In 1960 Dr Frank Drake attempted to create a way to guess how many intelligent civilisations like our own would arise in our galaxy and in the universe. The question of if we’re alone has driven astronomy and space exploration for centuries and is one of the greatest unanswered questions we have left.
What Drake wanted to do was to try and work out how likely it was that we’d ever make contact with another civilisation, or if the probability of intelligence arising was so remote that we’d spend our time very much alone. The result of his attempts is the now famous Drake Equation, the formula that essentially powers SETI and helps astronomers to focus their searches for artificial signals in the universe.
The Drake equation works backwards, looking at each factor that has lead to intelligent life evolving on Earth and then trying to predict how likely each event is to occur again elsewhere. By breaking down the factors leading to the evolution of intelligent life it is believed that we’ll be able to narrow down the search, hopefully with greater chances of success.
The Drake Equation is composed of seven parts, each thought to be a contributing factor as to how easy it is for an intelligent species to arise. A crucial factor to remember is SETI’s definition of intelligent life. The search is focusing on finding another species in the galaxy that is capable and willing to communicate through radio transmission or some other form of electromagnetic radiation. Using Earth as an analogy for life elsewhere it is assumed that as we leak radio transmission into the galaxy, then so would another species.
The problem with looking for radio transmissions is that it assumes two things. One is that an intelligent race will use radio for communication and be leaking it into the galaxy. The other is that they will continue to use it. It is actually very cost-ineffective to broadcast in all directions, and we can see this on Earth where broadband and fibre optic cables are now the new standard.
Soon we will leak very little information into space as we focus all communication in its intended directions. If we again use Earth as an example it could be said that any species just a few decades ahead of us in technological development would no longer leak radio into the cosmos, and a species a hundred years younger would not have developed radio yet. By looking for radio transmissions we’re looking for a narrow space in a species’ development, however it is assumed for the purpose of the equation that the alien life we’re looking for are willing communicators and would let themselves be known, much like us.
The Drake equation is written as N = R starred x fp x ne x fl x fi x fx x L. N is the number of civilisations in the Milky Way with which communication may be possible. The other figures are the factors that are considered vital for the appearance of intelligent life, based on observations in our own Solar System and what we know of life on Earth.
R starred is the rate of star formation in the galaxy. Currently the figure for this is put at seven per year. Life cannot exist without a star, and so this is seen as the first step. Fp is the fraction of stars that form that support planets. As we discover more exoplanets by the week this number is creeping up and up, current estimates put the figure at around 0.5, but the figure is certain to rise. Our current technology is only reliable at detecting Jupiter size planets, but once we start finding Earth analogs in other solar systems we’ll have a much better grasp of how many planets there are for potential aliens to evolve on.
Ne is the number of those planets where life can be supported. Life as we know it requires liquid water, which requires a certain temperature band known as the habitable zone. Life also requires an atmosphere, which only a planet meeting certain criteria will be able to hold. Based on observations of the Solar System we see only one planet orbiting that is capable of supporting liquid water on its surface and therefore, life. It is thought that Mars may once have had water on the surface, but due to its low mass it has lost must of its atmosphere and so water cannot stay as a liquid on the surface.
The next figure, Fl, is for how many of those planets that are capable of supporting life will develop life. On Earth we find life everywhere there’s water. Even without light at the bottom of the sea, or buried deep in layers of arctic ice, if we find water there’s at the very least bacteria. This suggests that if conditions are right then life is certain to arise, putting Fl at a value of 1.
Fi and Fc look at the chance of intelligent life evolving and then being willing to communicate with other planets. On Earth only one species in the four and a half billion year history of the planet has evolved intelligence and radio technology, which puts these figures quite low. Again, these guesses have a certain degree of bias as we only have one example to look at. It may prove that if life starts on a planet that the path to sentience is inevitable. It may also prove an extremely rare event, we won’t know for sure and so these figures are largely guesswork.
L is the number of years that the civilisation will last. Currently this figure is set at 10,000 years. Given that mankind has already developed the capability to destroy ourselves through nuclear weapons (developed just seven years after radio astronomy) it might be guessed that the self destructive ability goes hand in hand with interstellar communication. Our civilisation teetered on the brink of nuclear war during the Cold War, and the threat, while greatly reduced, has not entirely evapourated.
