journeys on Earth are pretty easy to plan and execute because we have mapped the surface quite extensively. Generally speaking, nothing moves and everything stays in the same relative position, which makes navigation pretty simple. In a car, for example, one needs only to follow the correct roads and the towns and cities we wish to reach will be at the end of them. Even in an aeroplane, once you’ve accounted for the Earth’s curvature, it is relatively straightforward to point in the direction you wish to go and you will get there. It is not quite that easy when it comes to travelling in space.
The real stumbling block when travelling around the Solar System is that it takes a significant period of time to get anywhere, so the relative position of an object will change. If you want to go to Jupiter, for example, if you simply aim your spacecraft at it, by the time you arrive it will have moved somewhere else. What you actually need to do is figure out where the planet will be at the calculated arrival time and aim for that spot instead. It is much like clay pigeon shooting: you have to aim at the place where the clay pigeon will be when the shot arrives, otherwise you miss. You must essentially intercept the moving object. Things get even more complicated for an epic journey like yours, as visiting more than one planet requires complex calculations and, ultimately, complex trajectories. But we can look back at the Voyager and Pioneer missions and learn from their calculations to help you plan your own flight path.
Over the years we have gained a very good understanding of movement within our Solar System which means at any given time we know where all the major bodies, and a great many of the minor bodies, are. Surprisingly perhaps, most of our knowledge about how the planets move comes from naked-eye observations over many hundreds of years. As we saw in the introduction, when mankind first started looking at the sky it became clear that a handful of ‘stars’ were unlike the rest, and these became known as the planets. Careful study helped to unlock the secrets of how they moved around the Solar System, revealing that their motion was not fully described by the Geocentric Model outlined by Ptolemy. The model was modified, and instead of the planets moving around the Earth in circular orbits, the idea of epicycles and deferents was proposed. In this system, all the planets orbited around a smaller circle known as an epicycle which itself completed a larger orbit around the Earth called the deferent. This went some way to explaining the strange wanderings of the planets, but there were still some anomalies.
In 1543, Nicolaus Copernicus proposed a different idea, which found no favour with the Catholic Church. In his new helio-centric model the Sun had found its way to the centre of the Solar System replacing the Earth, which became just one of the family of planets in orbit around it. It was a move that put Copernicus in direct conflict with the Church which believed at the time that the Earth was the most important object in the known Universe. They believed that it had been made by God and therefore must be at the centre of everything, in the most important position possible. Observations didn’t support that concept, but the idea of an Earth-centred Universe did not start to lose credibility until 1609, when Galileo published the initial observations he’d made with the newly invented device known as the ‘telescope’. In his work Sidereus Nuncius he reported craters on the Moon, the rings around Saturn and the four moons of Jupiter, and it was these observations that finally pushed the Catholic Church into a corner. They responded by accusing Galileo of heresy and he was placed under house arrest until he died. It wasn’t until 1992 that the Church finally released an official apology for this, several hundred years after his death.
In the same year that Galileo made his first observations, another astronomer, Johannes Kepler, had been analysing observations of the positions of the planets made by his now deceased tutor, a man named Tycho Brahe. This work led to Kepler publishing the first two of his well-known laws of planetary motion.
Kepler’s first law states that all planets move in elliptical orbits with the Sun at one of the points of focus of the ellipse. An ellipse is essentially a squashed circle, and you can imagine the two points of focus of the ellipse if you visualize a point at the centre of a circle. Now think about squashing the circle from the top and bottom and the central dot splitting in two and being moved outwards in different directions. In the case of the planets in the Solar System, the Sun is found at one of these points, and it is this that they all appear to orbit. His second law states that a line joining the Sun to a planet (known as the ‘radius vector’) sweeps out across equal areas of space over equal intervals of time. In other words, planets move faster when they are nearer to the Sun and slower when further away.
