DEPARTURE FROM EARTH is an emotional experience. As you stand at the foot of the Titan rocket, about to board, it dawns on you that the next breaths you take are the final few times fresh air will fill your lungs. You turn round to take one last look at the surface of Earth. All of a sudden the months you have spent training and preparing for this moment seem completely inadequate.
With your movement inhibited by the pressure space suit, which is an essential safety element of every launch, you find it a struggle to mount the steps then bend to get through the Titan’s tiny door. Inside, the conditions are very cramped. Assistants ensure you are connected up to the comms system and the life support system in case of depressurization on the way up. More importantly, they make double sure you are strapped in, because it’s going to be a pretty bumpy ride. At last the door closes and there is silence, except for the gentle hiss of the life support system and whirring and clicking from other equipment. Your epic journey has begun. At this moment, you feel more isolated than ever before.
The rocket engines fire. A thunderous noise fills the air and the cabin starts to shake violently. Eventually the words crackle through the intercom: ‘We have lift-off.’ You can feel it. The acceleration is incredible. You feel yourself being pressed hard against the seat: you are pulling about 3Gs on launch. With the noise and the shaking your senses are on full alert as they feel like they are being attacked on every level.
Most of the launch is automated, but there are one or two simple but critical tasks for you to perform, such as initiating the controls to separate the various stages of the rocket, each bringing with it a rather unnerving jolt. Almost as suddenly as it all started, though, you fall into silence as you reach Earth orbit. You feel strange, as though you are going over a humpback bridge but never landing. Weightlessness is going to take some getting used to. You unbuckle your belt and float almost gracefully out of your seat. This is really it!
Although for the launch itself you were required to wear the pressurized space suit, this cumbersome clothing can now be taken off and replaced with a more comfortable jumpsuit. Before long you can see out of the viewing window the International Space Station, with which you must dock to fill up with supplies: it is more efficient to launch with minimal payload and then top up somewhere else. After a brief stop, the Kaldi undocks and slips away; its living quarters start to spin up slowly and silently restore gravity, and a sense of normality begins to pervade life on board.
You glance out of the window to take a lingering look at Earth before departing on the long voyage. Seeing your home from space with such clarity you notice just how beautiful and vulnerable it appears. It looks perfectly circular but in reality Earth is an oblate spheroid, which means it is very slightly squashed at the North and South Pole as though being gently squeezed between finger and thumb. The diameter across the equator is 12,756 kilometres but measured through the poles it’s 41 kilometres shorter at 12,715 kilometres. Earth’s equatorial bulge is present because of its rotation, which we perceive as the rising and setting of the Sun, Moon, planets and stars. It is this rotation which gives us a measure of one day as twenty-four hours, although in reality it takes the Earth 23 hours, 56 minutes and 4 seconds to complete one rotation, and we just round it up. That discrepancy of 3 minutes and 56 seconds means that every day the stars seem to rise 3 minutes and 56 seconds earlier and slowly creep further towards the western horizon from day to day. You will remember from the last chapter that artificial gravity can be created by rotation. As the Earth rotates, material around its equator is forced into a curved path, but there is a resistance to this movement which results in the equatorial bulge.
The other thing you can see from your viewing window is the white light of the Sun. From Earth it always looks yellow, or possibly an orangey/red when lower down in the sky, but we can thank the Earth’s atmosphere for that illusion. The gas molecules that make up our atmosphere scatter light at the shorter end of the wavelength, which is why the sky looks blue, but the other end, around yellow, orange and red, remains less affected so the Sun appears more yellow than it really is. When viewed from space with optical protection and without the effects of the atmosphere, you can see it in its true colour, white.
As your first orbit of Earth continues, the Sun slowly drifts behind the Earth and you slip into our planet’s shadow. Another incredible sight appears as you see the stars, their crisp, pure light unaffected by the 100-kilometre-thick blanket of gas that surrounds our planet. With the Sun out of the way they appear brighter and clearer than you have ever seen them before. The atmosphere of Earth is constantly on the move as the energy of the Sun heats it, making it rise and fall. The rising and falling air creates pockets of low and high pressure which make the air move around in a horizontal fashion, producing wind. All of this movement – and this is a very simplistic view of an incredibly complex system – causes the incoming light from distant stars to bounce around and get disturbed. This, plus dust and pollutants in the atmosphere and of course the ever-present cloud, means the appearance of the stars is often distorted. From up here in orbit, though, they look incredible.