Given that some civilisations that develop the capability to destroy themselves will destroy themselves then it cannot be assumed that once intelligent life has evolved that it will last indefinitely. L is a guess, but is used to guess the window in which we have to communicate with a species from their development of communication to their eventual downfall.
Using Drake’s estimations the figure for the number of species able to communicate with us in the Milky Way is 10. The Milky Way contains between 200 and 400 billion stars, so the chances of finding an alien species are around one in thirty billion. Different estimates of parts of the equation will give higher values. For example if L is set to 50,000 years then there are 50 civilisations in the Milky Way. More optimistic outlooks can set the number of communicating aliens at 5,000 per galaxy.
The figure can change wildly depending on the number plugged into the equation, and at SETI they’re working on minimising the guesswork for each of the factors to help them narrow their search. With any luck we’ll find our first alien broadcast within the next few decades, and if we do, a lot of the success will be attributed to the Drake equation.
If you ask anyone if they know what Apophis 99942 is then the answer will likely be a blank one. Ask the same question again in 2029 and then in 2036 then people will most definitely be more aware. Apophis 99942 is a near-Earth asteroid and for a brief while was considered the most likely object to collide with Earth. In 2029 it will make a record-breaking near miss of our planet and in 2036 it will return with a 1 in 45,000 chance of hitting Earth.
Apophis is estimated to be 415m long and incredibly heavy, in 2029 it will pass within the range of geosynchronous satellites. Objects of this size are only thought to pass this close every 1,300 years. It will be visible to the naked eye as it makes its approach and will appear as a starlike object moving across the night sky. When the asteroid was first discovered initial estimates put the impact probability at 1 in 37, moving it up to four out of ten on the Torino scale, the highest anything has ever been.
However with more work on computing the orbit of Apophis astronomers were able to rule out an impact in 2029 and state that an impact in 2036 looked unlikely. The asteroid was discovered and named by Roy Tucker and David Tholen. Apophis was the Greek name for the Egyptian god Apep the Uncreator. Apep lived in the underworld and tries to swallow Ra as he passes. Despite a seemingly appropriate name it is rumoured that Tholen and Tucker named the asteroid after the character of the same name from their favourite TV show, Stargate SG-1, a being who sought to destroy the Earth.
Although in 2008 the likelihood of Apophis hitting the Earth has been proven to be very small it did for a while start to make scientists wonder what the effects would be should the asteroid actually strike our planet. An object of the size of Apophis would generate an explosion with the equivalent energy of 880 megatons of TNT. By comparison the eruption of Krakatoa was 200 megatons and the Tunguska event released 3-10 megatons of TNT equivalent energy. The impact would be severe and affect a huge area, but thought to be light enough that the Earth would avoid some of the more serious events, such as an impact winter.
Although Apophis is considered a low risk, its initial status of 4/10 on the Torino scale prompted talk of how we would deal with any future Earthbound asteroids. Many possible ideas have been looked at including deflection and nuking. Deflecting asteroids is seen as favourable to blowing them up as all that may do is create a stream of smaller asteroids that still hit the Earth. To deflect an orbit of an asteroid a small craft would drill into the surface of the rock and then eject material to slowly move the asteroid off-course.
For the moment we’re safe from asteroid impact, but NASA cannot possibly track every single body out there. Thankfully our atmosphere takes care of much of the smaller bodies and provides fantastic protection from asteroid impact. However, there is one body over 1km in diameter with a probability of hitting Earth. Asteroid 29075 1950 DA is rated 2/10 on the Torino scale and will make its pass in the year 2880. Fortunately for us its still a long way off and gives us ample time to work out an effective defense mechanism. An object the size of 1950 DA would have serious consequences for human civilisation and be very harmful for the climate and biosphere. Luckily for us we have nearly 900 years to work on a solution.
Black holes are among the strangest objects in the Universe. Hard to find, hard to explain, hard to understand and hard to imagine they defy the known laws of physics at their centre and provide mind boggling results at their edges. A black hole is impossible to see but there has been enough evidence to certify their existence. They are an important part of the solar life cycle and perhaps an even more important factor in galaxy formation.