The last of Kepler’s laws, published ten years later in 1619, explains that there is a mathematical relationship between the time it takes for a planet to complete an orbit and its distance from the Sun. In Kepler’s words, ‘The square of the orbital period of a planet is directly proportional to the cube of its mean distance from the Sun.’ This is a really useful relationship because we can measure how long an object takes to orbit the Sun simply from observation, and knowing that, we can calculate its average distance from the Sun with some accuracy. The same law applies to any other satellite objects, for example the moons of Jupiter. If we measure the time it takes for one of the moons to go around the planet, we can calculate its distance from the planet itself.
Kepler’s three laws helped to build a clear picture of the Solar System and allowed us to predict the movement of the planets, and this will be vital information for your journey. But Kepler is not the only figure in history who has helped us understand our planetary neighbourhood.
On 4 January 1643, Isaac Newton was born in a tiny village called Woolsthorpe-by-Colsterworth in Lincolnshire, England. Newton became one of the foremost scientific minds of all time with a real flair for physics and mathematics. The pinnacle of his career came in 1687 with the publication of Philosophiae Naturalis Principia Mathematica which included his law of universal gravitation. The law simply states that there is a force between any two objects in the Universe, and that force is defined by the mass of the two objects and the distance between them. Put more accurately, the force is calculated by multiplying the masses of the two bodies and then dividing the answer by the square of the distance between them. This means that if two objects retain the same distance between them but increase in mass, then the gravitational force between them will increase. Similarly, the force will increase if the masses are kept the same but the distance between them decreases. Not only does this describe the forces that caused the apple to fall from a tree in Newton’s garden which supposedly led him to ‘discover’ gravity, it also explains how the Moon is kept in orbit around the Earth, the Earth in orbit around the Sun, and even why the stars orbit the centre of the Galaxy. In fact, gravity (or ‘gravitas’ as Newton called it, which came from the Latin meaning ‘weight’) pervades the entire Universe, even binding galaxies into great clusters.
In his works, often shortened to Principia, Newton also articulated his three laws of motion. The first is the law of inertia which relates to an object’s resistance to changing its state of motion. The law states that any object in motion will stay in its current state of motion without accelerating until acted upon by an outside force. This is of great importance when it comes to space exploration as a spacecraft in flight will continue in its current direction and speed with zero fuel use unless it is acted upon by an external force, which might be the gravity of a planet, a meteoroid strike or its own rocket engines.
The second law explains that the speed at which an object accelerates will depend on its mass and the force exerted upon it. More accurately, the force referred to is the net force, which accounts for the fact that if you apply a force of 10 units to propel a spacecraft in one direction but simultaneously apply a force of 5 units in the opposite direction the craft will only accelerate by a value of 5 units. This is the net force and is the force Newton considered in his second law. The law gets a little more complicated when you consider the mass of the object in question, as increasing or decreasing the object’s mass will have an inverse effect on acceleration!
Newton’s third and final law is probably the most widely known: for every action there is an equal and opposite reaction. This means that for every interaction between two objects, there is a pair of matching but opposite forces acting upon the two bodies. There are some wonderful examples of this in nature, such as a bird in flight. Birds fly by flapping their wings, but for every beat of their wings downward, they exert a force on the air towards the ground. The interaction is between the bird and the air so there must be equal but opposite forces. With the bird pushing down on the air, there is an opposite force from the air, pushing the bird upward. The forces in play are equal, but the direction is opposite, and it is this that keeps the bird in the air.
With the amazing and insightful work of Kepler, Newton and a few other scientists along the way, the foundations were set for mankind’s exploitation of rocket flight and exploration of the Solar System. Your journey has been made possible by many of their discoveries.