Orbiting around the Earth at an altitude of about 400 kilometres, the continents and oceans come and go. Something that is less evident is the tilt of the Earth’s axis of rotation which has a very important part to play in our weather system. All of the planets, the Earth included, orbit around the Sun broadly in the same plane. This plane can be seen in the night sky as the Sun, Moon and planets all roughly follow the same path. You can imagine this by thinking of the Sun sitting at the centre of a giant sheet of heat-resistant paper and the planets orbiting it. As they spin, they all, with the exception of Uranus, tend to sit roughly upright with reference to the plane of the Solar System. They do all exhibit some tilt, however, and in the case of Earth the tilt is about 23.5 degrees to the vertical. This tilt is directly responsible for the seasons we experience. When the North Pole of Earth is tilted towards the Sun, its heat has to travel through less atmosphere in the northern hemisphere so we experience the warmer weather of the summer. When the North Pole is pointing away from the Sun then its energy has to pass through more atmosphere before hitting the surface so we experience the cooler weather of the winter. While northern hemisphere dwellers are experiencing summer, the southern hemisphere is pointing away from the Sun and experiencing winter.
The direction in space in which the axis of rotation points is known as the north celestial pole, and from Earth that point happens to lie very close to the star known as Polaris. But much like a spinning top, Earth is wobbling in space, completing one wobble every 26,000 years, so in a few thousand years’ time, Polaris will no longer be the North Pole Star.
Above the North Pole of the Earth it is possible to see an eerie auroral display in progress. This beautiful natural phenomenon is the result of the interaction between electrically charged particles from the Sun, the solar wind, and the atoms of gas in our atmosphere. The solar wind is a fairly steady stream of charged particles that emanate from the Sun but on occasion there is a burst of higher intensity from coronal mass ejections. Depending on where the wind has come from on the Sun, it will travel at either 400 or 750 kilometres per second so it takes a day or two for the outbursts to be felt on Earth. On arrival, they are channelled around the planet’s magnetic field, accelerating particles already within the field to higher speeds which then drop down into the denser regions of the atmosphere around the North and South Poles.
What happens next requires a little knowledge of atomic physics. We have already seen that atoms are made up of a nucleus containing protons and neutrons surrounded by a number of electrons in orbit. The electron orbits are at very specific distances from the nucleus and, if at all possible, the electrons like to sit in their rightful place. By giving the electrons some energy, they move into higher orbits (further from the nucleus), and as soon as they can, they dump the energy as light and drop back to their usual orbit. The light dumped by the electrons is what we see as the glow of light from the aurora borealis (known as aurora australis in the southern hemisphere). The displays are usually easy to see around the polar regions but less common the further away from the poles you are. If there is a particularly big burst of solar material and the conditions are right, then auroral displays can be seen at lower latitudes. They are an astonishing sight.
It is not uncommon for astronauts to see meteors as they plummet through the Earth’s atmosphere, although there is an element of luck in spotting them, as is the case from the Earth’s surface. Seeing a meteor from space must be a really graphic reminder of the dangers of space flight. These tiny pieces of rock, which usually measure only a few centimetres across, travel through space at speeds in excess of 90 kilometres per second. That is significantly faster than the fastest bullet, which travels at a mere 1.4 kilometres per second, so imagine one of those hitting a spacecraft, or worse, hitting you when you are out on a spacewalk.