In 1785 a man named Peirre Simon Laplace concluded that if enough mass were packed into a sufficiently small space that the gravity would be so intense that not even light would be able to escape. The idea is based on escape velocities being proportional to the mass of an object. For example, the energy needed to escape the gravitational pull of Earth is 19,700 mph greater than what is needed to break free of the Moon.
Laplace theorised that when gravity reaches a massively high value that the escape velocity would be faster than the speed of light, trapping light particles and rendering the object invisible. During the early 20th century physicists such as Oppenheimer, Volkoff, Snyder and notably Karl Schwarzschild would refine the theory and provide mathematical formulae to back it up.
The only known way for a black hole to form is from the death of a massive star, this is the only way that such a huge mass can be packed into such a small volume. During the main phase of their life stars are trying to explode through nuclear fusion but at the same time are held together by their own gravity. These two forces balance the volume of the star and will define its size.
When the star runs out of fuel for nuclear fusion then there is no outward force and so gravity causes it collapse in on itself. The collapse heats up the core to a massive degree and will cause a supernova explosion. After the supernova all that is left of the star is a highly compressed core, continually collapsing under its own gravity. The gravity becomes so strong that light cannot escape and so a black hole is formed.
Black holes effect spacetime in strangely different ways to other bodies. They cannot be seen by visible light, but have a number of interesting properties. The defining feature of black holes is the event horizon, a real point of no return. The event horizon is the point at which the gravity becomes so intense the the escape velocity reaches light speed. The horizon surrounds the singularity at the centre of the black hole and effectively forms the mouth of the black hole. From the outside, nothing inside the event horizon can be seen.
Outside the event horizon objects can orbit the black hole like they would any other large body of mass. The escape velocity will increase the closer you get to the horizon, but at a large enough distance the orbit will be no different to orbiting a star or planet
Crossing the event horizon will prove fatal to anyone that tried. In theory if one were able to cross an event horizon in a spacecraft it would seem initially unremarkable. Far away objects would appear distorted due to gravitational lensing but you would still be able to see out of the black hole into space. As your craft flies further from the horizon it would experience tidal forces so strong that eventually it would torn apart, right down to the atomic level. The time frame for this happening varies depending on the size of the singularity at the centre but is typically just a few seconds.
What can be confusing is thinking about what a distant observer would see as your doomed craft flew into the black hole. As the approach to the event horizon is made the light will take longer and longer to leave the craft because of the intense gravitational pull. This would make it seem as if the craft is slowing down in time until it reaches a point where the light takes an infinitely long time to reach the observer. Time would appear to stop for an outsider, and it is a property that gave black holes their original name of frozen stars.
There is also the fact that the gravity will cause a massive amount of time dilation to occur, meaning that time actually will pass more slowly for objects close to a black hole than those far away. Both theories are correct and it is one of the most intriguing properties of black holes. In reality an observer would see the craft slow right down and appear more red as the light is red-shifted.
Black holes cannot be seen from Earth but can be detected in a number of ways. One is through gravitational lensing. According to Einstein’s theory of general relativity gravity distorts both space and time. A star or a planet will cause an effect similar to a heavy ball on a rubber sheet, and it is this effect which causes the orbits we can see today. Gravity can also bend light and an massively intense gravitational pull will bend light much more and distort the light from objects behind it. Black holes have been detected from Earth by observing the effects on the light travelling from distant stars such as duplicate images or intense brightening.
Black holes are thought to exist at the centre of most galaxies. They are the only known objects that can reach a sufficient mass within the volumes observed. The mass of supermassive black holes believed to be at the centre of galaxies is measured by observing the orbit speeds of the objects surrounding it. It is a technique that is used to similar effect to work out the mass of bodies in the Solar System such as the Sun and Jupiter.
