YOU’VE LEFT BEHIND the warmth of the inner Solar System and you’re heading for an encounter with Mars, on average over 200 million kilometres away, making this the longest leg of the journey yet. And beyond Mars lies the asteroid belt, one of the most perilous parts of the trip. Is it possible to safely fly through it or is it like those scenes in science fiction movies where top pilots attempt to navigate through a veritable maelstrom of rocks? Time will tell. For now you must set out on the long haul away from the familiar surroundings of the inner Solar System.
During the months that will pass as you venture deeper into the Solar System there won’t really be an awful lot to do – and those months will soon run into years of being cooped up inside your spacecraft. Things can get quite claustrophobic. On long trips like this crew selection is of paramount importance, because you want to be sure that if anything does go wrong, at least the crew will remain strong and working well together. It is for this reason that long-haul space flight planners favour couples because they already have a good bond and are used to living in close proximity to each other. Although of course sending couples into space means that before long, natural urges will take over and they will want to demonstrate their love for each other in a rather more physical way.
Until the twenty-first century the subject of sex in space was considered taboo among many space agencies, but with astronauts spending more time on board the International Space Station, researchers started to investigate the impact weightlessness would have on this fundamental human activity, not to mention its effect on procreation and pregnancy (there are social concerns too, as the development of intimate relationships within a small crew can lead to issues with performance and even the safety or ultimate success of a mission, but here we’ll consider only the logistics of the physical act). For ships with artificial gravity it is not so much of an issue, but for those without it, sexual intercourse can be challenging. In the heat of passion, lovers will float around the room, often in opposite directions, leaving them floundering to get back to each other again.
In an effort to overcome the constant battle to stay together during moments of intimacy, the ‘2Suit’ has been developed, with velcro and zips aplenty, its primary purpose to ‘stabilize human proximity’. The suit allows its wearer either to attach himself to a work station to keep from floating away with minimal effort on his part or, and more appropriately, one 2Suit can be attached to another in such a way as to produce an almost cocoon-like garment, allowing its two occupants a chance for intimacy without unwanted interference from the laws of motion. And it’s not just amorous couples who can benefit from such a garment: families with children would be able to sit close while watching a film, for example, not constantly having to exert themselves against drifting apart.
But for our species to become truly space-faring in the future we need to have a good understanding of the effect weightlessness has on the process of fertilization and pregnancy. Studies have already been undertaken using rats and mice to see how a foetus develops in these conditions. Fertilization of mouse embryos seems to have been successful in microgravity, although fertilization rates were lower than normal. Once the fertilized eggs were implanted into mice back on Earth, they seemed to develop into normal healthy mice. In different experiments, rat embryo development was studied. Everything seemed to progress normally until the rats, some of which were born in microgravity, were exposed to gravity back on Earth, at which point the microgravity rats lacked the ability to stand up. To date, no study has examined the full process of fertilization and foetal development from start to finish while in space, but it looks as though the main issues will be experienced once those born in weightless conditions return to a planet with normal gravity. More research is required. If we are ever to reach out among the stars and colonize distant planets then procreation will be an essential part of the process and will clearly be paramount to the survival of our species.
As this leg of the journey continues, the Sun slowly grows fainter behind you and the intensity of its light begins to drop. By the time you reach the vicinity of Mars it appears just over half the size you’re used to seeing from Earth, and daylight on the surface of the red planet is similar to an overcast day back at home. The average distance from Mars to the Sun is about 228 million kilometres but its nearest approach takes it about 22 million kilometres closer. At that distance it takes just under 687 days to complete one full orbit of the Sun, which is about 1.88 times longer than an Earth orbit. Because of the orbital period of both planets, they experience a special alignment every two years and two months when the three bodies – the Sun, the Earth and Mars – line up, and this is known as ‘opposition’. It gets its name because the Sun and Mars are opposite each other in the sky with the Earth in the middle. It’s an alignment Earth enjoys with every one of the outer planets at some point. In the case of Mars, the distance between the two is usually about 90 million kilometres, and around the time of the next alignment there will be a flurry of spacecraft launched to head for the red planet.
An opposition of Mars means it is very well placed for observation from Earth, which is why it is without doubt one of the more popular planets to study among astronomers. This popularity goes back to the early days of telescopic activity around the 1600s. With basic and even crude telescopes, some very interesting observations of the red planet were recorded, and as the resolving power of telescopes improved some really quite remarkable detail could be seen. In September 1877, Giovanni Schiaparelli observed Mars with a 22-centimetre astronomical telescope during one of its particularly close oppositions and recorded a complex network of canals criss-crossing its surface. Schiaparelli actually gave the features the name canali, meaning ‘grooves’ in his native Italian, though the term was incorrectly translated at the time as ‘canals’. These ‘canals’ were soon misinterpreted as a great global network of waterways transporting much-sought-after water from the polar regions to the drier, arid equatorial regions occupied by a race of evidently supreme intelligence. As telescopes continued to improve it transpired that the features Schiaparelli had seen were actually just an optical illusion. He was detecting surface features on the planet but the relatively low resolving power of his telescope meant the detail was not great, so his mind had tried to make sense of the markings by conjuring up lines joining them together.