In order to be able to travel to all the planets in the Solar System it is necessary to make use of their gravity to affect changes in the velocity (speed and direction) of your spacecraft. Getting to the inner planets is actually pretty easy because a spacecraft that leaves Earth’s orbit and heads towards the Sun will be accelerated by the Sun’s gravity. This can make a fly-by of the inner planets really quite simple as long as the trajectory is right. Heading to the outer planets is a little more tricky, though, because any flight away from the Sun will be decelerated by its gravitational pull, causing your spacecraft to slow. Without any form of assistance it would take an incredible amount of fuel to get to the outer planets. So much, in fact, that it would be impossible simply to launch from Earth. This is because even for small changes in the trajectory of a spacecraft travelling at high speeds a large amount of fuel is needed. The more fuel on board, the heavier that spacecraft becomes, and the heavier it is, the more fuel you need to lift off in the first place – and so continues the problem. This is where the gravity of planets can be used to work in your favour, to assist you on your journey.
Gravity assist, or a ‘gravitational slingshot’ as the manoeuvre is known, was first used by the Mariner 10 mission that was launched to Mercury and Venus back in 1973. It has been used successfully on nearly every interplanetary mission since, including the historic Voyager and Pioneer projects. Another great example is the Cassini mission, which travelled to Saturn after acquiring the necessary speed by flying first around Venus (twice), Earth and Jupiter before arriving at Saturn. Similarly, the Mercury-bound Messenger spacecraft used the gravity of Earth, Venus and Mercury (three times) to reduce its velocity before it could drop into orbit at the innermost planet. The principle is simple. If you fly a spacecraft through the gravitational field of a planet then the spacecraft and planet will exchange energy. Depending on the mechanics of the fly-past, the spacecraft will either gain energy and speed up while the planet will lose a tiny amount, or vice versa. In the case of your grand journey around the Solar System, the purpose of a gravitational assist will be not only to adjust the direction of the spacecraft but also to speed it up.
Now there is a complication with this concept and it is articulated in the universal rule of the conservation of energy, which tells us that the total energy of a given system must always remain constant. So a spacecraft flying through the gravitational field of a planet should speed up on the approach but then slow down as it moves away again. For the conservation of energy to be upheld, this must be the case, and if observed from the point of view of the planet, this is indeed the case. Despite this apparent contradiction, planetary fly-bys are still of use to space missions. The planet is stationary when we consider the encounter from the planet’s point of view. However, the planet has significantly more mass than the approaching spacecraft so, on the approach, the spacecraft speeds up as it gains energy but it then loses the same amount of energy and slows down as it moves away. What does change is its direction, so the closer it flies to the planet, the greater the change in direction of the spacecraft, with the greatest possible direction change being 180 degrees as the spacecraft heads back in the direction it just came from. The total energy in the system is conserved but still, we have only changed direction, not speed.
Let us now consider the encounter from the point of view of the Sun. As we watch, the planet is moving, not stationary as it was when we considered it from the planet’s point of view. As the spacecraft swings past the planet, just like before, it speeds up when viewed from the Sun but it steals that energy from the orbital speed of the planet. This is how the speed increases from the point of view of the Sun, mathematically by adding the velocity of the planet to the velocity of the planet and the velocity of the spacecraft.
This may be a difficult concept to grasp, but a good way to imagine it is to think of a tennis player hitting a ball back to her opponent. If you were playing a Grand Slam champion then your shot might approach the champion at 20 kilometres per hour. She is much better than you at the game though and might swing her racket at a mighty 50 kilometres per hour. The champion’s racket will experience the ball approaching at a whopping 70 kilometres per hour, which comes from adding the speed of her racket to the rather feeble speed of your shot. As your opponent strikes the ball and it starts to head back towards you, her racket will still see the ball receding at the same speed that it approached, 70 kilometres per hour, but at the receiving end you will experience a ball travelling at an incredible 120 kilometres per hour. As far as your opponent’s tennis racket is concerned the ball approached and then receded at the same speed at the exact moment of impact, when the racket can be considered to be stationary. From your point of view, which can be aligned with the point of view of the Sun during a gravitational slingshot manoeuvre, the champion’s racket is moving and after impact the ball speeds up, but the racket will slow down by a tiny amount as a result of the interaction. It is the same for rockets flying around planets and it’s why the careful use of gravitational slingshots is such an integral part of space flight around the Solar System.