In mid-2008 there were some fears that the Large Hadron Collider would create a micro black hole that would destroy the Earth. In reality these black holes would be far to small to do this and would evaporate almost instantly. Despite the fears being ill-founded the thought of the planet being swallowed by a black hole showed how much these enigmatic objects have captured the public imagination. They’re often used as plot devices in science fiction, and the mysterious nature of the event horizon further adds to their intrigue. Black holes are certainly one of the most interesting phenomena in the Universe, and let’s hope that we never get too close to one.
Uranus and Neptune are known as the ice giants, huge blue planets located in the outer Solar System. They are similar to Jupiter and Saturn in that they are predominantly composed of gases and ice but differ in many key areas. While still dwarfing the inner planets the ice giants are smaller than the other gas giants. Due to their distance from Earth they were the last planets to be discovered, Uranus in 1781 and Neptune in 1846.
Uranus is the seventh planet from the Sun and orbits between 3,004,419,704 km and 2,700,938,461 km from the star. It has 27 known moons as well as a faint ring system. Uranus takes 17 hours to complete a rotation and 84 Earth years to complete one orbit of the Sun. The planet is named after the Greek God of the sky, the father of Saturn and grandfather of Jupiter.
What makes Uranus different from any other planet in the Solar System is that the axis is tilted onto its side. The tilt of Uranus is 97.7 degrees, meaning that the pole is facing the Sun, making its day/night cycles and seasons entirely different to any of the other planets. Each pole of Uranus gets 42 years of light followed by 42 years of darkness. What caused this extreme tilt is not known for sure but is thought to be due to a large planetary body striking the ice giant.
Uranus is composed of three layers, a rocky core, an icy mantle and an outer layer of gaseous hydrogen and helium. Uranus is the coldest planet in the Solar System, despite being closer to the Sun than Neptune. The lowest temperature ever recorded on Uranus was -224 degrees C, just 49 Kelvin. Why Uranus doesn’t emit as much of its own internal heat as other planets is not known for sure, but it has been speculated that the internal structure of the planet blocks much of the heat from the core from reaching the surface.
Neptune is the second ice giant, the eighth planet from the Sun orbiting at a distance of around four billion km. Neptune takes 164 years to orbit the Sun and takes 16 hours to complete one rotation, giving the planet short days and extremely long years when compared to Earth. After Pluto’s demotion from planet status Neptune has become the furthest planet from the Sun.
The structure of Neptune is similar to Uranus with its atmosphere making up around 10 percent of its mass, slowly becoming more solid as you reach the core where temperatures sit at around 5000 Kelvin. The atmosphere is scattered with clouds that form in the upper layers, believed to made of ammonia, ammonium sulphide, hydrogen sulfide and water. Neptune has a ring system, but it is much less prominent than Saturn’s.
Neptune has a much more active weather system than Uranus with many clouds and storms. The planet exhibits many similar features to those found on Jupiter, including its own great dark spot. The winds are thought to even reach speeds of up to 600 m/s, near supersonic speeds. The great dark spot was discovered in 1989 by the Voyager 2 probe. There are many other similar but smaller storms present on Neptune.
The ice giant has many moons but only one is large enough to be spherical, Triton, which orbits in the opposite direction to Neptune’s orbit, a unique property among the moons of the Solar System. Because of this it is believed that Triton is a captured body from the Kupier belt, a huge asteroid belt surrounding the Solar System. Pluto is one of the largest objects in the belt, bodies present in the Kuiper belt are known as Plutinos.
The ice giants are the least explored planets in the Solar System, Voyager 2 visited in the late 80s gathering images and scientific data. Neither planet is considered a high priority for human visitation or colonisation due to their immense distances from Earth. Their moons do not lend themselves well to settlements and the Saturnian and Jovian moons have much more to offer.
Saturn is the most visually striking planet in the Solar System. It is the second largest gas giant behind Jupiter and is surrounded by a spectacular series of rings. Saturn is the sixth furthest planet from the Sun at 1,400 million km away and is named after the Roman God Saturnus. The rings are made from icy particles and dust, Saturn has around 60 moons as well as the only moon with a stable atmosphere, Titan.