The incorrect identification of canals on Mars may have fuelled the imagination of many who believed in a race of Martians inhabiting the red planet, but we now know that intelligent life does not exist on Mars. That is not to say, however, that some form of very primitive life could not exist. Already we have found evidence that at some point there was running water on the surface of Mars and that water molecules are locked up in the sub-surface layers of the planet and also in the north polar region. Back in 1984, a meteorite known as ALH84001 was discovered in Antarctica and it is thought to have come from Mars: scientists analysed the tiny pockets of gas locked up inside the rock and found them to be identical to the unique chemical composition of the Martian atmosphere, as analysed by the Viking missions that landed on the planet’s surface in the late 1970s. Deep inside the meteorite are tiny little chains of chemical compounds known as polycyclic aromatic hydrocarbons (or PAHs for short) and these are thought to be the by-product of organic activity. So it seems that maybe, at least at some point in the past, there has been organic activity on the surface of Mars. But whether that activity ever led to the evolution of a more complex form of life like bacteria is still the subject of scientific debate.
Mars’s red colour will no doubt be familiar to you, particularly if you have looked at it through a telescope from Earth. You should also be able to see the tiny specks of light from the two Martian moons, Phobos and Deimos, the discovery of which makes for quite an amusing story. They were first talked about by Jonathan Swift in his famous book Gulliver’s Travels just over 150 years before their actual discovery in 1877. The invention of the telescope in the early 1600s had revealed four moons in orbit around the mighty Jupiter so, knowing that the Earth had one Moon, Swift postulated that Mars would probably have two. Hardly scientific reasoning, but he did turn out to be correct.
Phobos is the larger of the two with a diameter of 22 kilometres compared to the 12.6-kilometre diameter of Deimos. It is thought that they may be captured asteroids as their composition is similar to bodies found in the asteroid belt. However, initially they would have entered into a highly elliptical orbit around Mars which contradicts the very circular orbits they both now have. It is likely that mechanisms such as aerobraking from the Martian atmosphere could have decreased their orbital speed which would have allowed their orbits to become more circular, but in the case of the smaller Deimos there simply has not been enough time for this to take place, so it is a mystery that remains unsolved.
On a journey like yours around the Solar System, if there is one place that really calls out for a visit, it’s Mars. Walking on its surface and looking up at its two moons hanging in the sky is going to be an incredible experience. The planet itself is about half the size of Earth with a diameter of 6,779 kilometres but that does not stop people referring to it as Earth’s twin. It is perhaps more appropriate to talk of Venus as Earth’s twin, certainly based on its physical properties, but the conditions on the surface of Mars make it much more Earth-like.
As you approach, the white polar ice caps come into view. The atmosphere of Mars is mostly composed of carbon dioxide – similar to Venus, although it is much less dense so the surface temperature is nowhere near as high. Compared to Earth, the average surface pressure on Mars is about 0.6% so it would not sustain life and you’ll need to explore with the protection of a space suit. The suit will maintain an appropriate atmospheric pressure not only to aid breathing but also to ensure your body does not balloon up and make movement difficult. It achieves this by carefully pumping pure oxygen into its gas-tight confines.
From the surface, the moons are actually a little bit of a disappointment. Deimos is no more impressive than Venus when viewed from the Earth, but Phobos is a little more interesting. It appears about a third the size of the full Moon in the sky above the Earth and because its orbit around Mars is aligned with the Martian equator, it appears quite low in the sky from your landing spot just north of Hellas Basin in the southern hemisphere. If you had landed around the north or south polar regions, then Phobos would have been completely out of view. From here, though, both moons can be seen.
Observation of them over just a few hours reveals a startling difference between their orbital characteristics. Deimos is the outermost of the two moons and takes about thirty hours to complete one orbit, but when the rotation of Mars is taken into account, it takes about 2.7 days to slowly track across the sky from east to west. Phobos, on the other hand, is a little more nippy and orbits Mars so fast that it appears to rise in the west and set in the east with just seven hours between successive rises.
Both Phobos and Deimos present phases much like Earth’s Moon, although it requires a telescope to see the phases of Deimos because of its changing brightness. Phobos is much easier to see and, like the Moon, is tidally locked, meaning that it constantly presents the same face to Mars. Due to the proximity of Phobos and its fast orbital speed, tidal forces between it and Mars are actually beginning slowly to decelerate it in its orbit, which will eventually make it fall towards the surface. Before it crashes into the planet, though, it will reach the Roche limit where the tidal forces become so strong that they will rip the moon apart. There are a number of chain craters on Mars which are thought to have been caused by other young small moons that suffered such a fate. Deimos, on the other hand, is a little too far away and instead is being accelerated away from Mars.
Surveying the scene on the surface of Mars will no doubt remind you of the images sent back by the Viking landers. It feels like you’re in the middle of a vast red desert. Standing still is OK but moving around is challenging because of the lower gravity. It’s easy to lose your footing. The planet is about a tenth of the mass of Earth which means the force of gravity is just under 40% of what it is on Earth, so if you stepped on a set of scales on Mars you would weigh under half your normal body weight. The surface is not too dissimilar to the Moon’s with a powdery coating almost the consistency of talcum powder sitting on a basaltic rock upper crust, but there is one big difference: everything looks a salmony-red colour. The fine powdery coating is iron oxide, more commonly known as rust, exactly what is found on Earth on iron objects that have been left outside in the rain. On Mars the iron oxide formed billions of years ago when iron reacted with the then more plentiful supply of liquid water.
The water that existed on Mars billions of years ago also reacted with the carbon dioxide in the atmosphere and produced carbonate rocks, a process that extracted carbon dioxide from the atmosphere and then locked it away, slowly thinning it. The lack of tectonic activity on the planet means the carbon dioxide has remained locked away inside the rocks, leaving the Martian climate very different to how it was billions of years ago. The thin atmosphere and highly elliptical orbit of the planet are responsible for the pretty high temperature differences across Mars. Here on the rim of Hellas Basin at about 30 degrees south of the equator the midday temperature hits highs of about 10 degrees, but at night it plummets to around minus 60. The temperatures across the rest of the globe also vary widely, with highs of around 20 degrees in the northern hemisphere during the summer down to minus 150 degrees in the polar winters. The substantial temperature differences between the two hemispheres drive some quite powerful winds. The Viking landers recorded speeds of 30 metres per second (about 108 kilometres per hour) but because of the low atmospheric pressure the winds would not feel as strong as the same wind speed on Earth.