In order to complete your interplanetary voyage it is necessary to identify a route around the planets that will exploit as many gravitational slingshots as possible, to adjust your trajectory from one planet to the next. Maximizing the number of the slingshot manoeuvres will minimize the amount of fuel needed, ultimately reducing launch weight. There will only be a small number of opportunities for such planetary alignments. It’s a concept that was used very successfully during the Voyager missions. Both Voyager 1 and Voyager 2 executed slingshots around Jupiter and Saturn but the timing and mechanics of the Saturnian fly-past were slightly different for each. Following the encounter with Saturn, Voyager 1 was ejected from the Solar System while Voyager 2 went on to study Uranus and Neptune. For your mission to work, it is essential to identify a time when the planets will align perfectly to allow you to visit each of them in turn.
Once we are sure of the correct alignment and have a launch date, we can work out your flight path. A Titan Centaur rocket will set you on your journey, sending you first past the Moon and then on towards the inner Solar System for an encounter with the Sun. You could of course take in the inner planets en route but they will serve as useful fly-bys later on to adjust trajectory and increase velocity to get to the outer planets. Unfortunately you will gain nothing from a fly-by of the Sun in terms of velocity because all trajectories are relative to it. After just six months you will arrive at the closest point to the Sun and within the four months that follow you will fly by Mercury and then Venus. After the first Venus fly-by you will take in another orbit of the Sun before another fly-by of Venus just ten months later that will serve to increase your velocity relative to the Sun by 21,000 kilometres per hour. Following the second Venus encounter you will enjoy a rather emotional final look at Earth with a fly-by that will serve to set up your trajectory to the outer planets. You will arrive at Mars just five months later, and after a further four months you will take on the perilous crossing of the asteroid belt. Beyond the asteroid belt things will really start to slow down as you cruise to Jupiter, Saturn, Uranus, Neptune and finally Pluto, before setting off to explore the depths of the Solar System.
Are there any other giant planets orbiting beyond Pluto and the icy Kuiper Belt at the outer limits of our Solar System? If one does exist then the chances of spotting it are pretty slim, but your journey will continue through the so-called ‘termination shock’, where the influence of interstellar space starts to challenge the dominance of the Sun. A long way beyond this point your trip will eventually become interstellar as you reach the heliopause and the edge of the Solar System. After years of travelling your exploration around the familiar Solar System will come to an end, but the voyage won’t be over yet. As you return home the spaceship will leave the helio-pause behind and continue into deep space for a rendezvous with the theorized Oort Cloud, which will take a staggering 1,500 years. The final leg of the journey will take the now unmanned ship to its ultimate destination, Gliese 581, a star in the constellation of Libra at a distance of 20.2 light years, taking some 239,000 years.
Once you have a launch date and a flight plan we need to consider exactly how you’re going to make that journey. Rockets are obviously the right tool for the job, but what type of propulsion should we choose, and which is the most suitable spacecraft?
The first rocket engine employed a solid rocket fuel just like that used in the solid rocket boosters of the space shuttles. The ignition of the prepared solid fuel mixture produces the thrust to propel the shuttle upwards. The mixture in solid rocket boosters starts out as a thick liquid that can then be cast into various shapes as it cures. A typical solid rocket will be cylindrical in shape with a hollow tube running almost the entire length of the rocket. The ignition takes place within the hollow tube, and as the fuel burns it spreads outwards towards the casing of the rocket. Interestingly, if the shape of the channel inside the cylinder is changed to increase the surface area then the thrust can be increased. The shuttle boosters therefore have a star-shaped channel running through them to give maximum possible surface area. Solid rocket systems are cheaper to produce than the alternative liquid rocket propellant, but they are not as controllable because once the engine has been ignited it cannot be stopped or restarted.