Saturn has a small core made from ice and rock with the rest of the planet being chiefly hydrogen and helium. There are thought to be liquid metallic hydrogen and liquid hydrogen and helium layers surrounding the core before the gaseous outer layers. The inside of Saturn can reach extremely hot temperatures, with the interior temperature of 11,700 degrees C thought to be due to the Kelvin-Helmholtz mechanism of gravitational compression. The atmosphere of Saturn is not as strikingly active as the one on Jupiter, but it does display some of the same characteristics. The gases have a banded appearance and there is evidence of cloud layers and storms.
The rings of Saturn initially were a great source of confusion for early astronomers, lacking the high-powered telescopes to identify them. Galileo thought that Saturn was three bodies, but was lost for an explanation as to why the outer two would disappear only to reappear again later. It took until 1655 for Christiaan Huygens to correctly identify Saturn’s rings. The rings are mainly composed of water ice and they extend from 6,630km to 120,700km from the surface. The origin of the rings is not known for certain, with theories that they are the remnants of a moon ripped apart by tidal forces or a leftover material from Saturn’s formation both being considered.
Saturn has been visited by probes during the 80s and more recently in 2004. In 1980 and 1981 the Voyager probes, 1 and 2, both performed fly-bys of the gas giant on their way out of the Solar System. They provided high quality images of the planet and its rings. Voyager 1 flew close to Titan, obtaining more data about the atmosphere on the moon. The main probe to gather data about Saturn and its satellites was the Cassini-Huygens probe which reached the planet in July 2004. The probe gathered a lot of data on Titan including details of its lakes, coastlines and other geographical features. The craft then released the Huygens lander which became the first craft to land on a moon other than our own. Cassini would also go on to discover liquid water erupting from geysers on the surface of Enceladus, another of Saturn’s moons.
Titan dominates Saturn’s moons, making up 96% of their mass. Titan is unique among the moons of the Solar System as it has an extremely thick atmosphere, not dissimilar to the one found on Venus. The atmosphere of Titan, however, has a reverse effect of Venus as it has an anti-greenhouse effect which cools the surface where Venus’s atmosphere has heated the surface to scorching effect. The Cassini-Huygens probe discovered the liquid methane lakes on Titan, the first stable bodies of liquid found on any extra-terrestrial body. There are weather systems similar to Earth in effect on Titan with rain and high winds all present. The average surface temperature is between -179 and -290 degrees C, with atmospheric pressure higher than Earth and much lower gravity. Despite its low temperatures Titan is compared to early Earth with its high amount of organic chemical activity. The moon is thought to be one of the most likely places in the Solar System to find alien life.
The nature of gas giants means that they don’t lend themselves well for direct colonisation or human visits. However in Saturn’s case it may prove that Titan is an excellent place for a future colony if the low temperature and high pressures can be overcome. Any human landing on the planet-facing side of the moon would also be treated to some of the most spectacular views in the Solar System, with Saturn and its rings taking up a large portion of the sky.
Jupiter is the largest planet in the Solar System and is the innermost of the gas giants. Named after the Roman King of the Gods the planet is located beyond the asteroid belt. Jupiter orbits between 740 million and 778 million km from the Sun and is primarily composed of hydrogen and helium. It is the fourth brightest object in the sky behind the Moon, Sun and Venus and has an extremely fast rotation period, completing one every ten hours. The planet takes eleven years to complete an orbit of the Sun.
Jupiter is quite unlike Earth and its neighbours as it is mostly composed of gases rather than heavier elements. Jupiter is two and a half times more massive than all the other planets put together and is about as large as a planet of its type can grow. If it were to take on more mass the diameter would actually decrease from the increased gravity. If it were 75 times larger then it would be able to fuse hydrogen and become a star, this has lead some astronomers to label the planet as a failed star. However despite its humble status as a planet Jupiter does create a substantial amount of its own heat, generating more than it receives from the Sun.
The atmosphere of Jupiter is made of many distinctive bands of gas and light elements. Different areas of the planet have been shown to rotate at different speeds, and there are many storms present. The most notable storm on Jupiter is the Great Red Spot. This giant anomaly on the surface of the planet is larger than Earth and is a storm that has been raging ever since we first discovered the planet. Jupiter is peppered with similar storms and recently three large grey ones merged into one storm, similar to the red spot.