These winds were responsible for distributing the iron oxide around the planet and are still responsible for the global dust storms that engulf it, such as the event in 2001 which was monitored by the Mars Global Surveyor spacecraft, which was in orbit at the time. The fine nature of the dust that covers the surface is responsible for the rather strange and eerie pink sky too, because the individual particles are small enough for many to get left suspended in the thin Martian atmosphere. When the first pictures were received from the Viking probes they were corrected by the people who received them to show a blue sky. It was soon realized that the colour cards fixed to the side of the craft that allowed for a correct colour balance to be applied were now showing the wrong shades. Once they were corrected, a true colour image was seen and for the first time the alien pink sky of Mars was revealed.
The dust storms that often plague the planet are a real challenge for surface rovers and landers because they can easily deposit a fine layer on solar panels and optical instruments rendering them inoperative. As a direct result of the surface conditions, Mars landers have batteries that charge from solar panels to keep the power topped up during periods of reduced incident lighting when dust storms strike. Fortunately for you there is no dust storm under way today but the fine powdery nature of the dust looks like it could cause problems for ventilation systems. Certainly trying to survive the Martian surface without a space suit would more than likely be suicidal, not just because of the lack of air pressure, oxygen and extreme temperatures, but because the dust would, if unfiltered, lead to suffocation.
Dust storms seem to originate from certain areas of the planet and one such location is the Hellas Basin, which is the chief reason why this spot was chosen for your flight plan. This large impact crater measures a staggering 2,300 kilometres from side to side and is 7 kilometres deep. It’s the largest visible crater in the entire Solar System. At this depth, the atmospheric pressure rises to about 0.01 atm (Earth atmospheres), which is low but still higher than the average surface pressure. If the temperature could rise above about zero degrees then it is just possible that liquid water could exist on the surface. It may be responsible for some of the erosion features that have been seen, such as the many gullies that run to the north-east.
The similarities between Earth and Mars and its relative proximity to us make it the prime target for possible human colonization. In its current state, Mars is a great place for a short excursion to the surface but a bit more planning and preparation would be required before a human base could be properly established. Grand plans of terraforming Mars to make its surface capable of supporting life remain a very long way off, not to mention morally questionable: our exploration of the planet has so far shown up no conclusive signs of life, but until we can categorically state that Mars is completely devoid of any life, however primitive, then it would be wrong potentially to put any life forms at risk. But certainly setting up a human base in specially designed habitats to support life is within our grasp.
To make that vision a reality it would be prudent first to send some robotic missions to the planet to start transferring equipment and materials. The first few shipments could include inflatable habitation units and electricity production technology such as solar- or wind-driven units, along with the ability to store the energy created. Machinery and tools would also be needed to extract materials and minerals from the ground for production of food and drink, and later for building materials. There would also need to be equipment that could extract chemicals from the atmosphere to produce breathable air and rocket fuel. This is just the tip of the iceberg though.
Once enough infrastructure had been sent over and established as much as possible, the first human settlers would arrive. They would have to be people with a proven record of being able to live together, so again, it’s likely that couples will be the first to be sent. They would also need to be entirely self-sufficient so they would have to have a wide skill set, not only from a construction, engineering, medical and scientific perspective, but as the colony grows there will be a need for some form of governing process too, to ensure law and order. In essence, a new society would have to be created, one whose participants will probably have to be handpicked initially in order to give it the best chance of success. Longer term, such a colony would need an injection of people with more diverse skills to help the new society to flourish.
For now, though, your time on the planet has come to an end, and you are leaving behind what remains a desolate, barren world.
From the comfortable surroundings of the Kaldi you can once again look down on Mars which, like Earth and many other planets, has an axis of rotation which is tilted with respect to the plane of its orbit around the Sun. The axial tilt of Mars is 25 degrees which makes it very nearly the same as the Earth’s, which at 23.5 degrees is a little less. More crucially, and as a direct result of this tilt, Mars experiences seasons just like Earth, although there are some fairly significant differences. The orbit of Mars is just under two Earth years long, so the seasons are generally about twice as long. Mars also has a very eccentric orbit which means that its distance from the Sun varies by as much as 19%, so the seasons are not the same between the two hemispheres, unlike on Earth whose seasons follow the same but juxtaposed pattern in the northern and southern hemispheres. The seasons in the northern hemisphere of Mars bring temperatures that are on average about 30 degrees cooler than the same season in the southern hemisphere, but this has not always been the case. There is strong evidence that the tilt of Mars has changed over its history, having been much more extreme in the past. Deep under the surface there are vast reservoirs of water locked up in ice which are believed to be the result of a much larger polar ice cap that in previous millennia extended to more temperate latitudes on the planet. As the tilt reduced to the present value, the polar caps receded, leaving any evidence of their existence locked away underground.
An exciting discovery was made back in 2013 by the Opportunity rover which found evidence of neutral water on the planet’s surface. Locked up inside some Martian rocks the Opportunity found clay minerals that could not have formed from Mars’s rather more acidic version of water. Neutral water is close to what we would consider to be drinking water, so the evidence tantalizingly suggests that conditions on Mars a few billion years ago were conducive to the existence of life.