From Earth, these interplanetary pebble-sized chunks of rock usually become visible to us as they hit the upper layers of our atmosphere at an altitude of about 100 kilometres. As they fall they crash into atoms of gas, and the high-speed impact dislodges material from the meteor in a process known as ablation. The disturbed gas atoms momentarily have electrons stripped from them which produces a trail of positively charged atoms and negatively charged electrons. After a short while they recombine and give off a little light which we see as a bright trail behind the meteor – hence the name ‘shooting star’. It is not uncommon for larger meteors to plunge further into the atmosphere and compress the air so much that a shockwave builds up, producing a sonic boom. All but the largest meteors will completely disintegrate by the time they reach an altitude of about 50 kilometres. Those that do make it to the ground are known as meteorites and are surprisingly cool to the touch despite their violent journey through the atmosphere.
It’s a rather more worrying phenomenon up where you are. Meteors pose a real threat when it comes to space travel. There have been a few reports of impacts on the International Space Station and other spacecraft, but to date no one has been killed. Back in 2012, an impact event occurred to a viewing window in the Cupola module of the ISS. Just like a stone or piece of grit will chip the windscreen of your car, so will tiny pieces of space rock chip windows on spacecraft. In the case of the ISS the windows are made of fused silica and a material known as borosilicate which makes them literally bulletproof. Most modern spacecraft are now made from a very special multi-layered shell which includes a Kevlar-style woven fabric to reduce the velocity of any potential impactors. Astronauts outside this protective shell are vulnerable, however, and should a strike occur, the results would undoubtedly be fatal.
Time is of the essence on this mission so you cannot loiter around in Earth orbit any longer than necessary. It is now time to fire up the booster engines to take you to your first port of call, the Moon. Not firing the engines at the critical point will mean that you will more than likely arrive at the rest of the planets too late and they will have moved further along their orbit. Launch and rocket burn windows are more critical with chemical rockets as it takes a much greater amount of fuel to adjust trajectory compared to spacecraft powered by the VASIMR system. If we were to use that instead, it would lead to an orbit of Earth that would slowly spiral out towards the Moon. A similar system was used on the SMART-1 mission and from an orbit of just a few hundred kilometres it needed to increase it by about 384,400 kilometres – the distance of the orbit of the Moon. At the low thrust of the electric engine it took a little over a year to get there but it only used about 100 kilograms of fuel. It is an interesting contrast to the Apollo 11 mission which used almost 6,500 kilograms to get there in a little over three days. Clearly electric engines are significantly more efficient.
The coast to the Moon gives you plenty of time to get used to the experience of living in space. Weightlessness is a strange sensation that takes quite a few weeks to get used to but spinning up the habitable portion of the ship reinstates ‘gravity’ again. The rooms are arranged in a linear fashion around the doughnut-shaped portion of the craft so that the floor is actually the outer edge of the hull. There are sleeping quarters, living quarters, a kitchen, storage rooms, and even a bathroom.
The voyage will be very comfortable, but without the luxury of artificial gravity things would be very different. On a normal space flight vacuum power is used heavily to control the movement of things on board. It’s particularly useful for maintaining personal hygiene: for example, the toilets will use a vacuum flush system instead of water. Some toilets have handles to help the astronaut sit tight; some even have leg and foot restraints. Solid waste is simply sucked down into the toilet by rotating fans that produce suction through a tube connected to the bowl of the toilet. The stools are distributed into storage containers that are exposed to a vacuum to dry them before eventual transportation back to Earth. The vacuum tube also has a detachable urine receptacle with male and female adaptors, although unlike the solid waste the urine is collected and then dumped overboard. Interestingly, after launch, the lack of gravity causes all bodily fluids to spread evenly around the body. This movement is detected by the kidney which triggers a physiological reaction: astronauts must relieve themselves within a couple of hours of leaving Earth. For this reason there is a rigid diet and bowel movement regime in place for all launches. Thankfully, simulated gravity means you can enjoy normal bathroom activities for the duration of the flight.
The rooms on board the Kaldi are pretty similar to the ones back at home. The only difference is that they are not all on the same level: the rotating nature of the craft gives the sense of gravity on the inside of the outer edge so it is this face which acts as the ‘floor’. But all the comforts of home are present, from a cosy bed to a sitting area and flowing water for cleaning and cooking. What gives the rooms a surreal look is the pitch black outside the windows twenty-four hours a day, 365 days a year. All of this is in quite stark contrast to your recent experience on board the International Space Station which had no gravity, and where water did not flow in the conventional sense and personal hygiene tasks were much more difficult to carry out.