Jupiter has only been explored by automatic deep-space probes, there is no surface to land on and there have been no landings on any of its moons yet. The first probe to fly-by Jupiter was the Pioneer 10 in 1973. Pioneers 10 and 11 would gather a multitude of images and date about the planet, detecting the magnetic field and radiation belts. In 1979 Voyagers 1 and 2 passed Jupiter followed by Ulysses in 1992. The only probe to orbit Jupiter is the Galileo spacecraft, which reached the gas giant in 1995. The craft stayed in orbit for seven years, releasing an atmospheric probe in July 1995. The probe descended into Jupiter and sent back data for 57 minutes before it was crushed by the pressure. Eventually Galileo was sent into the planet in order to prevent it crashing into and contaminating any of Jupiter’s moons.
Jupiter has 63 moons, some of which are among the largest objects in the Solar System. The four largest are Io, Callisto, Ganymede and Europa, known as the Galilean moons after their discoverer Galileo. The moons of Jupiter are among the most interesting bodies in the Solar System with some striking geological activity and possible sites for life. The moon Europa may be the site of a liquid ocean underneath its icy crust and is a place that scientists are extremely interested in exploring. The moons of Jupiter are not as cold and lifeless as they may be otherwise due to tidal flexing providing friction on the inside of the planet and keeping them active. Io is the most geologically active body in the Solar System with over 400 volcanoes, Ganymede is the largest non-planet and Callisto is considered a possible site for extra-terrestrial microbial life.
Jupiter is not as openly considered for colonisation as our nearby planets Mars and Venus. The giant planet does however offer an inviting location for a stepping stone into the outer Solar System. Its many moons could make good places for colonies and may even provide economic benefits through mining. The future will see further missions to the moons to look for signs of life and liquid oceans.
Mars is the fourth planet from the Sun and is named after the Roman God of war. The planet is a distinctive shade of red and is the last of the rocky terrestrial planets. Mars is substantially smaller than Earth and Venus but larger than Mercury. The planet has been the subject of intense speculation about the existence of extra terrestrial life with evidence of water on the surface at the polar ice caps. While recent robotic explorations have shown that the presence of life is unlikely it has still not been conclusively ruled out.
Mars has a thin atmosphere, much thinner than Earth’s. It is kept this way by the solar winds stripping atoms from the top layer, Mars’s atmosphere lacks a magnetosphere to protect it. The surface pressure of the Martian atmosphere is less than one percent of that found on Earth. Despite the sparseness of the atmosphere however, it extends 5km higher than Earth’s due to lower gravity. Mars has a relatively low mass and the surface gravity is just 38 percent of Earth’s.
Mars is home to the largest mountain in the Solar System, Olympus Mons. It is three times higher than Mount Everest and the result of substantial volcanic activity with shallow slopes covering a massive area. Mars is also home to many other interesting geographical features such as canyons and valleys. Many of these features are attributed to running water, although it has been proved to no longer exist it is thought that at one time Mars may have had rivers on its surface. The Phoenix lander has found ice under the surface of the planet and Mars is the most water-rich location outside Earth that we have found so far.
Mars has two small moons, Phobos and Demios, both with irregular shapes. They are thought to be captured asteroids and the orbit extremely close to the planet. It is not fully understood how the moons have come to orbit Mars, however it is believed that Phobos is a relatively recent capture as it follows an unstable orbit and will collide with the Red Planet in around 50 million years.
Mars has been extensively explored by robotic spacecraft and probes by the USA, Russia, Europe and Japan. Probes began exploring Mars even before Man had landed on the Moon with the first flyby occurring in 1964. The Soviet were the first to successfully land objects on the planet, but they lost contact soon after arrival. In 1976 the NASA probes Viking 1 and 2 made it to the surface of Mars, spending several years there. They provided many images and helped to map the surface. The most recent probe to land on Mars is the Phoenix Lander. It arrived in May 2008 and began investigating the Martian soil, finding conclusive evidence of water ice. Phoenix landed much closer to the pole than any other spacecraft.