The polar ice caps of Mars are once again clearly visible from your vantage point, the southern cap looking smaller than the northern, so it must be summer in that part of the planet. The northern hemisphere is pointing away from the Sun causing it to experience winter, and as a result that polar region is plunged into permanent darkness. The drop in temperature causes carbon dioxide in the atmosphere to desublimate (a process where gas turns directly into a solid without going through its liquid phase first) into solid chunks of carbon dioxide ice that cap the polar regions. The ice caps have significant amounts of water ice too but these are generally covered by a layer of carbon dioxide ice, and together they form a polar cap that is between 2 and 3 kilometres thick. If all the water ice on Mars thawed, including that at the poles, there would be enough to cover the planet in a sea nearly 10 metres deep. Detailed images of the polar regions reveal strange almost spiral-shaped troughs in the ice which are sculpted by polar winds. This is a wonderful example of the Coriolis effect, which causes air to be deflected due to the rotation of the planet and is the same reason why the air circulates around regions of high and low pressure on Earth. With the change in the seasons and the onset of spring and summer, the poles warm, allowing the ice to sublimate back into the atmosphere, generating winds that blow from the poles.
There are many other large features that are prominent on the surface of Mars. The most stunning is Valles Marineris (‘Mariner’s Valley’), named after the Mariner 9 spacecraft that discovered it. It is a vast canyon system that runs for 4,000 kilometres across the Martian surface, at its widest part spanning 200 kilometres and descending 7 kilometres down into the Martian crust. Comparing those statistics to the Grand Canyon in Arizona, which is 446 kilometres long, 29 kilometres wide and 2 kilometres deep, makes you realize just how large it is. There are many cracks and fissures that run off the main valley, which is classed as a typical (albeit very big) rift valley. This type of feature is common in the Solar System. There are similar examples on Earth and on Venus, where a linear-shaped feature runs between highlands or mountain ranges and is the result of some kind of geological fault. The valley is thought to be the result of tectonic activity that appeared as Mars cooled early in its history. The crack then widened as the crust in the Tharsis region to the west started to rise and erosional forces began their assault. Closer inspection shows that some of the channels on the eastern side appear to have been carved by the erosive forces of running water.
The Tharsis region itself is a huge volcanic plateau that is home to three enormous shield volcanoes (domed-shaped volcanoes with gently sloping sides): Arsia Mons, Pavonis Mons and Ascraeus Mons. Just on the edge of the plateau is the tallest volcano in the Solar System, Olympus Mons, which towers 22 kilometres above the surface and dwarfs Earth’s largest volcano, Mauna Loa on the island of Hawaii, by 16 kilometres. The plateau has developed because it is over a hotspot in the underlying mantle known as a superplume, where vast quantities of hot, dense magma well up, forcing the crust to rise. The lack of individual plates on Mars means that this magma has built up over billions of years, unable to go anywhere and producing huge volcano complexes. Unfortunately there is unlikely to be a major eruption as a considerable quantity of the magma will slowly cool and solidify. It was once thought that the three volcanoes present on the Tharsis Plateau were individual structures but it is now believed that they represent one complex system – a theory which is supported by the many geological features that surround the region.
Olympus Mons is very much a separate system, but despite its large size it is actually pretty unimpressive from the surface. Being a shield volcano, the slope of its flanks is no more than about 5 degrees, so had you stood at the foot of this impressive feature you would actually have been unaware of it. The size of Mars too would have precluded any great views of Olympus Mons as the horizon is only 3 kilometres away and the extent of the volcano takes it beyond the visible horizon. Even standing on top of it you would still have been unaware of the nature of the feature below you as the ground would have just gently sloped down away from you.
What you would have experienced at the summit, though, is clouds. The atmosphere at the altitude of the peak is only 10% of the surface pressure but even so, the flow of air across the peak is sufficient to cause orographic uplift. The process by which clouds usually form begins with a parcel of air which sits on the surface and contains an amount of water in its gaseous form. Heating from the surface causes the cloud to rise and cool until it reaches its dew point when it can no longer hold the water gas. At this point, the air is saturated and water condenses out to visible droplets that we see as cloud. Orographic uplift is slightly different as it is the profile of the terrain that forces the air to rise – for example, air flowing across Olympus Mons gets forced up and cools, allowing cloud to form at the top. This process is often seen over mountains on Earth too and can be a real danger for climbers who suddenly find themselves engulfed in thick cloud.
The incredible size of Olympus Mons is largely due to the fact that there is no tectonic activity on Mars. A volcanic vent that sits over a hotspot will stay there, allowing molten lava to flow out, and it is very likely that the volcano’s sheer size is the cumulative effect of countless lava flows over billions of years that have simply built up. Unfortunately it is very unlikely that a lander will be sent to Olympus Mons because the atmospheric density at the summit is far too low to allow for parachutes to slow a lander’s descent. That said, a more complex landing technique could be employed like that demonstrated by the Mars Science Laboratory mission in 2012, where the lander was lowered on a winch from a rocket-powered hovering platform. For now we will have to be content with images and data gained from Martian orbit, and fortunately we can still learn a lot by using these gravitational mapping techniques.