‘Specific impulse’ is the term used to articulate how efficient a rocket propulsion system is and describes the force produced from a given amount of propellant over a given time period. Although they are cheap and still popular among military agencies, the solid fuel rocket systems have a low specific impulse. A higher specific impulse means a lower rate of propellant flow is required to produce a given amount of thrust. This is a very important factor as it determines the amount of fuel needed for your mission.
The alternative to solid rocket fuel was first tested by Robert Goddard in 1926 when he invented the liquid-propelled rocket. Instead of the slow and uninterruptible burn of a solid rocket, the new liquid rocket used gasoline and liquid oxygen to produce the required exothermic reaction. This time there was a significant difference though: the two chemicals were stored separately and injected into the combustion chamber. The rate at which the chemicals were injected dictated the amount of thrust produced, so for the first time rocket engines became controllable. The same principle was used on a grand scale in the mighty Saturn V rocket that took Neil Armstrong, Buzz Aldrin and Mike Collins to the Moon. The whole rocket assembly stood about 110 metres tall and weighed in at 6.5 million pounds – and of that, 5.6 million pounds was fuel. At launch the fuel economy was just 17.7 centimetres to the gallon, although that did improve drastically as the mission progressed. Compare that to the fuel economy of an average car – in the region of 44 kilometres to the gallon – and you’ll realize how expensive rockets are to run. Saturn V was actually composed of three different stages, all of them needed to get the astronauts up into Earth orbit, one firing after the previous had expired and been separated. Detaching each stage after it is spent is a more efficient way of getting into orbit, otherwise you have to carry the extra weight with you and that in turn means you need more fuel. The comparatively tiny command and service modules that actually went to the Moon sat on top of the Saturn V assembly and used small nozzles with compressed gas to make course corrections on the way. A final liquid rocket allowed them to leave lunar orbit and return to Earth.
The liquid fuel system employed by the Saturn V rockets used liquid hydrogen and liquid oxygen as the propellant mixture which has a higher specific impulse than the solid rocket fuels. Both technologies can generate huge amounts of thrust but have a relatively low specific impulse when compared to a new alternative concept known as the Variable Specific Impulse Magnetoplasma Rocket, or VASIMR for short. The VASIMR system has a much higher specific impulse making it highly efficient over long journeys but it generates very low levels of thrust. Imagine holding a sheet of A4 paper on the palm of your hand. The force the weight of the paper exerts on your hand is the same as the thrust generated by the VASIMR engine.
The principle behind the VASIMR engine is simple, and like all other rocket propulsion systems it exploits Newton’s third law of motion. The idea was developed by former astronaut Franklin Chang-Diaz who realized it would be possible to use magnetic fields to direct and channel superheated plasma out of the back of the rocket. This is an approach unlike conventional rockets which ignite chemicals in an exothermic reaction, as we have just seen.
To understand how it works we first need to understand the inner workings of an atom. Atoms are made up of a combination of particles in their nucleus called neutrons and protons. The protons carry a positive electrical charge and the neutrons, as their name suggests, are neutral and have no electrical charge at all. Surrounding the nucleus is a shell of electrons which have negative electrical charge and it is a combination of these and the neutrons and protons in the nucleus that determine the properties of the atom. A hydrogen atom, for example, has one positively charged proton in its nucleus and one negatively charged electron in orbit around it, while a helium atom has two protons, a number of neutrons depending on the type of helium, and two electrons. As you can see, generally the number of protons balances the number of electrons so the net electrical charge is neutral.
If you were to remove an electron from either of the atoms, then it would become positively charged; adding an electron would make it negatively charged. This process is known as ionization, and this is a key concept in the functioning of the VASIMR engine. Inside the rocket, hydrogen that carries no charge is injected into a magnetic field which strips away the electron, ionizing the hydrogen and making it positively charged. The ionized hydrogen is then moved to a second magnetic field where radio waves much like those in your microwave oven are used to heat it to temperatures in excess of 50,000 degrees Celsius. This heating turns the gas into a plasma, which is often referred to as the fourth state of matter, the others being solid, liquid and gas. Plasma is distinctly different from the other three states due to the presence of a significant quantity of charged particles. The plasma is then channelled to a third and final magnetic field which acts like a nozzle to expel the plasma which generates thrust to propel the rocket forward.