There are many future missions planned to Mars and in 2004 President George W. Bush announced that NASA’s vision for space exploration would be to launch a manned mission to the planet. NASA administrators believe that they will have successfully landed a man on Mars by 2037. Mars has been the subject of serious talk of eventual colonisation, as it is seen as the most suitable for life and the most habitable environment in the Solar System outside Earth. Mars has many of the elements needed for life present in its soil and with a carbon dioxide-rich atmosphere it is thought that with some terraforming that algae may be able to survive at the poles. Terraforming Mars is an area that is being looked into as a future destination for civilisation once the increasing heat of the Sun makes Earth uninhabitable.
Mars is a charismatic planet, inspiring countless works of fiction and a massive amount of speculation about the secrets it may hold. It has been a characteristic of our space exploration that many of the early romantic ideas of conditions on the surface have been dispelled and the planet has been found to be lifeless and largely barren. Martian thinking is now turning towards finding out its past and planning for its future.
Earth is unique among all known planets, ones native to our Solar System or otherwise. It is the only planet that has liquid water on the surface and is also the only world containing life. Earth is the third planet out from the Sun and is the densest and largest of the four rocky terrestrial planets.
Formed at the same time as the rest of the Solar System the Earth would accrue enough mass to maintain an atmosphere, composed chiefly of nitrogen and carbon dioxide. It is believed that the Earth collided with another planetesimal early in its history, the remains of this collision would form the Moon. Earth’s moon is its only natural satellite with its gravitational effects having great effects on the planet. The Earth was also just the right distance from the Sun for liquid water to form on the surface, filling the oceans and eventually covering 71% of the planet’s surface.
Half a billion years after the formation of the planet the first self-replicating molecules were formed, and by a process of natural selection would go on to evolve into all life as we know it. The impact of life has been significant on the Earth, the oxygen in the atmosphere and the ozone layer are attributed to early plant life. The ozone layer protects the surface from many harmful rays from the Sun and has allowed colonisation of the land by multicellular organisms. Life would go on to evolve intelligence and humans. We are still the only known life in the universe, and most likely the only intelligent forms of life within millions of light years.
The Earth is almost a perfect sphere, with a slight bulge at the equator due to its rotation. The planet is mostly composed of iron, oxygen, silicon, magnesium, sulfur, nickel, calcium and aluminum. The planet is formed from a mostly iron core, a molten mantle and a thin rocky crust, much like the other rocky planets. The surface of Earth is split into several continental plates which all move around on the surface. This geological activity gives rise to earthquakes, mountains, tsunamis and volcanoes.
Similar to Mars, the Earth has polar caps with ice at the Northern and Southern tips. The Earth has a strong magnetic field, which deflects much of the solar wind and protects the Earth from the harmful radiation. The Earth is tilted on its axis, meaning that some areas are closer to the Sun that others during orbit. This tilt gives rise the seasons.
The human population of Earth is expanding at an ever increasing rate and many concerns are being raised about the treatment of the planet and how long it will be until the finite resources are consumed. A lot of human activity is now going into finding alternative power sources and environmentally friendly methods of industry. The evolution of humans has been the first time in over four billion years that one species has had such a negative impact on the well-being of the rest of the planet.
Named after the Roman Goddess of love, the planet Venus is the second furthest from the Sun. It has been referred to as Earth’s twin planet due to its remarkably similar size. Venus is also a rocky planet with a gaseous atmosphere, however it differs from Earth in a number of crucial ways.
Venus is the closest planet to Earth and the brightest object in the night sky, with the exception of the moon, and has been referred to as both the Morning and Evening Star due to its luminosity. Venus is most striking for its thick atmosphere which covers the entire planet and obscures the view of the surface from space and Earth. The hidden surface of the planet was the subject of much curiosity and speculation before the myths were dispelled by probes and exploratory spacecraft. Science fiction writers in particular pondered what what be on the surface of Venus, speculating about possible life on the surface.
However, numerous probes sent to the planet would remove any lingering doubt over the possibility of life on Venus. The surface temperature averages at around 460 degrees C with the atmospheric pressure 92 times that of Earth’s, the same as that found 1km under the oceans. The extremely high temperature is caused by a runaway greenhouse effect. Venus’s atmosphere is predominantly composed of carbon dioxide with clouds of sulphur dioxide. Heat from the Sun becomes trapped and the planet gets warmer and warmer, to the point it is at today.