Images returned from the MSL show the flanks of the volcano to an incredible level of detail; intricate grooves, channels and ridges that have been forged by the lava flows can be seen. By studying the distribution of craters it is possible to age the surface to anything between 2 and 100 million years, which in geological terms is very recent, suggesting that Olympus Mons is still active but only in a very sedate fashion. The caldera of the volcano is made up of six overlapping craters, but not craters formed by impact: instead they are the result of surface collapse. As the molten magma seeps out of the volcano through its many vents, the surface becomes unsupported and collapses at the top. Therefore each caldera at the top of Olympus Mons represents one of the major expulsion events. In much the same way that the study of craters on the Moon can help us to determine roughly the age of the lunar surface, by studying these caldera it is possible to determine that the largest magma chamber lies about 32 kilometres under the surface. Each of the six caldera are thought to have formed within a million years of each other and are probably between 150 and 350 million years old.
All over the planet, from the Tharsis Plateau to the Valles Marineris, there is evidence of geological activity. The surface is likely to be one large plate, which accounts for the many surface features shown, although there is an alternative theory suggesting that Valles Marineris may be the result of two plates slowly moving apart. The crust itself is about 50 kilometres thick and composed mostly of silicon and oxygen which are locked up in silicate rock, but there are quantities of iron, magnesium, calcium and potassium. Deep underneath the crust are two concentric zones that represent the mantle and core of the planet. Differentiation is the process that drives the production of the very distinct zones on Mars and other rocky planets, and it’s a process that occurs because of the different behaviour and properties of the material. The mantle sits below the crust and is the region responsible for the production of many of the Martian geological features we can see today. Below that is the partially molten core which is thought to be 3,500 kilometres in diameter and made of iron and nickel with small amounts of sulphur.
But the most tantalizing aspect of Mars remains its potential suitability for long-term human habitation. Clearly at the moment it cannot support human life, chiefly because of the low atmospheric pressure, but extensive research into the possibility of humans inhabiting foreign worlds using artificial environments may just make it achievable. Experimentation into transforming the atmosphere of Mars has received serious consideration, but while this process is technically feasible it will take thousands if not millions of years to complete.
Should we ever manage to establish a human colony on Mars then something as seemingly simple as communication with Earth will be a little more problematic than you might think owing to the immense distances between the two planets. Any communications will be sent via radio waves and would therefore travel at the speed of light, which is 300,000 kilometres per second. At that speed it would take just three minutes for a message to traverse the space between the two planets when at their closest, or twenty-two minutes when they are furthest apart. When the two are on opposite sides of the Sun, communication will not be possible unless a satellite is placed some distance from Earth to relay the signal.
But still, there is a lot to be said for choosing Mars as a possible human outpost: the day is about the same length as an Earth day, a year is only about 320 days longer than Earth’s, and the seasons are broadly the same even though they are longer-lasting. A number of challenges will need to be met, but these could be overcome through the use of an artificial and self-contained environment on the surface of Mars inside which humans could live. Such environments have been tested on Earth already, like the Mars 500 mission where volunteers were locked up in a mini self-contained ecosystem to see what physical and emotional effects it had on them. It is quite possible to construct a habitable living environment on Mars just like this, and from there the first inhabitants would be able to explore and could slowly build a new human society.
An outpost on Mars would provide a great base from which missions could explore the outer Solar System. From there, not only are the large gas giants within reach but also the asteroid belt, which is your next destination. Since flying past the Earth on the way to the outer reaches of the Solar System we have travelled 100 million kilometres to get to Mars; to get to the inner boundary of the asteroid belt will take a further 100 million kilometres.
Before the discovery of the first asteroid, in 1766 the German astronomer Johann Titius scribbled a note suggesting that there was a numerical pattern in the distances between the planets. He realized that if you start off with the sequence of numbers 0, 3, 6, 12, 24, 48, 96 then add four to each and divide by ten, you end up with the following series: 0.4, 0.7, 1, 1.6, 2.8, 5.2 and 10. At first glance those numbers do not look too significant, but compare them to the average distance between the planets and the Sun in astronomical units (where 1AU is the average distance between the Earth and Sun) and you will find that Mercury is at 0.38AU, Venus at 0.72AU, Earth at 1AU, Mars at 1.5AU, Jupiter at 5.2AU, and finally Saturn is at 9.5AU. In 1768, another German astronomer, Johann Bode, referenced this number sequence in one of his publications but did not give credit to Titius so it became known as Bode’s Law. What made this series even more startling was that the discovery of Uranus in 1781 showed its average distance from the Sun to be 19.2AU, which almost matches the next number in the sequence, 19.6.
Look back at the numbers, though, and you’ll notice that there appears to be a gap. Mars has an average distance from the Sun of 1.5AU and Jupiter is at 5.2AU, but in the Titius–Bode sequence there is a number between them, 2.8. Titius even raised the question about another as yet undiscovered planet in orbit between Mars and Jupiter, and following the discovery of Uranus, in 1800 a team of astronomers led by Franz Xaver von Zach started to scour the sky in search of the missing planet. They were each allocated a portion of sky that spanned 15 degrees along the path that all the planets seemed to follow. In January 1801 Giuseppe Piazzi announced the discovery of a tiny moving object in the orbit predicted by Bode’s Law. Telescopic observation showed that it was a fast-moving object with a motion similar to that of the comets, although a lack of the fuzzy coma that surrounds comets led them to believe it was indeed a planet. Even under high magnification it was impossible to resolve the tiny object into a disc so it was only its motion that seemed to suggest it was any different from the surrounding stars. It was given the name Ceres after the Roman goddess of the harvest, and just over a year later, in March 1802, the discovery of a second object was announced and called Pallas. They became known as asteroids, from the Greek asteroeidas, which translates as ‘star-like’. Both share a very similar average distance from the Sun of 414 million kilometres, or 2.7 astronomical units.