One of the great benefits of this type of engine is that although its specific impulse is high, it can be adjusted even in flight. When higher levels of thrust are needed, its specific impulse could be reduced; when less thrust is needed and efficiency is more important – for example during the cruise – it could be increased. These properties make the VASIMR engine an excellent choice for your cruise around the Solar System because they can provide much longer periods of low acceleration than a conventional liquid- or solid-fuelled rocket. And there is one really quite wonderful added advantage to the VASIMR rocket: its use of hydrogen, which is a strategically good choice because it is one of the most common elements in the Universe, so even on your trip around the Solar System extra fuel will be easy to come by should the tanks need to be topped up.
On your trip you will actually make use of all of these rocket systems. To get off the surface of the Earth and into orbit you will utilize the high thrust of the solid- or liquid-fuelled system, then once under way you will look to the new technology of the VASIMR system to help speed your journey. Perhaps one of the most important benefits of such an engine is the low amount of fuel needed, which means you can keep your spacecraft light yet retain the means of providing a highly efficient form of thrust when you hit interstellar space. It is not unusual to utilize more than one different form of propulsion, as demonstrated by NASA’s Dawn mission to study Vesta and Ceres, the two largest bodies in the asteroid belt. Dawn was launched in 2007 by a Delta 7925-H rocket which used a combination of both liquid and solid fuel. Once in Earth orbit the Dawn probe was powered by an ion engine, which is very similar to the VASIMR concept.
With the path around the Solar System and the rocket technology identified, there are a few physiological issues to consider before you set off, and this is where choice of spacecraft and equipment becomes important. Living on the surface of a planet with an atmosphere and a magnetic field means we are protected from the harsh conditions in space. As soon as we travel beyond the protective confines of our ecosystem we can expect no air to breathe, no atmospheric pressure to stop our blood boiling, and a fatal dose of solar radiation. These are just some of the challenges facing human space explorers, and we can also throw into the mix the apparent loss of gravity and issues with bone and muscle density, not to mention the psychological rigours of the journey. Fortunately, spacecraft have been developed to provide a life-supporting environment that allows astronauts to live and breathe in space, while advanced space suits carry further life-sustaining systems that allow an astronaut to venture beyond the spacecraft itself.
Providing artificial environments is sufficient for short-term excursions into Earth orbit or trips to the Moon and back, but one of the biggest challenges facing astronauts who spend long periods of time in space is the slow reduction in muscle and bone density because of the weightlessness. On Earth, the pull of gravity holds us firmly against its surface and we experience that as our weight. Without that gravity a simple jump would see us float off into space. Astronauts seem to display this effect of weightlessness but the gravity of the Earth is still very much present; in fact it is in some way responsible for the floating experience. Orbiting spacecraft are in effect constantly falling towards Earth, pulled by the force of gravity, and it is their forward motion that gives their path a curved trajectory which essentially matches the curvature of the Earth, preventing them from falling to the ground. Astronauts on board the International Space Station live in this weightless environment where they, the space station and everything inside it are falling at the same rate.
We have all experienced a time when we have momentarily weighed a little less, for example when driving too fast over a humpback bridge. As we, the car and everything inside it drops down a little faster than usual, our bodies weigh a tiny bit less for a fraction of a second, giving that stomach-in-the-mouth experience. You can take this to the next level and experience a ‘zero G’ flight – these take place on board large converted commercial airliners. After a conventional aircraft-style take-off you climb to a high altitude, then the pilot puts the aircraft into a dive. The aircraft must dive at a very specific rate so that everything inside falls at the same rate, and then for a few moments you are floating around inside the cabin. Just as it is for astronauts in orbit, gravity has not been switched off, you are simply falling at the same rate as everything else around you.