For the first day en route to the Moon it looks no different to how you are used to seeing it from home, although it might seem more prominent against the inky blackness of space that is now no longer dulled by looking through the Earth’s atmosphere. The Sun, Moon and Earth are now all visible and they seem eerily similar to the diagrams you see in textbooks showing how their changing relative positions cause the appearance of the phases of the Moon. The Earth lies behind you, the Moon roughly ahead, and the Sun is off to the left or port side of the craft. In that configuration, the Moon would appear to those on Earth as a quarter Moon with half of the visible portion being illuminated and the other half in darkness. The line between the two where day meets night is known as the terminator, and the detail along this line always looks stunning. It is here that the shadows are at their longest, making surface features stand out.
Of all the surface features on the Moon probably the most well known are the craters and vast grey plains known as the seas, or maria (the plural of ‘mare’, the same as the Latin word for ‘sea’). Before the invention of the telescope it was generally believed that these grey patches were large bodies of water, but closer study (chiefly by Galileo) revealed that they are simply vast plains darker than the surrounding areas. The impacts of chunks of rock are to blame for the landscape of the lunar surface, with craters formed by smaller impacts and bigger collisions creating the large plains. When the Solar System was younger there were large pieces of rock left over from its creation and when they hit the Moon they cracked the crust and let molten lava flow through from the mantle below. This lava filled the impact crater and, over time, solidified leaving behind the plains we can see today. It is possible to roughly date the age of the lunar surface by studying the size and distribution of the craters. A very new feature, for example, is less likely to have been disturbed by subsequent impacts than an older feature.
You’ll enjoy seeing the familiar features of the Moon as you fly around it for the first time. Perhaps the most famous is the Sea of Tranquillity, where Armstrong and Aldrin took those historic footsteps. From here, up in space and without the distorting effects of the Earth’s atmosphere, it appears clearer now than ever before, somehow closer, as though you could reach out and touch it.
Then you will get to see something very few humans have seen, the far side. The concept of the far side of the Moon is a strange one but it is not uncommon among other natural satellites in the Solar System to have a hemisphere that remains forever turned away from the planet. Many people believe that the Moon’s far side is in permanent darkness, but in fact at some time or other the entire lunar surface experiences sunlight. It rotates once on its axis every 27.3 Earth days so common sense suggests that if you were standing on one spot on the surface of the Moon, you would see a sunrise every 27.3 days. Rather confusingly, you would actually see a sunrise every 29.5 days – a discrepancy created by its orbit around the Earth. In the time it has taken the Moon to spin once on its axis, the Earth has moved around the Sun a little, dragging the Moon with it. This means the Moon has to spin a little bit more – for 2.2 days as it turns out – to account for its slightly different position in space.
It isn’t a cosmic coincidence that the Moon takes 27.3 days to rotate once on its axis and also 27.3 days to complete one orbit of the Earth, and this is known as captured or synchronous rotation. Behind the scenes it is gravity that’s responsible for locking the rotational and orbital periods of the Moon, and it’s all linked with the effect we know as the tides.
The Moon, like all objects, exerts a gravitational pull on the Earth and that pull produces a slight bulge on the side of the Earth facing the Moon. As Earth rotates, any locations passing through this gravitational field will experience a rise in water levels, and to a much lesser extent a rise in land levels too. The rising water levels are what we know as high tides and they can be seen at nearly every location on the globe at some point: there are places on Earth where tides do not occur, for example at the North and South Poles at certain times. For the rest of the planet, there is another tidal bulge. Because the strength of gravity decreases with distance, another bulge is produced on the opposite side of the Earth. Effectively the material inside the Earth experiences a greater pull than the surface material (such as water) on the far side so it is more accurate to say that the Earth is being pulled away from the oceans, creating the second but slightly lower high tide roughly twelve hours later.