The new asteroids seemed to be orbiting the Sun at the right distance to fill in the gap in Bode’s Law, but the law has received much scrutiny over the years and it has been concluded that the sequence is actually nothing more than a mathematical coincidence. The discovery of Neptune in 1846 at a distance of 30.1 astronomical units compared to the Titius–Bode number of 38.8 further discredited the concept, suggesting coincidence rather than a real phenomenon.
Furthermore, Ceres and Pallas turned out to be just two of millions of asteroids that orbit the Sun between Mars and Jupiter. It is estimated that there are around 200 larger than 100 kilometres, nearly 750,000 that are larger than 1 kilometre, and perhaps millions that are smaller fragments or pieces of dust. Ceres is the largest of them all with a diameter of 950 kilometres, and Pallas the second largest at around 550 kilometres in diameter. When added to the mass of the third and fourth biggest, Vesta and Hygiea, the four of them account for half of the mass of the entire asteroid belt.
How this asteroid belt formed has been the cause of much controversy over the years, with theories ranging from exploding planets to planets that have been destroyed by cometary collisions, but the modern accepted theory is a little less dramatic. We have already seen how the Solar System formed when a large cloud of gas and dust slowly collapsed under the force of gravity, within which the temperatures and pressures became so extreme that nuclear fusion started to occur, giving birth to the Sun. Around the hot young Sun was an accretion disc of dust within which a number of random collisions took place, some of which caused the chunks to stick together. As the collisions continued and the chunks grew in both size and mass they started to become gravitationally dominant, pulling other chunks of rock towards them. Over time, the rocky inner planets and the gaseous outer planets formed, but between them another planet was struggling to form. The presence of the large and very dominant planet Jupiter prevented the further growth of any planetesimals in the belt and instead the remaining group of rocks continued to orbit the Sun together.
As the Solar System evolved and the majority of the planets slowly migrated inward from their initial positions, the increasing strength of Jupiter’s gravity on the asteroid belt caused many of them to accelerate as it adjusted their orbits. In most cases, this was the result of asteroids experiencing orbital resonances with Jupiter. Orbital resonances are not an unusual phenomenon in the Solar System and they exist where two or more bodies exert a regular gravitational interaction which over time has a cumulative effect on both of them. A good analogy to this is a parent pushing a child on a swing: regular pushing with the same force at a constant frequency will lead to the swing going higher and higher. It’s the same with objects in the asteroid belt and Jupiter: the regular tug experienced by the asteroid as it passes by Jupiter on each orbit will act to adjust the speed and therefore the orbit of the asteroid until it is in resonance with the orbit of Jupiter – for example an asteroid might complete two orbits for every one of Jupiter’s. In the majority of cases this is an unstable situation, and the continued gravitational interaction will eventually lead to the orbital resonance breaking down again.
This influence from Jupiter is also responsible for reducing the overall mass of the asteroid belt early in its history by a factor of a thousand. At the current observed inner boundary of the belt, just over 2AU from the Sun, an object in orbit will soon fall into a 4:1 resonance with Jupiter, completing four orbits for every one of Jupiter. With the cumulative effect of the regular tugging on the asteroid it will eventually become ejected from the belt into a new orbit outside the belt. Any asteroids that wander too close to the Sun will get swept up by the gravitational pull of Mars, whose furthest point in its orbit takes it to around 1.67AU. While Jupiter is responsible for inhibiting the growth of a planet within the belt, Mars and Jupiter together keep the components of the belt in check. If they try to wander too close or too far, they will either get ejected or put back in their place. A similar process takes place around Saturn where tiny moons in orbit around the planet keep particles of ice and dust in place, creating the majestic ring system.
The Sun has also influenced the development of the asteroid belt and the asteroids within it. During its early more formative years the temperature in the young Solar System would have been higher, and some of the larger asteroids would have partially melted. As they became molten, the heavier elements would have sunk and the lighter elements risen, leading to a body that is differentiated. It is quite likely that some would have experienced volcanic activity, with huge lakes of molten lava forming and then solidifying over millions of years. At the other end of the spectrum, any asteroids that formed at a distance greater than 2.7AU would have accumulated ices because at that distance from the Sun the temperature dropped sufficiently for water to freeze. In 2006, the discovery of a number of cometary nuclei in the outer reaches of the asteroid belt was announced, and it is thought that some of these may well have brought water molecules to the young Earth as it was forming, and that ultimately led to the existence of our oceans.
The asteroids within the belt broadly fall into three categories based on their composition. Around the inner region of the belt they tend to be rich in silicates, which is a particular type of rock composed of varying quantities of silicon and oxygen. Silicates are common in the inner Solar System and make up about 70% of the crust of the Earth. Silicate rocks are generally the result of geological processes such as partial melting or crystallization that have modified the original protoplanetary disc material out of which the asteroids formed. These types of asteroid are known as S-type asteroids, and many of them are also found to have traces of metals and carbon-based compounds. This is quite a contrast to the carbonaceous asteroids which are found around the outer rim of the belt, which as their name suggests are rich in carbon-based compounds. Unlike the S-type asteroids their composition is thought to be much more representative of the composition of the early Solar System. They make up the vast proportion of asteroids within the belt, numbering well over 70%, but though they are plentiful they are the hardest to see because of their low reflectivity.
The final common type of asteroid found within the belt is the metallic or M-type; as the name suggests, they are rich in metals like iron or nickel. They are fewer in number when compared to the other two types, making up only 10% of the overall population of asteroids in the belt, and their composition suggests that they originated from the core of differentiated asteroids, where the heavy metallic elements had settled to the core and been subsequently liberated by some form of collision. The theory of differentiation among the larger asteroids also predicts that they should form crusts composed mostly of basaltic rock as the lava from the partially molten asteroid rapidly cools. This in turn suggests that there should be much higher quantities of basaltic rock and basaltic asteroids in the belt than have been observed to date.