In this environment muscles and bones do not have to support body weight, so over time they weaken. Contrary to popular belief, an adult’s bones are not solid unchanging lumps of calcium, they are very much an evolving part of the body, constantly reshaping and renewing themselves based upon the stresses and forces imposed on them. Studies have shown that long-term space exploration can reduce bone mass by as much as 1–2% per month spent in space. That is a significant weakening of the body’s skeletal structure and it’ll be particularly noticeable in the legs and lower back. We are all probably a little more familiar with the concept of muscle degradation, often caused by lack of exercise, and just like bones, the body’s muscles will simply get weaker in a weightless environment. Of course this won’t be a problem in space but as soon as you return to Earth you will notice the difference.
There are two ways to solve this problem. The first is to exercise, a lot. To that end, astronauts in space spend a lot of their time on special gym equipment. This is doable on journeys into space over many months to a year or so, but even then it is not possible to exercise for enough hours in the day to retain full strength. For longer-term space exploration, such as the mission we are planning, a more effective solution is to try and simulate the force of gravity on board the spacecraft. Contrary to what many sci-fi films depict, we do not yet have a magic device that when turned on causes gravity suddenly to appear. There have been experiments, however, using extremely powerful magnets which generate equally powerful magnetic fields, and one such experiment managed to levitate an unsuspecting mouse. The system was able to counteract Earth’s gravity and was effectively producing a 1G (1G refers to the force of gravity that we feel on Earth) environment that suspended the mouse. Theoretically a similar magnetic system could be used in space to generate a 1G gravity field, but there is a problem with this approach. First we have no idea what impact such powerful magnetic fields would have on the human body, and second – and more practically – it takes phenomenal amounts of power to generate such huge fields.
There is, however, a more realistic solution that will help you on your journey. Instead of trying to generate a new gravity field, we can simulate the effects of gravity using other forces. You will have felt this already whenever you’ve been in a lift travelling upwards. As you stand in the lift and wait for it to start moving you will be experiencing 1G, but as the lift accelerates upwards, you will momentarily feel a little bit heavier. The acceleration of the lift means you get pinned against the floor a little bit harder, simulating a slightly higher pull of gravity. Just like this experience in an ascending lift, we could use linear acceleration to produce 1G. By constantly accelerating the spacecraft at the correct rate anything inside would be forced in the other direction, creating an experience similar to gravity, with the rear hull of the spacecraft becoming the floor upon which you could walk around as though you were on the surface of Earth.
Conventional technology using solid- or liquid-fuelled rockets is perfectly capable of simulating a 1G environment in this way, but the fuel would be used up in a matter of minutes. It might be possible to use a different fuel with a high specific impulse such as the VASIMR, although it is currently only capable of producing low levels of thrust. A more popular idea uses the concept of rotation, where inhabitants of a rotating spacecraft would experience gravity on the inside of the outer hull of the spacecraft. A large doughnut-shaped module could be rotated at a very specific speed to simulate gravity and the travellers on board could walk around and operate normally on the inside of the outer edge. Rotating any object at the right speed would cause simulated gravity on the outer portions of it. If you could rotate one of the rooms in your house at about seventeen revolutions per minute then you should be able to sit in a chair on the ceiling and read this book.