Crucially, the main bulge on the lunar side does not sit exactly on a line between the two objects. As the Earth spins, it drags the bulge with it, causing it to lie a little ahead of the line between the Earth and the Moon, and this misalignment of the tidal bulge means that the extra mass of the bulge results in an extra pull of gravity which tugs on the Moon making it accelerate in its orbit. This acceleration causes it to move further away from the Earth at a rate of about 3.8 centimetres per year.
With the Moon and Earth locked in an orbital dance around their common centre of gravity, known as the barycentre, it is perhaps reasonable to expect that on occasion they will block one another from view. For example, we see an eclipse of the Moon when the Earth lies directly between the Sun and Moon. It is also this rough alignment that gives us the full Moon phase, yet we do not see a lunar eclipse every time there is a full Moon. This is because the orbit of the Moon around the Earth is tilted by about 5 degrees with respect to the orbit of the Earth around the Sun so the three of them are not always in a perfect line. This means at most full Moon phases that the Moon will lie just above or just below the Earth–Sun line so sunlight will not be blocked by the Earth. Similarly we see a solar eclipse when the Moon blocks our view of the Sun, which only happens at new Moon, but on most occasions the Moon is either a little above or a little below the Sun. A perfect alignment of three celestial bodies like this is known as a syzygy. Solar eclipses are particularly spectacular because the Moon and Sun are both roughly the same size in the sky so usually the presence of the Moon in front of the Sun means it blocks out the Sun’s bright photosphere, revealing its beautiful yet faint outer atmosphere, known as the corona. There are also partial eclipses where either the Moon is only partly in the Earth’s shadow or it only partly blocks the Sun from view. And then there is the annular eclipse where the Moon is very slightly smaller than the Sun in the sky, which is a result of the elliptical nature of the Moon’s orbit around the Earth. When the Moon is at its furthest point from the Earth it appears at its smallest in the sky, and during an eclipse a ring (or ‘annulus’) of the Sun’s photosphere is visible behind it.
On arrival at the Moon there may be a need to execute a long fire of the VASIMR engine to adjust the trajectory and send you on to the first planet on your journey. Firing engines on arrival (or close to arrival) at destinations is not unusual, whether it is to adjust the path of the spacecraft a little so that it follows the necessary trajectory to slingshot correctly on to its next destination, or to slow the spacecraft so that it can drop into orbit. If the velocity of an approaching spacecraft is too high then it will simply slingshot around. If the velocity is too low then it will crash into the surface, so getting it right is essential.
This was beautifully demonstrated in 2011 by the Gravity Recovery and Interior Laboratory spacecraft which was composed of two lunar orbiters. During a carefully choreographed four-month slow cruise to the Moon, the two spacecraft managed to achieve a separation of about 100 miles which was crucial to the success of the mission. Their purpose was to map the interior structure of the Moon by monitoring their relative positions. The two spacecraft were equipped with a system that used electromagnetic waves to tell them exactly how far apart they were and how far away from Earth they were too, to an accuracy of a hundredth of the width of a human hair – equivalent to about 1 micron. The systems on board emitted electromagnetic waves whose frequencies were monitored from Earth and by each other. As they tracked above the lunar surface at an altitude of about 23 kilometres, the frequencies monitored shifted as the spacecraft were tugged downward and accelerated or drifted up and slowed down. The changing speed was the result of a changing gravitational field caused by ‘heavier’ material below them in the interior of the Moon. By measuring the changing speed it was possible to weigh the Moon at that location and therefore build a picture of its internal structure. After many orbits over the four-month period a gravity map of the entire lunar surface was completed, giving an unprecedented view of the inside of the Moon. Other missions have helped to develop this knowledge. For example, magnetometers have been carried to the Moon on various missions and are used to measure the magnetic field at different locations, while lunar landers have studied rock compositions. By piecing all this information together we now have a good understanding of the internal structure of our nearest astronomical neighbour – and that can help tremendously when planning missions.