With the composition of the asteroids so well understood, an exciting possibility exists to mine them for minerals, either to bring back to Earth or to use for the construction and running of space exploration and colonization activities. Theoretically, at least, iron, nickel, titanium, oxygen and hydrogen could all be mined from asteroids within the main belt, or indeed from any asteroids wandering around the Solar System. Whether they should be used to replenish the dwindling and finite reserves on Earth, which are forecast to run out by 2100, is an ethical dilemma, and an operation that would come at an enormous financial cost, but the idea of using them as supplies for space exploration seems to have received much more support. It is not just the asteroids, though: there are thought to be many exhausted, almost extinct cometary nuclei which could be mined for oxygen by passing spacecraft and which could be used to produce rocket fuel or air to breathe, while the heavier metals from the asteroids could be used to build spacecraft in space, or repair damaged craft. The possibilities are endless.
Choosing which asteroids to mine would be the first challenge to overcome, regardless of the intended use of the mined material. One of the first considerations is the location and orbital parameters of the asteroid, because put simply, some are easier to get to than others. The considerations will be different depending on the point of destination. For example, when travelling through space on a voyage such as yours the velocity of the spacecraft is likely to be high so it may take a considerable amount of fuel to be able to adjust the velocity sufficiently to match that of the asteroid. Expeditions from Earth, however, need to make sure that travel to the asteroid can be achieved using the Hohmann Transfer orbit, which is among the most fuel-efficient ways of travelling to a destination. During this type of journey there are two bursts from the rocket engine, one to increase the velocity of the spacecraft and send it into a highly elliptical orbit to intercept the destination, and another to move it off the transfer orbit and on to an orbit around the Sun that matches the orbit of the destination. The process can be reversed to get back to Earth but this time by firing the rocket engines in the opposite direction to slow the spacecraft, first to return it on to the Hohmann Transfer orbit and again on arrival at Earth to drop into Earth orbit. The concept of the Hohmann Transfer orbit is pretty straightforward and was first discussed in 1925 by the German scientist Walter Hohmann, after whom it is named.
How essential it is to plan the spacecraft’s trajectory was beautifully demonstrated with the Rosetta mission that arrived at Comet 67P/Churyumov/Gerasimenko in 2014: ten years were spent carefully adjusting its trajectory in order to match its orbit around the Sun with that of the comet. The huge effort was worthwhile, though, as it meant the probe was travelling around the Sun at the same speed as the comet so that the relative speed between the two was as low as it could be, which made the landing so much easier.
So we know we can do it, and all the types of asteroid we have looked at offer something in terms of mining activities. The C-type carbonaceous asteroids have high abundances of water which could be used for life support; the oxygen and hydrogen molecules could also be dissociated to provide rocket fuel. By their very nature they are high in organic compounds too which can be used for production of fertilizers to help with food production. The S-type asteroids contain very little water or organic compounds but are rich in metals including iron, nickel and even gold and platinum. The M-type asteroids are few and far between but are worth hunting down as they contain up to twenty times more metal than the S-types, making them a great source of material for repairing equipment or even building new spacecraft.
Deciding which asteroids to mine and getting to them is one thing; actually doing the mining is a whole different challenge. A number of different approaches have been described in various works of science fiction. In the film Alien, the crew of the mining spaceship Nostromo returned to Earth after a specific mission with almost 20 million tonnes of material. In reality, acquiring such a haul from our own asteroid belt would be a tall order. Even if you were to capture the largest of the asteroids and drag it all the way back to Earth you would only gain about 1,000 tonnes of material, and that would still need to be refined. The practicalities of mining asteroids are a little different from what we see in the movies.
Perhaps the biggest challenge is the lack of a significant gravitational field to hold spacecraft, equipment and even astronauts on the surface of asteroids. Unlike the Moon, where astronauts were able to walk or hop around, if you tried that on any of the asteroids you would more than likely just float off into space. It would be necessary to use hooks and tethers to attach anything that needs to stay where it is in order to stop it floating off. As a prelude to such an exercise, NASA have been training astronauts under the surface of the Atlantic Ocean, not only to simulate the experience but also to test techniques and equipment they might use.
This may sound pretty straightforward, but as the European Space Agency found out when they tried to drop the Philae lander from the Rosetta probe, the process is fraught with difficulties. After touchdown on the comet nucleus, a jet was supposed to fire to push the lander against the surface while some screws worked their way in to attach the craft; at the same time, harpoons should have fired to secure it firmly to the surface. Both the jet and the harpoons failed, leaving Philae only tenuously attached to the surface, just one false move needed to send the tiny little craft careering off into space again.
Assuming it is possible to attach the members of a mission to the surface, several different mining approaches could be employed. Many of the asteroids are thought to have rubble-like surfaces so a simple scoop or claw might be used to scrape off the surface material. Those with a high metal content like the M-type asteroids may be covered in metallic grains or granules which could be harvested with a magnet that sweeps over the surface. Perhaps the most adventurous and challenging approach, though, is to dig a mine shaft and extract minerals from deep within the asteroid. This is by far the most technically complex operation, not just from an engineering and logistics point of view but also in terms of working out exactly where to drill in order to get to the minerals.