At its closest Venus passes 41.8 million km away from Earth, takes 225 Earth days to orbit the Sun and 243 days to complete one rotation. Venus’s orbit is the closest of any planet to being circular, with an eccentricity of less that 0.01. The extremely slow rotation of Venus makes it stand out from the other planets, taking much longer to complete. Venus also spins in the opposite direction to Earth. Venus has no moons.
Venus has been the subject of a lot of the initial ideas of colonisation by Man of the Solar System. The discovery of the conditions on the surface seemed to all but rule out any human settlement on the surface, the extreme temperature and pressure would make life impossible. There has been some talk, however, of possible colonisation of the Venetian atmosphere. 50km above the surface the conditions are much more favourable and Earth-like. Floating cities filled with human-breathable air would float in the dense atmosphere, at that height the atmospheric pressure is the same as that of Earth and temperatures range from 0-50 degrees. A different approach could be to artificially cool Venus using giant mirrors or manipulating dust clouds. In just a few decades the planet would cool significantly enough to give conditions similar to Earth on the surface.
Venus is an interesting planet given its similar properties to Earth. It is also a living example of what a runaway greenhouse effect can do to a planet. The excessive CO2 content in the atmosphere has massively heated the planet beyond what it would be otherwise, a stark reminder of what could eventually happen to Earth.
Mercury is the closest planet to the Sun and the smallest of the rocky terrestrial planets in our solar system. Named after the Roman God of commerce, travel and thievery the planet is a harsh world of extreme temperatures about which relatively little is known. Mercury orbits the Sun at a distance of 46-70 million km and passes within 77.3 million km of the Earth at its closest pass. It takes the small planet just 88 Earth days to complete an orbit round the Sun and 58 days to complete one rotation, a very slow rotation compared with the other planets, only Venus is slower.
Mercury’s structure is typical of that of a terrestrial planet, containing a core, a mantle and then a rocky outer crust. Mercury is the second densest planet in the Solar System, right behind Earth. Mercury’s core is larger in comparison to the diminutive size of the planet giving it the high density. It is believed that at some point in its history Mercury was struck by a planetesimal, stripping away much of the mantle and crust and leaving the planet with a relatively large core.
The surface of Mercury is unforgiving and harsh. It is quite similar to that of the Moon, pockmarked with craters and the remains of meteorite impacts. With no erosion to wear them down the craters last for millions of years. In sunlight temperatures can reach 450 degrees C and drop as low as -170 degrees C at night. The huge fluctuations are due to Mercury’s distance to the Sun and its lack of a real atmosphere. The gravity on the planet is not strong enough to retain a permanent atmosphere and so much of it is lost to the solar winds. There is evidence of ice on Mercury, despite its extreme heat. There are deep craters at the poles of the planet which never see sunlight and so are permanently cold enough for layers of ice to form.
Exploration of Mercury has been slow compared to the other planets. The planet is the hardest to reach by spacecraft as the speed required is extremely high, given its closeness to the Sun. This is combined with the fact that any spacecraft looking to orbit Mercury would be acted upon by the Sun’s gravity. Probes must use an excessive amount of fuel to brake so as not to overshoot Mercury. Landing on the planet is also tricky because of the lack of atmosphere meaning that parachutes and aerobraking are unlikely options. Mercury is not considered a strong candidate for terraforming and colonisation either because of the extreme temperature fluctuations. Mars and Venus as well as some of the gas giant’s moons are seen as more likely options.
Despite the relative difficulties of sending probes to Mercury there have been some observations made from its orbit. The Mariner 10 probe was the first to arrive in March 29th 1974. Mariner 10 did not orbit Mercury, but the Sun instead and used Venus’s gravity to slingshot a path to Mercury. Mariner 10 would fly by Mercury twice more in 1975, photographing 45 percent of the surface. The mission led to the discovery of the Earth-like magnetic field and the surface details of the planet, showing extensive amounts of meteorite impact. The second mission to Mercury, MESSENGER, is currently en-route and will enter orbit in 2011.