We have learned that the Moon has a solid iron-rich inner core with a diameter of 480 kilometres which is surrounded by a liquid iron outer core that has a diameter of about 610 kilometres. Surrounding the outer core is a partially molten boundary layer which is an estimated 200 kilometres thick and which separates the core from the mantle and crust. The formation of these outer layers is thought to have followed a period of fractional crystallization of a magma ocean that covered the entire lunar surface. This process is one of the most important in the evolution of planetary geology. Silicates were removed from the molten magma, and eventually about three-quarters of the magma crystallized, creating a mantle rich in magnesium, iron, pyroxene and olivine while lower-density materials such as plagioclase and anorthosite (minerals of the feldspar family, the latter igneous in nature, having formed from the cooling of magma) rose to the surface forming the crust that we see today, which is thought to be about 50 kilometres thick.
The fractional crystallization process in the magma ocean is strong evidence that the Moon had a violent birth. A very large object about the size of Mars is thought to have struck Earth around 4.5 billion years ago and sent debris flying out into space. Over millions of years, the material suspended in orbit around Earth coalesced into the Moon we see today. The vast amounts of energy involved in the impact meant that large portions of the Moon were liquidized, leading to the formation of the magma ocean.
Thanks to other orbiting spacecraft such as the Lunar Reconnaissance Orbiter (LRO), which used laser altimeters, we now also have a huge amount of detail about the surface topography. Cameras on board were even able to take pictures of some of the Apollo lunar landers which still sit dormant on the surface. Such topographical maps were not available for the Apollo missions so NASA used surface drawings created by Sir Patrick Moore for their Moon landings, such was the quality of his work. Because of orbiters like LRO we not only have detailed maps of the near side of the Moon but of the far side features too, and there are quite significant differences between them. The lunar maria you looked at earlier are found almost entirely on the near side of the Moon and account for just over 30% of the surface features; only 2% of the far side is covered in maria. It is thought that a thinner crust and a concentration of heat-producing elements under the crust on the near side are responsible for this difference. As the Moon formed and the crust solidified, elements that are incompatible and were separated in the liquid became trapped together in the region between the crust and mantle. Uranium and thorium are just two of these elements, and their radioactive properties produced a lot of heat. The heating caused a partial melting of the upper layers of the mantle which were subsequently brought to the surface through volcanic eruption, or through cracks and fissures in the crust from meteoric impacts.
The remainder of the lunar surface is made up of the highlands, named simply because the surface level is generally higher than the plains, not because of rolling hillside and mountain chains. As we know, the craters are formed by meteoric impact, and if you have ever looked at the Moon through a telescope you will know there are thousands of them. Estimates suggest there are just over a quarter of a million with a diameter of a kilometre or more, and that’s only on the near side; there’s at least the same number on the far side. There are some craters near the polar regions on the Moon whose bases are almost permanently in shadow, and it is in these craters that water ice has been discovered.
Water in its liquid form cannot exist on the surface of the Moon chiefly because of the exposure to solar radiation. Liquid water on Earth is protected from such an onslaught, but when exposed to the radiation on the Moon, water molecules soon decompose in a process known as photodissociation – essentially a chemical reaction that breaks down the hydrogen and oxygen in water. It has long been thought that water in its ice state may well exist in deep craters and in the sub-surface layers, having been deposited on the Moon by cometary impacts. In 1998, an instrument known as the neutron spectrometer on board the Lunar Prospector spacecraft revealed high concentrations of hydrogen in the top metre of the lunar surface, indicating the presence of sub-surface water, similar to permafrost found in regions on Earth. Study of rocks returned by the Apollo missions also revealed evidence of water, so it seems that water is a common commodity on the Moon. This has since been confirmed by the Chandrayaan-1 spacecraft, which in 2008 found conclusive evidence of water ice in polar craters that received no sunlight. This is all great news for potential lunar habitation.