With millions of asteroids to choose from you would think that finding a suitable one would be an easy task, but remember, the four largest asteroids account for nearly half of the mass of the entire belt. Identifying the exact mass of an asteroid is done in much the same way as for other objects in the Solar System, through the examination of their gravitational interaction. In the case of Ceres, the largest of the asteroids, it weighs in at 940 billion billion kilograms. That does sound quite a lot, but when you think that it would take twenty-five of them to equal the mass of the Moon, which is itself quite small, that puts the figure into perspective. It measures just under 1,000 kilometres in diameter and because of its large mass it has achieved hydrostatic equilibrium, which you will recall means it is roughly spherical in shape. (Even though this is one of the criteria for the definition of a planet as laid down by the International Astronomical Union, Ceres has not become gravitationally dominant in its orbit so it is officially classed as a dwarf planet with all other objects in the belt classified as asteroids.)
Unlike the planets we have looked at so far, Ceres is unusual because its body is not differentiated, which means the metals have not had a chance to separate from the rocks. The core is believed to be a rocky composition and observations suggest this may be surrounded by an icy mantle. Spectroscopic studies have identified nearly 400 million trillion gallons of water ice in the mantle of Ceres compared to an estimated 326 million trillion gallons on Earth. There is a problem with this theory, however, because Ceres presents a rocky surface, and a rocky surface overlaying an icy mantle is an unstable situation as the force of gravity would attempt to drag the rock down through the ice. This would leave significant salt deposits on the surface which have so far remained elusive to spectroscopic studies. There does seem to be a very tenuous atmosphere on Ceres, and although there is some evidence of water ice on the rocky surface, the low atmospheric pressure means that the water soon turns into a gas through sublimation and escapes into space.
Pallas was the second asteroid to be discovered, and with an average diameter of 544 kilometres is generally referred to as the second largest of the known asteroids. Vesta is a very similar size with an average diameter of 525 kilometres, and their irregular shapes mean there is often contention over which is larger. Pallas just about beats Vesta on size, but when we compare the mass of the two, Pallas is about 30% less massive. Its composition somewhat resembles some of the meteorites that have been found on the surface of the Earth which are high in carbon compounds, and spectroscopic studies suggest a surface with high concentrations of silicate rocks. The orbit of Pallas makes it unusual among the asteroids because it is highly elliptical with an eccentricity of 0.231 compared to Earth’s almost circular orbit and eccentricity of 0.02 (‘eccentricity’ refers to the roundness of a circle: a perfect circle has an eccentricity of 0 and the number increases as the ellipticity increases). Most of the asteroids in the main belt have broadly circular orbits too, but the orbit of Pallas takes it from a perihelion distance of 315 million kilometres all the way out to an aphelion distance of 510 million kilometres. When these figures are compared to the main bulk of asteroids within the belt then Pallas swings from the inner boundary of the belt, which is around 309 million kilometres from the Sun, to a point about 20 million kilometres beyond the outer limits. Not only does it have a highly elliptical orbit, but its orbit is more inclined to the plane of the Solar System than any other asteroid. Generally, the maximum orbital inclination of individual asteroids is no more than about 30 degrees, but Pallas follows a path which is tilted by a little over 34 degrees. Another unique property of Pallas is the angle of tilt of its axis of rotation, which is estimated to be around 70 degrees. This means the asteroid is almost rolling around the Solar System in much the same way that the giant planet Uranus does.
Like the vast majority of asteroids, Pallas has an irregular shape and has not achieved hydrostatic equilibrium, which means it is resigned to the classification of an asteroid rather than a dwarf planet. With millions of these asteroids in orbit around the Sun you would think that flying through the belt is a hazardous activity. Certainly if science fiction films are to be believed you are currently sweating at the Kaldi’s controls, trying to navigate your way through, swerving and dodging around to avoid impact after impact. Thankfully, the reality of travelling through the asteroid belt is a little less exciting than these Hollywood rollercoaster rides. Traversing the 180-million-kilometre-deep belt is still a pretty nerve-racking experience, and it is a journey that takes just over a month to complete, but even though there are millions of rocks out there, they are spread over an area spanning around a trillion square kilometres so the chances of being hit by anything significant enough to cause a catastrophic failure are slim. In fact it’s unlikely that you’ll even see any of them directly from the Kaldi. The larger pieces are well studied and their orbits are well known, but there is always the likelihood that there are other big pieces that have simply not yet been discovered, and many of the asteroids are really quite dark so spotting them against the blackness of the sky is difficult. Furthermore, any object on a collision course with you will have a fixed relative position to the Kaldi so will remain stationary in your field of view, making it even harder to spot – it will just get bigger and bigger as it moves closer.
And it’s worth remembering that even the smallest items can cause damage. The space shuttle had to have its windows replaced on more than one occasion after impacts that cracked the outer layers. On investigation it was revealed the damage was caused by nothing more than flecks of paint travelling at speeds in excess of 28,000 kilometres per hour. At those speeds even the smallest objects are a serious danger. Things are extremely cluttered around Earth’s orbit so a lot of effort has gone into developing protection strategies. For instance, the International Space Station uses a multi-layered skin as protection from the half a million or so pieces of debris about the size of a marble that are flying about. The outer layer is an aluminium alloy and is the first layer of protection, but anything that gets through that would then hit a thick woven fabric much like Kevlar which would absorb most of the energy, slowing the speed of the impactor to a low enough velocity to prevent it from penetrating the interior aluminium skin.
This multi-layering of the shell of spacecraft is common nowadays, but larger obstacles require special treatment, and that largely comes down to getting out of their way. Using radar technology it is possible to track accurately the position of the larger, more damaging pieces of rock and ultimately make the necessary changes to the trajectory of the spacecraft to avoid collisions. And rest assured, the Kaldi’s radar will be running 24/7 to pick up any moving objects long before they pose a threat to you.