There are unwanted side effects of simulating gravity by rotation, and the main consideration in this respect is the Coriolis effect. You may well have heard of this before but it is a concept which is often misunderstood. Claims that it causes water to spin down a plughole in different directions in different hemispheres are wrong, but it does have an effect on our atmosphere. A great example is the movement of a parcel of air across the surface of the Earth in the northern hemisphere. Let us consider such a parcel moving from the polar regions towards the equator: it does not follow a straight line with respect to a weather watcher on the ground; instead, the rotation of the Earth causes it to turn in a clockwise direction with reference to its direction of travel, and it is this effect which gives us the rotational nature of high and low pressure systems. The apparent force from the Coriolis effect acts at right angles to the rotation axis, so an astronaut moving towards or away from the rotational axis would experience a force pushing towards or away from the direction of the spin. This would lead to feelings of dizziness and nausea, and the only way to overcome these would be to reduce the rate of spin to lower than about two revolutions per minute, but that would require a much larger rotating doughnut to produce the required 1G effect.
If we could simulate gravity then muscle and bone mass loss would be minimized if not eradicated and the whole experience of a long-duration flight around the Solar System would be much more agreeable.
The psychological challenges are huge, not the least among them being the necessity of getting a decent night’s sleep on board. The varying levels of lighting on board the International Space Station make it difficult for the mind to hook into night and day because the usual cues to help regulate the body clock – a blue sky, or a daily sunrise – are just not there. In fact astronauts on board the ISS see about fifteen sunrises and sunsets every day. Even something as simple as looking out of the window before sleep can send the wrong signals to the brain, leading to a disturbed night’s sleep.
Probably the biggest psychological issue to overcome, however, will be the sense of isolation – and that’s regardless of whether you are on your own or not. Humans are by nature sociable creatures so to be separated from the rest of mankind will take its toll. Studies on astronauts have shown that the longer the mission, the greater the feeling of isolation. This effect is worsened when the Earth is out of view, as experienced by Apollo astronauts when they were on the far side of the Moon. This is known as the ‘Earth-out-of-view’ problem, and it will only become more of an issue as you travel further into the Solar System.
This anticipated isolation is just one of the reasons why astronauts must pass a series of psychological assessments, as well as character and personality tests. One of the most important factors for consideration when selecting astronauts is for them to have ‘the right stuff’, to be calm and rational under stress and great at working in a team as well as when alone. Certainly in order to undergo this mission you will need to be strong in body and mind.
Preparing psychologically and physically for your epic journey through the Solar System is therefore essential. Whatever the mode of transport in space, you first have to get there, and to do that you need to go fast – about 11 kilometres per second to escape Earth’s gravity. On launch, you will experience a force of 3G, which is three times the force of gravity experienced at the surface of the Earth. Some fairground rides will exert this kind of force on the body but a better training approach is to spend some time on a human centrifuge. These installations are used by professional astronauts and simply comprise a large device with a long arm which has a capsule at the end. You sit inside the capsule and the whole thing rotates around at speed. The device is capable of producing as much as 20G but astronauts experience only about 3G. Fighter pilots can experience higher G-forces for shorter periods of time. With a special G-suit on, a trained person can sustain about 9G, maybe 10G, but not much more. So with a human centrifuge the launch experience can be simulated and it is possible to prepare for the sensation of taking off and the physical experience of space flight itself.
As I am sure you have gleaned from this chapter, there are many challenges facing you as you embark on your journey around the Solar System. Astronauts have even reported missing seeing colours on long-term postings to the International Space Station, so even that aspect needs to be considered. And we haven’t even touched on the issues of food and hygiene for the journey, although we will do so along the way.
It seems, then, that our voyage is set. The spacecraft will be launched into Earth orbit on board a conventionally fuelled liquid-/solid-fuelled rocket before using an ion engine for low, long-term thrust. Your craft will be called the Kaldi after the Ethiopian goat herder who is said to have discovered coffee – the other fuel that will keep you going on your long journey. It will use VASIMR to power it into interstellar space and it will rotate at less than two revolutions per minute to provide a simulated 1G environment. Finally, so that you can take a proper look at the planets of the Solar System as you visit them, we will have to momentarily forget about the laws of physics and employ a device called the Reality Suspension Unit, or RSU. This will enable you to get up close to the planets and explore them in all their glorious detail. Which is, after all, one of the main purposes of your mission.