You will find that the craters on the Moon look stunning close up. Even through a telescope from Earth it is amazing just how much detail can be seen. It’s surprising, too, to see how different they can all appear, from the single young craters that have pierced the pristine lava plain of the maria and the old craters that have almost been obliterated by newer impacts through to the complex crater chains seen stretching across the surface. We have already touched on the cause of craters and the fact that on Earth the atmosphere acts as a protective shield against rock impacts. That is why our planet is so devoid of craters – that and the constant erosive effects of the weather, which over the centuries gently erase signs of impact. Although there are a few craters left on Earth like the majestic and aptly named Meteor Crater in Arizona, on the Moon the lack of weathering from an atmosphere means that craters remain for millennia, unless another impact obliterates them. Even the footprints of the twelve Apollo astronauts who have walked on the surface remain for all to see, and will do so for millions of years to come.
One of the most prominent craters is Tycho in the southern lunar highland region, named after Johannes Kepler’s old tutor, the Danish astronomer Tycho Brahe. Not only is it an almost perfect example, it also exhibits two quite remarkable features. The first is particularly noticeable when the Moon is full, which is not the usual time to make lunar observations: usually the features look best when the Sun is low so they appear more prominent when along the terminator. At full Moon, Tycho’s stunning ray system bursts into view, stretching for 1,500 kilometres across the surface and centred on the crater itself, which measures 86 kilometres in diameter. Theories about the origin of ray systems have varied over the years but have included salt deposits from water through to deposits of volcanic ash. What we now know is that their nature is intrinsically linked to the formation of the craters themselves. As the impactor strikes the surface, the energy released blasts quantities of the lunar regolith (the unconsolidated layer that sits on the bedrock) and maybe even fragments of the impactor itself outwards from the impact site. This material then settles back on to the surface as the ray systems that we can see, and it is clear from their bright colour that they tend to be from relatively newly formed craters. In the case of Tycho, that’s about 108 million years old – young in geological terms.
Another very common crater feature which you’ll notice is a central almost mountain-like peak. They are more difficult to observe than the ray systems and are best seen when the crater sits on the terminator. For Tycho, this means when the Moon is about nine days old as the incoming light from the Sun casts long and deep shadows on the floor of the crater so the peak looks quite prominent. The origin of the peaks found inside craters is known to be the result of the energy involved in the impact when the crater forms. As the rock slams into the surface of the Moon the release of energy is sufficient to partially melt the surface material. The force of the impact sends a shockwave outward which rebounds back into the centre, taking with it some of the molten material which then forms into a peak as it solidifies.
Mare Crisium is a great example of how craters and maria are linked. If you look around the edge of Crisium you’ll see that it has a very jagged appearance and is circular in shape. Clearly this was once a crater that has since been filled by molten lava. Across the surface of the Moon you can see other examples of curved jagged mountains which are actually ancient crater walls that have since been modified by more impacts. When looking at the Moon it is actually quite hard to find much undisturbed material, but there are some areas where it is possible to see what the surface is really like.
As you swing past the Moon, this is your first chance to try out the Reality Suspension Unit, which will allow you to pop down to the surface and have a walk around. ‘The surface of the Moon is fine and powdery’ are among the many words uttered by the first human visitors, Neil Armstrong and Buzz Aldrin, and those words will come to mind now as you take your first steps. Walking on the Moon for the first time is an incredible experience and the view from the surface really is that of an alien world. It is as far removed from the views we get on Earth as we can possibly imagine. Unlike Earth, which is teeming with life, on the Moon there is no sign of it, anywhere. It is a completely desolate and inhospitable world. It is a humbling and emotional experience to be among the first living creatures to set foot on this landscape since it formed over 4 billion years ago.
The surface that Armstrong and Aldrin described, and which you are now walking on, is known as the lunar regolith and consists of countless pieces of tiny particles the same size as sand and silt. The particles have been created over billions of years by a bombardment from rocks that pulverized the surface material over and over again, much like a chef with a pestle pounds whole spices into a powder in a mortar. As you might expect, there are larger chunks mixed in with the finer particles, but overall it has a powdery feel. The material covers the Moon’s entire surface to depths ranging from 5 to 15 metres, but it gets quite compacted just a few inches down. Armstrong even reported that it had almost snowlike properties, sticking to their boots as they moved around. The Moon’s regolith is not unique in the Solar System, though, as we shall see. A fine powdery surface exists on Mars too, as well as a number of asteroids and even a few other planetary moons.
Something else that is making your Moon walk quite an alien experience is the noticeable absence of atmospheric perspective. If you look at a scene on Earth, perhaps a range of mountains in the distance, they appear faded and almost a little more blue than your immediate surroundings. This fading effect is caused by dust in the atmosphere which tends to block some of the light coming from that distant scene; it’s one of the key ways in which our brain is able to determine that things are far away. The contrast too seems to reduce over distance, bringing those distant objects closer to the apparent colour of the sky. The presence of the dust particles in the atmosphere means that some of the already scattered blue light through the atmosphere gets reflected back towards you, the observer. Having an atmosphere that can suspend dust in it is essential for this perspective effect to occur. On the Moon there is very little atmosphere. In fact it is so rarefied it is almost a vacuum, and its virtual absence means no suspension of tiny particles and therefore no atmospheric perspective. It is for this reason that it is very difficult to judge distances to lunar landmarks such as craters and mountains, which always seem a lot closer than they really are.
The lack of an atmosphere on the surface of the Moon would make it a wonderful base from which to carry out astronomy. The Earth’s atmosphere has a number of effects that thwart astronomers in their attempts to get sharp images of the night sky. The most noticeable is known as scintillation. You are already familiar with it as the twinkling of stars in the night sky. As light from a distant object passes through the atmosphere it gets deflected from its path, making the object seem to jump around. If you have ever looked through a telescope then you will already know that the image of a distant object can be spoiled terribly when the atmosphere is particularly turbulent. This is one of the reasons why professional observatories are often placed at the top of high mountains, above as much atmosphere as possible. The atmosphere also blocks out some of the wavelengths of the electromagnetic spectrum coming from distant objects so we are unable to study them from the surface of the Earth. This has a profound effect on our ability to fully understand the nature of objects in the Universe since we must study them in all wavelengths of light. A great analogy is the sound coming from an orchestra: to fully appreciate it we must listen to the music from all the instruments, not just one or two of them. There are ways to counteract the effects of scintillation but some level of detail will always be lost. We have come to accept that the blocking effects of the atmosphere are not something that can be worked around, which is why astronomers have taken to launching telescopes into space.
The Moon would be an ideal alternative to this expensive and challenging operating environment. Its lack of atmosphere means that even ground-based telescopes would get fantastic views. The fine dust might be a problem but there is very little movement of the regolith material because there is no wind. Astronomy could even be done during daylight hours because the sky is black not blue during the lunar day. There are craters near the poles of the Moon whose bases are nearly constantly in the dark and therefore very cold, making them ideal locations for infrared telescopes, and the far side of the Moon would be great for radio telescopes as they would be protected from the constant chatter of noise from our terrestrial radio transmissions.
Like certain craters whose floors remain permanently in the dark, there are some mountain peaks around the Peary Crater at the Moon’s North Pole that remain constantly illuminated. These are known rather romantically as Peaks of Eternal Light, and there are other such places on other bodies in the Solar System. Not all planets have these points though. For areas on a body to remain permanently lit they must be high altitude, and if the axial tilt is small then they must be near the polar region. The tilt of the axis of the rotation of the Moon is small, at just 1.5 degrees, but Earth’s is over 20 degrees so there are no regions like this on Earth.
The low surface gravity on the Moon also makes it a great base camp for an exploration of the outer planets. To launch a spacecraft into space from Earth requires a speed of 11.2 kilometres per second, and in the case of the Apollo missions this required 5.6 million pounds of fuel, but the return trip was far more efficient because of the Moon’s lower gravity. The escape velocity of the Moon is just 2.4 kilometres per second so the Apollo missions required just 40,000 pounds of fuel to take off and return home. Add into the mix that there is a great supply of liquid fuel available on the Moon in the form of water and it makes a lot more economic and logistical sense to explore the Solar System from the Moon.
Fortunately for you, though, your lunar excursion can be brought to an end simply by returning to the ship. Back on board, your trajectory is adjusted very slightly by the gravity of the Moon and you swing around on course for an encounter with the Sun, the star that is the powerhouse of our Solar System.